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Coordination Chemistry Reviews 257 (2013) 1728– 1763

Contents lists available at SciVerse ScienceDirect

Coordination Chemistry Reviews

jo u r n al hom ep age: www.elsev ier .com/ locate /ccr

eview

ecent advances in dysprosium-based single molecule magnets: Structuralverview and synthetic strategies

eng Zhanga,b, Yun-Nan Guoa,b, Jinkui Tanga,∗

State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, PR ChinaUniversity of Chinese Academy of Sciences, Beijing 100039, PR China

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17291.1. Dysprosium (DyIII) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17291.2. Single ion anisotropy of dysprosium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1730

1.2.1. DyIII-DOTA and [DyIII(hfac)3-(NIT-R)2] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17312. Single ion magnet (SIM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1731

2.1. The SIM with square antiprism environment (SAP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17322.1.1. [DyPc2] series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17322.1.2. Dy-�-diketones series . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1733

2.2. Organometallic and low-symmetric SIMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17342.3. The SIMs with equatorial LF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17352.4. Magnetic dilution in mono-DyIII compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1736

3. Dy2 systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17363.1. Radical-bridged coupling systems: N2

3−–Dy2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17363.2. Typical Dy2 complexes with ferromagnetic coupling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1736

3.2.1. Asymmetric Dy2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17363.2.2. Ferromagnetic [Tb2Pc2obPc] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1737

3.3. Typical Dy2 complexes with antiferromagnetic coupling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17383.3.1. The Dy2 with a salen-type ligand. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17383.3.2. Planar DyIII

2CoIII2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1739

3.4. Other Dy2 complexes with different bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17393.4.1. Dy2 bridged by R-COO− . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17393.4.2. Dy2 complexes based on non-oxygen-bridging ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1740

4. Synthetic strategies for multinuclear Dy-based SMMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17414.1. Ligand design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1743
4.2. Molecular assembly based on special bridging ligand (Fig. 28) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1743
2−
4.2.1. Oxalate/CO3 bridged Dyn (n = 2–6) complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1743 4.2.2. Dy complexes based on R-COO−/R-O− bridging ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1744
4.3. Molecular assembly based on o-vanillin Schiff base ligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17474.3.1. Dy2 complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17474.3.2. Dy3 complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17494.3.3. Planar Dy4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1750

Abbreviations: SMM, single molecule magnet; QTM, quantum tunneling of magnetization; TM-SMM, transition metal single molecule magnet; SCM, sin-le chain magnet; MRI, magnetic resonance imaging; MS, effect of the magnetostriction; [DyPc2], phthalocyanine double-decker Dy complexes; DyIII-DOTA,Na{Dy(DOTA)(H2O)}]·4H2O; H4DOTA, 1,4,7,10-tetraazacyclododecane N,N′ ,N′′ ,N′′′-tetraacetic acid; EPR, Electron Paramagnetic Resonance; hfac− , hexafluoroacetylaceto-ate; NIT-R, 2-R-4,4,5,5-tetramethylimidazolidine-3-oxide-1-oxyl; SIM, single ion magnet; LnPOM, lanthanide complex with polyoxometalates; SAP, square antiprism;F, ligand field; Pc, unsubstituted phthalocyaninate; Pc(�-OC5H11)4, 1,8,15,22-tetrakis(3-pentyloxy)phthalocyaninate; TClPP, meso-tetrakis-(4-chlorophenyl)porphyrinate;cac, acetylacetonate; ac, alternating-current; dc, direct-current; TTA, 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedionate; Cp*, C5Me5

− (pentamethylcyclopentadienide);OT, C8H8

2− (cyclooctatetraenide); HPz, pyrazole; obPc, dianion of 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine; T(p-OMe)PP, tetra-p-methoxyphenylporphyrinato;2valdien, N1,N3-bis(3-methoxysalicylidene)diethylenetriamine; 3-H2tzba, 3-(1H-tetrazol-5-yl)benzoic acid; Acc, 1-amino-cyclohexanecaboxylic acid; btaH, 1H-,2,3-benzotriazole; NH2pmMe2, 2-amino-4,6-dimethylpyrimidine; Cp′ , �5-C5H4Me; 2,2-bptH, 3,5-bis(pyridin-2-yl)-1,2,4-triazole; H2bmh, 1,2-bis(2-hydroxy-3-ethoxybenzylidene)hydrazone; Hmsh, 3-methoxysalicylaldehyde hydrazone; HSAB, Hard and Soft Acids and Bases; H3Bpz, hydrotris(pyrazolyl)-borate; 3-bpp,

,6-di(pyrazole-3-yl)pyridine; Chp, 6-chloro-2-hydroxypyridine; py, pyridine; mdeaH2, N-methyl-diethanolamine; Ph2acac, dibenzoylmethanide; TBP, trigonal bipyramidal;2ovph, o-vanillin picolinoylhydrazone; H2hpch, 2-hydroxylbenzaldehyde (pyridine-4-carbonyl) hydrazone; H4TBSOC, p-tert-butylsulfonylcalix[4]arene; TBC, [4]p-tert-utylcalix[4]arene.∗ Corresponding author. Tel.: +86 431 85262878; fax: +86 431 85262878.

E-mail address: [email protected] (J. Tang).

010-8545/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.ccr.2013.01.012

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P. Zhang et al. / Coordination Chemistry Reviews 257 (2013) 1728– 1763 1729

4.4. The isolation of Dy-SMMs based on hydrazone ligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17504.4.1. Monohydrazone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17504.4.2. Dihydrazone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1753

4.5. Molecular assembly from macrocyclic ligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17554.6. DyIII system supported by calix[4]arene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1755

5. Dyn systems with higher nuclearity (n > 5) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17575.1. Dy systems from Dy3 triangle building blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17575.2. Dy systems from Dy4 cubane building blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17585.3. Dy systems from Dy5 pyramid building blocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17595.4. Dy systems with cyclic structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1759

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1760Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1760Appendix A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1760References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1762

r t i c l e i n f o

rticle history:eceived 24 September 2012ccepted 8 January 2013vailable online 25 January 2013

eywords:ysprosiumingle molecule magnetynthetic strategiesnisotropyagnetic interaction

a b s t r a c t

The last few years have seen a huge renaissance in the study of the magnetism of lanthanide coordinationcomplexes, especially in the field of single molecule magnets (SMMs) due to the large inherent anisotropyof lanthanide metal ions. It has led to intense activity on the part of synthetic chemists to producesystems suitable for detailed study by physicists and materials scientists, thus synthetic development hasbeen playing a major role in the advancement of this field. In this review, we demonstrate the researchdeveloped in the few years in the fascinating and challenging field of Dy-based SMMs with particularfocus on how recent studies tend to address the issue of relaxation dynamics in these systems fromsynthetic point of view. In addition, the assembly of multinuclear Dy SMMs using various ligands issummarized, showing that several typical motifs are favorable structural units which could be exploitedin the formation of new Dy-based SMMs and supramolecular architectures.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

The interest in nanoscale magnetic materials has been driveny the rapid growth in high-speed computers and high-densityagnetic storage devices with the promise of a revolution in infor-ation technology [1,2]. The discovery of single molecule magnet

SMM), where slow relaxation and quantum tunneling of the mag-etization result from a molecular-based blocking anisotropy [3], isecognized as an important breakthrough in the field of nanomag-etism [4]. It opens up a popular avenue to nanoscale electronicevices, sensors and high-density data storage media at the molec-lar level (the ultimate size limit) [5–7]. Further, SMMs providenique opportunities to observe quantum effects (quantum tun-eling of magnetization (QTM) and quantum phase interference)8–11], because they straddle the interface between classical anduantum mechanical behavior and all magnetic particles, basedn tailor-made molecules, are identical and monodisperse [11].agnetic properties of SMMs depend strongly on their intrin-

ic magnetic characteristics, such as spin ground state (S) andagneto-crystalline anisotropy (D), which lead to a spin-reversal

arrier (Ueff) for the slow relaxation of magnetization, Ueff = |D|S2

nd |D|(S2 − 1/4) for a transition metal single molecule magnet (TM-MM) with integer and half integer spins, respectively [3,8]. D muste negative in order to give rise to this spin-reversal barrier. Aegative D is indicative of Ising type magnetic anisotropy.

The initial study was just confined to the realm of coordinationomplexes based on 3d metals, including SMMs (the first wavef SMMs, Scheme 1) [3,12–19] and single chain magnets (SCMs)20–22]. In particular, the discoveries of CoII-organic radical and

nIII–NiII chains in 2001 and 2002 provide the experimentalonfirmation of Glauber’s prediction for SCMs [23]. However,ecently particular emphasis has been placed on the design of newMMs applying 4f metal ions [24,25], as a result of their significantagnetic anisotropy arising from the large, unquenched orbital

SMMs [5,26–29]. Herein, the DyIII ion seems to be especially usefulin this respect. DyIII-radical family (4f–2p) was considered and usedover the last years as a bench for understanding the magnetismof the lanthanide ions and has given rise to many groundbreakingresults in SMMs [5,30] and SCMs in recent years. Owing to thestrong Ising type anisotropy of DyIII ion, DyIII-radical chains arevery appealing candidates for constructing SCMs [31,32]. Researchinto heterometallic dysprosium complexes has also led to a floodof intriguing SMMs, including Cr–DyIII [33], Mn–DyIII [34–38],Fe–DyIII [39–41], Co–DyIII [42–44], Ni–DyIII [45–47] and Cu–DyIII

[48–50], even a 5d–DyIII complex [51].Since a triangular Dy3 cluster with toroidal arrangement of

magnetic moments on the dysprosium sites (spin chirality on theground states) shows SMM behavior of thermally excited spinstates [27,52,53], pure DyIII-based SMMs with different topologieshave really been the center of attention for chemical, physi-cal, and material scientists and yielded a flood of remarkableresults [26,54,55] such as the highest relaxation energy barriersfor multinuclear clusters (Ueff = 528 K for Dy5) [29] and the highesttemperature at which hysteresis has been observed for any singlemolecule magnet (T = 8.3 and 14 K for Dy2 and Tb2, respectively)[5,56]. Our recent efforts have been considerably dedicated to thisfield [28,57–59]. Herein, we provided a critical discussion on themost up-to-date achievements associated with DyIII-based SMMs,regarding the synthesis, structural motifs (Scheme 2) and magneticstudies of Dy single ion magnet to complicated Dy26 cluster, with anemphasis focusing on the synthetic strategies to DyIII-based com-pounds with the aim to shed light on the design of to DyIII-basedcompounds with specific magnetic properties.

1.1. Dysprosium (DyIII)

The applications of dysprosium to some advanced materials,such as MRI (magnetic resonance imaging), MS (effect of the

ngular momentum. As shown in Scheme 1, the discovery thatanthanide mononuclear complexes can exhibit slow relaxation ofhe magnetization has initiated intensive interest in the SMMs con-aining lanthanide metals (4f/3d–4f), leading to the second wave of

magnetostriction), and SMMs, suggest its great potential in thefield of magnetism [1,60–62]. In MRI, DyIII-based complexes areconsidered to be ideal negative contrast agents at high magneticfields compared with GdIII-based complexes [63]. In MS, metal

1730 P. Zhang et al. / Coordination Chemistry Reviews 257 (2013) 1728– 1763

Scheme 1. Time axis of the development of SMMs. 1. Mn12 (the first SMM) in Ref. [3]; 2. Mn4 (the second type of SMM) in Ref. [12]; 3. Fe8 SMM in Ref. [14]; 4. Mn84 (thelargest Mn cluster) in Ref. [15]; 5. Mn19 (the largest ground state. In conventional SMMs the dipolar coupling energy is much smaller than the anisotropy energy while inMn19 both energies are probably of the same order of magnitude, thus its behavior is dramatically influenced by the intermolecular dipolar interactions) in Ref. [16]; 6. Mn6

( ononu[ tivly).

dlrttpo[Tctbtwm

c(lm±TcKrKd

the largest barrier for transion metal SMM, TM-SMM) in Ref. [17]; 7. Fe (the first m19]. The Ln-based SMMs will be introduced below (9–14 in Refs. [25–29,5], respec

ysprosium has a huge saturation MS �s value about 100 timesarger than for Fe, Ni and Co [62]. Especially for SMMs, DyIII ionepresents a very ideal ion, because, as a number of the late lan-hanides (4fn, n > 7), it can provide stronger magnetic anisotropyhan the early lanthanides (n < 7) despite the weak exchange cou-ling [64,65]. In particular, the DyIII ion possesses an odd numberf electrons (n = 9), thus insuring the Kramers doublet ground state66], a critical factor in the presence of typical SMM properties.his has been indicated in a series of mononuclear lanthanideompounds, discovered by Ishikawa, with a trigonal (D3) symme-ry where DyIII, ErIII, and YbIII complexes show field-induced SMMehavior, while TbIII, HoIII and TmIII complexes are non-SMMs dueo fast quantum tunneling [67]. All those suggest that dysprosiumill play a crucial role in the exploitation of molecular magneticaterials.The free DyIII ion is characterized by f9 configuration, which, in

ombination with spin–orbit coupling effects, give rise to 6H15/22S+1LJ) multiplets with 16-fold (2J + 1) degeneracy [65]. Neverthe-ess, the surrounding crystal field can remove those degenerate

ultiplets into new sublevel structure characterized by mJ = ±15/2,13/2, ±11/2, ±9/2, ±7/2, ±5/2, ±3/2, ±1/2 (high symmetry).he doubly degenerate ±mJ state cannot be separated by therystal field because of the feature of spin-parity effect for aramers ion (odd numbers of 4f electrons) [64]. This is due to time
eversal symmetry, and the pairs of degenerate levels are calledramers doublets [68]. From the point view of magnetism, theoublets demonstrate the decisive role for the occurrence of SMM
Scheme 2. The basic structural motifs in Dyn complexes (n < 6).

clear TM-SMM) in Ref. [18]; 8. Co (mononuclear TM-SMM under zero field) in Ref.

behavior. Furthermore, a suitable crystal field can place the highermJ states (±15/2 or ±13/2) as the ground state, and make a largeseparation between the ground state with other mJ states, whichdefines the energy required to relax the spin and will further afforda high relaxation barrier [26,69,70]. In result, two underlying condi-tions for SMM behavior, i.e. doubly-degenerate ground states witha high ±mJ value and a large separation between ground states andexcited states, are easy to be achieved for a Dy-based compound[64].

1.2. Single ion anisotropy of dysprosium

Persistent axiality of DyIII ions in lanthanide compoundsis an important feature enabling them functioning as SMMsand responsible for the unusual magnetic behavior such asmultiple relaxation processes and spin chirality due to thenoncollinearity of their single-ion anisotropic axes [28,52,55].For highly symmetric mononuclear systems, such as phthalo-cyanine double-decker Dy complexes ([DyPc2]) [26] with thelocal symmetry of D4d, the unaxial anisotropy (C4) seems tobe clear. In particular, Long and coworkers have developed asimple model to explain and predict the presence of signifi-cant single-ion anisotropy, based on the shape variation of thef-electron charge cloud [64]. However, in polynuclear DyIII sys-tems or DyIII monomers with low-symmetry environments, thedetermination of single-ion anisotropy should be rather tricky,where the simple analysis of anisotropy may be misled by over-simplification associated with the idealized symmetry, as seenin Dy-DOTA (1, [Na{Dy(DOTA)(H2O)}]·4H2O, H4DOTA = 1,4,7,10-tetraazacyclododecane N,N′,N′′,N′′′-tetraacetic acid) [71]. There-fore, complicated accurate measurements should be taken intoconsideration for a deep insight into the magnetism of DyIII ion.The Electron Paramagnetic Resonance (EPR) technique suitablefor transition-metal ions is not available for characterization ofits magnetic anisotropy, because anisotropic rare earths presentvery fast electronic relaxation that broadens the EPR signal, ham-pering a precise determination of the g tensor [30]. Recently,post-Hartree–Fock ab initio calculations have proved to be aninvaluable method for predicting the magnetic anisotropy ofDyIII ions in some multinuclear DyIII systems (triangular Dy3and planar Dy4). Besides energies of the multiplets, directions ofthe anisotropy axes and the g tensors for the lowest Kramersdoublets of each dysprosium site can be obtained, providing a
theoretical prediction of the single-ion anisotropy [52,55]. Toexplore the nature of single dysprosium anisotropy in low-symmetry environments, Sessoli and coworkers initiated thesingle-crystal magnetic measurements in molecular magnetism,

P. Zhang et al. / Coordination Chemistry R

Fig. 1. The crystal structure of Dy-DOTA (1, the violet rod represents the orientationof the experimental easy axis of the magnetization, while the pale blue rod repre-sents the calculated one, left); angular dependence of the magnetic susceptibility ofD

R

wi[RfMMbs

1

ctsbm(

riscwsiiebtut

Fr

R

yIII-DOTA.

eprinted with permission from Ref. [71]. Copyright (2012) Wiley-VCH.

hich straightforwardly confirmed the presence of local anisotropyn mononuclear 4f systems, such as Dy-DOTA (1) [71] andDy(hfac)3-(NIT-R)2] (2, hfac− = hexafluoroacetylacetonate; NIT-

= 2-R-4,4,5,5-tetramethylimidazolidine-3-oxide-1-oxyl) [30]. Inact, single-crystal susceptibility work has been performed by R.

ason, S. Mitra and others before SMMs were known [72,73].oreover, an interesting toroidal arrangement of the spins has

een simultaneously confirmed by single-crystal magnetic mea-urements and ab initio calculations [52].

.2.1. DyIII-DOTA and [DyIII(hfac)3-(NIT-R)2]The coordination of a H2O molecule breaks the square antiprism

oordinated environment in the first coordination sphere, and theetragonal symmetry is also suppressed in a larger coordinationphere due to the coordinated Na+ ion [74]. Thus, simple analysisased on the idealized symmetry of the lanthanide complex can beisleading. Here the triclinic symmetry is present in DyIII-DOTA

1), which is suited for single-crystal magnetic measurements [71].The single-crystal magnetic measurements on Dy-DOTA (1)

eveal the high angular dependence of the magnetization, suggest-ng a strong magnetic anisotropy of DyIII ion. The diagonalization ofusceptibility tensor (�) can provide the principal values of the sus-eptibility and their orientation: g// = 17.0 for an effective Seff = 1/2,hich confirms the Ising anisotropy of the DyIII ion in the low-

ymmetry environment. The orientation of the easy axis (violet rod)s shown in Fig. 1 left. To ensure the accuracy, ab initio calculationsncluding all the atoms of the complex have been performed. Theffective g tensor of the ground-state doublet of the 6H15/2 has also
een estimated at 18.6, approaching the Ising limit. The orienta-ion of anisotropy (pale blue rod) is close (10◦, within experimentalncertainty) to the experimental one, and is almost perpendicularo the idealized symmetry axis. In addition, the calculations were
ig. 2. The crystal structure of [Dy(hfac)3-(NIT-R)2] (2, the yellow rod represents the orepresents the calculated one, left); angular dependence of the magnetic susceptibility of

eprinted with permission from Ref. [30]. Copyright (2009) American Chemical Society.

eviews 257 (2013) 1728– 1763 1731

performed on several reduced models, revealing that the hydro-gen bonds on coordinated water molecule play a key role in theunexpected orientation of the easy axis.

In addition, the single-ion anisotropy of DyIII ion in DyIII-radicalcomplex [Dy(hfac)3-(NIT-R)2] (2) [30] was investigated as early as2009 (Fig. 2). The complex crystallizes in the triclinic space groupwith only one Dy ion located in a distorted square-antiprismaticenvironment. The analytic outcomes of angle-resolved magneticmeasurements show a strong Ising-type anisotropy for the com-plex. The largest g component of ground-state doublet obtainedfrom ab initio calculations is about 18, and its orientation is veryclose (7◦) to the experimental one.

The above examples provide experimental and theoretical evi-dence of the local magnetic anisotropy of the DyIII ion in alow-symmetry environment. It is obvious that the determina-tion of magnetic anisotropy of lanthanide ion in a low-symmetryenvironment is fraught with difficulties and simple magneto-structural correlations based on the coordination environment arenot enough. Detailed information on the type of anisotropy and onthe orientation of the principal axes of the magnetic tensors in themolecular structure can only be obtained by a joint effort, combin-ing single-crystal magnetic studies with ab initio calculations.

2. Single ion magnet (SIM)

In fact, some single anisotropic ions can provide a sufficient con-dition for the establishment of a SMM with thermal barrier forreversing the magnetization and for observing quantum tunnelingeffects, provided there is a suitable ligand field environment, evenin mononuclear actinide or transition metal compounds [18,26,75].Owing to their single-ion features, a family called Single Ion Mag-nets (SIM) has rapidly developed containing 3d-, 4f- and 5f-SIMs[18,26,75–77]. To date, the lanthanide-based SIMs can be viewedas the most extensive class in this family. How to realize an easy andhigh accurate forecast of the performance for a pre-designed Dy-based SIM has been the issue of current study. The investigations onexisting lanthanide-based SIMs provide important implication thatthe coordination environment of the lanthanide ion plays a crucialrole in tuning their SMM property.

In general, the axial crystal fields can promote the unaxialanisotropy of lanthanide ions, as indicated through a simple modeldeveloped by Long et al. For DyIII, TbIII and HoIII with the oblatef-electron charge cloud, a sandwich-type crystal field should bepreferable to maximize the anisotropy of an oblate ion, which hasbeen explained through the examples of [LnPc2] in the perspective
[64]. Besides the most commonly observed square-antiprismaticgeometry in most Dy-based SIMs such as [DyPc2] [26,78] andDy-�-diketones [70], axially enforced ligand field (LF) as foundin organometallic and low-symmetric SIMs is a new issue for
ientation of the experimental easy axis of the magnetization, while the blue rod [Dy(hfac)3-(NIT-R)2].


F Ref. [6s

A

dstIlEfi

2

vSf[fdbltb4[sttIHatLcT

2

d

ment, increasing the splitting between the lowest and the secondlowest sublevels. Interestingly, the effective barrier height Ueffof [{Pc(OEt)8}2Dy]+ (4) increases about twice as large as that of

ig. 3. (a) Details of the relevant structural parameters in a square antiprism in

quare-antiprism polyhedron.

dapted from Ref. [85].

esigning novel Dy-based SIMs recent years [76,79]. Moreover,everal examples with macrocyclic ligands reflect that the equa-orial LF may not favor the design of Dy-based SIMs [80,81].n contrast, such equatorial fields should favor prolate 4f ionsike ErIII and this hypothesis has found some success, with therZn3 complex [80] being a typical example of such equatorialeld-induced SMM reported.

.1. The SIM with square antiprism environment (SAP)

Herein, the classic LF (ligand field) models can provide a lot ofaluable information about applying the anisotropy to constructMMs in spite of its numerous limitations [66]. For example, in theamily of SIMs, the foremost species are the [LnPc2] [26,78], LnPOM82] and Dy-�-diketones [70] series, where eight coordinate atomsorm an approximately square-antiprismatic coordination polyhe-ron with close to D4d symmetry. As discussed in a tutorial reviewy Dante Gatteschi, the determination of electronic structure of

anthanide-based systems depends mainly on two crucial parame-ers of square antiprism (Fig. 3a): (skew angle) and (the angleetween the C4 axis and a RE-L direction); ˚, corresponding to5◦ and 54.74◦, respectively, for a regular square antiprism (SAP)66]. For ˛, the deviation from 54.74◦ stands for the compres-ion/elongation of SAP along the C4 axis (Fig. 3b), which may affecthe energy difference between the two lowest lying states even ifhe changes of ground states do not occur, as in LnPOM and [DyPc2].n contrast, a variation of angle is an important perturbation foramiltonian due to the change of symmetry, where the transversenisotropy is introduced, then promoting the quantum tunneling ofhe magnetization in these systems. The discussions about classicF models based on symmetry should be very helpful for applyingoordination chemistry to design SIMs with high energy barrier.he representative examples will be presented below.

.1.1. [DyPc2] seriesBecause of the high symmetry of [LnPc2] with a double-

ecker structure, some [TbPc2] complexes have shown better SIM

6], Copyright 2011 Royal Society of Chemistry; (b) Longitudinal contraction of a

behavior compared with [DyPc2] complexes. Therefore, it isnecessary to introduce the relationship between structures andproperties for [TbPc2] complexes. For TbIII ion, as it is a non-Kramers ion, thus only a high symmetry can guarantee a bistableground state requisite for SMMs [64].

The structure of the phthalocyanine complex is shown inScheme 3. The high-order axial coordination field promotes the sin-gle ion anisotropy of lanthanide ions, leading to an easy axis of themagnetization. The relationship between magnetic properties withthe 4f-electronic structures has been reviewed before [83].

[{Pc(OEt)8}2Tb]+ (3) and [{Pc(OEt)8}2Dy]+ (4) were subse-quently reported in 2007 and 2008, resulting from the significantincrease of the barrier energy for magnetization reversal of [LnPc2]species by a longitudinal contraction of the coordination space,as seen in Fig. 3b [84,85]. Here a redox reaction on the ligandside was performed, leading to a longitudinal contraction of thesquare-antiprism coordination polyhedron (the increase of ˛). Thelongitudinal contraction strengthens the interaction between thesingle lanthanide ion electron density and the crystal field environ-

Scheme 3. Schematic diagram of the [Pc2Ln] (left) and [Ln2Pc2obPc].


F -decker complexes (6 and 7, left, Ref. [86]), mixed phthalocyanine/porphyrin Tb triple-d Crystallographic data available in original reference.

[8

bsoddaD(a7OTaglmattt

atoTs

2

[sD

icimsbttpslaidats

Fig. 5. The relaxation time (�) of the Dy3+ complex at different concentrations in[Y(acac) (H O) ] in the temperature range 2–12 K (�, undiluted sample; ©,1:20

anisotropy barriers of 136 and 187 K with the large auxiliary groups.Both activated barriers have been among the highest effectiveenergy barriers for reported DyIII-containing SIMs (8 and 11). That

ig. 4. The crystal structure of mixed (phthalocyaninato)(porphyrinato) Dy doubleecker complexes (middle, Ref. [88]) and [Tb2(obPc)3] compound (right, Ref. [89]).

{Pc(OEt)8}2Dy]− (5), while the increase of the Ueff value was only% for the isostructural Tb complexes.

For a non-Kramers ion TbIII, the degeneracy of the dou-let mJ = ±6 can be removed easily upon breaking down theymmetry of D4d, resulting in the fast quantum tunnelingf magnetization. In contrast, for DyIII ion the situation isistinct. Recently, mixed (phthalocyaninato)(porphyrinato) DyIII

ouble-decker complexes (Fig. 4 left) were synthesized by Jiangnd coworkers [86]. The three complexes, Dy(Pc)(TClPP) (6),y{Pc(�-OC5H11)4}(TClPP) (7a) and DyH{Pc(�-OC5H11)4}(TClPP)

7b), show the slow relaxation behavior in the similar temper-ture ranges, but much stronger QTM were observed in 7a andb than that in 6 [Pc = unsubstituted phthalocyaninate, Pc(�-C5H11)4 = 1,8,15,22-tetrakis(3-pentyloxy)phthalocyaninate, andClPP = meso-tetrakis-(4-chlorophenyl)porphyrinate]. This may bescribed to the fact that the introductions of four 3-pentyloxyroups onto the phthalocyanie non-peripheral positions lead to thearger deviation from the ideal square antiprism molecular sym-

etry associated with the larger deviation of twist angle (˚) in 7and 7b than that in 6, which give the first direct evidence to theheoretical inference about the perturbation of the twist angle onhe fourfold axis symmetry ligand field and in turn the quantumunneling of corresponding SIMs [66,86].

In addition, the triple-decker Pc complexes with the f–f inter-ction have been investigated by Ishikawa and coworkers, wherehe f–f interaction makes a great difference to suppress the effectf the quantum tunneling of magnetization [87–89] (Fig. 4 right).he detailed investigation for the dynamical magnetism of such aystem will be discussed below.

.1.2. Dy-ˇ-diketones seriesIn 2010, the magnetic study of a neutral mononuclear

Dy(acac)3(H2O)2] (8, acac = acetylacetonate) complex was pre-ented by Gao et al., which initiated intensive interest in SIMs ofy-�-diketones series [70].

As seen in Fig. 5 inset, the eight oxygen atoms form an approx-mately square-antiprismatic coordination polyhedron, where therystal field with the nearly D4d local symmetry promotes the singleon anisotropy of DyIII ions. The alternating-current (ac) magnetic

easurement indicates the complex behaves as a typical SMM inpite of the quantum tunneling under low temperature. By com-ining both dilution and a direct-current (dc) field, the quantumunneling effect has been effectively suppressed in the low-emperature range. The calculation using the package CONDONresents the fact that the DyIII complex has an Ising-type groundtate, with the quantum number |mJ| of the ground state (13/2)arger than that of the first excited state (11/2). The ligand fieldnalysis via CONDON is limited to some extent due to its approx-mations, compared with ab initio calculations (MOLCAS/CASSCF)
eveloped by Chibotaru and coworkers [90,91]. In fact, the square-ntiprismatic symmetry around DyIII ion is considerably distorted,hus the direction of anisotropy axis will not lie along anticipatedymmetry direction. Liviu Chibotaru et al. have moved forward the
3 2 2

dilution; �, 1:50 dilution). Inset: the crystal structure of [Dy(acac)3(H2O)2] complex.

Adapted from Ref. [70].

calculations of anisotropy directions, energy levels, g values and,importantly, exchange coupling values for Dy clusters in a mostrigorous manner. Traditionally it is difficult to separate exchangecoupling contributions from ligand-field/Zeeman level thermaldepopulation effects in orbitally degenerate Ln systems, such asDyIII.

Recently, the group of Liao and our group further advanced thestudies on [Dy(acac)3(LA)] (LA = auxiliary ligands) system [92,93].Compared with [DyPc2] complexes, it seems to be easy to alterthe coordination geometry through the replacement of the aux-iliary ligands, thus making it a promising system to probe thedynamics of magnetization. New SIMs (9–11) with approximatelysquare-antiprismatic symmetry were isolated through replacingtwo coordinated H2O with the phen and its large aromatic deriva-tives, as seen in Fig. 6. Complexes 9 and 10 show much higher

Fig. 6. Structures of the �-diketone-based DyIII complexes. Adapted from Refs.[92,93].


Fig. 7. Structures of the TTA-based DyIII complexes. Crystallographic data availablei

mtaalar

1hfissttsSbwa

b[dmal

tem (15), exhibiting multiple relaxation pathways arising from asingle metal center [77]. The molecular structure consists of two

n Ref. [94].

ay be due to the fact that the use of large aromatic groups leadso the alteration of angle, thus enhancing the SAP ligand fieldround DyIII and resulting in a strong uniaxial magnetic anisotropynd a higher thermal barrier. Herein, the careful choice of auxiliaryigand induces the increase of anisotropy barriers, which represents

promising new route toward generation of record anisotropy bar-iers.

Another Dy-�-diketones system with TTA (TTA = 4,4,4-trifluoro--(2-thienyl)-1,3-butanedionate) and bpy/phen (12 and 13, Fig. 7)as been developed by our group with the goal of identifying

eatures relevant to modulating relaxation dynamics of single-on magnets [94]. The O6N2 environment forms an approximatelyquare-antiprismatic coordination polyhedron (Fig. 7), with theimilar angles of 57.2 and 56.4◦ and remarkable difference inhe skew angle of 39.7 and 42.1◦ for 12 and 13, respectively. Here,he ligand-field analysis through the program CONDON demon-trates a typical Ising-type splitting which is responsible for theMM behavior under zero dc field. But the faster tunneling rate haseen observed in complex 12, which may be naturally associatedith its larger rotation of SAP, leading to much more transverse

nisotropy.Other Dy-�-diketones systems (Fig. 8), have also been exploited,

ut the square-antiprismatic coordination polyhedron is lacking95–97]. The coordinated atoms form distorted square antiprism,odecahedral (DD), or bicapped trigonal prism (TPRS), whereuch more transverse anisotropy is introduced and the single-ion

nisotropy of Dy ion cannot be exploited to the full, leading to theoss of effective energy barriers.

Fig. 8. The coordination geometry of Dy ion in oth

Fig. 9. The crystal structure of two mono-Dy organometallic compounds (a, 14,adapted from Ref. [98]; b, 15, adapted from Ref. [77]). Crystallographic data availablein original reference.

2.2. Organometallic and low-symmetric SIMs

The magnetic investigation of organometallic compounds con-taining f elements is initiated by the relative recent work on actinide(5f) compounds [75]. In 2011, the discovery of an organometal-lic ErIII-based SIM has extended the field into the study oforganometallic lanthanide-based SIM [76]. Subsequently, this kindof SIMs with other lanthanide ions was also discovered, such asthe systems developed by Gao and coworkers (14, Fig. 9a) [98]and Murugesu and coworkers (15, Fig. 9b) [77], respectively. Thefirst system, (Cp*)Ln(COT) (14, Ln = TbIII, DyIII, HoIII, ErIII, TmIII, YIII;Cp* = C5Me5

−; COT = C8H82−) features a lanthanide ion sandwiched

by two unparallel aromatic ligands with a slight tilt angle, an over-riding factor in determining the quantum tunneling decoherencerelaxation time. Research indicates that the bending of the axisinduced by asymmetric intermolecular interactions increases withthe increment of ionic radius of lanthanide ions, thus leading to thefaster quantum tunneling of magnetization combining with otherperturbation sources. Consequently, the ErIII compound behaves asthe best SIM among them, with two high effective barriers Ueff = 323and 197 K. Furthermore, the DyIII and HoIII compounds also exhibitfield-induced SIM behavior.

Murugesu reported the second organometallic lanthanide sys-

COT ligands �8-bound to the central DyIII ion with a Dy–C bonddistance range of 2.6–2.7 A (Fig. 9b). Ac magnetic susceptibility

er Dy-�-diketones systems in Refs. [95–97].


ceptibility data of DyZn complex (16). Adapted from Ref. [79].

dporqtCdt

mFwpsm

wbisitaDbftro�(darr(io

2

scts

(di[n

Fig. 10. The crystal structure and alternating current sus

ata reveal strong frequency-dependent out-of-phase (�′′) and in-hase (�′) signals below 14 K under zero dc field, and a barrier wasbtained to be 18 K. Interestingly, the application of a static fieldesults in the appearance of a second peak in �′′ at much lower fre-uencies, whose intensity increases with the field (100–600 Oe) athe expense of the high frequency peak, as seen in DyIII-DOTA andl-bridged Dy2 (Figs. 1 and 22c). In addition, the anisotropic barrieremonstrates an increase from 18 to 43 K with the dc field from 0o 600 Oe.

Actually, Kajiwara and co-workers had previously proposed aultiple relaxation pathways in a DyZn coordination complex (16,

ig. 10) [79]. However, the multiple relaxations overlapped andere unclear in the ac data in contrast with the results for com-lex 15, which marks compound 15 as the first clear example of aystem showing multiple relaxation pathways arising from a singleetal center in a lanthanide complex.The DyZn complex (16) with low symmetry reported by Kaji-

ara affords one extremely high barrier of 333(6) K, which maye ascribed to the strongly axial coordination environment, lead-

ng to an extremely large LF splitting between two lowest Starkub-levels (>400 K) [79]. Fig. 10 shows its structure with a Dy ionn an axially enforced LF, which should result from the coordina-ion of three phenoxo oxygen atoms with strong negative chargest axial positions. The deduction can be seen from their shortery–O bond lengths (2.189(3)–2.329(3) A) than other metal–oxygenonds (2.406(3)–2.591(3) A). Ac susceptibility data exhibits strongrequency dependence below 40 K under zero dc field, suggestinghe strongly “freezing” of the spins by a high anisotropy bar-iers. A frequency dependent tunneling relaxation process wasbserved below 10 K, and a high energy barrier, Ueff = 333(6) K with0 = 1.1 × 10−9 s, was estimated in the thermal activated region>12 K). To reduce the quantum relaxation process, an externalc field of 1000 Oe was applied. Therefore, a second thermalctivated region was observed below 20 K, with an anisotropic bar-ier of 90.8(7) K (�0 = 1.38 × 10−5 s). Here the barrier is enhancedemarkably compared with that of isostructural Dy–Cu complexUeff = 36 K), where the exchange interactions between Dy and Cuons should make a great difference, leading to the further splittingf the ground sub-level of DyIII ion [99].

.3. The SIMs with equatorial LF

To date, lanthanide-based SIMs with strictly equatorial LF aretill lacking. However, a series of LnZn3 compounds with macro-yclic Schiff base can be roughly treated as such a system and dueo the diamagnetic nature of zinc ions, they will be discussed in thisection.

A family of LnZn3 compounds with macrocyclic Schiff baseL1 and L2), deriving from the combination of 1,4-diformyl-2,3-
ihydroxybenzene and aliphatic diamines, has been assembled
n 2011 (Fig. 11) (a, 17-DyZn3; b, 18-ErZn3; 19-Er(Yb)Zn3)80,100,101]. For compound 17 (a) and 18 (b), the lanthanide ion isine-coordinate with six phenoxo oxygen atoms of the macrocyclic

Fig. 11. The crystal structure of wheel-shaped LnZn3 compounds (a, 17-DyZn3 inRefs. [100,101]; b, 18-ErZn3 in Ref. [80]). Crystallographic data available in originalreference.

Schiff base ligand at equatorial positions and three oxygen atomsfrom water and a nitrate anion at axial positions. Compound 19exhibits a similar structure to 17 with the water molecule replacedby a nitrate anion. In virtue of the differences of ligand L1 and L2,the ErZn3 complex 18 from ligand L2 shows a more planar struc-ture than other LnZn3 compounds 17 and 19 with ligand L1. Incompound 18 (b, ErZn3), it can be concluded that the six phe-noxo oxygen donors form a strong equatorial LF supported by theestimation of the Mulliken charges of the donor oxygen atoms.The similar equatorial LF should also exist in other LnZn3 com-pounds (17 and 19) with ligand L1 in spite of the small deviationsfrom equatorial plane. In result, complex 17 containing oblate Dyions shows only very weak out-of-phase ac susceptibility, reflect-ing that the equatorial LF may not favor the design of Dy-basedSIMs. Nevertheless the strong equatorial LF can favor the easy-axis magnetic anisotropy of prolate 4f ions such as ErIII accordingto Long’s model [64]. Indeed, ErIII- and YbIII-based complexes (18and 19) exhibit SMM behavior under an external dc field. Suchan ErIII- or YbIII-based SMM is quite rare. The strong equatorialLF should have a dominant contribution to the presence of theirSMM behavior in complex 18 with more planar structure, whichis consistent with the deduction from the model developed byLong. However, several LnCu3 compound (3d–4f) with equatorialligand field show slow relaxation of the magnetization in zerodc field [81,102], which is obviously contrary to the prediction,where Tb is of oblate, implying that this simple model is only par-tially successful. Such 3d–4f systems will not be discussed here indetail.

For non-axial systems containing lanthanide ions, the SMMbehavior was observed mainly for DyIII-based SMMs due to theinternal nature of DyIII ion: a Kramers ion can insure a doublydegenerate ground state [30,103,104].


F eld de(

2

ubsct[pwq[wiHatbiep

3

iiQrTo

vcataieii[

iSmab(

ig. 12. Structure of N23−–Dy2 (21, left), crystallographic data available in Ref. [5]; Fi

2011) American Chemical Society.

.4. Magnetic dilution in mono-DyIII compounds

To suppress the quantum tunneling induced by the intermolec-lar dipole–dipole interactions, the methodology of dilution haseen extensively applied in mononuclear lanthanide complexes,uch as in [DyPc2], Dy-�-diketones and trigonal prismatic DyIII

omplexes [26,70,105]. The application of magnetic dilution leadso the shift of the peaks of �′′/�M to higher temperatures inLnPc2] compounds [26]. In the mononuclear Dy-�-diketones com-lexes reported by Gao, the obvious quantum tunneling regimeas observed below 8 K for undiluted samples, but, remarkably, the

uantum tunneling was decreased effectively by dilution (8, Fig. 5)70]. In addition, in complex Dy(H2BPzMe2

2)3 (20, HPz = pyrazole)ith almost D3h point symmetry, no magnetic relaxation behav-

or was observed even in a dc field of several hundred Oe [105].owever, with the increase of Dy:Y molar ratios (1:1, 1:15, 1:65,nd 1:130), the gradually increasing ac signal was observed up tohe ratios 1:65, and, significantly, two relaxation regions becomeetter resolved within the frequency range measured. The results

ndicate that intermolecular interactions clearly have a profoundffect on dynamic relaxation, even to obscure the two relaxationrocesses.

. Dy2 systems

Compared with transition metal SMMs, the rate of tunnel-ng relaxation is much faster in lanthanide-based SMMs, whichs mainly due to the internal nature of lanthanide ion. The fastTM can cut down the height of the effective barrier (Ueff) to spin

eversal, as indicated in some lanthanide-based SMMs [105–107].herefore, the suppression of QTM is very crucial for the increasingf Ueff.

In fact, the fast quantum tunneling mainly arises from a trans-erse anisotropy introduced by a geometrical distortion of theoordinated sphere and hyperfine or intermolecular dipolar inter-ction, which gives rise to the perturbation Hamiltonian allowinghe mixing of the two states [8,108,109], thus the Ising symmetry is

condition to create slow relaxation [64]. In addition, recent stud-es have revealed that intramolecular magnetic interactions canffectively suppress QTM at low temperatures, because the tunnel-ng splitting (�T) can be decreased effectively when the exchangenteraction is higher than the temperature domain of QTM (Fig. 14)91,110].

Therefore, in order to better understand the origin of magnetismn SMMs, the constructions of the simplest polynuclear DyIII-basedMMs, Dy2 systems, have been the focus of research in the field of
olecule magnetism, due to its advantages compared with SIMs
nd the simple structural motif. One main aspect is that their SMMehavior can be tuned effectively by the magnetic interactionsdipolar interactions and exchange coupling) [91]. From the point

pendence of the normalized magnetization for N23−–Tb2 (right, Ref. [56]), Copyright

view of synthesis, the choice of different kinds of bridging ligands isa key factor to achieve strongly coupled systems. Here some typicalDy2 systems will be highlighted.

3.1. Radical-bridged coupling systems: N23−–Dy2

In contrast with other lanthanide complexes bridged by O, N andCl atoms, N2

3− radical-bridged Tb2 complex behaves as the hardestmolecular magnet based on discrete molecules to date, indicatingthe ability of the N2

3− radical ligand to mediate the strong exchangecoupling between lanthanide ions [5,56].

The radical-bridged Dy2 complex {[(Me3Si)2N]2(THF)Dy}2(�-�2:�2-N2) (21) is readily generated through reduction of theN2

2−-bridged complex [5]. Each five-coordinate DyIII ion is lying ina pseudotetrahedral environment, with one vertex being occupiedby the bridging N2

3− ligand (Fig. 12). Fitting direct current (dc) mag-netic susceptibility data of the isomorphic N2

3−–Gd2 reveals thestrongest magnetic coupling of J = −27 cm−1 between a GdIII centerand paramagnetic ligand observed for a gadolinium compound. ForN2

3−–Dy2 complex, relaxation times were extracted through fittingCole–Cole plots for ac susceptibility collected at a 0 Oe applied dcfield. An Arrhenius fit to the data gives an effective relaxation bar-rier of Ueff = 123 cm−1 with a pre-exponential factor of �0 = 8 × 10−9

s. In contrast, by measuring variable-frequency ac magnetic sus-ceptibility of the non-radical N2

2− bridged DyIII compound, anenergy barrier of Ueff = 18 cm−1 was shown with �0 = 2 × 10−6 s. Theobvious difference indicates that the blocking originates from theconcerted effect of the anisotropic DyIII ground states and the strongmagnetic coupling through the radical N2

3− bridge. Moreover, theisomorphic N2

3−–Tb2 possesses extraordinarily slow molecularmagnetic relaxation properties, with a highest measured blockingtemperature of 13.9 K (Fig. 12 right) [56]. The results demonstratethat a joint contribution, combining strong magnetic coupling withsingle-ion anisotropy, may ultimately lead to higher relaxationbarrier SMMs capable of retaining their magnetization at morepractical temperatures.

3.2. Typical Dy2 complexes with ferromagnetic coupling

3.2.1. Asymmetric Dy2Recently, an asymmetric Dy2 ([Dy2ovph2Cl2(MeOH)3]3·MeCN,

22) has been reported by some of us and thus serves as an excel-lent candidate for probing simultaneously the contributions ofboth the magnetic interaction and single-ion anisotropy to therelaxation dynamics of polynuclear lanthanide systems [91]. Theeight-coordinate Dy1 center exhibits (Fig. 13) a hula hoop-like
geometry, while the seven-coordinate Dy2 center has a near per-fect pentagonal bipyramidal coordination. The one-dimensionalsupramolecular chain arrangement is held together by strong intraand intermolecular hydrogen-bonding interactions.


Fig. 13. Temperature dependence of the in-phase (�′) and out-of-phase (�′′) partsof the ac susceptibility for asymmetric Dy2 under zero-dc field. Inset: orientations ofl

RS

cbaaeocgac

Iiceasfrclra

Ft

ocal anisotropy axes (dashed lines) and of ground-state local magnetizations (22).

eprinted with permission from Ref. [91]. Copyright (2011) American Chemicalociety.

Ac susceptibility measurements reveal that both �′ and �′′

omponents for the complex cascade like avalanches below thelocking temperature and nearly vanish as the temperaturepproaches 2 K. This signals the “freezing” of the spins by thenisotropy barriers and can be taken as a clear indication of thefficient suppression of the zero-field tunneling of magnetizationccurring in this complex. In addition, the significantly increasingoercivities with decreasing temperature in compound 22 sug-est sluggish quantum tunneling. This is associated with the highxiality and Ising exchange interaction as approved by ab initioalculations (Fig. 14).

Moreover, the lowest exchange multiplets are two exchangesing doublets, separated by 2.85 cm−1, each showing a tunnel-ng splitting of the order of 10−8 cm−1 as revealed by ab initioalculations (Fig. 14). The negligible tunneling splitting in bothxchange doublets points to an almost net Ising exchange inter-ction between Dy sites and is the reason for the observed stronguppression of relaxation rate at low temperatures [91]. There-ore, the specific progress of this representative work was the clearealizing, on the basis of combined experimental and theoreti-
al investigations, of the crucial ingredients necessary to achieveong relaxation times in dinuclear lanthanide complexes and theirelation to the molecular structure. These are (1) the high axi-lity of lanthanide sites leading to a preponderant Ising type of
ig. 14. Plot of normalized magnetization (M/Ms) versus �0H. The loops are shown at diffhe anisotropy axis of the ground exchange doublet.

eviews 257 (2013) 1728– 1763 1737

interaction between them, (2) the almost parallel local anisotropyaxes and (3) the non-negligible exchange interaction between theDy ions. These three ingredients together contribute to the suppres-sion of the quantum tunneling of magnetization, a preconditionfor the use of these complexes as elements in nanoelectronics[111].

The effect of intermolecular and intramolecular magnetic inter-actions on the blocking depends on the temperature domain whereit is studied [110]. For temperature higher than the exchange inter-action, the later will enhance the tunneling of magnetization whichis in full analogy with the effect of intermolecular interaction inthe paramagnetic regime. However, in compound 22, we were inan opposite regime where the temperature was lower than theexchange interaction. In that regime the effect of exchange inter-action is opposite, i.e. enhancing the blocking of magnetization ashas been seen here [91,110].

3.2.2. Ferromagnetic [Tb2Pc2obPc]The triple-decker Pc complexes (23, Scheme 3) with asymmet-

ric TbIII sites, containing [Tb, Tb], [Tb, Y] and [Y, Tb], were reportedby Ishikawa et al., thus providing a rare opportunity to carry outexperimental studies on the dynamic magnetism of coupled 4f elec-tronic systems [87]. Direct current magnetic susceptibility datareveals the ferromagnetic dipolar interaction between Tb ions.Fig. 15 shows the temperature dependence of ac susceptibilitiesof the three complexes under a zero and nonzero dc magnetic field,respectively. By contrast, the �′′

M/�M peaks of the [Y, Tb] and [Tb, Y]complexes shift to higher temperatures when a dc field was applied,while in the [Tb, Tb] complex the positions of the two �′′

M/�Mpeaks do not change, suggesting that the quantum tunneling ofmagnetization has been suppressed by the f–f interaction betweentwo TbIII ions. In addition, another triple-decker Pc complexes withsymmetric TbIII sites, [Tb2(obPc)3] (24, Fig. 4 right; obPc = dianionof 2,3,9,10,16,17,23,24-octabutoxyphthalocyanine), has also beenreported by Katoh, showing only a single relaxation peak at 1488 Hzand 24 K in zero dc field, but two magnetic relaxation processes inthe low-temperature region in the presence of a dc magnetic field
[89].
Recently, a similar mixed-ligand bis-TbIII complex,(Pc)Tb(Pc)Tb(T(p-OMe)PP) (25, T(p-OMe)PP = tetra-p-methoxyphenylporphyrinato), was provided by Ishikawa et al.,

erent sweep rates at 0.04 K. Zeeman diagrams calculated for the field applied along


F d [TbR

wsmtm

3

3

SmtcTsrci

F2

R

ig. 15. Plots of �′′M/�M against temperature T for [Y, Tb] (top), [Tb, Y] (middle), an

ef. [87].

ith not only a SAP coordination site but also a well-definedquare-prismic (SP) site (Fig. 4 middle) [88]. Similarly, [Tb, Tb] hasuch longer � than [Tb, Y] and [Y, Tb] with Hdc = 0 Oe because of

he strong hindrance of quantum tunneling path in SAP site byagnetic dipolar interaction between the two ions.

.3. Typical Dy2 complexes with antiferromagnetic coupling

.3.1. The Dy2 with a salen-type ligandAn antiferromagnetic superexchange coupled Dy2

MM, [Dy2(valdien)2(NO3)2] (26) (H2valdien = N1,N3-bis(3-ethoxysalicylidene)diethylenetriamine), was obtained through

he use of a polydentate bridging Schiff base ligand from theondensation reaction of o-vanillin and diethylenetriamine [90].he complex provides an ideal model for the elucidation of
low relaxation of the magnetization mechanism of an antifer-omagnetic lanthanide system. The centrosymmetric dinuclearomplex is composed of two eight-coordinate DyIII ions in anntermediate coordination polyhedron between square antiprism
ig. 16. Structure and the calculated anisotropy axes on Dy sites of antiferromagnetic Dy2

–25 K (middle); Field dependence of the normalized magnetization for antiferromagnet

eprinted with permission from Ref. [90]. Copyright (2011) American Chemical Society.

, Tb] (bottom) measured in zero dc-field (left) and nonzero-dc field. Adapted from

(D4d) and dodecahedron (D2d). Ab initio calculations confirmthe weak antiferromagnetic Ising interaction between Dy ionswith JDy–Dy = −0.21 cm−1 (Fig. 16). Ac measurements exhibit that inquantum regime (below 4 K), a drop of the imaginary susceptibility,�′′, is observed, which most likely originates from an antiferromag-netic interaction between the DyIII ions. In addition, an “S-shaped”hysteresis is observed with a large step at H = ±0.3 T, which candirectly be associated with a spin flip of the antiferromagneticallycoupled DyIII spins.

In addition, for polynuclear Ln-SMMs, it is difficult to elucidatehow the dilution affects the slow relaxation of the magnetiza-tion owning to the complicated component in diluted polynuclearsamples. The Dy2 SMM diluted in a diamagnetic Y2 matrix,[Y2(valdien)2(NO3)2], was investigated firstly by Murugesu andcoworkers (Fig. 17) [112]. Herein, the effective barriers (Ueff)
increase with an increase in the species DyY, and are much largerthan that of the parent 100% Dy2 complex (76.0 K), which sug-gests that the single ion anisotropy is the primary contributorto the barrier. The single-crystal hysteresis loop measurements,
(26, left); Out-of-phase susceptibility �′′ vs. frequency � in the temperature rangeic Dy2.


Fig. 17. Molecular structure of the centrosymmetric complex [Y2(valdien)2(NO3)2]a

RS

cgievi

3

C(daosapi

FC

Fr

nd Core unit of the diluted YDy and Dy2 complex.

eprinted with permission from Ref. [112]. Copyright (2011) American Chemicalociety.

arried out on the diluted samples, show the QTM when H = 0 Tradually fades away with the presence of increasing Dy2 species,mplying the effective suppression of zero-field QTM by the weakxchange-biased interactions within the molecule. This study pro-ides a means to understand the nature and the strength of thenteraction.

.3.2. Planar DyIII2CoIII

2Very recently, a planar tetranuclear (butterfly)

oIII2DyIII

2 complex (27), [DyIII2CoIII

2(OMe)2(teaH)2(O2CPh)4MeOH)4](NO3)2·MeOH·H2O, that behaves as a dinuclear Dy2erivative because CoIII is essentially diamagnetic, also displays

non-magnetic coupled ground state that leads to suppressionf QTM [113]. Structure and the calculated anisotropy axes on Dy
ites are shown in Fig. 18. The metallic core is best described as
planar butterfly motif, with the DyIII ions occupying the bodyositions and the CoIII ions the outer wing-tips. The two DyIII

ons are eight coordinate with distorted square-antiprismatic

ig. 18. Structure and the calculated anisotropy axes on Dy sites of planar CoIII2DyIII

2 comhemical Society.

ig. 19. The crystal structure of some Dy2 complexes bridged by two R-COO− bridges (2eference.

eviews 257 (2013) 1728– 1763 1739

geometries. In the asymmetric unit of crystals of this compoundthere were equal amounts of a similar but not identical cluster[DyIII

2CoIII2(OMe)2(teaH)2(O2CPh)4(MeOH)2(NO3)2]·MeOH·H2O.

Dilution of the Dy complex into an isostructural diamagneticyttrium matrix allowed the relaxation mechanism within thesystem to be evaluated. These systems with their non-magneticground states and suppressed quantum tunneling are of futureimportance in the development of qubits and related devices.

3.4. Other Dy2 complexes with different bridges

3.4.1. Dy2 bridged by R-COO−

Firstly, some Dy2 complexes are bridged by two R-COO−

bridges in a �-�2:�1 or �-�1:�1 fashion. In 2009, a acetate bridg-ing Dy2 complex (28, [Dy2(OAc)6(H2O)4]·4H2O) was reported byPowell and coworkers, but shows no slow relaxation behavior(Fig. 19a) [114]. More recently, another isostructural Dy2 com-plex (29, [Dy2(BuCOO)6(MeOH)2(H2O)2]) bridged by n-butyric acidligand was obtained, only showing the weak slow relaxationbehavior of the magnetization [115]. In addition, Deng and cowork-ers obtained a Dy2 cluster, [Dy2(3-Htzba)2(3-tzba)2(H2O)8]·4H2O(30, 3-H2tzba = 3-(1H-tetrazol-5-yl)benzoic acid), through in situhydrothermal synthesis with lanthanide ions as catalyst, charac-terized as a typical SMM with an energy barrier of 53.7 K, which israther high in Dy-SMMs bridged by R-COO− group [116]. Here anO8 coordination sphere around Dy ion forms the bicapped trigonalprismatic coordination geometry (Fig. 19c).

Secondly, three Dy2 complexes bridged by four carbox-ylate groups have been discovered by the groups of Longand Luo, respectively [117,118]. In Fig. 20a, the complex,[Dy2(Acc)4(H2O)8]·Cl6·5.89H2O (31), was prepared by the reaction
of DyCl3·6H2O and Acc (1-amino-cyclohexanecaboxylic acid) inthe presence of NaOH [117]. Each DyIII center locates in the cen-ter of a square antiprism geometry. The Dy2 displays a frequencydependent out-of-phase signal but without maximum. Another
plex (27). Reprinted with permission from Ref. [113]. Copyright (2012) American

8–30) in Refs. [114–116], respectively. Crystallographic data available in original


[117]; (b) 32 in Ref. [118]. Crystallographic data available in original reference.

trboTcitwa

3

sftpoh[mloob

3ob(rN(stwaim

[

Fig. 20. Dy2 complexes bridged by four carboxylate groups. (a) 31 in Ref.

wo Dy2 complexes (32a and 32b, Fig. 20b) demonstrate theeversible single-crystal-to-single-crystal transformation (SCSC)etween 32a and 32b by dehydration/rehydration cycle, which wasbserved for the first time in 4f-based molecular magnets [118].he phase-transition results in the Dy-O bond cleavage/formation,ausing the change of local coordination surrounding around DyIII

ons from the nine-coordinated monocapped square-antiprismatico eight-coordinated square-antiprismatic coordination geometry,hich consequently alters the ligand field, and magnetic anisotropy

nd then changes energy barrier from 28.8 K to 5.9 K.

.4.2. Dy2 complexes based on non-oxygen-bridging ligandsThe single-ion anisotropy of 4f ion, owing to the strength and

ymmetry of the local crystal field, is probably the most importantactor for SMM behavior, but the intramolecular magnetic interac-ions should hold the key to moderating the magnetic relaxation ofolynuclear DyIII complexes [5]. Consequently, exploring the non-xygen-bridging complexes may be a useful method for probingow the exchange coupling affects the dynamic magnetic behavior119]. Hitherto, the non-oxygen-bridging DyIII SMMs are still rare

ainly due to the difficulty in synthetic approach, considering thatanthanide ions are easier coordinated by O atoms based on the the-ry of HSAB (Hard and Soft Acids and Bases). Currently, there arenly several N-, Cl- and S-bridged DyIII complexes, showing SMMehavior (33–38) [119–122].

.4.2.1. Heterocycle-bridged Dy2 SMMs. In 2010, tworganometallic dimers of dysprosium: [{Cp2Dy(�-bta)}2] (33,taH = 1H-1,2,3-benzotria-zole) and [{Cp2Dy[�-N(H)pmMe2]}2]34, NH2pmMe2 = 2-amino-4,6-dimethylpyrimidine) wereeported by Winpenny and coworkers, where the [bta]− andH2pmMe2 demonstrate the role of bridging ligands, respectively

Fig. 21a) [120]. In result, the benzotriazolide-bridged Dy2 complexhows typical SMM behavior below about 12 K, which representshe firstly discovered Dy2 SMM containing N-Bridging ligand,hile the Dy2 cluster bridged by �-amido ligands only shows

weak increase in �′′ below 4 K. This suggested that exchange
nteractions mediated by the �-amido ligands could influence the
agnetic dynamics.Subsequently, a rare pyrazine-bridged Dy2 complex,

Dy2(hfac)6(H2O)4pz]2·pz (35, Fig. 21b), was constructed by

Fig. 22. The crystal structure of Dy SMMs bridged by �-chloro (a, b, 36; c, 37

Fig. 21. The crystal structure of heterocycle-bridged Dy2 complexes, (a) 33 and(b) 35 from Refs. [120,121], respectively. Crystallographic data available in originalreference.

some of us through applying the ligand, hexafluoroacetylaceto-nate (hfac), with the strong electron-withdrawing effect, whichfacilitates the coordination of 4f metal ions to non-chelatingnitrogen-containing donors [121]. The dinuclear DyIII core has aninversion center of symmetry, in which the two [Dy(hfac)3(H2O)2]units are linked by a pyrazine ligand with a Dy· · ·Dy separationdistance of 8.09 A and each DyIII ion is characterized by a tricappedtrigonal prismatic environment. The ac susceptibilities under zerodc field reveal that above 9 K the relaxation follows a thermallyactivated mechanism affording a large energy barrier of 110 K witha pre-exponential factor (�0) of 8.4 × 10−10 s. We present herea means of developing polynuclear lanthanide SMMs using thestrong electron withdrawing ligand, hfac.

3.4.2.2. Cl-bridged Dy2. Besides N-bridged DyIII-based SMMs,recently three new organometallic DyIII-based SMMs bridged by�-chloro have been reported, which allows the exciting studyfor DyIII-SMMs bridged by other non-oxygen-bridging ligand[122]. The centrosymmetric dimer 36a (Fig. 22a) contains eight-coordinate dysprosium ions coordinated by two �5-Cp ligandsand two �-chloro ligands with Dy–Cl–Dy angle of 96.62(5)◦ andthe Cl–Dy–Cl angle of 79.91(16)◦, while polymer chains (36b,Fig. 22b) consist of {Cp2Dy} units bridged by �-chloro ligands withmuch greater Dy(1)-Cl(1)-Dy(1A) and Cl(1)-Dy(1)-Dy(1A) anglesof 123.15(7)◦ and 89.82(4)◦, respectively. Furthermore, the dimer
36a and the polymer 36b co-crystallize in a 3:1 ratio in the crystal,which is supported by a powder X-ray diffraction experiment, andboth show slow relaxation of magnetization with reversal barriersof Ueff = 37.9 K and 97.6 K, respectively. The ac data at zero-dc field
) from Ref. [122]. Crystallographic data available in original reference.


F

R

r2cctbtftosrbt

3Srh�Feaclgli4a

Fc

ig. 23. The crystal structure of the first sulfur-bridged [{Cp′2 Dy(�-SSiPh)}2] (38).

eprinted with permission from Ref. [119]. Copyright 2012 Wiley-VCH.

eveal two independent relaxation processes involving ca.75% and5% of �, respectively, which corresponds to 36a and 36b, and isonsistent with the powder X-ray diffraction. The structure of theentrosymmetric dimer 37 (Fig. 22c) is similar to that of 36a andhe main difference between them arises from the complexationy thf, which raises the coordination number of each dysprosiumo nine. Field-dependence studies of 37 revealed different featuresrom most DyIII-SMMs: the increasing magnetic field shifts �′′

maxo higher frequencies, leading to the observation of a second peakf the out-of-phase (�′′) component of the ac susceptibility, ashown in Dy-DOTA (1) [74]. In addition, a micro-SQUID study of 37evealed that quantum tunneling of the magnetization is exchangeiased, which may be ascribed to the weak intramolecular interac-ions mediated by the �-chloro ligands.

.4.2.3. S-bridged Dy2 compound. Recently, the first sulfur-bridgedMM, [{Cp′

2Dy(�-SSiPh)}2] (Cp′ = �5-C5H4Me) (38, Fig. 23), waseported by Layfield and coworkers [119]. The complex showsigher blocking temperature and effective barrier compared with-bta (33) and �-chloro (37) bridged Dy2 compounds (Fig. 21a and

ig. 22c). Herein, magnetic exchange between DyIII ions could benhanced due to the presence of soft donors (S), as indicated byb initio calculations. For compound 38 and 37, while the dipolarouplings are similar, the exchange interaction in 38 is twice asarge as that in 37. In addition, the calculated values of transverse

values on the DyIII ions (gx and gy) are one order of magnitude
arger in 37 relative to 38. Therefore, the excellent SMM behav-or was observed in 38, with frequency dependence up to about0 K. The exploration of non-oxygen-bridging SMMs might present
promising new route to obtain SMMs with high energy barrier.

ig. 24. Structure and the calculated anisotropy axes on Dy sites of Dy3 triangles (39, leoupling Dy3 triangles (40, right), crystallographic data available in Ref. [54].

eviews 257 (2013) 1728– 1763 1741

Besides the important breakthroughs of SIMs and Dy2 SMMsas shown above, the assembly of polynuclear structures has beenof increasing interest, with the aim of improving the currentknowledge of the structure-property relationship in lanthanide-containing SMMs [28].

4. Synthetic strategies for multinuclear Dy-based SMMs

Multinuclear DyIII-based SMMs were gotten into the researchlater than their SIM rivals (beginning with the discovery of Dy3 tri-angles in 2006), but the field evolved by leaps and bounds recentyears, mainly because such systems might conserve precious non-collinear spin architectures at molecular level, such as toroidalmagnetic moment (Dy3 [52]), and bring about some special mag-netic phenomenon as multiple relaxation processes [91]. It couldtherefore be possible to store the information in the spin chiralityof the triangular unit with the advantage of much weaker dipolarfields and a reduced sensitivity to external fields [24]. Further theexploration of special magnetic phenomenon will also lead to a use-ful understanding of complicated magnetic mechanism, such as indefect-dicubane Dy4 [55] and linear Dy4 [28].

In particular, it is clear that purely lanthanide-based polynu-clear systems are an important avenue to explore in the pursuitof SMMs with high anisotropic barriers. Recently, the anisotropicbarrier records have toppled like dominoes for polynuclear lan-thanide SMMs, such as in defect-dicubane Dy4 (170 K) [55], linearDy4 (173 K) [28] and pyramid Dy5 (528 K) [29]. Here it seems tobe difficult to obtain large ground spin state through promotingmagnetic interactions, as in transition metal SMMs, between thelanthanide ions due to the internal nature of lanthanide ions [65].Therefore, the rational arrangement of single ion anisotropic axesof Dy corresponding to the careful choice of ligand may hold thekey to obtaining lanthanide-only SMMs with high blocking tem-perature, as seen in coupling Dy3 units (i.e. Dy6) [54], planar Dy4(42) [123] and defect-dicubane Dy4 [55].

The magnetic investigations of Dy3 triangles (39), discovered byPowell and coworkers, have been performed through the static anddynamic magnetic measurements as well as ab initio methodology,revealing that the anisotropy axes of the ground Kramers doublet atthe three dysprosium sites form an almost perfect equilateral trian-gle and lie practically in the Dy3 plane (toroidal moment), as shownin Fig. 24 left [27,52,53]. Here in spite of the almost non-magneticground state, typical features of SMM behavior are observed withan effective barrier of 61.7 K, possibly stemming from the thermallypopulated excited state. In 2010, an antiferromagnetic linking oftwo Dy3 to form Dy6 (40, Fig. 24 right) gave a spectacular increase in
the temperature at which slowing of the magnetization is observedfrom 8 to 25 K, representing a promising strategy to increase theblocking temperature of lanthanide-based SMMs [54]. To retain thefunctionality of Dy3 triangle in Dy6, the careful design of ligands
ft, Ref. [53]), Copyright 2009 Royal Society of Chemistry; The crystal structure of


Fig. 25. Main anisotropy axes (dashed lines) on Dy ions and local magnetizations(arrows) in the ground state in 41.

R

stctoesDsmptDmhit

ba(sc�toapab

SbbttDoi

parallel to each other for the opposite ions and point almost radi-ally to each Dy site. The axes on Dy1 and Dy2 sites make an angleof 28.78 and 3.18◦ with the Dy4 plane and are almost orthogonal


hould be crucial. Compared with ligands in other Dy6 SMMs, herehe simple reduction of the aldehyde to an alcohol makes the leasthange for the ligand in Dy3. Even though it is inevitable to break therigonal symmetry of Dy3 in Dy6, which results in a small deviationf the easy axis of Dy3 from the plane of the triangle, the calculatednergies as a function of the magnetic field applied in the plane stillhow a non-magnetic ground doublet state of Dy6 as in the case ofy3 and a weakly magnetic state at only 0.6 K above the ground

tate (compared with about 10 K in Dy3). Alternating-current (ac)easurements show that the application of a field of 1 kOe has

ractically no effect on the relaxation dynamics, suggesting thathe quantum tunneling in zero field is suppressed effectively iny6, possibly associated to the linking of the two triangles. Further-ore, the presence of two different relaxation processes with one

igh anisotropy barrier of 200 K may arise from the peculiar changen magnetic anisotropy from easy plane to easy axis on populatinghe excited states based on the whole of this cluster.

In continuation of our interest in toroidal spin topologies,y employing a polydentate Schiff-base ligand, we were able torrange the Dy3 triangles in a robust edge-to-edge arrangement41, Fig. 25) whilst perfectly retaining the nonmagnetic groundtate as revealed by magnetic measurements and ab initio cal-ulations [124]. Such arrangement and the strong couplings via4-O2− ion stabilize similar arrangement of toroidal moments in

he ground states of each triangle, making the toroidal momentf the complex maximal possible. The present results provide

promising strategy for enhancing the toroidal magnetisms ofolynuclear lanthanide-based compounds via fine-tuning of therrangements of the lanthanide ions and strengthening couplingsetween lanthanide ions.

In addition, Tong and coworkers have reported a novel {Dy4}MM, [Dy4(�3-OH)2 (�-OH)2(2,2-bpt)4(NO3)4(EtOH)2] (42, 2,2-ptH = 3,5-bis(pyridin-2-yl)-1,2,4-triazole) (Fig. 26a), where thept− ligand with two bidentate chelating sites is, for the firstime, used to produce lanthanide-based complexes and displayshe role of bridging ligands [123]. In contrast with other planary4, Dy–N–N–Dy linking leads to the almost perfect arrangement
f those distorted SAP coordination spheres (Fig. 26b), thus result-ng in a toroidal magnetic moment in the ground state. This stands
Fig. 26. The crystal structure of planar [Dy4(�3-OH)2(�-OH)2(2,2-bpt)4(NO3)4(EtOH)2] (42). Crystallographic data available in Ref. [123].

for the first case of the {Dy4} family with a nonmagnetic groundexchange state, as seen in Dy3 triangle.

In 2009, a defect-dicubane Dy4 (43, Fig. 27 top) with anisotropybarrier of 170 K, which was considerably higher than any pre-viously reported Ueff value for polynuclear SMMs, was reportedby the group of Murugesu [55]. The four DyIII ions are coplanar,bridged by two �3-OH ligands, four phenoxide oxygen atoms andtwo diaza bridging groups. Alternating-current (ac) measurementsdisplay the out-of-phase ac signals with maxima at 30 and 9 K for1500 Hz (Fig. 27 bottom), indicating the occurrence of multiplerelaxation. Ab initio calculations reveal the Ising-type anisotropyaxes on each Dy site (g‖ = 19.5 for Dy1, g‖ = 19.2 for Dy2). They are

Fig. 27. The crystal structure of defect-dicubane Dy4 complex (43, top); out-of-phase ac signals at different frequency (bottom).

Reprinted with permission from Ref. [55]. Copyright 2009 Wiley-VCH.


in Ln

ttw,tto(c

pSett

4

elbmbpHltoasm

mot

Scheme 4. Four classes of ligands used

o each other, suggesting the weak exchange interaction betweenhem. The energy difference between the two lowest lying statesas predicted to be 83 and 199 K for Dy1 and Dy2, respectively

which supports the large effective barrier (Ueff = 170 K), indicatinghat the main contribution to the barriers of blocking of magnetiza-ion in this compound come from the blockages of magnetizationsf individual dysprosium ions. The two maxima of the out-of-phase�′′) ac signals indicate the presence of two modes of relaxation,orresponding to two types of dysprosium ions.

As a result, those examples demonstrate that the Dy-based com-ounds with more nuclearity may have many advantages over theIM for the discovery of the novel magnetic phenomenon. It isssential to develop the multinuclear Dy-based systems in ordero move closer to future applications. The synthetic strategies ofhem will be discussed in detail below.

.1. Ligand design

In brief, the interplay between the ligand field effect, the geom-try, and the strength of the magnetic interaction between theanthanide sites will govern the SMM behavior of lanthanide-ased SMMs [90]. Based on above fact, we can intentionally alterolecular structure to probe the effect on relaxation dynamics

y ligand replacement or modification as for mono-DyIII com-lexes, such as in [DyPc2], Dy-�-diketones complexes [86,92].owever, for polynuclear DyIII complexes, the changes of molecu-

ar structure with the variations of ligands should be rather difficulto grasp. Thus, we can design different multidentate ligands tobtain polynuclear DyIII complexes with distinct structural motifs,nd then study the SMMs behavior. Nevertheless, there are stillome reasonable agreements between ligand design and structuralotifs.
The synthetic approaches to build polynuclear Dy-based SMMs
ay take four respects into consideration. (1) Based on the the-ry of HSAB (Hard and Soft Acids and Bases), the O atoms tendo coordinate with the lanthanide ions with hard Lewis acidity

-SMMs based on the structural motifs.

[117]. Therefore, it is reasonable to design the ligands with theO-donors, such as hydroxy, carbonyl or carboxy group. (2) Theintroduction of some bridging ligands, containing N, Cl or radi-cal, can fine-tune the exchange coupling between lanthanide ions[5,121,122], which seem to be of great importance for modulatingthe relaxation dynamics. (3) To hamper the 3D magnetic order-ing, it is necessary to design the ligands favoring the formation ofisolated molecule or to apply the bulk co-ligands with long spac-ers in network-structured complexes [125,126]. (4) It is noticeablethat, by designing the ligands, suitable modulation of inter- orintramolecular hydrogen bonds or �–� interactions may have anunexpected effect on magnetic relaxation dynamics [91,127].

To our knowledge, the number of metal centers for mostlanthanide-based SMMs is less than six, and the basic structuralmotifs extracted as building blocks were shown in Scheme 2. Theligands used in those SMMs can be mainly divided into four classesbased on the structural motifs: �-diketone and macrocyclic ligand,o-vanillin Schiff base ligand, hydrazone based ligand and bridgingligand, as listed in Scheme 4. The complexes with �-diketone andmacrocyclic ligand and the Dy2 system with different bridging lig-ands have been discussed above. Recently, considerable efforts ofour group have been dedicated to the assembly of DyIII-SMMs withhydrazone and some special bridging ligands [91,121]. In combi-nation with the excellent works of other groups, we will make asystemic review on multinuclear DyIII-SMMs to provide a snapshotof their development.

4.2. Molecular assembly based on special bridging ligand (Fig. 28)

4.2.1. Oxalate/CO32− bridged Dyn (n = 2–6) complexes

To date, only one oxalato-based Dy2 SMM, [Dy2(HBpz3)4(�-ox)]·2CH3CN·CH2Cl2 (44, Fig. 29a), was successfully synthesized by
some of us in 2009 [59]. In fact, as an excellent linker, the potentialof the oxalate anion has been clearly demonstrated in the searchfor the magnetic coordination materials with multifunctionality[128]. It can provide an efficient pathway for the super-exchange

1744 P. Zhang et al. / Coordination Chemistry R

Fl

itptcrvo

ac(deSF

Fi

ig. 28. The structural motifs of multinuclear Dy compounds from different bridgingigands.

nteractions. In compound 44, the dc magnetic susceptibility in lowemperature suggests the fact that the bridging oxalate anion mayropagate a weak ferromagnetic coupling between Dy ions. In addi-ion, the bulky capping ligand, hydrotris(pyrazolyl)-borate (Bpz3−),an minimize undesirable intermolecular magnetic interactions. Inesult, the Dy2 complex behaves as a typical SMM. This work pro-ides a promising strategy to design new lanthanide SMMs basedn the versatile oxalate ligand.

In contrast with oxalate anion, the bridging ability of carbon-te ion has been seen with a growing number of DyIII clustersonstructed by CO3

2− ions, such as Dy4 (45) [129], Dy6, Dy874, 75) [130] complexes. The carbonate ions are presumably
erived from the fixation of atmospheric CO2 or the pres-nce of sodium carbonate. The bridging modes are listed incheme 5. Complex [Dy4(3-bpp)3(CO3)6(H2O)3]·DMSO·18H2O (45,ig. 29b, 3-bpp = 2,6-di(pyrazole-3-yl)pyridine) has been reported,
ig. 29. The crystal structure of Oxalate/CO32− bridged Dy complexes. (a) Oxalato-based

n Ref. [129]. (c) carbonato-bridged Dy2 complex anions 46 in Ref. [131]. (d) CO32−-bridge

eviews 257 (2013) 1728– 1763

containing a metal-CO32− core obtained from fixation of atmo-

spheric CO2 [129]. Thereinto, six CO32− ions display two modes of

bonding, as depicted in Scheme 5. The four DyIII ions form a Dy4trigonal pyramid structure, where the basal Dy ions with distortedsquare-antiprismatic geometry are bridged to each other by threeCO3

2− ions, forming the triangular plane. Further the three DyIII

ions are then bridged to the apical DyIII ion with a distortedtri-capped trigonal prismatic geometry via the other three CO3

2−

ions. Eventually, a 3D network is generated with the tetranuclearcomplex being the node and connected to six others via the �–�interactions. The ac susceptibility data show the weak frequencydependence within the measured frequency range for the com-plex. Recently, a carbonato-bridged Dy2K2 compound (46, Fig. 29c)has been synthesized using the 2, 3-quinoxalinediolate ligand,showing a high effective barrier of Ueff = 39.1 K [131]. Here thestructure of Dy2 compound consists of discrete carbonato-bridgeddinuclear DyIII complex anions, [Dy2L6(�-CO3)]8−, and coordi-nated potassium cations, together with tetramethylammoniumcountercations and both coordinated and free water molecules.

Another CO32−-bridged DyIII complex, Dy6(teaH)2(teaH2)2

(CO3)(NO3)2(chp)7(H2O)](NO3) (47, Fig. 29d), was reportedby Murray and coworkers, where teaH2 and chp representmonodeprotonated forms of triethanolamine and 6-chloro-2-hydroxypyridine, respectively [132]. Four coplanar DyIII ions (Dy1,Dy4, Dy5, and Dy6) form a trapezoid, with the final two ions (Dy2and Dy3) lying above and below the plane of the longest rectangu-lar edge. Remarkably, the carbonate ion appears to be directing thepositions of these Dy ions. An increase in �′′

M is suggestive of thepresence of slow relaxation of the magnetization, but the absenceof any frequency-dependent peaks may be due to fast zero-fieldquantum tunneling.

4.2.2. Dy complexes based on R-COO−/R-O− bridging ligandsThe survey shows the strongly bridging ability of the nega-

tively charged carboxylate bridges (R-COO−) enables the design ofa myriad of DyIII-based SMMs containing Dy2 to Dy26 with vari-ous motifs and SCM [114,133]. However, their magnetic propertiesare not very fascinating, only a Dy7 complex partly bridged by

Dy2 compound 44 in Ref. [59]. (b) The Dy4 compound 45 bridged by six CO32− ions

d Dy6 complex 47 in Ref. [132]. Crystallographic data available in original reference.


Scheme 5. The bridging modes in Dy4 (Ref. [1

Fa

R�octips

4Fwped

F

ig. 30. The crystal structure of linear Dy3 complex (48). Crystallographic data avail-ble in Ref. [135].

-COO− showing a high energy barrier Ueff = 140 K (>100 K) with0 = 7.2 × 10−9 s [134]. In contrast, until now the Dy-SMMs basedn alkoxide bridging ligand (R-O−) are still rare for the lanthanideomplexes with small-nuclearity (n < 6), which may be ascribedo the lack of structural stability with the decreasing in nuclear-ty number. Nevertheless, the energy barrier record (530 K) ofolynuclear DyIII-SMMs, held by the alkoxide-bridged Dy5 with aquare-based pyramid, still has not been broken [29].

.2.2.1. Linear Dy3. In 2011, a luminescent linear Dy3 complex (48,ig. 30) was obtained through hydrothermal reactions of Dy2O3
ith organic ligand, and shows the multifunctional properties ofhotoluminescence and magnetism [135]. All the DyIII centers areight-coordinated but surrounded by different donor atoms, withifferently distorted triangular dodecahedron. Alternating current
ig. 31. The crystal structure of two discrete linear Dy4 (a, 49; b, 50) compounds in Refs.

29]) and Dy6, Dy8 (Ref. [130]) complex.

(ac) magnetic susceptibility data at zero dc field shows the fre-quency dependent peaks and the crossover between thermallyactivated relaxation and direct tunneling. An energy barrier ofUeff = 65 K with �0 = 1.5 × 10−5 s−1 was obtained by the Arrheniusfitting.

4.2.2.2. Linear Dy4. Two discrete linear Dy4 (49 and 50) compoundshave been prepared by some of our group, through employingSchiff-base ligand H2L1 (Fig. 31a) and HL2 (Fig. 31b), respectively[136,137]. Strikingly, distinct bridging fashions can be found forthe carboxylate bridges. In 49, three different binding modes canbe observed for the polydentate Schiff-base ligand in its zwitteri-onic and di-deprotonated forms. The flexible binding modes lead toa linear metal array of four DyIII ions, and the distinct coordinationsphere of two asymmetric DyIII ions. In contrast, for 50, the presenceof ligand HL2 can provide effective hindrance to prevent the for-mation of extended structures (1D chain), resulting in the isolationof a discrete linear Dy4 with Dy–Dy–Dy angle of 175.673(2)◦. Twounique crystallographic DyIII ions show similar distorted bicappedtrigonal-prismatic geometry. In spite of the different coordinationgeometry in 49 and 50, the almost equivalent energy barriers canbe obtained by fitting � values using an Arrhenius law: Ueff = 20 Kfor 49 and 17.2 K for 50.

4.2.2.3. Planar Dy4. In 2010 and 2011, Powell’s and Murray’sgroup reported two planar Dy4 complexes displaying slow mag-netic relaxation by applying different ligands [138,139]. In bothcomplexes, the ancillary ligand, pivalic acid (piv), provides the�-�1:�1 bridging fashion so as to stabilize the planar structure. In
Fig. 32a, the Dy4 complex (51) was synthesized by the use of ligand,N-methyl-diethanolamine (mdeaH2), providing an example ofDy-SMMs partly bridged by R-O−. Two asymmetric DyIII ions areeight-coordinate, but with different coordination surrounding. Dy1
[136,137], respectively. Crystallographic data available in original reference.


F 138])

r

htdpstTaat�

4c[(dwgvtsDatauhbt[ct

F

ig. 32. The crystal structure of planar Dy4 complexes from Powell’s (a, 51, Ref. [eference.

as a geometry somewhat between dodecahedral and bicappedrigonal-prismatic, while Dy2 has a distorted dodecahedral coor-ination environment. Ueff was estimated to be 6.9 K with there-exponential factor, �0 = 4.8 × 10−5 s. The micro-SQUID mea-urements were performed on single-crystal sample, showing aypical SMM behavior, the opening of hysteresis loops below 1.1 K.he two unique DyIII ions in compound 52 (Fig. 32b) are eight-nd nine-coordinate, displaying distorted square-antiprismaticnd tri-capped trigonal prismatic geometries, respectively. Fittinghe data to an Arrhenius law afforded values of Ueff = 6.25 K and0 = 3.75 × 10−5 s.

.2.2.4. Cubic Dy4. In 2009, the first example of a dis-rete Dy4 cubane showing slow magnetic relaxation,Dy4(�3-OH)4(isonicotinate)6(py)(CH3OH)7](ClO4)2·py·4CH3OHpy = pyridine) (53, Fig. 33a), was reported by our group [140]. Aistorted cubane-like Dy4 core is formed by four �3-OH bridges,hereas six isonicotinate ligands through bridging carboxylate

roups encapsulate the core to sustain the cubic array. The Ms. H/T data at different temperatures shows nonsuperposi-ion plots and a rapid increase of magnetization at low field,uggesting the anisotropy and the crystal-field effect at theyIII ion. Furthermore, ac susceptibility measurements reveal

frequency-dependent out-of-phase signal below 6 K, indica-ive of slow relaxation of magnetization. Subsequently, in 2010,nother Dy4 cubane (54, Fig. 33b) was obtained by some ofs from a potentially tetradentate Schiff-base ligand, 2-{[(2-ydroxy-3- methoxyphenyl)methylidene]amino}benzoic acid,ut it shows absence of SMM behavior [141]. A closer look at the
wo cubic arrangements reveals important disparities. In 53, theDy4(�3-OH)4] core was encapsulated by six peripheral bridgingarboxylate groups (�-COO−), while only four �-bridges exist andwo of them are �-OH− bridge in 54, leading to the significant
ig. 33. The crystal structure of Dy4 cubane complexes, (a) 53, (b) 54, (c) 55 from Refs. [1

and Murray’s (b, 52, Ref. [139]) group. Crystallographic data available in original

disparities between the two cubic arrays in the Dy–O–Dy angles(103.65(16)–110.11(15)◦ in 53; 99.00(14)–109.50(15)◦ in 54),which should be responsible for the different magnetic behaviorobserved.

Recently, a cubane-like Dy4 cluster (55, Fig. 33c) was alsoreported by Kong et al. This Dy4 compound was prepared using thesimilar procedure as described above for the synthesis of the Dy2complex (31, Fig. 20a), but using Dy(ClO4)3 in place of DyCl3 [117].The four Dy ions form a nearly perfect tetrahedron with each of theedges bridged by a carboxylate group of the Acc ligand and withthe Dy–O–Dy angles in range 101.6(2)–109.9(2)◦. The slow mag-netic relaxation behavior of this Dy4 compound resembles that of53.

4.2.2.5. Dy4 compounds based on R-O− bridge. In addition, a nearlylinear Dy4 complex, bridged by phenol groups and alkoxide armsfrom the ligand with four chelating alcohol groups (56, Fig. 34a),was reported by Yang and coworkers, which shows both lumi-nescent and slow magnetization relaxation (SMMs-like) behavior[142]. As mentioned above, the Dyn SMMs bridged by R-O− withsmall-nuclearity (n < 6) are rare. The combination of four chelatingalcohol groups with a phenol group might stabilize the lineararray of four Dy ions, leading to the formation of this Dy4 com-plex. All DyIII ions are eight-coordinates with a highly distortedsquare antiprism geometry. The out-of phase ac signals show aseries of frequency-dependent peaks at the oscillating frequencyup to 10,000 Hz, suggesting its SMM behavior. Only a small effec-tive energy barrier of 1.5 K can be obtained, however, it representsa rare example of alkoxido bridged Dy-SMMs.

In 2011, a Dy4 complex (57, Fig. 34b) with an unprecedentedzigzag structure was reported by Liao and coworkers throughpyridine-2,6-dimethanol ligand [143]. Four Dy ions are all eight-coordinate in a distorted square-antiprismatic geometry with the

40,141,117], respectively. Crystallographic data available in original reference.


F s and

s

Didpad

4Dt[Pa[mmlwabprie�emmteofmo

ci

F

ig. 34. (a) The crystal structure of linear Dy4 complex 56, bridged by phenol grouptructure in Ref. [143]. Crystallographic data available in original reference.

y–O bond lengths in the range of 2.270(16)–2.517(7) A, and Dy–Nn the range of 2.480(10)–2.514(8) A. Ac magnetic susceptibilityata show frequency-dependent out-of-phase signals but withouteaks below 10 K, indicating the onset of slow magnetization. Thepplication of a dc field has no obvious effect on the relaxationynamics.

.2.2.6. Dy5 compounds. To the best of our knowledge, they5 compounds, exhibiting slow relaxation of the magne-

ization, are still rare. In 2008, a dysprosium compound,Dy5(�4-OH)(�3-OH)4(�-�2-Ph2acac)4(�2-Ph2acac)6] (58,h2acac = dibenzoylmethanide; Fig. 35b), displaying slow relax-tion of the magnetization was reported by Powell and coworkers144]. The five dysprosium atoms adopt the square-based pyra-

idal arrangement with each triangular face capped by one �3-Ooiety and with four dysprosium atoms in the square-based face

inked by one �4-O atom. An approximate energy gap of 33 Kas afforded by fitting the data of ac susceptibilities. In 2011,

nother Dy5 compound, [Dy5(�5-O(OiPr)13], (59, Fig. 35a) bridgedy iso-propoxide was reported, showing a similar square-basedyramid [29]. However, a closer look at the two Dy5 pyramidseveals important disparities: here each DyIII ion is six-coordinaten octahedral environment, while the DyIII ions of the former areight-coordinate; here five DyIII ions are linked by the central5-oxide, defining a clear unique anisotropy axis, while only �4-Oxists in the former; each triangular face is capped by one �3-OiProiety in place of the �3-O moiety. Ac magnetic susceptibilityeasurements show that the maximum in �′′ is observed at

emperatures as high as 41 K for 1400 Hz, giving the high thermalnergy barrier of Ueff = 528 ±11 K with a pre-exponentional factorf �0 = 4.7 × 10−10 s, which is by far the largest barrier yet observedor polynuclear Dy compounds. Here the presence of �5-oxide

ay be an overriding factor in determining the fourfold symmetry
f crystal field, leading to the excellent SMM behavior.
Very recently, an unprecedented trigonal bipyramidal (TBP) Dy5luster, [Dy5(�3-OH)6(Acc)6 (H2O)10]·Cl9·24H2O (60, Fig. 35c), wassolated through a similar reaction as depicted in the preparation of

ig. 35. The crystal structure of three Dy5 compounds, (a) 59, (b) 58, (c) 60 from Refs. [29

alkoxide arms in Ref. 142. (b) The crystal structure of Dy4 complex 57 with a zigzag

Dy2 complex (31, Fig. 20a), by increase the DyIII/ligand ratio [145].All five DyIII ions are eight-coordinate. Three DyIII ions in the equa-torial plane exhibit the square-antiprism coordination geometry,while the two DyIII ions occupying the apical positions display thedicapped trigonal prism. To stabilize the structure, six bridging R-COO− encapsulate the TBP Dy5 core. Ac magnetic susceptibilitymeasurements show that only an increasing in �′′ was observed,without maximum.

4.3. Molecular assembly based on o-vanillin Schiff base ligand

One current outstanding class in the field of coordination chem-istry is the clusters with Schiff base ligands, because of their simplesynthetic methodology and versatile coordination modes, espe-cially for the Schiff base ligands containing o-vanillin (Fig. 36). Therecent years have seen a huge increase in the number of com-plexes with o-vanillin Schiff base ligand, containing 3d-, 3d–4f- and4f-clusters, indicating their important contribution to the field ofmagnetic assemblies [24]. In addition, many isolated complexesincluding this kind of ligand can act as the useful building blocks tosupport the formation of bulk magnets, even molecular superpara-magnets [146]. In the field of magnetic study of lanthanide-basedcompound, the o-vanillin ligand was firstly used by Costes et al.with GdIII ions and a triangular Gd3 cluster similar to Dy3 wasobtained, initially showing its strongly coordinated ability for lan-thanide ions [147]. In this chapter, the discovered Dy-based SMMscontaining o-vanillin Schiff base ligand will be discussed with theaddition of two compounds with salicylaldehyde.

4.3.1. Dy2 complexesSalen-type ligands can provide a suitable coordination pocket to

accommodate a lanthanide ion by modifying the central diaminemoiety. As shown in Fig. 37, the N3O2 and N2O2 coordination pock-
ets were afforded from two kinds of dimamines. Therefore, twodistinct Dy2 compounds (26 and 61) were obtained by the group ofMurugesu and coworkers [90,148]. In Fig. 37a, the N3O2 pocket islarge enough to hold a Dy ion, while in Fig. 37b, the N2O2 pocket is
,144,145], respectively. Crystallographic data available in original reference.


ased S

snsw(cTgtmtacfib(

4wti

F

Fig. 36. Schematic diagram of the Dy-b

mall, thus a ancillary ligands is necessary to facilitate its coordi-ation. The former Dy2 compound (26) shows a centrosymmetrictructure with antiferromagnetic interaction between two Dy ions,hich has been discussed above [90]. The second Dy2 compound

61) consists of two Dy ions (Dy1 and Dy2) with unsymmetricaloordination environments, leading to two slow relaxation modes.he Dy1 is eight-coordinate, adopting a distorted square antiprismeometry, while a distorted capped trigonal prism is found forhe seven-coordinate Dy2 ion. Direct current (dc) susceptibility

easurements reveal weak intramolecular ferromagnetic interac-ions between the DyIII ions. The temperature dependence of thec susceptibility, under an optimum field of 1000 Oe, shows twolear peaks, indicating the presence of two relaxation processes. Bytting the relaxation times at different temperature, two energyarriers were obtained, i.e. Ueff = 80 K (�0 = 8.3 × 10−8 s) and 36 K�0 = 4.2 × 10−7 s).

.3.1.1. Helical Dy2 compounds. Three Dy2 compounds (62–64)ith helical structure have been designed by Murugesu though

he careful choices of three Schiff base ligands with two coordinat-ng ends and a rigid spacer [149]. The formulation of those ligands

ig. 37. The crystal structure of two different Dy2 complexes (26 and 61) from salen-type

MM from o-vanillin Schiff base ligand.

(Fig. 38) not only accommodates high coordination numbers of Dyion, but also prevents the chelate effect to encapsulate a singleDyIII ion so as to form the helical structure. Three compounds dis-play the similar structure with two eight-coordinate DyIII ions inan intermediate geometry between a square antiprism (D4d) and adodecahedron (D2d). However, with increasing the length and flex-ibility of the spacer in the ligand, it is expected that the distancebetween the metal centers will increase (10.81 A in 62; 14.87 A in63; and 15.30 A in 64) as well as subtle differences in Dy–O/N bondlengths will result around each metal center. Therefore, this leadsto the different angles between the anisotropic axes (55.10◦ (62),52.14◦ (63) and 85.14◦ (64)) determined by ab initio calculations,and their varying relaxation processes. Under zero dc field, threecompounds demonstrate the similar frequency dependence of theout-of-phase signal (�′′) with no peaks, indicating their fast quan-tum tunneling rates. Under an optimum dc field, distinct relaxationprocesses were observed for them. The variation of �′′ as a function
of temperature exhibits two unique relaxation processes for 62,while only one for 63 and 64. In addition, under an applied field,an opening of a hysteresis loop can be observed below 1.1 K for 62and 5 K for 63 and 64, indicating field-induced SMM-like behavior.
ligands in Ref. [90 and 148]. Crystallographic data available in original reference.


F ical stm

Thu

4

fEtdHeD1tbtampaft

rt

F

A

single-crystals show unusual three-step shaped hysteresis loopsbelow 1.1 K, indicating the typical SMM behavior.

ig. 38. The crystal structures of three Dy2 compounds (62, 63, and 64) with heleso-helical structure. Crystallographic data available in Ref. [149].

hose results suggest the successful synthetic strategy employedere i.e. it is possible to rationally design and synthesize SMMssing ligands which dictate specific structural motifs.

.3.2. Dy3 complexesActually, ligand oximation can open out the Dy3 triangle (39),

orming a linear trinuclear dysprosium complex (65, Fig. 39) [150].ach Dy ion almost retains the dodecahedral coordination geome-ries as those of Dy3 triangle in spite of the small deviation fromodecahedron for two outer DyIII ions (Dy2 and Dy3). A postartree–Fock ab initio calculation estimated the nature and ori-ntation of the single-ion magnetic anisotropy tensor of the threeyIII ions. The Ising axis of central DyIII ion (Dy1) forms angles of7.4 and 11.8◦ to the easy axes of Dy2 and Dy3, respectively, whilehe last two are angled only 5◦ to each other. Those axes seem toe essentially collinear in contrast with the situation in the Dy3riangular cluster, as a result of a favorable dipolar interaction. Inddition, ac measurements show complex slow relaxation of theagnetization with an energy gap (Ueff) of 28.7 and 69.3 K and a

re-exponential factor (�0) of 6.3 × 10−5 and 5.9 × 10−8 s, belownd above 6 K, respectively. This work presents an effective methodor modifying the arrangements of anisotropic axes, then affecting
he magnetic behavior.
Recently, three ferromagnetic coupled Dy3 compounds wereeported by Tong and coworkers [151], and show the similar struc-ural motif only with some small differences compared with the

ig. 39. The crystal structure of linear (65) Dy3 compound from planar ligand.

dapted from Ref. [150].

ructure. 62, 63 show the quadruply-stranded lanthanide helicates and 64 shows

linear Dy3 mentioned above. Ac magnetic susceptibility data forthree Dy3 complexes do show frequency-dependent out-of-phasesignals.

4.3.2.1. Dy3Zn2. Due to the presence of diamagnetic Zn ion,DyIII–Zn complexes behave magnetically as pure DyIII-based SMMs.However, the DyIII–Zn complex affords a good candidate for inves-tigating the nature of the overall exchange interaction between theparamagnetic transition metals (TM) and DyIII ions for isostructuralTM–DyIII complex. To the best of our knowledge, DyIII–Zn com-pounds are still rare, only a few of them, such as DyZn3 [100], DyZn[79], Dy2Zn2 [152] and Dy3Zn2 [153] show SMM behavior.

In 2009, a family of four isostructural Ln3Zn2 complexes witha V-shaped structure (66, Fig. 40) was reported by the group ofMurugesu through the reaction of a salen-type ligand, ZnCl2 andLn(NO3)3 under the presence of NaN3 and NEt3 [153]. A frequencydependent out-of-phase ac signal was observed below 2.6 K bythe ac magnetic measurements, and an effective barrier of 13.4 K(�0 = 3.3 × 10−7 s) was obtained. Micro-SQUID measurements on

Fig. 40. The crystal structure of Dy3Zn2 complexes 66 with salen-type ligands. Crys-tallographic data available in Ref. [153].


F ligandr

4

aaAaotibsTf

capls

ig. 41. The crystal structure of two planar Dy4 complexes with different Schiff baseeference.

.3.3. Planar Dy4In 2008, two slightly different planar Dy4 compounds (67

nd 68; Fig. 41a) with Schiff base ligand (o-vanillin and 2-minoethanol) were synthesized by Powell and coworkers [103].ll DyIII ions are eight-coordinate, displaying a distorted square-ntiprismatic geometry and are linked together by a combinationf �3-hydroxo group. The unique difference between 67 and 68 ishe coordinated anion at one DyIII ion: two chlorides in 67 hav-ng been replaced by azides in 68, which leads to the difference inond lengths. Therefore, ac magnetic susceptibility measurementshow different magnetic slow relaxation behavior for 67 and 68.he maxima of �′′ are clearly observed from 100 to 1500 Hz onlyor 68, with a small barrier, Ueff = 7 K (�0 = 3.8 × 10−5 s).

A similar planar Dy4 compound (69, Fig. 41b) was also dis-overed by Sun and coworkers, through a salen-type ligand and
n ancillary �-diketonate ligand [106], where the ancillary ligandushes DyIII ions into the N2O2 coordination pocket of the salen
igand. All eight-coordinate DyIII ions are located in a distortedquare-antiprismatic geometry. A frequency-dependent signal was

Fig. 42. The crystal structure of the Dy2 SMM (70) with momohyd

s. (a) 67 in Ref. [103]; (b) 69 in Ref. [106]. Crystallographic data available in original

observed in the �′′ vs. T plot below 10 K, but with no maxima.Thereby, an optimum field of 1400 Oe was applied to suppress thequantum tunneling. This results in a signal of �′′ with one broadpeak below 10 K, and an effective barrier of 22 K was obtained with�0 = 3.66 × 10−6 s.

4.4. The isolation of Dy-SMMs based on hydrazone ligand

4.4.1. Monohydrazone4.4.1.1. Dy2 system. In 2008, a Dy2 SMM (70) with momo-hydrazone ligand was obtained through applying the ligand(2-hydroxy-3-methoxyphenyl)methylene(isonicotino)hydrazine(H2hmi) (Fig. 42), showing the typical SMM features with anenergy barrier of 56 K [125]. In addition, further coordination ofthe pyridyl N atoms results in the formation of a two-dimensional
network of the Dy2 complexes, which can display the similarmagnetic properties with Ueff = 71 K.
Meanwhile, in our group, considerable attempts have been car-ried out in this respect so as to design the molecules displaying

razone ligand. Crystallographic data available in Ref. [125].


of H2

dwaacoaf

sfupwtnTo

Fo

Scheme 6. Reversible deprotonation and base-assisted keto-enol tautomerism

istinct anisotropic centers through assembly hydrazone ligandsith DyIII ions. Several interesting systems, such as linear Dy4

nd asymmetric Dy2 compounds have been isolated and discussedbove [28,91]. It has been noticed that such a rigidly linear ligandan provide different kinds of coordination modes in considerationf those multichelating sites and the tautomeric maneuver of theroylhydrazone ligand (Scheme 6, Fig. 45), which are especiallyavorable for the isolation of polynuclear Dy SMMs [154].

In the case of H2ovph (o-vanillin picolinoylhydrazone), the ver-atility of the coordination to DyIII has been shown in Scheme 6,rom the keto-enol tautomerism and deprotonation of H2ovphnder different base condition. Naturally, the use of ovph2− ligandromotes the formation of four Dy2 systems (22, 71–73; Fig. 43)ith distinct structural features, which are mostly responsible for

he distinct relaxation dynamics observed [91,154]. The main, sig-ificant disparities between the four Dy2 cores have been listed inable 1, in which the different bridge atoms in combination withther effects should affects significantly the Dy–O–Dy angles, thus

ig. 43. The comparison of structure and magnetism for four Dy2 complexes with H2ovriginal reference.

ovph and the corresponding coordination modes in complexes 22 and 71–73.

leading to different magnetic interactions between two Dy ions,while the different coordination geometry of each DyIII ion is alsoresponsible for the nature or directions of the easy axes throughthe ligand fields. Firstly, the high energy barriers suggest a fact thatsuch a hula-loop-like geometry may be a suitable and robust LF forslow magnetic relaxation of DyIII ions, as seen in complex 22 and71. Complex 22 demonstrates the strong axiality at each DyIII site,leading to an efficient blocking of magnetization, which has beenrevealed by ab initio calculations. However, in complex 72 and 73,the interposition of another O atom into the coordination spherebreaks the hula-loop-like geometry, thus leading to the weak or dis-appearing relaxation behavior. Secondly, the magnetic interactionsare drastically different among 22 and 71–73 (Fig. 43). These dispar-ities must be caused by crucial structural differences between the
[Dy2(�-O)2] (22, 71, and 72) and [Dy2(�-O)3] (73) cores. In complex73, the additional �-OH bridge results in the variation of Dy-O-Dy angles from only 92.14 to 100.08◦, which are more than 10◦
smaller than those observed in 22, 71 and 72. Those variable angles

ph ligand (22 in Ref. [91]; 71–73 in Ref. [154]). Crystallographic data available in


Table 1The significant disparities between the four Dy2 cores (22 in Ref. [91]; 71–73 in Ref. [154]).

22 71 72 73

Bridge atoms Hydrazone-O (2) Hydrazone-O (2) Hydrazone-O (1)Phenoxide-O (1)

Phenoxide-O (2)�2-OH− (1)

Geometry Dy1 7-Coordinate pentagonal bipyramidal 8-Coordinate hula hoop-like 9-Coordinatemonocappedsquare-antiprismatic

9-Coordinatedistortedmonocappedsquare-antiprismatic

Dy2 8-Coordinate hula hoop-like Same as above Same as above Same as above

69 K

wibiacaitblb

4eehbmwt[ab(lswmiBwcctaz

aitfb

FDbbH[[

of �MT values at low temperature. The ac susceptibility datareveals both complexes show slow magnetic relaxation withanisotropic barriers of 17.14 K and 41.29 K, respectively. Veryrecently, a rare �4-OH centered square Dy4 molecule (79, Fig. 45b),

Ueff 198 K, 150 K

ill modify the overlap of the magnetic orbitals, thus affecting thentradinuclear magnetic interactions. In addition, the difference ofridging O atoms (hydrazone-O and phenoxide-O) might exert an

nfluence on the intradinuclear magnetic interactions with the vari-tions of Dy–O bonds. Eventually, the distinct magnetic behavior ofomplex 22 and 71–73 could be due to the unusual combination ofll those effects such as different coordination geometry and bridg-ng atoms. Such systems provide a promising strategy for enhancinghe single molecule magnet properties of polynuclear lanthanide-ased complexes via fine-tuning of the local environments of the

anthanide ions as well as the intradinuclear magnetic interactionsetween metal centers.

.4.1.2. Molecular assembly from Dy2 building block. Given thexcellent SMM behavior of Dy2 system with hula hoop-like geom-try, our recent efforts have been dedicated to the assembly ofigh-nuclearity lanthanide SMMs using this Dy2 as buildinglocks. Therefore, a Dy6 with a triangular prism arrange-ent (74) and a Dy8 with a tub conformation (75) (Fig. 44),ere obtained by the introduction of carbonate (CO3

2−) fromhe different sources [130]. Obviously, the Dy6 compound,Dy6(ovph)4(Hpvph)2Cl4(H2O)2(CO3)2]·CH3OH·H2O·CH3CN (74),rises from the linking of three Dy2 building blocks by two car-onato ligands in a rare �3-�2:�2:�2-tridentate bridging modeScheme 5). The bridging mode makes the carbon atoms of CO3

2−

ie on the triangular prism quasi-C3 axis and the CO32− ligands

ymmetrically fixed in the center of the Dy6 cluster (Fig. 44),hich insures a rational arrangement of those anisotropic axes. Acagnetic susceptibility measurements show that the maximum

n �′′ is observed at temperatures as high as 15 K for 1500 Hz.y fitting the data, a high effective energy barrier Ueff of 76 Kith �0 = 1.2 × 10−6 s was obtained, which may result from the

ontribution of three well-preserved Dy2 “skeletons”. The Dy8ompound (75) displays an interesting tub conformation, wherehe Dy2 units undergo large distortions. Thus the low-temperaturec susceptibility only shows the increasing in �′′ below 15 K inero dc field.

In addition, a Dy6 compounds with similar structural motifs haslso been assembled by applying a pyrazine hydrazone ligand (X = Nn Scheme 6) [155]. Because of the slight difference in ligand andhe resulting structures, similar relaxation behavior was observedor Dy6 complexes, only with a small reduction in the anisotropicarrier (76–56 K).

Other DyIII compounds with hydrazone ligand are listed inig. 45. In 2011, a new alkoxido-bridged linear tetranuclearyIII aggregate, [Dy4(L)4(MeOH)6]·2MeOH (76), showing SMMehavior with a remarkably large energy barrier of 173 K has
een assembled by some of us using a rigid ligand (H3L), where3L represents the ligand 2-hydroxy-3-methoxybenzoic acid
(2-hydroxy-3-methoxyphenyl)methylene]hydrazide (Fig. 45a)28]. The centrosymmetric complex has a nearly linear Dy4 core,

∼1.5 K No ac signal

characterized by Dy–Dy–Dy angles of 149.99(1)◦. Two crystallo-graphic Dy1/Dy2 ions are eight and nine-coordinate and displaya distorted bicapped trigonal-prismatic and a nearly perfectmonocapped square-antiprismatic geometry, respectively. The �′′

vs. plot clearly indicates two peaks, indicating the occurrence ofa multiple relaxation associated with distinct anisotropic centers.

Furthermore, two Dy2 compounds (77 and 78, Fig. 45c)with remarkably distinct magnetic interactions were synthe-sized by some of us, through using two hydrazone ligands fromthe reactions of 1-hydroxy-2-naphthaldehyde and benzohy-drazide/picolinohydrazide [156]. The Dy2 compound bridgedby hydrazonge-O shows intramolecular weak ferromagneticinteractions, with a larger Dy–O–Dy angle than that of thephenoxido bridging Dy2 complex, which displays a decrease

Fig. 44. Highly axial Dy2 SMM building block and representation of the self-assembly of Dy6 (74) and Dy8 (75) aggregates.

Adapted from Ref. [130].


F ), (b),

r

[HztfdoopdHst

4

ahifittasF4bfcLa[

peaks.The first example of a discrete Dy4 grid (81) exhibiting slow

magnetic relaxation was reported by some of us from the ligand

ig. 45. The crystal structure of Dy complexes from different hydrazone ligands. (aeference.

Dy4(�4-OH)(Hhpch)8)]·(ClO4)3·2CH3CN·MeOH·4H2O (where2hpch = 2-hydroxylbenzaldehyde (pyridine-4-carbonyl) hydra-one), was presented by some of us, displaying field enhancedhermally activated mechanism [107]. Here the �4-OH definesour Dy ions in a square arrangement with the shortest Dy· · ·Dyistance of 3.536 A. All Dy sites are nine-coordinate in the geometryf quasi mono capped square-antiprism. Frequency dependencesf ac susceptibilities in zero and 1 kOe static field both reveal theossible multiple relaxation processes, possibly due to the slightifferences in the local coordination spheres at the DyIII centers.owever, in the high temperature region, the effective barrier is

ignificantly enhanced up to 92 K under 1 kOe field compared withhat of 30.3 K in zero dc field.

.4.2. DihydrazoneIn this field, the dihydrazone ligands are applied rarely to date,

s listed in Scheme 7. The coordination chemistry of L1 and L2as been explored before by the group of L. K. Thompson, prov-

ng to be very effective in producing large numbers of grids withrst-row transition-metal ions [157]. In addition, we see that theridentate pockets in these ligands are favorable for the forma-ion of lanthanide complex. Thus a linear Dy3 complex (80) wasssembled by ligand L1 [158], while two different [2 × 2] LnIII

4quare grids (82 and 83) were obtained by using ligand L2 [159].or ligand L3, two Dy4 compounds (planar and tetrahedral Dy4,3 and 84) have been assembled, where the planar Dy4 (43) haseen discussed above. The ligands (L4–L6) were applied extensivelyor the construction of Dy-SMMs in our group. In this way, heli-
al Dy2 and Dy3 compounds (86–88) were obtained by the ligand6 [160,161], while the approximate planar and grid-like Dy4 (81nd 85) were assembled by the L4 and L5 ligands, respectively162,163].
(c) from Refs. [28,107,156], respectively. Crystallographic data available in original

4.4.2.1. The Dy-based compounds from ligand L1, L2 and L3. A lin-ear Dy3 compound (80, Fig. 46) was obtained from the ligandL1. The putative grids were not formed probably due to the factthat the larger size of the Ln ions compared with transition-metal ions [158]. Two ligands bind the three DyIII ions in thesame relative tridentate coordination pockets in each ligand,thus the central Dy ions are bridged to the outer Dy ionsthrough �-O hydrazone bridges, with bridge angles in the range116.55(9)–117.78(9)◦. Alternating-current (ac) magnetic measure-ments display a frequency-dependent ac signal below 10 K in zerodc field, indicating the presence of slow relaxation, but without full

Fig. 46. The crystal structure of linear Dy3 compound (80) with ligand L1 .

Reprinted with permission from Ref. [158]. Copyright (2011) American ChemicalSociety.


des o

L�ctDcsfioda1atpsfimdfobafi

ions. The calculated directions of the anisotropy axes for the four

Fo

Scheme 7. The coordination mo

4, with the motif shown in Fig. 47a [163]. The ligand adopts a3-�4:�2:�1 fashion (Scheme 7) in one side to bridge three Dy

enters through their hydrazone-O and phenoxide-O atoms, thushe self-assembly of four Dy ions and four ligands leads to they4 unit with a windmill-type motif. Another two grid-like Dy4omplexes (82 and 83, Fig. 47b) were prepared by L.K. Thomp-on et al. from the ligand L2, where the four Dy ions in 83 werexed by an additional �4-oxide group [159]. For 82, four Dy ionsccupy both tridentate N2O ligand pockets, and are bridged by foureprotonated hydrazone oxygen atoms and four �-OH bridges withverage Dy–Ohydrazone–Dy angle of 108.4◦ and Dy–OH–Dy angle of13.5◦. The structure of 83 has similar square grid arrangements that of 82, but the OH bridges are replaced by �-N3

− withhe Dy–N3–Dy angles in the range of 99.5–99.3◦, and an unex-ected �4-O (oxide) ion occupies the central position within thequare. In addition, in 83 Dy–O–Dy angles to the central oxide ionall in the range 88.9–91.4◦, indicating that the Dy4(�4-O) subunits almost planar, and square. Alternating-current (ac) measure-

ents on 81 and 82 showed no ac signal (�′′) (<20 K) in zeroc fields, suggesting the absence of a SMM behavior. However,or 81, under 900 Oe dc field, the peak in the �′′ as a functionf the frequency is observed, indicating the field induced-SMM
ehavior. For 83, alternating current (ac) measurements showed
clear frequency dependent signal in �′′ below 35 K in zero dceld, with a large shoulder around 20 K, which confirms the SMM

ig. 47. The crystal structure of three Dy4 grid complexes, (a) 81 from ligand L4 in Ref.riginal reference.

f different dihydrazone ligands.

behavior. Two effective barriers, 51 K (�0 = 3.0 × 10−9 s) and 91 K(�0 = 4.5 × 10−7 s), were obtained by Arrhenius law. Furthermore, alarger barrier, Ueff = 270 K (�0 = 4.0 × 10−10 s), was shown by apply-ing a dc field of 1600 Oe. The remarkable disparities in magneticdynamics among 81–83 may be driven by the unusual combina-tion of �2-O hydrazone, �2-N3 and �4-O bridges. In particular, thecentral �4-O should be an important feature in defining the clearunique anisotropy axis of each DyIII ion and creates possibilities forboth ferromagnetic and antiferromagnetic exchange respectivelybased on Dy–O–Dy angles close to 90◦ and 180◦.

In 2011, Murugesu’s group reported the first example of a tetra-hedral Dy4 SMM with coordination-induced chirality (84, Fig. 48)[164], where four octacoordinate DyIII ions arrange in a distortedtetrahedral fashion around the central �4-O atom with a Dy–Odistance of 2.9 A. The synthetic methodology is critical in the for-mation of tetrahedral Dy4 core. The in situ condensation reactionpromotes the formation of fully formed beh2− (L3, bis(2-hydroxy-3-ethoxybenzylidene)hydrazone) and partially formed esh− ligands(3-ethoxy salicylaldehyde hydrazone). An exchange parameterJ = −0.3 cm−1 was obtained by ab initio calculations, confirmingthe overall antiferromagnetic exchange coupling between the DyIII

DyIII sites are shown in Fig. 48. The observed step in the hysteresisloop that occurs at 0.6 T may be associated with the antiferromag-netic exchange between DyIII ions.

163; (b) 82, (c) 83 from ligand L2 in Ref. [159]. Crystallographic data available in


F III gnetica

R

4poitueob

pa(DbaLtectoecodt

l�

Ft

ig. 48. Structure and the calculated anisotropy axes on Dy sites of antiferromantiferromagnetic Dy4.


.4.2.2. The Dy-based compounds from ligand L5 and L6. A Dy4 com-ound (85) from L5 reported by our group demonstrates the linkingf two edge-sharing Dy3 triangle units through two �3-O, sim-lar to the planar Dy4, but with the dihedral of 154.5◦ betweenhem (Fig. 49) [162]. Any out-of-phase ac signal was not observednder a zero-dc field within the available frequency range. How-ver, the application of a dc field H = 1200 Oe induces the presencef two-relaxation signal, indicating the possible multiple relaxationehavior.

Recently, one area of attention is the investigation of the com-lexes with helical structures, containing single-, double-, triple-nd even quadruply-stranded helicates [149]. Three complexesFig. 50), a Dy2 with triple-stranded helicates (86) [160] and twoy3 with circular helicates (87 and 88) [161], have been preparedy some of us from the ligand L6. Here the ligand design should be

key for the formation of helical structures. By comparing ligand5 and L6 (Scheme 7), we can see the phenoxide-O group leads tohe formation of two coordination pockets (O2N), which are largenough to encapsulate two Dy ions. Thus this rigid ligand with twooordinating ends is very favorable for constructing helical struc-ures. In addition, the helical structures could result in the presencef two chiral configurations i.e. � and , but the mixture of bothnantiomers in the single crystal leads to a loss of chirality. For Dy2ompound (86), alternating current (ac) measurements show nout-of-phase ac signal under zero dc field, but an obvious frequencyependent ac signal is detected under a 700 Oe dc field, indicating
he onset of the slow magnetization relaxation.
For two Dy3 compounds (87 and 88), a remarkable differenceies in the �3-bridging atoms: the Dy3 triangle is capped by two

3-methoxy oxygens in complex 87, while in complex 88 by one

ig. 49. The crystal structure of Dy4 compound (85) with two edge-sharing Dy3

riangle units from ligand L5 . Crystallographic data available in Ref. [162].

tetrahedral Dy4 (84, left); Field dependence of the normalized magnetization for

�3-OH and one �3-N3− above and below the plane. (Fig. 50b)

This leads to their dramatically different dynamic behavior, com-bined with the distinct coordination environments. Complex 87contains two distinct metal centers, with one nine-coordinate Dyin a distorted tricapped trigonal-prismatic surrounding and twoeight-coordinate DyIII centers in the distorted bicapped trigonal-prismatic geometry. Alternating current (ac) measurements showobvious temperature-dependent ac signal with a broad peakbetween 2 and 8 K in the range 100–1200 Hz and a tail of the peakbelow 2 K, indicating the presence of multiple relaxation processes.In contrast, for complex 88 all three DyIII ions are the same nine-coordinate with a distorted tricapped trigonal-prismatic geometry.Thus only a temperature-dependent ac signal without maxima isobserved below 7 K, indicating the onset of slow magnetizationrelaxation.

4.5. Molecular assembly from macrocyclic ligand

Because of the limited internal size of planar macrocycle, onlymixed 3d–4f complexes such as LnZn3 have been isolated and theirstructure and magnetism have been discussed in detail above. Herewith increasing flexibility of ligand through the simple reduction ofSchiff base C N double bonds, we were able to encapsulate threeDy ions inside the cavity of a macrocycle ligand, obtaining twonovel triangular Dy3 complexes (89 and 90; Fig. 51) [165]. Twocomplexes have the almost identical Dy3 arrangement with eacheight-coordinate DyIII ion in distorted dodecahedral geometry. Thedifferences arise from the different coordinating atoms: in com-pound 89, Dy1 and Dy2 are coordinated by one mono-dendatenitrate anion and one H2O molecule in axial positions on oppo-site sides of the macrocycle, while Dy3 is completed by two H2Omolecules; but in compound 90, one SCN− anion and one H2Omolecule coordinate Dy1 and Dy2 in axial positions on oppositesides of the macrocycle, and two SCN− anions complete Dy3. There-fore, the distinct magnetic behavior was observed in compound89 and 90. Compound 89 shows the obvious temperature- andfrequency-dependent ac signals below 7 K with no peaks underzero dc field, and an application of dc field has no effect on itsrelaxation behavior. In contrast, compound 90 shows no obviousout-of-phase ac signal under zero dc field. However, an applicationof dc field leads to the presence of two-step relaxation behavior.This work represents a successful rational design of Dy3 trianglestrapped in a macrocyclic ligand showing slow magnetic relaxationperturbed by axial ligands.

4.6. DyIII system supported by calix[4]arene

Calix[4]arene-type ligands have been applied extensively intransition metal and 3d–4f systems [166]. However, the purely


Fig. 50. The crystal structure of Dy2 and Dy3 compounds (86–88) with helical structure from ligand L6 in Refs. [160,161]. Crystallographic data available in original reference.

acroc

lskii

lsitsan

F

Fig. 51. The crystal structure of two Dy3 complexes with m

anthanide-based complexes supported by this kind of ligand aretill unusual, especially in the field of SMM. To the best of ournowledge, only several DyIII-based compounds (91–94) contain-ng calix[4]arene-type ligands were reported in this field, as shownn Fig. 52.

In 2009, the first Dy4 compound (91) with the calix[4]arene-typeigands was reported by Liao and coworkers [167]. The structure ishown in Fig. 52a, revealing a square arrangement for four DyIII

ons, which are bonded by two tail-to-tail thiacalix[4]arene ligandso form a sandwichlike entity. The � -OH occupies the center of Dy
4 4quare, which is similar to the two Dy4 grids in Figs. 45b and 47c.c susceptibilities data only shows the temperature-dependent sig-als with no peaks for the Dy4 complex.
ig. 52. The crystal structure of four Dy-based complexes with Calix[4]arene-type ligand

yclic ligands. Crystallographic data available in Ref. [165].

The other Dy4 compound (92) with a cubane structure wasreported by Liu et al., through applying a different calix[4]areneligand, H4TBSOC (p-tert-butylsulfonylcalix[4]arene) (Fig. 52b)[168]. Here all four Dy ions are eight coordinate and bridged by four�3-OH bridges to form the [Dy4(OH)4]8+ core with Dy–Ohydroxy–Dyangles in the range of 105.7(4)–107.9(3)◦ (>99◦). Surprisingly, allfour Dy atoms have been presumed in disordered sites with thesame occupancy factor of 0.5. The ac susceptibility data indicate thetypical SMM features. An effective barrier of 22.9 K was obtainedwith � = 1.1 × 10−8 s. Therefore, the Dy compound represents the
0 4first SMM with a holistically disordered metal cluster core.
In 2011, a Dy6 compound (93) with two TBC[4] ligands i.e. p-tert-Butylcalix[4]arene was reported by Brechin and coworkers,

s, (a) 91, (b) 92, (c) 93 and (d) 94. Adapted from Refs. [167–170], respectively.

istry Reviews 257 (2013) 1728– 1763 1757

stbr(b[ectiowaefi

5

usiCtwuDc

Fi

P. Zhang et al. / Coordination Chem

howing a rare octahedral arrangement connected internally bywo �4-O, as presented in Fig. 52c [169]. Regretfully, its magneticehavior did not show any signs of slow magnetic relaxation, thusuling out the possibility as a SMM. A very similar Dy6 compound94) was reported through the same ligand by Liao and coworkers,ut with the Dy ions in different coordinated geometry (Fig. 52d)170]. The former has two nine-coordinate apical Dy ions and fouright-coordinate DyIII ions on the periphery, while three uniquerystallographic Dy ions are seven-, seven- and eight-coordinate forhe latter Dy4 compound. Therefore, the distinct relaxation behav-or was observed between two Dy6 compounds. For the latter Dy6,bvious frequency-dependent both in- (�′) and out-of-phase (�′′)ere demonstrated under a zero dc field ranging from 0.4 to 15 K,

nd the broad peak with a shoulder at 1500 Hz suggests the pres-nce of multi-relaxation processes. This example represents therst single molecule magnet of pure lanthanide clusters of TBC [4].

. Dyn systems with higher nuclearity (n > 5)

For most Dy systems with high nuclearity, the structures aresually constructed from assembly of several stable building blocks,uch as Dy3 triangles, Dy4 cubanes and Dy5 pyramids. Those build-ng blocks are bridged by some small bridging ligands, such as OH−,O3

2−, and NO3−. We summarize these high nuclearity DyIII sys-

ems here. However, their SMM properties are not so appealing,
hich may be ascribed to an effective compensation of the individ-al magnetic contributions from all DyIII ions in the polynuclearyIII compounds [159]. In addition, a few examples (n > 5) withircular arrangement of all Dy ions have been also presented.
ig. 54. The crystal structure and sketch maps of different Dy-based complexes built fromn Ref. [174]; (e) Dy10 in Ref. [172]; (f) Dy12 in Ref. [173]. Crystallographic data available i

Fig. 53. The crystal structure of Dy6 complex (95) reported by Murugesu. Crystal-lographic data available in Ref. [171].

5.1. Dy systems from Dy3 triangle building blocks

The most typical examples are the construction of three Dy6compounds (40, 41 and 95) reported by Powell and coworkers,respectively [54,124,171]. For 40, the reduction of the aldehyde toan alcohol for one of the three o-vanillinato ligands in Dy3 archetypeprovides the R-O− bridge, thus leading to the head-to-head linkingof two Dy3 triangles, as shown in Fig. 24 [54].

In the second example, the linking of two Dy3 archetypes
arises from an in situ aldol condensation reaction between someof the o-vanillin molecules and acetone, forming a new aldolbridging-ligand (95, Fig. 53) [171]. Below 1 K, the unusual two-stephysteresis loops were observed, possibly arising from the toroidal
Dy3 building blocks. (a) and (b) Dy6 in Refs. [57,58]; (c) Dy8 in Ref. [143]; (d) Dy7

n original reference.


Fig. 55. (Top) The construction of Dy complex (96) from Dy cubane from Ref. [141]. (Bottom) The construction of Dy net from the dumbbell {DyIII } moieties (97) in Ref.[

atFDtbt

F[

8 4

175]. Crystallographic data available in original reference.

rrangement of the magnetic moments on the DyIII centers inhe triangular units. The third example is the Dy6 SMM (41,ig. 25) with enhanced toroidal magnetization, where the twoy3 triangles are arranged in a robust “edge-to-edge” fashion

hrough a strong �4-O2− linkage [124]. Other examples haveeen presented in Fig. 54 containing their sketch maps from Dy3riangles [57,58,143,172–174].

ig. 56. The construction of Dy-based complexes from cubic Dy4 building blocks. (a), (b)133]. Crystallographic data available in original reference.

4 4 2

5.2. Dy systems from Dy4 cubane building blocks

In Fig. 55a, the application of different DyIII salts and the sameligand under several different solvents leads to the transforma-
tion from Dy4 cubane (54) to Dy8 compound (96) composed oftwo Dy4 cubanes [141]. The formation mechanism for these dif-ferent architectures is not clear as yet. In Dy8 compound, two
Dy8 in Refs. [177,178]; (c) Dy11 in Ref. [178]; (d) Dy12 in Ref. [176]; (e) Dy26 in Ref.

istry Reviews 257 (2013) 1728– 1763 1759

sccpft

stTifcsliwsD

5

wtpagAactTt

Fd

FC


ymmetry-related [Dy4(�3-OH)4] cubanes doubly bridged by �-arboxylato groups, belonging to two shared ligands. The Dy4ompound exhibits no SMM behavior, while an obvious out-of-hase ac signal was observed in the Dy8 complex, which may ariserom the larger Dy–O–Dy angles (102.5(4)–109.3(4)◦) in Dy8 thanhat in Dy4 compound (99.00–109.50◦), as discussed above.

Another representative example is the construction ofupramolecular two-dimensional networks of Dy4 cubane fromhe dumbbell {DyIII

4}2 moieties (97), as shown in Fig. 55b [175].he assembly of a supramolecular compound from intermolecularnteractions greatly hinges on the ligand design, leading to theormation of hydrogen bonds or �–� bonds. In this example, thelose electron-rich functional triazole aromatic rings in crystaltructure results in the strong �–� interactions between them,eading to the 2D organization of the Dy8 molecules. Here anncrease of the out-of-phase component of the ac susceptibility

as observed at 1500 Hz and 1.8 K, suggesting the presence of alow relaxation of the magnetization. Other examples, Dy8, Dy11,y12, Dy26, have been presented in Fig. 56 [133,176–178].

.3. Dy systems from Dy5 pyramid building blocks

To date, two sandglass-like Dy9 compounds (98 and 99, Fig. 57)ere reported by our group and Stamatatos and coworkers, respec-

ively [179,180]. Both are constructed by two square pyramidalentanuclear units via the apical metal center DyIII ion. The lig-nds in two Dy9 complexes appear to be similar with the R-OHroup bridging two close DyIII ions located in the square-based face.dditionally, nine DyIII atoms were held together by two �4-OHnd eight �3-OH. In contrast with Dy5 pyramid archetype, two Dy9
ompounds show no SMM behavior, possibly arising from the head-o-head arrangement of two square pyramidal pentanuclear units.wo more examples, Dy6 and Dy7 compound, shows the bottom-o-bottom linking of two Dy5 pyramids, as shown in Fig. 58a and b
ig. 58. The crystal structure of Dy6, Dy7 and Dy14 compound from Dy5 building blocks.ata available in original reference.

ig. 59. The crystal structure of Dy6, Dy7 and Dy10 compound with cyclic structure. (rystallographic data available in original reference.

Fig. 57. The crystal structure of two sandglass-like Dy9 compounds (98 and 99) fromRefs. [179,180]. Crystallographic data available in original reference.

[181,182]. A larger Dy14 cluster is also shown in Fig. 58c, adoptingthe two linking fashions (head-to-head, bottom-to-bottom) [183].

5.4. Dy systems with cyclic structure

Three examples (Dy6, 100; Dy7, 101; and Dy10, 102), where allDy ions show a circular arrangement, have been exhibited in Fig. 59.The Dy7 compound (101, Fig. 59b) presents a perfect discs-likearrangement, showing single molecule magnet (SMM) behaviorwith a large energy barrier of 140 K [134]. The central Dy ion liesjust out of the Dy6 plane, thus being disordered over two sites and
is bound to the peripheral Dy ions by six �3-OH groups, alter-nating above and below the Dy7 plane. This motif appears to bevery helpful for the arrangement of anisotropic axis of each Dy ion.Therefore, ac susceptibility measurements display the frequency
(a) Dy6 in Ref. [181]; (b) Dy7 in Ref. [182]; (c) Dy12 in Ref. [183]. Crystallographic

a) 100, Dy6 in Ref. [184]; (b) 101, Dy7 in Ref. [134]; (c) 102, Dy10 in Ref. [185].

1 istry R

dbiwr[tda

6

aitetmwtTtcfiescttitam

760 P. Zhang et al. / Coordination Chem

ependence of in-phase (�′) and out-of-phase (�′′) componentselow ∼28 K, with multiple peaks at some frequency, indicating

ts SMM behavior. In addition, the Dy6 and Dy10 compound withheel-like structure (100, Fig. 59a and 102, Fig. 59c) have been

eported from two different ligands with multiple alcohol–O atoms184,185]. Recent investigations indicate that the high symmetry ofhe Dy6 complex (100) in combination with strong intramolecularipolar interactions between Dy ions leads to the net toroidalrrangement of the local magnetic moments [186].

. Conclusions

Remarkably, a strong interest has been developed for lookingt the design and the relaxation mechanism of lanthanide SMMsn the chemistry and physics communities, which is evident fromhe sheer number of research papers published and the ever-xpanding scope of the research. Many examples have indicatedhat lanthanide elements, especially Dy, display the superiority in

agnetism over transition metal as a result of their ground statesith highly anisotropic angular momentum. For instance, from

he first single molecule magnet discovered in 1993 (Hysteresis = 3 K in Mn12 [3]) to present, the highest hysteresis tempera-ure increased only 50% (T = 4.5 K in Mn6 [17]) for transition metallusters, while approximately 370% (T = 14 K in N2

3−–Tb2 [56]) for-elements [64]. Therefore, the interest in lanthanide-based SMMs being continued with the goal of obtaining the SMM with higherffective barrier and blocking temperature, where the synthetictrategy should be the most crucial factor. It provides a tailoredhemical environment (ligand field) to trap anisotropic ions andhe alteration of the ligating groups available within the ligando favor certain electronic states for the ion/aggregate will in turnnfluence the magnetic relaxation. The simplest strategy is to find
he relation between the chemical environment of metal centersnd the magnetic anisotropy of the compound/fragment, like theodel developed by Long and coworkers, which could further give
Index Complex

1 Dy-DOTA (Fig. 1)

2 [Dy(hfac)3-(NIT-R)2] (Fig. 2)

3 [{Pc(OEt)8}2Tb]+

4 [{Pc(OEt)8}2Dy]+

5 [{Pc(OEt)8}2Dy]−

6 Dy(Pc)(TClPP) (Fig. 4 left)

7a Dy{Pc(�-OC5H11)4}(TClPP)

7b DyH{Pc(�-OC5H11)4}(TClPP)

8 [Dy(acac)3(H2O)2] (Fig. 5)

9 [Dy(acac)3(dpq)] (Fig. 6)

10 [Dy(acac)3(dppz)] (Fig. 6)

11 [Dy(acac)3(phen)] (Fig. 6)

12 [Dy(TTA)3(bpy)] (Fig. 7)

13 [Dy(TTA)3(phen)] (Fig. 7)

14 (Cp*)Ln(COT) (Fig. 9a)

15 Ln(COT)2 (Fig. 9b)

16 DyZn (Fig. 10)

17 [Zn3Dy(LPr)(NO3)3(MeOH)3]·4H2O, [Zn3Dy(LPr)(NO3)3

18 [ErZn3(L)(OAc)(NO3)2(H2O)1.5(MeOH)0.5] (Fig. 11b)

19 Zn3Er(LPr)(NO3)3·3H2O·2MeOH, Zn3Yb(LPr)(NO3)3·7H20 Dy(H2BPzMe2

2)3

21 {[(Me3Si)2N]2(THF)Dy}2 (�-�2:�2-N2) (Fig. 12)

22 [Dy2ovph2Cl2(MeOH)3]3·MeCN (Fig. 13)

23 Tb2Pc2obPc (Scheme 3)

24 [Tb2(obPc)3] (Fig. 4 right)

25 (Pc)Tb(Pc)Tb(T(p-OMe)PP) (Fig. 4 middle)

26 [Dy2(valdien)2(NO3)2] (Fig. 16)

27 [Dy2CoIII2(OMe)2(teaH)2(O2CPh)4(MeOH)4](NO3)2·M

28 [Dy2(OAc)6(H2O)4]·4H2O (Fig. 19a)

29 [Dy2(BuCOO)6(MeOH)2(H2O)2] (Fig. 19b)

30 [Dy2(3-Htzba)2(3-tzba)2(H2O)8]·4H2O (Fig. 19c)

31 [Dy2(Acc)4(H2O)8]·Cl6·5.89H2O (Fig. 20a)

eviews 257 (2013) 1728– 1763

rise to effective targeting of new single molecule magnets withhigh anisotropy barriers [64]. However, the determination of thisrelationship is fraught with difficulties because of the extremelycomplex electronic structure of lanthanide ions. As a result, themodels of complexes with typical SMM behavior demonstrate thecrucial role in designing the new lanthanide-based SMMs, suchas Dy-based SIMs with D4d symmetry, Dy2 complex with hulahoop-like geometry and the Dy3 archetype. They provide signifi-cant insight for the rational design of molecule-based magnets interms of the coordinated geometry around Dy ion and the assemblyof multinuclear Dy compound. In addition, with recent develop-ments in the successful application of ab initio calculations toopen-shell systems, it would seem that the methods represent aunique tool because they allow an accurate prediction of the localanisotropy [53,54,150]. Further high level ab initio or DFT inves-tigations could provide useful clues to predict and elucidate therelationship between the electronic structure and relaxation pro-cess.

It is envisaged Dy-based SMM could be harnessed to providequantum-based spintronic devices and high-density data stor-age media. The studies so far have provided some very valuableindicators for the structural features required to optimize the con-tribution of an Ising type spin to a molecular magnet, but this taskis far from straightforward, thus requiring a multi- and interdisci-plinary vision of scientific research by bringing together syntheticand physical chemists as well as physicists to achieve the final goal.

Acknowledgement

We thank the National Natural Science Foundation of China(Grants 91022009, 21241006 and 21221061) for financial support.

Appendix A.

Ref.

[71,74][30][84][85][85][86][86][86][70][92][92][93][94][94][98][77][79]

(H2O)3]·2H2O (DyZn3, Fig. 11a) [100,101][80]

2O (Er(Yb)Zn3) [101][105][5][91]
[87][89][88][90]
eOH·H2O (Fig. 18) [113][114][115][116][117]

istry Reviews 257 (2013) 1728– 1763 1761

A

Ref.

[118][118][120][120][121][122][122][122][119][27]

hydroxymethyl-6-methoxyphenol) [54][124][123][55][59][129]

droquinoxaline-2,3-dione) [131][132][135]

N-(2-carboxy-phenyl)salicylidenimine) [136][137][138]

b) [139]H (Fig. 33a) [140]2H2O (Fig. 33b) [141]

[117]H3 = [2,6-Bis[N,N-di(2-hydroxyethyl)aminomethyl]phenol]) [142]4b, pdmH2 = pyridine-2,6-dimethanol) [143]) [144]

[29][145]

e) -o-phenylenediamine) [148][149][149][149]

anox = o-vanillin oxime) [150][153]

a, hmmpH2 = 2-[(2-hydroxyethylimino)methyl]-6-methoxyphe-nol) [103][103][106][125][154]

[154][154]

. 44) [130][130][28][156][156]

) [107][158]

) [163][159][159][164]

p = 2,6-(picolinoylhydrazone)pyridine) [162][160]

) [161]b) [161]

[165][165]

g. 52a, H4PTC4A = p-phenylthiacalix[4]arene) [167][168]

) [169]] (Fig. 52d) [170]CO (Fig. 53) [171]5a) [141]cpt = 4-(4-carboxyphenyl)-1,2,4-triazole ligand) [175]) [179]


ppendix A (Continued )

Index Complex

32a [Dy2(phen)2(L)6]·2H2O (HL = �-naphthoic acid, Fig. 20b)

32b [Dy2(phen)2(L)6]

33 [{Cp2Dy(�-bta)}2] (Fig. 21a)

34 [{Cp2Dy[�-N(H)pmMe2]}2]

35 [Dy2(hfac)6(H2O)4pz]2·pz (Fig. 21b)

36a [(�5-Cp)2Dy(�-Cl)]n (n = 2, Fig. 22a)

36b [(�5-Cp)2Dy(�-Cl)]n (n = ∞, Fig. 22b)

37 [(�5-Cp)2(thf)Dy(�-Cl)]2 (Fig. 22c)

38 [{Cp′2Dy(�-SSiPh)}2] (Fig. 23)

39 [Dy3(�3-OH)2(o-vanillin)3Cl2(H2O)4] (Fig. 24)

40 [Dy6(�3-OH)4(o-vanillin)4L2(H2O)9Cl]Cl5·15H2O (Fig. 24, L = 2-41 [Dy6L4(�4-O)(NO3)4(CH3OH)]·CH3OH (Fig. 25)

42 [Dy4(�3-OH)2(�-OH)2(2,2-bpt)4(NO3)4(EtOH)2 (Fig. 26)

43 [Dy4(�3-OH)2(bmh)2(msh)4Cl2] (Fig. 27)

44 [Dy2(HBpz3)4(�-ox)]·2CH3CN·CH2Cl2 (Fig. 29a)45 [Dy4(3-bpp)3(CO3)6(H2O)3]·DMSO·18H2O (Fig. 29b)

46 (Me4N)6{K2(H2O)4[Dy2L6(�-CO3)]}·nH2O (Fig. 29c, L = 1,4-dihy47 Dy6(teaH)2(teaH2)2(CO3)(NO3)2(chp)7(H2O)](NO3) (Fig. 29d)48 [Dy3(HL)5L2(phen)3] (Fig. 30, H2L = salicylic acid)

49 [Dy4(L)4(HL)2(C6H4NH2COO)2(CH3OH)4]·5CH3OH (Fig. 31a, L =50 [Dy4(L)2(C6H5COO)12(MeOH)4] (Fig. 31b)

51 [Dy4(�3-OH)2(mdeaH2)2(piv)8] (Fig. 32a)

52 [Dy4(�3-OH)2(o-vanillin)4(piv)4(NO3)2]·CH2Cl2·1.5H2O (Fig. 3253 [Dy4(�3-OH)4(isonicotinate)6(py)(CH3OH)7] (ClO4)2·py·4CH3O54 [Dy4(HL)4(C6H4NH2COO)2(�3-OH)4(�-OH)2(H2O)4]·4CH3CN·155 [Dy4(�3-OH)4(Acc) 6(H2O)7(ClO4)](ClO4)7·11H2O (Fig. 33c)

56 [Dy4(dhampH3)4(NO3)2]·2(NO3)·3CH3OH·H2O (Fig. 34a, dhamp57 [Dy4(pdmH)2(pdm)4(PhCO2)2(PhCO2H)4]·3CH3OH·3H2O (Fig. 358 [Dy5(�4-OH)(�3-OH)4(�-�2-Ph2acac)4(�2-Ph2acac)6] (Fig. 35b59 [Dy5(�5-O(OiPr)13] (Fig. 35a)

60 [Dy5(�3-OH)6(Acc)6(H2O)10]·Cl9·24H2O (Fig. 35c)

61 [Dy2(L)2(acac)2(H2O)]·2CH2Cl2 (Fig. 37b, L = N,N-bis(salicyliden62 (NEt4)2[Dy2(L62)4]((CH3)2CO)0.25 (Fig. 38)

63 (NEt4)2 [Dy2(L63)4](H2O)(DMF)0.5 (Fig. 38)

64 (NEt4)2[Dy2(L64)4](Et2O)2((CH3)2CO)1.5 (Fig. 38)

65 [Dy3vanox2(Hvanox)4(EtOH)2](ClO4)·1.5EtOH·H2O (Fig. 39, H2v66 [Zn2Dy3(�-salen)3(N3)5(OH)2] (Fig. 40)67 [Dy4(�3-OH)2(hmmpH)2(hmmp)2(Cl)4]·3MeOH·MeCN (Fig. 4168 [Dy4(�3-OH)2(hmmpH)2(hmmp)2(N3)4]·4MeOH (Fig. 41a)

69 [Dy4(�3-OH)2L2(acac)6]·2H2L·2CH3CN (Fig. 41b)

70 [Dy2(hmi)2(NO3)2(MeOH)2] (Fig. 42)

71 [Dy2(ovph)2(NO3)2(H2O)2]·2H2O (Fig. 43)

72 [Dy2(Hovph)(ovph)(NO3)2(H2O)4]·NO3·2CH3OH·3H2O (Fig. 43)73 Na[Dy2(Hovph)2(�2-OH)(OH)(H2O)5]·3Cl·3H2O (Fig. 43)74 [Dy6(ovph)4(Hpvph)2Cl4(H2O)2(CO3)2]·CH3OH·H2O·CH3CN (Fig75 [Dy8(ovph)8(CO3)4(H2O)8]·12CH3CN·6H2O (Fig. 44)

76 [Dy4(L)4(MeOH)6]·2MeOH (Fig. 45a)

77 [Dy2(HL77)4(CO3)]·4H2O (Fig. 45c)

78 [Dy2(L78)2(NO3)2(CH3OH)2]·4CH3CN (Fig. 45c)

79 [Dy4(�4-OH)(Hhpch)8)]·(ClO4)3·2CH3CN·MeOH·4H2O (Fig. 45b80 [(2pomp-2H)2Dy3(NO3)5(DMF)](DMF) (Fig. 46)

81 [Dy4(HL4)4(MeOH)4]2·7CH2Cl2·MeOH (Fig. 47a, L4 in Scheme 782 [Dy4(H2L2)2(HL2)2(OH)4]·Cl2·8H2O (Fig. 47b, L2 in Scheme 7)

83 [Dy4(H2L2)2(HL2)2(N3)4(O)]·14H2O (Fig. 47c, L2 in Scheme 7)

84 [Dy4(�4-O)(�-OMe)2(beh)2(esh)4]·3MeOH (Fig. 48)

85 [Dy4(�3-OH)2(php)2(OAc)6(H2O)2]·4MeOH·nH2O (Fig. 49, H2ph86 [Dy2(L6)3](ClO4)3·6CH3OH (Fig. 50a, L6 in Scheme 7)

87 [Dy3(�3-OCH3)2(HL6)3(SCN)]·4CH3OH·2CH3CN·2H2O (Fig. 50b88 [Dy3(�3-N3)(�3-OH)(H2L6)3(SCN)3](SCN)·3CH3OH·H2O (Fig. 5089 [Dy3L(�3-OH)2(NO3)2(H2O)4]·2NO3·6MeOH·H2O (Fig. 51a)

90 [Dy3L(�3-OH)2(SCN)4(H2O)2]·3MeOH· 2H2O (Fig. 51b)

91 [Dy4(�4-OH)(PTC4A)2Cl3(CH3OH)2(H2O)3]·4.7CH3OH·2H2O (Fi92 [Dy4(OH)4(TBSOC)2(H2O)4(CH3OH)4]·4H2O (Fig. 52b)

93 [Dy6(TBC[4])2O2(OH)3.32Cl0.68(HCO2)2(DMF)8(H2O)0.5] (Fig. 52c94 [Dy6(�4-O)2(TBC[4])2(NO3)2(HCOO)2(CH3O)2(DMF)4(CH3OH)4

95 [Dy6(�3-OH)4(ovn)4(avn)2(NO3)4(H2O)4](NO3)2·(H2O)3·(CH3)2

96 Dy8(HL)10(C6H4NH2COO)2(�3-OH)8(OH)2(NO3)2(H2O)4] (Fig. 597 [Dy4(�3-OH)2(�3-O)2(cpt)6(MeOH)6(H2O)]·15H2O (Fig. 55b, H98 [Dy L (� -OH) (� -OH) (CH OH) ](OH)(CH OH) (Fig. 57 left
9 8 3 8 4 2 3 8 3 3
99 [Dy9(OH)10(hmp)8(NO3)8(DMF)8](OH)·1.6H2O·0.6CH2Cl2 (Fig. 57 right) [180]100 [Dy6(teaH)6(NO3)6]·8MeOH (Fig. 59a) [184]101 [Dy7(OH)6(thmeH2)5(thmeH)(tpa)6(MeCN)2](NO3)2 (Fig. 59b, thmeH3 = tris(hydroxymethyl)ethane; tpaH = triphenylacetic acid) [134]102 Dy10(OC2H4OCH3)30 (Fig. 59c) [185]

1 istry R

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762 P. Zhang et al. / Coordination Chem

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