Inorganica Chimica Acta 361 (2008) 3663–3668

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Inorganica Chimica Acta

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Synthesis, structural, magnetic and high frequency EPR studies on ahexametallic Fe(III) complex with a highly rhombic S = 5 ground state

Paul Cooper a, Floriana Tuna a, Muralidharan Shanmugam a, Lorenzo Sorace b, Sarah L. Heath a,David Collison a, Eric J.L. McInnes a,*, Richard E.P. Winpenny a,*

a School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UKb Laboratorio di Magnetismo Molecolare, Dipartimento di Chimica, Polo Scientifico, Via della Lastruccia 3, 50019 Sesto Fiorentino, Italy

a r t i c l e i n f o a b s t r a c t

Article history:Received 10 January 2008Received in revised form 18 March 2008Accepted 18 March 2008Available online 23 March 2008

Dedicated to Prof. Dante Gatteschi.

Keywords:Iron clustersHigh spin ground statesMagnetic studiesHF-EPR

0020-1693/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.ica.2008.03.081

* Corresponding authors. Tel.: +44 161 275 4469;McInnes).

E-mail address: eric.mcinnes@manchester.ac.uk (E

The synthesis, crystal structure and magnetic properties of the hexametallic Fe(III) cluster½FeIII

6 O2ðOHÞ2fðC4N2H2SMeÞ2CðOHÞOg2ðtBuCO2Þ10� (1) are reported. Compound 1 was prepared by reactionof mixed-valent [Fe3O(tBuCO2)6(py)3] with one equivalent of the new ligand di-2-(4-methylthio)pyrim-idyl ketone (DTPK) in MeCN solution. Compound 1 consists of a planar array of six Fe(III) ions comprisingtwo [Fe3(l3-O)]7+ subunits that are related by an inversion centre and linked at two of their apices, eachvia one l-hydroxide and two l-carboxylate groups. Variable temperature magnetic susceptibility and lowtemperature magnetization measurements show a well isolated S = 5 electronic spin ground state. Vari-able temperature W-band EPR measurements on a polycrystalline sample give well resolved spectra andsimulations confirm a negative zero-field splitting of D = �0.224 cm�1 but also reveal a very rhombic nat-ure (E/|D| = 0.27). Although 1 has a well isolated S = 5 ground state with a negative magnetic anisotropy, itis not a SMM due to this large rhombicity.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

The observation by Gatteschi and co-workers that individualmolecules can retain their magnetization in the absence of an ap-plied magnetic field [1] gave rise to the class of materials now called‘single molecule magnets’ (SMMs) [2]. The energy barrier to relaxa-tion of magnetization is caused by a large electronic spin groundstate S with a significant negative (D < 0) zero-field splitting (ZFS)and zero or small rhombicity (E/D) as defined byH ¼ DbbS2

z � SðSþ 1Þ=3c þ EðbS2x � bS2

yÞ. The two most studied SMMsto date are the mixed-valent ½MnIII

8 MnIV4 O12ðO2CRÞ16ðH2OÞ4� family,

originally prepared by Lis [3a] (R = Me) then by Christou andco-workers (R = Ph) [3b], and ½FeIII

8 O2ðOHÞ12ðtacnÞ6�Br8 ðtacn ¼1;4;7-triazacyclononaneÞ first prepared by Wieghardt [4a] thenrecognized as an SMM by the Gatteschi group [4b]. Although thesespecies both have S = 10 ground states they differ in that the formerare approximately axially symmetric (E/|D| � 0), while the latter aresignificantly rhombic, with E/|D| � 0.19. This is important becausethe transverse anisotropy gives rise to efficient quantum tunnelingof magnetization (QTM) through the energy barrier and hence signif-icantly different relaxation dynamics [2]. Many SMMs have beenreported since these first examples [5].

ll rights reserved.

fax: +44 161 275 4616 (E.J.L.

.J.L. McInnes).

In this work we report a hexametallic Fe(III) cluster, based on thenew ligand di-2-(4-methylthio)pyrimidyl ketone (DTPK) which isrelated to the well known [6,7] di-2-pyridyl ketone (DPK). The hexa-metallic iron(III) cluster comprises two co-planar {Fe3O}7+ triangles– a well known structural type [8] – and in common with otherexamples has an S = 5 ground state. We present the first EPR studyon this class of cluster, which confirms the magnitude of S and provesthe negative sign of its ZFS parameter D in contrast to conclusionsfrom magnetization studies on previous examples [8]. More impor-tantly, these studies also reveal the large rhombicity of this systemhaving E/|D| = 0.27, approaching the rhombic limit of 1/3. This isthe reason for the non-SMM behaviour of this class of clusters de-spite the well isolated, large spin ground state with negative D.

2. Experimental

2.1. Synthesis

DTPK was synthesized in a two step reaction from commerciallyavailable 4-chloro-2-methylthio-pyrimidine (Scheme 1).

2.1.1. Synthesis of 4-iodo-2-methylthio-pyrimidine4-Chloro-2-methylthio-pyrimidine (8.99 g, 5.6 mmol) was

added dropwise to a rapidly stirred solution of HI (57 wt.%,50 ml) and the mixture stirred for 48 h at room temperature.Water (50 ml) was added, the mixture filtered and the filtrate

N N

Cl

S

N N

I

S

N N N N

S S

O

HI, 48hrs, RT iPrMgCl, (EtO)2CO

-78 C, dry toluene

Scheme 1. Synthesis of DTPK.

3664 P. Cooper et al. / Inorganica Chimica Acta 361 (2008) 3663–3668

brought to pH 7.5 by slow addition of solid NaHCO3. The mixturewas extracted into CHCl3 (2 � 40 ml) and the organic layer driedover MgSO4. After filtration and evaporation the solid was redis-solved in light petroleum (200 ml), refluxed over activated char-coal (10 g), hot filtered and placed in a refrigerator to crystallize.Yield 15.7 g (82%); m.p. 51–53 �C. Elemental Anal. Calc. forC5H5N2SI: C, 23.82, H, 2.00, N, 11.11, S, 12.72. Found: C, 24.03, H,1.86, N, 11.18, S, 12.40%.

2.1.2. Synthesis of di-2-(4-methylthio)pyrimidyl ketone (DTPK)4-Iodo-2-methylthio-pyrimidine (50.4 g, 200 ml) in anhydrous

toluene (420 ml) was treated with iPrMgCl (200 mmol) in THF(100 ml) at �78 �C then stirred for 2.5 h at �10 �C. The resultingbrown solution was transferred via canula to diethylcarbonate(72 ml, 594 mmol) and stirred for a further 2.5 h at �10 �C. A col-our change from brown to deep red was observed after 15 min.The reaction mixture was then quenched with a saturated aqueoussolution of NH4Cl and allowed to warm to room temperature. Theyellow aqueous layer was partitioned in a separating funnel withethyl acetate. The organic layers were run off, dried over MgSO4,and evaporated to dryness. The resulting product was purifiedvia column chromatography on silica gel [40–63 lm particle size;60 Å pore size] using an initial eluent ratio of 4:1 hexane:ethyl ace-tate and then 3:1 after the first fraction had been collected. The de-sired product was in the second fraction, Rf = 0.21. Yield 80%.Elemental Anal. Calc. For C11H10N4S2 O: C, 47.47, H, 3.62, N,20.13, S, 23.04. Found: C, 47.06, H, 3.83, N, 19.87, S, 22.55%.

2.1.3. Synthesis of ½FeIII6 O2ðOHÞ2fðC4N2H2SMeÞ2CðOHÞOg2ðtBuCO2Þ10�

(1)A solution of DTPK (0.1 g, 0.36 mmol) and [Fe3O(tBuCO2)6(py)3]

[9] (0.34 g, 0.36 mmol) in MeCN (20 ml) was stirred for 20 min andfiltered. Orange plate-like crystals of 1 � 2MeCN grew from the fil-trate within 1 month (32%). Elemental Anal. Calc. forFe6O28N8S4C72H114: C, 43.26, H, 5.55, N, 5.60, S, 6.42, Fe, 16.76.Found: C, 42.80, H, 5.63, N, 5.26, S, 5.99, Fe, 16.86%.

2.2. X-ray crystallography and solution of structure

Data collection, structure solution and refinement used SHELXL

[10]. Empirical formula C76H118Fe6N10O28S4, Mw = 2083.14, orangeblocks 0.25 � 0.1 � 0.1 mm, T = 100(2) K, k = 0.71073 Å, triclinic,space group P�1, a = 12.9616(7) Å, b = 15.0280(8) Å, c =15.2878(14) Å, a = 108.7760(10)�, b = 99.1640(10)�, c = 115.2940(10)�,V = 2391.0(3) Å3, Z = 1, qcalc = 1.447 Mg m�3, absorption coeffi-cient = 1.051 mm�1, F(000) = 1088, h = 2.36–30.51�, 27606 reflec-tions collected, 14194 independent [Rint = 0.1535], full-matrixleast-squares refinement on F2, data/restraints/parameters 14194/98/595, goodness-of-fit on F2 0.957, final R indices [I > 2r(I)]:R1 = 0.0661, wR2 = 0.1714, R indices (all data): R1 = 0.0858,wR2 = 0.1822, largest difference in peak and hole 1.287 and�1.253 e Å�3. Data were absorption corrected using semi-empirical

methods using repeated and equivalent reflections. Although thestructure determination establishes the molecular connectivityunambiguously, refinement of all aspects of disorder, for exampletBu groups of carboxylate ligands, has not quite converged.

2.3. Magnetic and EPR studies

Magnetic measurements were performed with a Quantum De-sign MPMS-XL7 SQUID magnetometer, on a polycrystalline sampleof 1 in an applied magnetic field of 1 T from 300 to 2 K. Pascal con-stants were used to calculate the diamagnetic correction. Variabletemperature W-band (94 GHz) EPR measurements were performedon polycrystalline samples of 1 on an ELEXSYS E600/E680 seriesspectrometer with E600-1021H Tera Flex W-band EPR resonatorand Oxford magnet. Magnetic data were fitted using CLUMAG[11]; EPR spectra were simulated using Hanson’s XSophe software[12,13].

3. Results and discussion

DPK is an excellent polynucleating ligand in its gem-diol formpresent in protic solvents. On deprotonation it can adopt a varietyof coordination modes, as illustrated by the elegant work of Perle-pes and co-workers [6] and Pecoraro et al.’s manganese-DPK SMM[7]. We prepared DTPK (Scheme 1) in order to introduce furthercoordinating sites.

Aerobic reaction of mixed-valent [Fe3O(tBuCO2)6(py)3] andDTPK in a 1:1 ratio in MeCN gave orange crystals of 1 � 2MeCN,suitable for X-ray diffraction, in good yield. This reveals 1 to be[Fe6O2(OH)2{(C4N2H2SMe)2C(OH)O}2(tBuCO2)10] (Fig. 1) where(C4N2H2SMe)2C(OH)O� is the mono-deprotonated gem-diol formof DTPK, (C4N2H2SMe)2C(OH)2. The compound crystallizes in thetriclinic space group P�1 and the molecules have crystallographicCi point symmetry with the asymmetric unit containing only threeiron ions. The cluster core (Fig. 1b) consists of two ½FeIII

3 ðl3 � OÞ�7þ

units linked at two of their apices, hence forming a planar Fe6 ar-ray. Each of these links is by one l3-OH� [O3(A)] and two l-piva-lates. Although the protons on O3(A) are not found directly, bondvalence sum calculations [14] support their assignment as hydrox-ide, as also required by charge balance.

The iron ions within each triangular unit are bridged by two l-pivalates between Fe(1) and Fe(2), and a l-pivalate and a l-alkox-ide (O1) arm of the activated DTPK (see below) between Fe(1) andFe(3). The octahedral coordination environment at Fe(1) is com-pleted by a pyrimidyl group, which therefore forms a five-mem-bered chelate ring at Fe(1). In each triangular unit Fe(2) andFe(3) are directly connected only by the l3-O2� (O2). The Fe3 trian-gles are scalene with Fe� � �Fe distances of 2.958, 3.605 and 3.240 Å.

The DTPK ligand has undergone the common activation ofDPK-like molecules to the gem-diol species di-2-(4-methyl-thio)pyrimidyl-diol [(C4N2H2SMe)2C(OH)2] in solution. However,

Fig. 1. X-ray crystal structure of 1 (left) and its core (right). Scheme: orange (Fe), blue (N), red (O), black (C), yellow (S), H omitted for clarity.

P. Cooper et al. / Inorganica Chimica Acta 361 (2008) 3663–3668 3665

upon coordination only one of the gem-diol arms is deprotonated(O1), bridging Fe(1) and Fe(2), the other remaining protonatedand unbound, leaving a �1 charge on the ligand. Only one of thepyrimidine rings is coordinated, to Fe(1). Selected bond distancesand inter-bond angles are in Table 1.

The core of 1 is similar to the previously reported [Fe6O2(O-H)2(O2CMe)10(L)2] [LH = 1,1-bis(N-methylimidazol-2-yl)-1-hydrox-yethane (2)[8(a)] or 2-(N-methylimidazol-2-yl)-2-hydroxypropane(3) [8b]], [Fe6O2(OH)2(O2CR)10(hep)2] [hepH = 2-(2-hydroxyethyl-pyridine); R = tBu (4),[8(c)] Ph (5) [8d]] and [Fe6O2(OH)2 (O2CR)10(di-paH2)2] [dipaH3 = di-isopropanolamine (6)] [8e], all of which also

Table 1Selected inter-atomic distances (Å) and angles (�) for 1

Fe(1)–O(1) 1.996(2) Fe(2)–O(7) 2.027(2)Fe(1)–O(2) 1.897(2) Fe(3)–O(1) 2.038(2)Fe(1)–O(8) 2.009(2) Fe(3)–O(2) 1.969(2)Fe(1)–O(9) 1.983(2) Fe(3)–O(3A) 1.933(2)Fe(1)–O(10) 2.027(2) Fe(3)–O(11) 2.037(2)Fe(1)-N(1) 2.260(3) Fe(3)–O(12) 2.044(2)Fe(2)–O(2) 1.8905(19) Fe(3)–O(13) 2.073(2)Fe(2)–O(3) 1.968(2) O(3)–O(3A) 3.199(5)Fe(2)–O(4) 2.045(2) Fe(1)–Fe(2) 3.240(2)Fe(2)–O(5) 2.046(2) Fe(1)–Fe(3) 2.958(6)Fe(2)–O(6) 2.038(2) Fe(2)–Fe(3) 3.605(8)O(7)–Fe(2)–O(6) 86.21(10) Fe(1)–O(2)–Fe(2) 117.60(6)O(2)–Fe(2)–O(4) 1.68(8) Fe(1)–O(2)–Fe(3) 99.79(8)O(3)–Fe(2)–O(4) 97.24(9) Fe(1)–(O1)–Fe(3) 94.31(7)O(7)–Fe(2)–O(4) 83.93(9) Fe(2)–O(2)–Fe(3) 138.09(11)O(6)–Fe(2)–O(4) 168.61(10) Fe(3)–O(3A)–Fe(2A) 123.02(4)O(7)–Fe(2)–O(6) 86.21(10) Fe(1)–O(2)–Fe(2) 117.60(6)O(2)–Fe(2)–O(4) 1.68(8) Fe(1)–O(2)–Fe(3) 99.79(8)O(3)–Fe(2)–O(4) 97.24(9) Fe(1)–(O1)–Fe(3) 94.31(7)O(7)–Fe(2)–O(4) 83.93(9) Fe(2)–O(2)–Fe(3) 138.09(11)O(6)–Fe(2)–O(4) 168.61(10) Fe(3)–O(3A)–Fe(2A) 123.02(4)O(7)–Fe(2)–O(6) 86.21(10) Fe(1)–O(2)–Fe(2) 117.60(6)O(2)–Fe(2)–O(4) 1.68(8) Fe(1)–O(2)–Fe(3) 99.79(8)O(3)–Fe(2)–O(4) 97.24(9) Fe(1)–(O1)–Fe(3) 94.31(7)O(7)–Fe(2)–O(4) 83.93(9) Fe(2)–O(2)–Fe(3) 138.09(11)O(6)–Fe(2)–O(4) 168.61(10) Fe(3)–O(3A)–Fe(2A) 123.02(4)O(7)–Fe(2)–O(6) 86.21(10) Fe(1)–O(2)–Fe(2) 117.60(6)O(2)–Fe(2)–O(4) 1.68(8) Fe(1)–O(2)–Fe(3) 99.79(8)O(3)–Fe(2)–O(4) 97.24(9) Fe(1)–(O1)–Fe(3) 94.31(7)O(7)–Fe(2)–O(4) 83.93(9) Fe(2)–O(2)–Fe(3) 138.09(11)O(6)–Fe(2)–O(4) 168.61(10) Fe(3)–O(3A)–Fe(2A) 123.02(4)O(7)–Fe(2)–O(6) 86.21(10) Fe(1)–O(2)–Fe(2) 117.60(6)O(2)–Fe(2)–O(4) 1.68(8) Fe(1)–O(2)–Fe(3) 99.79(8)O(3)–Fe(2)–O(4) 97.24(9) Fe(1)–(O1)–Fe(3) 94.31(7)O(7)–Fe(2)–O(4) 83.93(9) Fe(2)–O(2)–Fe(3) 138.09(11)O(6)–Fe(2)–O(4) 168.61(10) Fe(3)–O(3A)–Fe(2A) 123.02(4)

contain two co-planar {Fe3O}7+ units connected by one l-OH andtwo l-O2CR. As a consequence these materials all display similarmagnetic properties (see below).

4. Magnetic properties

The room temperature value of vMT (vM = molar magnetic sus-ceptibility) is 10.2 cm3 K mol�1, considerably lower than the valueof for six uncoupled high spin Fe(III) ions (26.3 cm3 K mol�1 withg = 2.0), indicating strong anti-ferromagnetic interactions. vMT de-creases slowly on cooling, passing through a minimum at 240 K,after which it increases reaching a maximum of 13.8 cm3 K mol�1

at 21 K. On further cooling it decreases rapidly. This behaviour istypical for systems with competing anti-ferromagnetic interac-tions, resulting in incomplete compensation of the spins and amoderately high spin ground state. Moreover it is qualitativelysimilar to structurally related Fe6 clusters consisting of linked,co-planar {Fe3O}7+ cores [8], which have been shown to haveS = 5 ground states. This ground state is consistent with the maxi-mum in vMT of 13.8 cm3 K mol�1 at 21 K for 1 (vM

T = 15.0 cm3 K mol�1 calculated for S = 5 with g = 2.0). The lowtemperature collapse in vMT is due to ZFS of the ground state(see later). The S = 5 ground state is confirmed by low temperaturemagnetization versus applied field (Fig. 3) measurements, whichsaturate at 9.3lB.

The molecular symmetry requires in principle four distinctmagnetic exchange interactions in 1: three within the scalene Fe3

triangles, and a further one between the ions linking the triangles[Eq. (1), Scheme 2]:

Fe2

Fe3

Fe1 Fe1A

Fe2A

Fe3A

J1

J1 J2

J2

J3 J3

J4

J4

Scheme 2. Exchange interactions in 1 and representation of ground state spinconfiguration.

0 1 2 3 4 5 6 70

2

4

6

8

10

M /

μ Β

H / T

Fig. 3. Magnetization (M) vs. applied magnetic field (H) measurements for a pol-ycrystalline sample of 1 at 2 K (black) and 4 K (red). Solid lines: calculated for S = 5,D = �0.206 and E = 0.062 cm�1 (E/|D| = 0.30) with g = 1.9.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 280

1

2

3

4

5

χM /

cm3 m

ol-1

T / K

6

8

10

12

14

χM T

/ cm3 K

mo

l -13666 P. Cooper et al. / Inorganica Chimica Acta 361 (2008) 3663–3668

bH ¼ �J1ðbS1bS2 þ bS1a

bS2aÞ � J2ðbS1bS3 þ bS1a

bS3aÞ � J3ðbS2bS3 þ bS2a

bS3aÞ

� J4ðbS2bS3a þ bS3

bS2aÞ ð1Þ

The vMT data have been fitted to Hamiltonian (1) in the temper-ature range 300–25 K (in order to avoid the effects due to groundstate ZFS, see below). The best fit curve was obtained with:J1 = �48.5, J2 = �7.2, J3 = �98.0 and J4 = �21.8 cm�1 with g = 1.93(Fig. 2). We have also attempted to use a simpler spin-Hamiltonianwhere J1 = J4 considering that Fe(1)� � �Fe(2) and Fe(2)� � �Fe(3a) havesimilar bridging arrangements [two pivalates and a (hydr)oxide]with similar Fe–O(H)–Fe angles (117.6� and 123.1�, respectively).Fitting gives: J1,4 = �47.0 cm�1; J2 = �9.5 cm�1; J3 = �70.0 cm�1

with an equally good fit as that in Fig. 2. Both these sets of param-eters give a well isolated S = 5 ground state: for example, the threeJ model gives lowest lying excited states of S = 4 and S = 3 at 61.6and 126.5 cm�1, respectively.

On this basis, and from the temperature dependence of themagnetic data, we can safely conclude that at low-temperatureonly the ground state is significantly populated hence we at-tempted to fit the 2 and 4 K M(H) data considering an S = 5 statewith g, E and D as variable parameters, with both positive and neg-ative D. Best fit values for negative D were D = �0.206 cm�1,E = 0.062 cm�1 (E/|D| = 0.30), g = 1.9 (Fig. 3), and for positive Dwere D = +0.232 cm�1, E = 0.061 cm�1 (E/|D| = 0.26), g = 1.9(Fig. S1, see ESI). The fit assuming negative D is slightly superiorin the low-field region. This significant ZFS can also account forthe low-temperature downturn in vMT(T): we have calculated thesusceptibility (as M/H) from 2 to 25 K assuming population of justthe ground state. The experimental data can be reproduced with asimple axial model with D and g as variables, givingD = �0.23 cm�1 and g = 1.93 (Fig. 4). Note this fit is insensitive tothe sign of D. We conclude the decrease in vMT below 25 K is dueto ground state ZFS and not due to intermolecular interactions.

The magnetic behaviour of the co-planar linked-triangularFe(III)6 topology has been studied in depth by Gatteschi, Hendrick-son and co-workers [8b] and by Christou and co-workers [8d].Hendrickson had previously noted (for compound 2) that theS = 5 ground state must arise from the competing anti-ferromag-netic interactions, where one of the exchange interactions withinthe triangles must differ significantly from the other two [8a].Gatteschi and Hendrickson later quantified this as part of a widerstudy on Fe(III) hexametallics [8b] fitting the vMT(T) data for com-

0 50 100 150 200 250 300

7

8

9

10

11

12

13

14

T / K

χ MT

/ cm

3 K m

ol-1

Fig. 2. vMT vs. T of a polycrystalline sample of 1 in 1 T applied magnetic field(black), and calculated for 20–300 K data using Hamiltonian (1) with J1 = �48.5,J2 = �7.2, J3 = �98.0, J4 = �21.8 cm�1 and g = 1.93 (red; see Scheme 1).

Fig. 4. Low temperature vM and vMT vs. T of a polycrystalline sample of 1 (1 Tapplied magnetic field), and calculated with S = 5, D = �0.23 cm�1 and g = 1.93(solid lines).

pound 2 above 20 K (i.e. above the maximum) to a Hamiltonianequivalent to (1) but with the approximation that J1 = J3 (i.e. the tri-angles are isosceles). They found J1,3 = �76, J2 = �11.2 andJ4 = �15 cm�1 with g = 2.0. Christou later reported J1 = �59.8,J2 = �7.0, J3 = �78.2 and J4 = �34.8 cm�1 with g = 1.94 for complex5 [8d]. [Values converted to be consistent with our Hamiltonian.]These patterns are similar to those found for 1: within the trianglesthere is one exchange interaction (J2) much weaker than the othertwo. This, crudely, leads to the spin alignment in Scheme 2 andhence an S = 5 ground state. Christou has estimated that this spinconfiguration accounts for ca. 50% of the ground state wavefunc-tion [8d]. He also noted the importance of the inversion symmetryin determining this ground state: if the two equivalent weak inter-actions are on the same side of the molecule then S = 0 results.

In these studies several attempts were made to model low tem-perature M(H) data in order to determine the ground state ZFSparameters, as we have above. Hendrickson found D = +0.22 cm�1

with g = 1.94 for 2 [8a] and D = +0.32 cm�1 with g = 2.0 for 3 [8b],both assuming an axial ZFS; Christou reports D = +0.458 and

P. Cooper et al. / Inorganica Chimica Acta 361 (2008) 3663–3668 3667

E = 0.046 cm�1 (E/|D| = 0.10) with g = 1.96 and D = +0.69 andE � 0 cm�1 with g = 1.89 for 4 and 5, respectively;[8c] Murray givesD = +0.25 with g = 1.94 for 6 assuming axial ZFS [8e]. Note that allof these studies give a positive value for the axial ZFS parameter, D.This has been offered as the explanation for the absence of SMMbehaviour in this class of molecules despite the well isolated highspin ground state [8c]. In contrast to 2–6, our modeling of M(H) for1 marginally favours a negative D and with a very strong rhombic-ity (E/|D| = 0.30, see above). In common with 2–6, complex 1 doesnot behave as an SMM – there is no hysteresis in M(H) nor an out-of-phase ac susceptibility signal down to 1.8 K. Hence, there is stillambiguity regarding the nature of the ground state in these mate-rials. Therefore, in order to define the ground state ZFS parametersdirectly we have undertaken HF-EPR experiments. EPR studieshave not been performed in any of the previous studies of this classof compound.

5. EPR spectroscopy

Polycrystalline samples of 1 give well resolved W-band EPRspectra below ca. 30 K (Fig. 5) caused by ZFS of the S = 5 groundstate. Below 20 K only this state is significantly populated. At 5 Ka clear progression is observed spanning from ca. 1.2 to 5.4 T, withabsorption-like lineshapes and separations of ca. 0.45 T. The low-field transitions in this progression increase in relative intensityas the temperature decreases. In the high-field limit there are 2Sallowed EPR transitions separated by 2|D|/glB or |D|/glB for theparallel (z) and perpendicular (xy) orientations, respectively, in ax-ial symmetry [15]. In this case, enhancement of the lowest fieldparallel transitions on decreasing temperature is a signature ofnegative D [15]. Thus, at first sight the EPR spectra of 1 appear likean S = 5 with an axial ZFS of D � �0.2 cm�1.

0 10000 20000 30000 40000 50000 60000

yy yy z

zzz

5 K

10 K

15 K

20 K

H / G

Fig. 5. 94 GHz EPR spectra of a polycrystalline sample of 1 (black) and simulations(red) with S = 5, giso = 2.0, D = �0.224 cm�1, E/|D| = 0.27 and Gaussian linewidths of600 G. The ‘‘doubled” zy peaks are highlighted in the 20 K spectrum.

However, on closer inspection each of the ‘‘parallel” features isin fact doubled (Fig. 5). In addition, the high-field features do notshow perpendicular-type lineshapes, and a considerable intensityin the g = 2.0 region is still observed even at 5 K. These facts areindicative of the ZFS tensor not being axially symmetric butapproaching the rhombic limit of E/|D| = 1/3. D and E are definedin terms of the principal elements of the diagonal D-tensor asD = Dzz � (Dxx + Dyy)/2 and E = (Dxx � Dyy)/2. In the rhombic limit(E/|D| > 1/3 implies a redefinition of axes) the Dii elements are re-lated by Dzz = �Dyy and Dxx = 0. Hence, in this case the z and y tran-sitions will be coincident, and the sign of D is meaningless beingsimply a choice of definition of axes. An important consequenceis that if E/|D| = 1/3 then calculated EPR spectra are independentof the sign of D.

Excellent simulations of the experimental spectra for 1 areachieved with: giso = 2.0, D = �0.224 and E = 0.061 cm�1 (Fig. 5).Here E/|D| = 0.27 and is approaching, but not at, the rhombic limitsuch that |Dzz | and |Dyy| differ slightly (and that Dxx is not exactlyzero; |Dzz| = 0.149, |Dyy| = 0.136 and |Dxx| = 0.013 cm�1). This resultsin the ‘‘doubled” zy peaks which are evident at the higher temper-atures (Fig. 5). Fig. 6 shows a Zeeman diagram calculated usingthese parameters for H parallel to z, together with the simulationof the powder W-band spectrum at 5 K, illustrating the enhance-ment of the z-component of each low-field zy pair on decreasingtemperature. In Fig. 7, we show corresponding variable tempera-ture powder simulations for the same parameters but positive D.This also results in enhancement of transitions at low magneticfield on decreasing temperature. However, here the other peak(y) of each zy pair is enhanced, illustrated by the Zeeman diagramfor positive D and field parallel to y shown in Fig. 8. Thus, in 1 thesign of D is meaningful and can be determined.

As noted above, 1 is not an SMM despite the well isolated S = 5ground state and in related clusters this has been blamed on a po-sitive D determined by modeling of M(H) [8]. Here we show that Dis in fact negative, at least for complex 1, but more importantly theZFS is highly rhombic. The absence of SMM behaviour is likely dueto the large transverse anisotropy introduced by E which mixesstates differing in MS by ±2.

This has been discussed at some length [16] for [Fe8O2(OH)12

(tacn)6] Br8. As noted in the introduction this cluster has anS = 10 ground state with negative D (�0.205 cm�1), as doesMn12, and behaves as an SMM. In contrast to Mn12, there is a

-10000 0 10000 20000 30000 40000 50000 60000 70000

-30

-20

-10

0

10

20

30

En

erg

y / c

m-1

H / G

Fig. 6. Energy level diagram calculated with S = 5, D = �0.224 and E = 0.061 cm�1

(E/|D| = 0.27), with giso = 2.0 and with magnetic field parallel to z. Arrows mark theallowed (94 GHz) EPR transitions for this orientation, corresponding to the mainpeaks in the simulated 5 K powder spectrum (red).

0 10000 20000 30000 40000 50000 60000

z yy

yz

zyz

5 K

10 K

15 K

20 K

H / G

Fig. 7. 94 GHz EPR spectra of a polycrystalline sample of 1 (black), and simulationswith S = 5, giso = 2.0, D = +0.224 cm�1, E/|D| = 0.27 and Gaussian linewidths of 600 G(blue). The doubled zy peaks are highlighted in the 20 K spectrum. Note in thesesimulations with positive D it is the y-component of each pair that is being enha-nced on decreasing temperature, see Fig. 8.

0 10000 20000 30000 40000 50000 60000-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

25

30

yy

y

zz

zEn

erg

y / c

m-1

H / G

Fig. 8. Zeeman diagram calculated for the same parameters as Fig. 6 but with po-sitive D and with magnetic field parallel to y. Arrows mark the allowed (94 GHz) EPRtransitions for this orientation, corresponding to the main peaks in the simulated5 K powder W-band spectrum for these parameters (blue). For comparison thepowder simulation for negative D is shown (red), highlighting the enhancement oflow-field z or y resonances depending on sign of D. The similarity of the Zeemanplots and calculated spectra in Figs. 6 and 8 follows from Dzz � �Dyy for E/|D| � 1/3.

3668 P. Cooper et al. / Inorganica Chimica Acta 361 (2008) 3663–3668

significant rhombicity (E/|D| = 0.19) which has two consequences.Firstly, the axial picture of degenerate pairs of |MS| states is onlyvalid up to ‘‘MS” = ±6; above this the states are heavily mixed bythe rhombic ZFS term and the top of the ideal |D|S2 barrier to rever-sal of magnetization is destroyed. Secondly, the weaker mixing be-tween the states of larger |MS| still provides a pathway for

enhanced relaxation via QTM. For example, in the analogousmixed-counter-ion species [Fe8O2(OH)12(tacn)6] Br4.3(ClO4)3.7, thetransverse anisotropy was found to be ca. 10% larger (E/|D| = 0.21) resulting in a large enhancement of QTM rates via thelarger tunnel widths [17].

The rhombicity of 1 (E/|D| = 0.27) is significantly larger than in[Fe8O2(OH)12(tacn)6]Br8 (E/|D| = 0.19) and the ground state spin issmaller. This results in more extensive mixing of the ground MS

states. The eigen-values and -vectors from diagonalisation of theground state ZFS matrix for 1 (Table S1, ESI) show that the approx-imation of degenerate pairs is only even vaguely valid for ‘‘MS” = ±5and ±4. Above these states there are three non-degenerate levelsand two pairs of quasi-degenerate levels. Hence, there is no effec-tive barrier to relaxation of magnetization in 1.

Acknowledgements

We thank the EPSRC (UK) and the EC (MagMaNet Network ofExcellence) for funding.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.ica.2008.03.081.

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