Crystal engineering of lanthanide–transition-metal coordination polymers

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Journal of Molecular Structure 887 (2008) 56–66

Crystal engineering of lanthanide–transition-metalcoordination polymers

Xiaojun Gu a, Dongfeng Xue a,*, Henryk Ratajczak b,*

a State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, School of Chemical Engineering,

Dalian University of Technology, 158 Zhongshan Road, Dalian 116012, PR Chinab Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland

Received 23 October 2007; received in revised form 27 November 2007; accepted 30 November 2007Available online 14 January 2008

Abstract

Crystal engineering allows us to predict and control the packing of molecular building units in solid state, which has been attractingmuch attention due to its exploitation for the synthesis of crystalline materials with novel structures and promising properties. The crys-tal engineering strategies toward the synthesis of high-nuclearity lanthanide clusters and three-dimensional (3D) lanthanide–transition-metal (Ln–M) coordination polymers were well discussed in the present work. It has shown that the high-nuclearity lanthanide clusterscan be rationally synthesized by surface modification strategy. On the basis of the different coordination nature of lanthanide and tran-sition-metal ions, the multifunctional organic ligands with mixed coordination sites such as isonicotinate have been elaborately selectedto rationally construct a series of homochrial and achiral 3D Ln–M coordination frameworks built from inorganic heterometallic chainswith improved thermal stability. Furthermore, novel 3D Ln–M coordination frameworks have been built from discrete lanthanide clus-ters (or cluster polymers) and transition-metal clusters (or cluster polymers) by faultlessly harmonizing the subtle relationship betweenthese two different types of metal cluster or cluster polymer units. The current work offers us great potential toward the pursuit ofrational synthesis of Ln–M coordination assemblies on the basis of crystal engineering principles.� 2007 Elsevier B.V. All rights reserved.

Keywords: Crystal engineering; Lanthanide–transition-metal coordination polymers; Lanthanide clusters; Inorganic heterometallic chains; Surfacemodification

1. Introduction

Crystal engineering, a flourishing and rapidly growingsubject in modern chemistry, lies at the intersection ofsolid-state chemistry and supramolecular chemistry and ispracticed by scientists with interest in the design, synthesis,and applications of crystalline solids with predefined anddesired aggregation of molecules and ions [1–6]. Thegrowth of crystal engineering has coincided with theadvances in our understanding of intermolecular interac-tions and structure–function relationships. Currently, anincreasing number of scientists are directing their attention

0022-2860/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.molstruc.2007.11.052

* Corresponding authors.E-mail addresses: [email protected] (D. Xue), henryk.ratajczak@

gmail.com (H. Ratajczak).

toward crystal engineering both as a means of developingsophisticated devices, and to learn how to control theself-assembly and molecular recognition. The rapid devel-opment of materials science and chemistry inspires us tosearch for new crystal materials, and thus, the concept ofcrystal engineering has been widely used to makes crystalswith a state-of-the-art or applied purpose. Recently, thedesign and synthesis of metal–organic coordination poly-mers (MOCPs) in the field of the coordination chemistryand crystal engineering has been great interest in view oftheir intriguing variety of architectures and topologiesand potential applications in catalysis, ion exchange,molecular adsorption, charity, fluorescence, nonlinearoptics, and magnetism [7–21]. Consequently, a variety ofMOCPs with one-dimensional (1D), two-dimensional(2D), and three-dimensional (3D) network structures have

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X. Gu et al. / Journal of Molecular Structure 887 (2008) 56–66 57

been successfully designed and synthesized by the selectionof suitable metal centers and organic building blocks aswell as reaction pathways [22–36]. The pioneering researchof Zaworotko [3], Robson [8], Fujita [14], Yaghi [7], andothers best illustrates the utility of this synthetic approachin the preparation of interesting MOCP materials with pre-determined structures and properties.

Generally speaking, the MOCPs can be divided into twomain categories: (i) those based on transition-metal motifsand (ii) those based on lanthanide motifs. The success inproducing such assemblies depends on understanding andcontrolling the topological and geometrical relationshipbetween molecular modules along with the coordinationnature of lanthanide and transition-metal ions. Up tonow, much work on MOCPs has focused on the construc-tion of transition-metal or lanthanide homometalliccoordination system [7–37], while chemistry toward hetero-metallic analogue remains largely unexplored [38–40].Recently, lanthanide–transition-metal (Ln–M) heterome-tallic coordination polymers have attracted increasinginterest owing to their aesthetically appealing topologiesand significance of discovering new materials in the fieldof chemical sensor, luminescence, catalysis, and magnetism[41–52]. Among the available Ln–M coordination com-plexes, most of them possess discrete or low-dimensionalstructures in the crystallographic viewpoint [53–58]. Theassembly of high-dimensional Ln–M coordination poly-mers, especially 3D analogue, based on crystal engineeringprinciples is currently a formidable task due to the highand variable coordination numbers of lanthanide ionsand their small energy difference among various coordina-tion geometries, and the complicated interactions amongtwo different types of metal centers and organic moiety.Furthermore, controlling the assembly of target com-pounds in three dimensions is still a challenge to chemists,as can be seen from Roald Hoffmann’s logion [59] – But intwo or three dimensions, it’s a synthetic wasteland. Themethodology for exercising control so that one can makeunstable but persistent extended structures on demand isnearly absent. Or to put it in a positive way – this is acertain growth point of the chemistry of the future.

Currently, we have begun the studies on the crystal engi-neering of homochiral and achiral 3D Ln–M coordinationframeworks [60–65]. Meanwhile, we also focused on under-standing the role of different metal building units in theconstruction of such assemblies. Some properties, whichpoint toward possible long-term applications, have alsobeen studied. Here, we present our strategy toward therational design and synthesis of such fascinatingassemblies.

2. Design of organic linkers

The general strategy for the synthesis of Ln–M coordi-nation complexes is self-assembly of mixed metal ionsand organic ligands containing mixed-donor atoms [38–40]. Besides the consideration of the nature of metal ions,

the selection or design of ligands containing appropriatecoordination sites is crucial to construct high-dimensionalheterometallic coordination polymers. It is well-knownthat lanthanide ions behave as hard acid, while transi-tion-metal ions are soft or borderline acids [66]. As a result,they have different affinities for donors, which provide us aunique opportunity for the design of organic ligands to linklanthanide and transition-metal building units. Further-more, the subtle relationship between lanthanide and tran-sition-metal building units might play a key role in theconstruction of high-dimensional Ln–M coordinationpolymers. Our approach is to introduce a linear multifunc-tional organic ligand with mixed donors to link these twodifferent building units based on the hard-soft/acid-baseclassification, leading to unusual 3D heterometallic coordi-nation frameworks with useful physical–chemical proper-ties and intriguing structure features. Moreover, thesecondary organic species can also be introduced into thiscoordination system to tune the fine structures of targetcomplexes. Based on the above, isonicotinic acid (HIN)with nitrogen and oxygen donor atoms is selected as abridge between lanthanide and transition-metal buildingunits, giving rise to high-dimensional designed polymers(shown in Fig. 1). Moreover, other similar ligands suchas nicotinic acid (HNIC) and pyridine-3,4-dicarboxylicacid (H2PYDC) can also be used as multifunctionalorganic linkers to carry out the blueprint.

3. Design of 3D Ln–Ag coordination frameworks

3.1. Homochiral 3D Ln–Ag coordination frameworks byspontaneous resolution

The design of chiral MOCPs is an area of intenseresearch [16,17,23,67–70]. In general, two approaches havebeen adopted for the synthesis of such complexes: (i) enan-tioselective synthesis using chiral species, generating enan-tiopure products, and (ii) spontaneous resolution uponcrystallization without any chiral auxiliary, which yields aconglomerate. To date, most of the homochiral coordina-tion polymers have been obtained by the use of chiral spe-cies, which transfer their chiral information to the wholecoordination framework. In contrast, homochiral coordi-nation polymers generated by the second method are stillrare since the laws of physics determining spontaneous res-olution are not fully understood. Furthermore, the studieson homochiral coordination polymers have focused onhomometallic system, whereas the chemistry toward homo-chiral Ln–M coordination complexes by spontaneous reso-lution remains largely unexplored.

Our interest is to design and synthesize high-dimen-sional Ln–M coordination polymers by using achiralligands. As is known, the control of helicity is a fascinatingchallenge, and the helicity can be induced by conformationrestriction of macromolecules, hydrogen bonds, or coordi-nation to metal ions. As an exploration, the helical chains,which exhibit an axial chirality, can be selected as the trans-

N

O

O

Transition-metal building unit Lanthanide building unit

Fig. 1. Our strategy on the assembly of two different types of metal building units.

58 X. Gu et al. / Journal of Molecular Structure 887 (2008) 56–66

fer medium of chiral information in the formation ofhomochiral coordination frameworks [71,72]. Our strategyis to select soft Ag(I) ion, which can induces the formationof helical structures, as a transition-metal ion to constructhelical chains with the help of achiral organic ligands, andthen these helical chains can be used as chiral building unitsto construct high-dimensional homochiral Ln–Ag coordi-nation polymers through homochiral interactions. [60]Based on this strategy, two homochiral Ln–Ag coordina-tion polymers LnAg(OAc)(IN)3 [Ln = Nd (1), Eu (2);HOAc = acetic acid] have been successfully obtained fromthe reaction of Ln2O3, AgNO3, HIN and HOAc underhydrothermal conditions. Both polymers are built frominfinite right-handed homochiral helical chains with Ln–O–Ag connectivity (Fig. 2), representing the first homochi-ral Ln–M coordination polymers with a 3D coordinationframework by spontaneous resolution. In both structures,the helical chain provides the chiral units, and the coordi-nation bonds between IN ligands and two types of metalcenters afford the homochiral interactions to realize thehomochiral assembly. To our knowledge, only two homo-chiral helical chains with Ln–O–M connectivity have beenreported [45]. Our present strategy is expected to designand synthesize more homochiral Ln–M coordination poly-mers with interesting structures and function of enantiose-lective separation.

Fig. 2. Illustration of the assembly of homochiral 3D Ln–Ag

3.2. 3D Ln–Ag coordination frameworks constructed from

inorganic heterometallic chains

It has been already proved that there is a tight relation-ship between compound structures and functions, whichhave also been embodied in the field of Ln–M coordinationcomplexes. Our interest is how to design and synthesizehigh-dimensional Ln–M coordination polymers with highthermal stability. It can be found that the available Ln–M complexes are constructed from either single metal poly-hedra or isolated clusters [38–40]. These low dimensionalbuilding units reduce their performance and thermal stabil-ity required for practical applications to some degree. Oursynthetic strategy is to increase the dimensionality of thesebuilding units, resulting in dramatic improvement of theirthermal stability. At the same time, the existence of thesepolymeric building units can lead to the improvement ofthe possibility to control framework structures of 3D Ln–M coordination polymers.

In order to carry out our design, we chose mulitifunc-tional PYDC and IN species as organic linkers to constructpolymeric building units. Moreover, the secondary ligandwith carboxyl group is introduced into the Ln–M–IN sys-tem due to the weak chelating effect of IN ligand with lan-thanide ions. As is known, multidentate carboxylates areessential in chelating lanthanide ions to form chainlike

coordination framework from 1D inorganic helical chains.


units with Ln–O–Ln connectivity [73], and the active oxy-gen atoms in the 1D inorganic chain provide one possibilityto bond second transition-metal ions, resulting in inorganicheterometallic subunit. [64] Based on this design, four tar-get complexes LnAg(PYDC)2(NIC)0.5 [Ln = Nd (3), Eu(4)], LnAg(IN)2(NICO)�0.5H2O [Ln = Nd (5), Eu (6)] havebeen successfully synthesized from the reaction of Ln2O3,AgNO3 and H2PYDC, and Ln2O3, AgNO3, HIN and 2-hydroxynicotinic acid (H2NICO) under hydrothermal con-ditions, respectively. Compounds 3 and 4 exhibit a 3Dcoordination framework built from 1D inorganic hetero-metallic subunits and PYDC linkers (Fig. 3a). To ourknowledge, they represent the first examples of 3D open-framework 4d–4f coordination polymers. Compounds 5and 6 also possess a 3D coordination framework with 1Dinorganic heterometallic chains (Fig. 3b), which are thefirst 3D Ln–Ag coordination frameworks based on mixedlinear and nonlinear ligands with same donors. Interest-ingly, the thermal stability of these compounds has beenimproved, which expands the application scope of thesecompounds as potential functional materials.

3.3. 3D Ln–Ag coordination frameworks based on

lanthanidecarboxylate subunits

On the basis of hard–soft/acid–base classification, theAg(I) ion behaves as a soft acid and possesses a strong ten-dency to coordinate nitrogen donors, while the hard lan-

Fig. 3. Illustration of the assembly of 3D Ln–Ag coordination frameworks bas

thanide ions prefer oxygen to nitrogen donors. Thus, theligands containing O-donor atoms such as benzenecarb-oxylate easily bonded to lanthanide ions can induce the for-mation of lanthanide–carboxylate polymers. If theselanthanide–carboxylate building units are introduced intoLn–Ag–IN coordination system, the resulting complexesmight exhibit remarkable different structure features. Inother words, the Ln–O bonds construct lanthanide–car-boxylate subunit, while M–N (M = transition-metal ions)bonds direct the assembly of heterometallic coordinationframework [62]. On the basis of the above, we have intro-duced 1,2-benzenedicarboxylic acid (1,2-H2BDC) and 1,3-benzenedicarboxylic acid (1,3-H2BDC) as the secondaryligands to the Ln–Ag–IN system and successfully con-structed three novel 3D coordination frameworks underhydrothermal conditions, [Ln4(H2O)2Ag(1,3-BDC)4(IN)5]�nH2O (Ln = Nd (7), n = 0.25; Eu (8), n = 0) and [Nd(H2O)Ag(1,2-BDC)(IN)2] (9). Compounds 7 and 8 exhibit a 3Dcoordination framework constructed by Ag(IN)4 com-plexes and 3D supramolecular lanthanide–carboxylate net-works based on 2D wavelike lanthanide-cluster-based layersubunits (Fig. 4). Both compounds represent the first exam-ple of 3D 4d–4f coordination frameworks, in which thetransition-metal complexes bond to 3D lanthanide–carbox-ylate supramolecular framework with nanoscale channels.Compound 9 displays an unusual 3D coordination frame-work constructed from nanoscale Nd2Ag2(IN)4 rings(Fig. 5). The successful synthesis of these three heterome-

ed on 1D inorganic heterometallic chains with improved thermal stability.

Fig. 4. The assembly of nanoscale metal rings into 3D Ln–Ag coordination framework.

Fig. 5. The assembly of supramolecular lanthanide-cluster network with nanoscale channels and Ag–IN complexes into one 3D coordination framework.


tallic compounds shows that the secondary ligands play animportant role in the formation of Ln–M coordinationpolymers, which might lead to a lot of novel heterometalliccoordination polymers with interesting structures andpotential applications.

The study on the photoluminesce of 2, 4, 6, and 8 showsthe characteristic transitions of Eu3+ ions are present,implying that the ligand-to-europium energy transfer ismoderately efficient under experimental conditions. Thisobservation suggests that these four compounds may beexcellent candidates for potential photoactive materials.

4. Design of 3D Ln–M cluster-based coordination

frameworks

High-nuclearity clusters are gaining continuing interestbecause of their intriguing architectures and potentialapplications in magnetism, optics, electronics, catalysis,and so on [74–77]. Up to now, many large transition-metalclusters such as molybdenum [78,79], silver [80], and man-

ganese clusters [81], have been successfully synthesized,while the analogous chemistry of lanthanides is still under-developed due to some of the intrinsic characteristics oflanthanide ions [82–86]. The general strategy for the syn-thesis of high-nuclearity lanthanide clusters is to controlthe hydrolysis of metal ions in the presence of supportingligands, and some high-nuclearity lanthanide clusters havebeen reported [82–86]. Although there have been manyattempts to synthesize high-nuclearity lanthanide clusters,most of them still possess discrete structures in the crystal-lographic viewpoint. The synthesis of high-nuclearity lan-thanide-cluster polymers remains a big challenge. Inaddition, transition-metal-cluster polymers have receivedmuch attention owing to their remarkable structures andproperties [26,87,88]. It is worth noting that lanthanideclusters (or cluster polymers) and transition-metal clusters(or cluster polymers) possess different coordination prefer-ences and potential applications. If one or two types ofthese clusters (or cluster polymers) can be assembled intoone crystallographic frame, the as-obtained Ln–M coordi-


nation polymers may exhibit novel structure features andproperties. However, little progress has been made on thecombination of high-nuclearity lanthanide clusters andtransition-metal clusters to prepare high-dimensional het-

Fig. 6. Illustration of three different types of cluster-based building units and p

Fig. 7. Illustration of the surface modification strategy for the constructionclusters.

erometallic coordination frameworks due to the synthesischallenge of these two different types of clusters (or clusterpolymers), especially high-nuclearity lanthanide clusters(or cluster polymers).

ossible assembly modes of such units in Ln–M coordination frameworks.

of large lanthanide clusters and two tetramers constructed from {Ln26}

Fig. 8. Illustration of three types of assemblies from discrete lanthanideclusters and transition-metal clusters.


In general, there are three types of metal-cluster-basedbuilding units according to the classification of theirdimensions, namely, discrete, 1D, and 2D metal clusters.Thus, there are six different types of assemblies basedon these building units (Figs. 6a–f): (i) assembly of twodifferent types of metal clusters, (ii) assembly of 1Dmetal-cluster polymers and the other type of metal clus-ters, (iii) assembly of two different types of 1D metal-clus-ter polymers, (iv) assembly of 2D metal-cluster polymersand the other metal clusters, (v) assembly of 1D metal-cluster polymers and 2D metal-cluster polymers, and (vi)assembly of two different types of 2D metal-cluster poly-mers. If the fine assembly is considered, three types ofcoordination polymers with 1D, 2D, and 3D structurescan be further obtained with mode a. Four types of coor-dination frameworks, 2D and 3D coordination frame-works constructed from transition-metal clusters and 1Dlanthanide-cluster polymers, or constructed from lantha-nide clusters and 1D transition-metal cluster polymers,can be obtained with mode b. The 2D and 3D coordina-tion frameworks can be obtained with mode c. Two typesof 3D coordination frameworks constructed from transi-tion-metal clusters and 2D lanthanide-cluster polymers,or lanthanide clusters and 2D transition-metal clusters,can be obtained with mode d. Two types of 3D coordina-tion frameworks constructed from 1D transition-metalcluster polymers and 2D lanthanide-cluster polymers, or1D lanthanide-cluster polymers and 2D transition-metalcluster polymers, can be obtained with mode e. Therefore,there are fourteen types of coordination assembly modesbased on these cluster-based building units. Certainly, ifthe supramolecular interactions such as hydrogen bondsand inter-molecular interactions are considered, therewould be more assembly modes. Our interest has focusedon the design and synthesis of high-nuclearity metal clus-ters and cluster polymers, especially high-nuclearity lan-thanide clusters and cluster polymers, and the crystalengineering of high-dimensional Ln–M coordinationframeworks based on these cluster-based building units.These high nuclearity lanthanide clusters and high-dimen-sional cluster-based Ln–M coordination polymers areexpected to exhibit fascinating structures and propertiesremarkably different from those in other reported Ln–Mcoordination polymers.

4.1. Design of high-nuclearity lanthanide clusters

To date, despite the synthesis progress of high-nuclearitylanthanide clusters, the mono-route on the ligand-con-trolled hydrolysis seems inappropriate to remarkablyimprove the dimension of these cluster cores, due to thefact that it is difficult to incorporate more lanthanide cen-ters into one cluster to satisfy the cooperativity among dif-ferent lanthanide centers. Furthermore, high-nuclearityclusters possess a high positive charge and are very unsta-ble in solution, which easily leads to the formation of poly-meric lanthanide hydroxide precipitates.

Our strategy is to incorporate surface modifiers such assmall multidentate anion ligands into a cluster core back-bone, resulting in the decrease of positive charge in thecluster core. Thus, the cluster core surface expands, gen-erating lanthanide cluster with higher nuclearity with thehelp of supporting ligands [65]. On the basis of this strategy,we have used NO3 ligand with trigonal planar geometry assurface modifier to successfully synthesized two novel lan-thanide cluster compounds from the reaction of Dy2O3,AgI, HIN and HNO3

� under different hydrothermalconditions, Dy30I(l3-OH)24(l3-O)6(NO3)9(IN)41(OH)3(H2O)38

(10) and Dy104I4(l3-OH)80(l3-O)24(NO3)36(IN)125(OH)19

(H2O)167 (11). In both cluster compounds, the building blockof [Dy26(l3-OH)20(l3-O)6(NO3)9I]36+, {Dy26}, is the largestcage-shaped lanthanide cluster core known. Compound 10 isconstructed from the {Dy26} and cubane [Dy4(l3-OH)4]8+


clusters (Fig. 7a), representing the first tetramer based onthe linkage of two different types of high-nuclearity lan-thanide clusters and IN ligands, while compound 11 rep-resents the first tetramer constructed by {Dy26} clustersand IN linkers (Fig. 7b). Interestingly, compound 10

exhibits a slow relaxation of magnetization, suggestingits possible Single-Molecule Magnet behavior. The presentstudy demonstrates that the synthesis of materials basedon high-nuclearity lanthanide clusters can be achievedwith the surface modification strategy and can lead tointeresting magnetic properties. On the other hand, thistype of high-nuclearity lanthanide clusters can be usedas building units to construct Ln–M coordinationpolymers.

Fig. 9. The assembly of nanoscale metal rings with {Cu4I4

Fig. 10. The assembly of Ln2Cu2 clusters

4.2. Design of 3D Ln–Cu coordination frameworks

constructed from two different types of metal cluster or

cluster polymers

To date, many Ln–M coordination complexes have beenreported; however, almost these assemblies are based onthe single metal ions or heterometallic metal clusters [38–40]. Our interest is to design and synthesize high-dimen-sional Ln–M coordination polymers based on discretehigh-nuclearity lanthanide clusters (or cluster polymers)and transition-metal clusters (or cluster polymers). In orderto carry out such an assembly, one problem must be solved,that is, how to harmonize the subtle relationship betweenthese two different types of metal cluster-based building

} clusters into one 3D Ln–M coordination framework.

into one 3D coordination framework.


units in one coordination framework. Moreover, how tocontrol the formation of cluster-based building units inone coordination framework, is also an important factor.In all, many factors can influence the formation of suchassemblies, which provides us more challenges.

The successful synthesis of Ln–Ag coordination polymersaffords us a valuable understanding of heterometallic coordi-nation chemistry, which can help us study the crystal engi-neering of cluster-based Ln–M coordination frameworks.Although silver and copper elements belong to the samegroup in the periodic table, they possess different coordina-tion nature. It is well-known that copper iodide is capableof meeting the special requirements of bridging ligands toconstruct high-dimensional coordination polymers, andare excellent inorganic functional modules possessing richcoordination forms and rich photophysical properties. [89–92]. Therefore, the copper–iodine clusters can be selectedas transition-metal building units for constructing cluster-based heterometallic coordination frameworks. Moreimportant, the soft metal copper(I) ions can rather easilycoordinate to the nitrogen atoms in IN and NIC ligands.

In the exploration, we have successfully prepared a 3Dcoordination framework from the reaction of Er2O3, CuIand HIN under hydrothermal conditions, Er4(l3-OH)4(H2O)2

Cu6I5(IN)9�(H2O)2 (12), which is built from {Cu6I5} and cub-ane-like {Er4O4} clusters through IN linkers (Fig. 8c). Itshould be noted that there are three types of fine coordina-tion assembly modes based on two different types of metalclusters. It can be seen that the 3D assembly is present inthe formation of 12, while the 1D and 2D assemblies areundergoing (Fig. 8a and b).

Fig. 11. Illustration of the assembly of 2D lanthanide-cluster polymers and

In the course of the exploration of other cluster-basedassembly, we have obtained three novel 3D Ln–M coordi-nation polymers, [Nd(H2O)2(CuI)2(NIC)3]�H2O (13) and[LnCu(IN)2(OX)]�H2O [Ln = Nd (14), Eu (15); H2OX =oxalic acid], from the reaction of Nd2O3, CuI and HNIC,and Ln2O3, CuCl2, HIN and HCl under hydrothermal con-ditions, respectively [63]. Compound 13 exhibits a 3D het-erometallic coordination framework based on the Cu4I4

clusters (Fig. 9), representing the first 3D Ln–M coordina-tion framework, in which discrete cubane transition-metalclusters covalently bonded to the lanthanide centersthrough linear ligands. Compounds 14 and 15 exhibit anunusual 3D coordination framework constructed from tet-ranuclear Ln2Cu2 clusters (Fig. 10).

Compared to 12, compounds 13–15 are constructedfrom one type of metal clusters or heterometallic metalclusters. From this, it can be concluded that the assemblyof the cluster-based Ln–M coordination frameworks underhydrothermal conditions is heavily influenced by many fac-tors such as the structural characteristics of the ligand,coordination nature of metal ions, the pH values of thesolution, and the ratio of metal to ligand [20].

Compared to the assembly of discrete metal clusters,there is more challenge in the assembly of high-dimensionalcoordination frameworks from two types of metal-clusterpolymer units [61]. To our knowledge, no efforts have beendevoted to this study. By using IN ligand as bridge, wehave realized this design and successfully synthesized two3D coordination polymers from the reaction of Ln2O3,CuI, HIN and HOAc under hydrothermal conditions,Ln4(l3-OH)2Cu6I5(IN)8(OAc)3 [Ln = Nd (16), Pr (17)].

1D transition-metal-cluster polymers into one coordination framework.


Both polymers are constructed from 2D lanthanide clusterpolymer and 1D copper–iodine cluster polymer units(Fig. 11). Success in producing such assemblies dependson the understanding and controlling of the topologicaland geometrical relationship between these two differenttypes of cluster-based units along with the coordinationnature of different metal ions.

Aside from the above mentioned cluster-based coordi-nation frameworks, other types of cluster-based assembliesare expected to be synthesized by the selection of suitableorganic species and fine-tuning of reaction conditions fromthe viewpoint of crystal engineering.

5. Conclusion

In summary, we have outlined several successful strate-gies toward high-nuclearity lanthanide clusters and 3D Ln–M coordination polymers. We have demonstrated the fea-sibility of crystal engineering of Ln–M coordination poly-mers by exploiting the different coordination nature oflanthanide and transition-metal ions. The key to our suc-cessful approaches lies in the utilization of multifunctionalligands with mixed coordination sites to bridge the lantha-nide and transition-metal units based on the hard–soft/acid–base classification. We have also shown that the sur-face modification strategy can lead to large lanthanide clus-ters with remarkable improvement of cluster-coredimension. Inorganic heterometallic chains can be usedas building units to construct homochiral and achiral 3Dcoordination frameworks. Finally, lanthanide clusters (orcluster polymers) and transition-metal clusters (or clusterpolymers) can be assembled into one coordination frame-work by faultlessly harmonizing the subtle relationshipbetween these two different types of building units. Fur-thermore, some heterometallic coordination complexeshave a favorable combination of improved thermal andchemical stability and luminescence that will find potentialapplications. Although we can make use of the principle ofcrystal engineering to make crystals with a purpose, wecannot generate the target Ln–M coordination polymersfreely. The outcome of such designed syntheses will beexpected to have tremendous importance to surpramolecu-lar chemistry and crystal engineering, and the explorationof advancing these strategies to other high-dimensionalcluster-based Ln–M coordination assemblies is currentlyunder development.

Acknowledgements

This work was financially supported by the Program forthe New Century Excellent Talents in University (No.NECT-05-0278), the National Natural Science Foundationof China (No. 20471012), and the Foundation for theAuthor of National Excellent Doctoral Dissertation ofP.R. China (No. 200322).

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Crystal engineering of lanthanide–transition-metal coordination polymers

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