Cu(I) Coordination Frameworks through Linear Ligands - American

Nov 24, 2006 - heterometallic coordination polymers, [Nd(H2O)2(CuI)2(nic)3]‚H2O (1) (Hnic ) nicotinic acid) and [LnCu(inic)2(ox)]‚H2O [Ln ). Nd (2...
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Incorporating Metal Clusters into Three-Dimensional Ln(III)-Cu(I) Coordination Frameworks through Linear Ligands Xiaojun Gu and Dongfeng Xue* State Key Laboratory of Fine Chemicals, Department of Materials Sciences and Chemical Engineering, School of Chemical Engineering, Dalian UniVersity of Technology, Dalian 116012, P. R. China

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1726-1732

ReceiVed NoVember 24, 2006

ABSTRACT: By using linear multifunctional ligands with different geometries, three novel three-dimensional (3D) Ln-M heterometallic coordination polymers, [Nd(H2O)2(CuI)2(nic)3]‚H2O (1) (Hnic ) nicotinic acid) and [LnCu(inic)2(ox)]‚H2O [Ln ) Nd (2), Eu (3); Hinic ) isonicotinic acid, H2ox ) oxalic acid], have been synthesized under hydrothermal conditions. Compound 1 exhibits a novel 3D coordination framework constructed by Cu4I4 clusters, Nd centers, and nic ligands. Our present work represents the first example of 3D Ln-M heterometallic coordination framework incorporating discrete cubane transition metal clusters covalently bonded to lanthanide centers through linear ligands. An unusual chemical rearrangement from Hinic to ox occurs in the formation of 2 and 3. Compounds 2 and 3 are isostructural and possess the first 3D coordination framework based on the linkage of twodimensional layers constructed by tetranuclear Ln2Cu2 clusters and inic ligands. Furthermore, the luminescent properties of 3 were studied. Introduction Crystal engineering of lanthanide-transition metal (Ln-M) heterometallic coordination complexes has attracted increasing interest due to their potential applications in various aspects such as magnetism,1 catalysis,2 molecular adsorption,3 and photochemical sensors,4 as well as their intriguing molecular topologies associated with various metal-ligand coordinations.5,6 A variety of heterometallic complexes have been reported so far; however, most of them only possess discrete structures in the crystallographic viewpoint.7-14 The assembly of extended structures with three-dimensional (3D) Ln-M heterometallic coordination frameworks is currently a formidable task because of some of the intrinsic characteristics of lanthanide ions such as the high and variable coordination numbers and the small energy difference among various coordination geometries.5 Furthermore, it has been found that most of the available 3D Ln-M heterometallic coordination frameworks are built up from single metal polyhedra.1-5 In contrast, rational synthesis of 3D Ln-M coordination polymers constructed by discrete metal clusters (including transition metal clusters, lanthanide clusters, or heterometallic clusters) remains largely unexplored due to its great synthetic challenges, and especially the cooperativity among these clusters. In fact, 3D cluster-based Ln-M coordination polymers are rarely reported in the literature.6,15 Transition metal clusters and lanthanide clusters possess different coordination preferences and potential applications.16 If one or two types of these clusters can be assembled into one crystallographic frame, the as-obtained Ln-M coordination polymers may exhibit novel structures and properties. The different affinity for oxygen and nitrogen donors of lanthanide and transition metal ions provides the impetus to construct unusual 3D Ln-M coordination frameworks based on metal clusters. In the current work, we chose nicotinic acid (Hnic) and isonicotinic acid (Hinic) as organic ligands, according to the following considerations: (i) Both are linear bridging ligands with nitrogen and oxygen donor atoms. The carboxylate group * To whom correspondence should be addressed. Tel/Fax: +86-41188993623. E-mail: [email protected].

can bond to lanthanide ions, while the nitrogen atoms can bond to transition metal ions; especially, Hinic ligand has been demonstrated to be an excellent spacer in the construction of novel 3D Ln-M coordination polymers.6c,d,17 (ii) The different geometries of both ligands can be used to understand the effect of the organic molecules on the whole framework. On the other hand, different transition metal clusters also play an important role in the formation of 3D Ln-M coordination frameworks. It is well-known that copper halides, whose anion can be used as an essential element of frameworks, are capable of meeting the special requirements of bridging ligands to construct highdimensional coordination polymers.18-21 Various structural motifs of copper halide such as rhomboid Cu2X2 dimers,19 cubane Cu4X4 tetramers,20 and Cu6X6 clusters21 have been reported. As an exploration, the copper halide clusters may be selected as building units for constructing Ln-M coordination frameworks, since they are excellent inorganic functional modules possessing rich coordination forms and rich photophysical properties.18-21 More importantly, the soft metal copper(I) ions can rather easily coordinate to the nitrogen atoms in inic and nic ligands; therefore, novel 3D Ln-M coordination frameworks incorporating copper halide clusters can be obtained. Herein, we report three novel 3D Ln-M heterometallic coordination polymers, [Nd(H2O)2(CuI)2(nic)3]‚H2O (1) and [LnCu(inic)2(ox)]‚H2O [Ln ) Nd (2), Eu (3); H2ox ) oxalic acid]. Compound 1 exhibits a 3D heterometallic coordination framework based on the Cu4I4 clusters. To the best of our knowledge, it represents the first example of a 3D Ln-M coordination framework in which the discrete cubane transition metal clusters covalently bonded to the lanthanide centers through linear ligands. It is surprising that an unusual chemical rearrangement from Hinic to ox occurs in the formation of 2 and 3. Compounds 2 and 3 exhibit an unusual 3D coordination framework based on the linkage of 2D layers containing tetranuclear Ln2Cu2 clusters by inic ligands. Moreover, the luminescent properties of 3 were studied. Experimental Section Materials and General Methods. All chemicals were of reagent grade and used without further purification. Distilled water was used in all reactions. The elemental analyses (C, H, and N) were carried out

10.1021/cg060836z CCC: $37.00 © 2007 American Chemical Society Published on Web 08/09/2007

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Table 1. Crystal Data and Structure Refinement for 1-3 1 empirical formula fw cryst system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Fcalcd (g cm-3) µ (mm-1) F(000) θ range (deg) reflns collected/ unique GOF R1a (I > 2σ(I)) wR2b largest diff peak and hole (e Å- 3) a

2

Table 2. Selected Bond Lengths (Å) and Angle (deg) for 1-3

3

C18H18Cu2I2N3NdO9 945.47 orthorhombic Ibam 9.3247(16) 17.011(3) 31.447(6) 90 4988.1(15) 8 2.518 6.269 3544 2.39-29.14 14920/3189

C14H10CuN2NdO9 558.02 monoclinic C2/c 21.148(3) 9.2771(16) 17.264(3) 106.404(3) 3249.1(9) 8 2.282 4.525 2152 2.01-26.38 8796/3326

C14H10CuN2EuO9 565.74 monoclinic C2/c 21.296(5) 9.203(2) 17.063(4) 105.939(3) 3215.3(14) 8 2.337 5.245 2176 1.99-29.12 9737/3949

1.044 0.0283 0.0733 0.912, -0.899

1.056 0.0201 0.0490 0.795, -0.583

1.014 0.0235 0.0577 1.027, -1.245

R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ){∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2.

on a Perkin-Elmer 240C elemental analyzer. The infrared spectra were recorded (400-4000 cm-1 region) on an Alpha Centaurt FT/IR spectrophotometer using KBr pellets. Thermogravimetric analyses (TGA) were performed on an SDT Q600 instrument in flowing nitrogen atmosphere with a heating rate of 10 °C/min. Fluorescence spectroscopy data were recorded on an SPEX FL-2T2 luminescence spectrometer equipped with a 450W xenon lamp as the excitation source. Synthesis of [Nd(H2O)2(CuI)2(nic)3]‚H2O (1). A mixture of Nd2O3 (0.168 g, 0.5 mmol), CuI (0.095 g, 0.5 mmol), Hnic (0.123 g, 1.0 mmol), and H2O (15 mL) was stirred for 30 min in air and then sealed in a 23 mL Teflon-lined stainless steel vessel, which was heated at 155 °C for 9 days. The mixture was cooled down to room temperature at a rate of 5 °C/h. Yellow crystals of 1 were obtained (yield: 0.097 g, 41% based on Cu). Anal. Calcd for C18H18Cu2I2N3NdO9 (%): C, 22.98; H, 2.02; N, 4.51. Found: C, 22.85; H, 1.90; N, 4.44. IR (KBr, cm-1): 3393 (m), 1610 (s), 1567 (s), 1415 (s), 1201 (w), 1040 (w), 852 (w), 758 (m), 690 (m), 553 (w), 425 (w). Synthesis of [LnCu(inic)2(ox)]‚H2O [Ln ) Nd (2), Eu (3)]. A mixture of Ln2O3 (Nd2O3, 0.168 g, 0.5 mmol; Eu2O3, 0.176 g, 0.5 mmol), CuCl2‚2H2O (0.085 g, 0.5 mmol), Hinic (0.123 g, 1.0 mmol), HCl (0.009 g, 0.25 mmol), and H2O (15 mL) was stirred for 30 min in air and then sealed in a 23 mL Teflon-lined stainless steel vessel, which was heated at 155 °C for 9 days. The mixture was cooled down to room temperature at a rate of 5 °C/h. Yellow crystals of 2 and 3 were obtained (yield: 0.036 g, 13% based on Cu for 2 and 0.034 g, 12% based on Cu for 3). Anal. Calcd for C14H10CuN2NdO9 (%): C, 30.22; H, 1.89; N, 5.15. Found: C, 30.11; H, 1.79; N, 5.02. IR (KBr, cm-1) for 2: 3430 (m), 1602 (s), 1543 (s), 1405 (s), 1312 (m), 1219 (w), 1056 (w), 861 (w), 785 (m), 700 (m), 510 (w), 442 (w). Anal. Calcd for C14H10CuN2EuO9 (%): C, 29.81; H, 1.89; N, 5.04. Found: C, 29.70; H, 1.77; N, 4.95. IR (KBr, cm-1) for 3: 3432 (m), 1602 (s), 1544 (s), 1405 (s), 1313 (m), 1218 (w), 1056 (w), 865 (w), 789 (m), 700 (m), 510 (w), 443 (m). X-ray Crystallographic Study. The collection of crystallographic data was carried out on a Bruker SMART Apex CCD diffractometer using graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å) at 293 K. Empirical absorption correction was applied. These structures were solved by direct method and refined by the full-matrix leastsquares methods on F2 using the SHELXTL crystallographic software package.22 Anisotropic thermal parameters were used to refine all nonhydrogen atoms. The hydrogen atoms for C-H were placed in idealized positions. The Cu1 atom in compound 2 is disordered. The crystal data and structure refinement of compounds 1-3 are summarized in Table 1. Selected bond lengths and angles for compounds 1-3 are listed in Table 2. CCDC reference numbers: 622069 for 1, 621953 for 2, and 621952 for 3.

Nd(1)-O(2) Nd(1)-O(3) Nd(1)-O(5) Cu(1)-I(1)#2 O(2)-Nd(1)-O(1)#1 O(1)-Nd(1)-O(1)#1 O(3)-Nd(1)-O(5) O(4)-Nd(1)-O(2)#1 N(1)-Cu(1)-I(1)#2

Compound 1a 2.367(3) Nd(1)-O(5) 2.493(4) Nd(1)-O(4) 2.593(5) Cu(1)-N(1) 2.6732(8) Cu(1)-I(1)#3 150.77(14) O(5)-Nd(1)-O(1)#1 108.71(16) O(3)-Nd(1)-O(4) 120.18(15) O(2)-Nd(1)-O(3) 118.05(16) N(1)-Cu(1)-I(1) 106.36(11) I(1)-Cu(1)-I(1)#2

2.593(5) 2.513(4) 2.035(4) 2.7164(8) 70.46(10) 143.57(14) 79.22(13) 108.26(10) 115.85(2)

Compound 2b Nd(1)-O(1) 2.5022(19) Nd(1)-O(2) 2.510(2) Nd(1)-O(4) 2.5015(19) Nd(1)-O(6)#1 2.4184(19) Nd(1)-O(7)#4 2.526(2) Nd(1)-O(8)#4 2.756(2) Nd(1)-O(5)#2 2.466(2) Cu(1)-N(2) 1.944(4) Cu(1)-N(1) 1.922(4) Cu(1)-O(4) 2.422(4) O(4)-Nd(1)-O(6)#1 153.18(7) O(7)#5-Nd(1)-O(8)#4 48.95(6) O(4)-Nd(1)-O(1) 92.25(7) O(2)-Nd(1)-O(8)#4 146.49(6) O(6)#1-Nd(1)-O(7)#4 97.54(8) O(1)-Nd(1)-O(8)#3 95.21(6) O(8)#4-Nd(1)-O(7)#4 122.51(7) N(1)-Cu(1)-N(2) 150.4(2) N(2)-Cu(1)-O(4) 108.74(16) N(1)-Cu(1)-O(4) 95.97(14) Eu(1)-O(1) Eu(1)-O(3) Eu(1)-O(7) Eu(1)-O(6) Cu(1)-N(2)#2 O(5)-Eu(1)-O(4) O(1)-Eu(1)-O(4) O(5)-Eu(1)-O(7) O(7)-Eu(1)-O(8)#1 N(2)#2-Cu(1)-O(4)#3

Compound 3c 2.455(2) Eu(1)-O(2) 2.456(2) Eu(1)-O(5) 2.481(2) Eu(1)-O(8) 2.419(2) N(1)-Cu(1) 1.911(3) Cu(1)-O(4)#3 152.43(7) O(7)-Eu(1)-O(8) 92.84(8) O(2)-Eu(1)-O(8)#1 96.13(9) O(1)-Eu(1)-O(8)#1 122.35(8) N(1)-Cu(1)-N(2)#2 110.951(8) N(1)-Cu(1)-O(4)#3

2.476(2) 2.379(2) 2.775(2) 1.914(3) 2.454(4) 49.06(7) 146.59(7) 94.48(8) 154.20(15) 94.912(9)

a Symmetry codes for 1: (#1) x, y, -z; (#2) 1 - x, y, 1/ - z. b Symmetry 2 codes for 2: (#1) -x, -1 + y, -z + 1/2; (#2) x, -y + 2, z + 1/2; (#3) x + 1/ , y - 1/ , z; (#4) -x - 1/ , -y + 3/ , -z + 1. c Symmetry codes for 3: 2 2 2 2 (#1) -x, -y + 4, -z - 1; (#2) -x - 1/2, y - 1/2, -z - 1/2; (#3) x, 3 - y, 1/ + z. 2

Results and Discussion Syntheses. Our present aim is to construct 3D Ln-M heterometallic coordination polymers incorporating metal clusters. We tried to isolate these single crystals from a conventional solution method (using lanthanide salts with a mixture of copper salts and organic ligands); however, only uncharacterized precipitates were obtained. Hydrothermal techniques show us some great advantages over other methods for the synthesis of high-dimensional coordination compounds.23 Moreover, some unusual reactions such as in situ ligand syntheses occur under these conditions.24 In our previous work, lanthanide oxides have been proven to play an important role in the formation of 3D Ln-M coordination polymers.17 Therefore, we use lanthanide oxides as lanthanide source to synthesize novel 3D cluster-based Ln-M coordination frameworks under hydrothermal conditions. In the course of preparing 2 and 3, an unusual chemical rearrangement was found, in which Hinic ligands were partly decomposed into the ox moiety. To our knowledge, only one example of this similar reaction has been reported under hydrothermal conditions.25 Pressure under hydrothermal conditions is a necessary factor for this kind of rearrangement. It is noteworthy that compounds 2 and 3 can also be obtained by using mixed H2ox and Hinic ligands. The reaction among Ln2O3, CuCl2, Hinic, and H2ox at 155 °C for 9 days afforded yellow crystals of 2 and 3. Structure of [Nd(H2O)2(CuI)2(nic)3]‚H2O, 1. Single-crystal structure analysis reveals that compound 1 exhibits a 3D coordination framework composed of Cu4I4 clusters, Nd centers, and nic ligands, crystallizing in orthorhombic space group Ibam. The asymmetry unit consists of one crystallographically unique copper atom, one-half neodymium ion, one iodine atom, one

1728 Crystal Growth & Design, Vol. 7, No. 9, 2007

Gu and Xue Chart 1. Coordination Modes of nic Ligands in 1 (a, b), and inic (c, d) and ox (e, f) Ligands in 2 and 3

Figure 1. ORTEP plot of the asymmetric unit of 1 (50% probability ellipsoids). All hydrogen atoms are omitted for clarity. Symmetry codes: A (x, y, -z), B (1 - x, -y, z), C (1 - x, y, 1/2 - z), D (-1/2 + x, 1/2 - y, z), E (1 - x, 1 - y, z).

Figure 2. View of the 3D heterometallic coordination frameworks along the a-axis in 1. The hydrogen atoms and terminal nic ligands are omitted for clarity.

and one-half nic ligands, and one coordination and one-half noncoordination water molecules. An ORTEP view of 1 is shown in Figure 1. The Cu center is tetrahedrally coordinated by three I atoms and one nitrogen atom from the nic ligand. The Cu-N and Cu-I distances are 2.034(4) and 2.6732(8)2.7162(9) Å, respectively. Each Cu(I) center links three neighboring I atoms to form a distorted cubane Cu4I4 unit, similar to those reported for Cu4I4 tetramer units.20 The CuCu distances are 2.6506(13)-2.8213(13) Å, which are comparable to those found in other structurally characterized Cu4I4 complexes.20 The Nd center is coordinated by four oxygen atoms from four bridging nic ligands, two oxygen atoms from two terminal ligands, and two oxygen atoms from two water molecules. The 8-fold coordination polyhedron of Nd can be described as a slightly distorted square antiprism. The Nd-O bond lengths fall in the range of 2.367(3)-2.593(5) Å, similar to those of other Nd(III)-carboxylate complexes.17,26 The O-Nd-O bond angles range from 70.46(10)° to 150.77(14)°.

The nic ligands exhibit two types of coordination modes; as shown in Chart 1, one acts as a bridging ligand to connect one Cu center and two Nd centers (Chart 1a), while the other acts as a terminal ligand to connect two Nd centers (Chart 1b). It is interesting that the linkage between Cu4I4 clusters and Nd centers through nic ligands gives rise to an unusual coordination framework with 1D channels (Figure 2). These channels are occupied by lattice waters, coordination waters, and terminal nic ligands. In the 3D coordination framework, each Cu4I4 cluster links eight Nd centers by four bridging nic ligands, while each Nd center links four Cu4I4 clusters (Figure 3a,b). Therefore, two Cu4I4 clusters and two Nd centers are linked together by four nic ligands to form a nanosized ring unit (Figure 3c). The distances of the opposite Cu‚‚‚Cu and Nd‚ ‚‚Nd are 13.934 and 9.671 Å, respectively. Therefore, the structure can also be regarded as the assembly of these nanosized ring units. Due to the difficulty in synthesizing multinuclear copper halide clusters, rare examples of 3D metal-organic polymeric structures constructed from multinuclear copper halide clusters have been reported; moreover, only two threedimensional Ln-Cu coordination frameworks built up from Cu2Cl2 dimer or Cu3Cl4 trimer have been reported so far.6c,d To our knowledge, compound 1 is the first 3D Ln-M heterometallic coordination framework based on the linkage of Cu4I4 cubane cluster and lanthanide centers by nic Ligands. Structure of [NdCu(inic)2(ox)]‚H2O, 2. Single-crystal structure analysis reveals that compound 2 displays a 3D coordination framework constructed by tetranuclear Nd2Cu2 clusters and mixed inic and ox ligands, crystallizing in monoclinic space group C2/c. There are one Nd(III) center, one Cu(I) center, two inic ligands, one ox ligand, and one water molecules in the asymmetry unit. An ORTEP view of 2 is shown in Figure 4. The Nd(III) center is coordinated by four oxygen atoms from two ox ligands and five oxygen atoms from four inic ligands, leading to a distorted monocapped square-antiprism geometry. The Nd-O bond lengths range from 2.4184(19) to 2.756(2) Å, comparable to those of compound 1 and other Nd(III)carboxylate complexes.17,26 The O-Nd-O bond angles range from 48.95(6)° to 153.18(7)°. In the structure of 2, the Cu center has an oxidation state of +1, attributed to a reduction reaction

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Figure 4. ORTEP plot of the asymmetric unit of 2 (50% probability ellipsoids). All hydrogen atoms are omitted for clarity. Symmetry codes: A (-x, 2 - y, 1 - z), B (-x, y, 1/2 - z), C (-1/2 - x, 3/2 - y, 1 - z), D (1/2 + x, -1/2 + y, z), E (x, 2 - y, 1/2 + z), F (-x, -1 + y, 1 /2 - z).

Figure 5. View of the Nd2Cu2 tetranuclear cluster in 2. Figure 3. (a) View of the connectivity of Cu4I4 cluster in 1. (b) View of the connectivity of Nd center in 1. (c) View of the ring consisting of two Nd centers and two Cu4I4 clusters in 1.

involving the Hinic and ox ligands. This observation can also be judged by the yellow color of the crystals of 2. Each Cu center is coordinated by two nitrogen atoms from two different inic ligands (Cu-N, 1.922(4) and 1.944(4) Å) and one oxygen atom from one ox ligand (Cu-O, 2.422(4) Å) to furnish a trigonal geometry. Two inic ligands in the asymmetric unit adopt two coordination modes, as depicted in Chart 1c,d. In mode c, the nitrogen atom coordinates to one Cu center and the carboxylate connects two Nd centers through a bimonodenate coordination. In mode d, the nitrogen atom coordinates to one Cu center and the carboxylate connects two Nd centers through a chelating and bridging coordination. Two ox ligands also adopt two coordination modes; one coordinates to two Nd centers through a chelating coordination, while the other coordinates to two Nd centers and two Cu centers through a chelating and bridging coordination (Chart 1e,f).

On the basis of bridging connectivity of inic and ox ligands, a Nd2Cu2 tetranuclear cluster forms. As many as eight inic ligands (four with mode c and four with mode d) and four ox ligands (two with mode e and two with mode f) coordinate to the Nd2Cu2 core through the pyridyl nitrogen atoms and carboxylate oxygen atoms (Figure 5). To our knowledge, the Ln2M2 heterometallic clusters have never been reported to date. It is interesting that the tetramuclear clusters are connected by inic ligands to form a 3D pillared coordination framework (Figure 6), in which the inic ligands act as pillars bonded to tetranuclear clusters. Due to the little hindrance of inic ligands, small channels occupied by water molecules emerge along the b- and c-axes (Figures 6 and S3 of the Supporting Information). Due to the strong chelating effect of ox ligands, the connection between Nd centers and ox ligands generates a 2D Nd-ox layer in the bc-plane (Figure S4, Supporting Information). This analogous arrangement can be found in a homometallic lanthanide oxalate framework;27 however, more important

1730 Crystal Growth & Design, Vol. 7, No. 9, 2007

Gu and Xue

Figure 8. Solid-state emission spectrum for 3 at room temperature (excited at 320 nm). Figure 6. View of the 3D heterometallic coordination framework along the c-axis in 2.

Figure 7. View of the 2D Nd-Cu layer in 2 along the a-axis. The hydrogen, carbon, and oxygen atoms in inic ligands are omitted for clarity.

is that this arrangement provides active coordination sites to bond Cu(I) ions to form 2D Nd-Cu heterometallic layers (Figure 7). Therefore, the 3D coordination framework can also be regarded as the assembly of 2D heteometallic layers and inic ligands. It should be noted that these two types of inic ligands play different roles in the construction of the 3D coordination framework. The inic ligands in mode c link the Cu centers and Nd centers in the same layer, while the inic ligands in mode d link adjacent layers to form a 3D pillared framework (Figure S5, Supporting Information). To our knowledge, compound 2 is the first 3D Ln-M coordination framework built up from 2D layers containing tetranuclear Ln2Cu2 clusters and mixed organic ligands. Since the crystal structure of 3 is isomorphous with that of 2, there is only a small difference between them. Since the radius of Eu(III) is smaller than that of Nd(III), most Eu-O bonds in 3 are slightly shorter than the corresponding ones in 2, as shown in Table 2. As a result, the cell volume of 3 is smaller than that of 2. The structures of 1 and 2(3) are remarkably different owing to the different geometries between inic and nic ligands as well

as the different source of copper ions. Crystal engineering has provided us a useful paradigm to develop rational approaches to design new coordination complexes. However, it is difficult to control the assembly and 3D structure of the target compound.28 The in situ reaction of Hinic ligand in the formation of 2 and 3 can well reveal the difficulty of controlling the structure of the target compound. IR Spectroscopy. The IR spectrum of compound 1 shows the characteristic bands of carboxyl groups at 1567 cm-1 for asymmetric stretching and at 1415 cm-1 for symmetric stretching. For 2 and 3, the characteristic bands of carboxyl groups are shown in the range 1543-1602 cm-1 for asymmetric stretching and at 1405 cm-1 for symmetric stretching. The absence of the characteristic bands at around 1700 cm-1 in 1-3 attributed to the protonated carboxyl group indicates that all carboxyl groups of organic moieties in 1-3 have been deprotonated.29 The broad band at around 3430 cm-1 is assigned to the vibration of water molecules in these compounds (Figures S6-8, Supporting Information). Thermal Analysis. Owing to the structural similarity of 2 and 3, compound 3 was selected to study their thermal stability. TG curves have been obtained in N2 for crystalline samples of 1 and 3 in the temperature range of 30-800 °C. It can be seen from the TG curve of 1 that the weight loss of 5.92% from 30 to 380 °C corresponds to the removal of the noncoordination and coordination water molecules (calcd 5.71%). The second weight loss between 380 and 800 °C is attributable to the loss of all nic ligands. The TG study of 3 shows that the first weight loss of 3.03% from 30 to 350 °C corresponds to removal of water molecules (calcd 3.19%). The second weight loss between 350 and 800 °C is attributable to the loss of all organic components (Figures S9 and S10, Supporting Information). Photoluminescent Properties. Due to the excellent luminescent properties of Eu ions, the luminescence of 3 containing Eu ions was investigated. The emission spectrum of 3 (Figure 8) at room temperature upon excitation at 320 nm exhibits the characteristic transition of Eu ions, which can be attributed to 5D f 7F (J ) 1, 2, 3, 4) transitions, i.e., 591 nm (5D f 7F ), 0 J 0 1 615 nm (5D0 f 7F2), 646 nm (5D0 f 7F3), and 696 nm (5D0 f 7F ). The present intensity ratio I(5D f 7F )/I(5D f 7F ) is 4 0 2 0 1 equal to ca. 3.6, which suggests a noncentrosymmetric coordination environment for Eu(III) ions in 3.30 The most intense transition is 5D0 f 7F2, which implies the red luminescence of 3. For the H2ox ligand, the luminescent emission peak is located at 370 nm, while the luminescent emission peak is located at 436 nm for the Hinic ligand (Figures S11 and S12, Supporting

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Crystal Growth & Design, Vol. 7, No. 9, 2007 1731

Information). There is a ligand-based emission in the current spectrum, which suggests that the energy transfer from the ligand to lanthanide center is not quite effective. (7)

Conclusions In summary, we have successfully synthesized three novel cluster-based Ln-M heterometallic coordination frameworks by using linear multifunctional ligands with different geometries. The structure of 1 possesses a 3D coordination framework constructed from cubane Cu4I4 clusters, Nd centers, and nic ligands. To the best of our knowledge, it represents the first example of 3D Ln-M heterometallic coordination framework incorporating transition metal cubane clusters. Both compounds 2 and 3 display an unusual 3D coordination framework based on tetranuclear Ln2Cu2 clusters and inic ligands, representing the first 3D Ln-M coordination framework built up from 2D layers containing tetranuclear heterometallic clusters and mixed organic ligands. These results demonstrate that the metal clusters can be incorporated into the 3D heterometallic coordination frameworks by selecting multifunctional ligands with particular coordination sites. This synthetic approach may be applicable to prepare other novel 3D cluster-based Ln-M coordination polymer materials for particular prospects. Acknowledgment. This work was financially supported by the Program for New Century Excellent Talents in University (Grant No. 05-0278), the National Natural Science Foundation of China (Grant No. 20471012), and the Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (Grant No. 200322).

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Supporting Information Available: X-ray crystallographic file (CIF) for compounds 1-3, IR spectra, TG curves, and additional figures (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org.

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