Solvent Dependent Crystallization of Isomeric Chain Coordination

Solvent Dependent Crystallization of Isomeric Chain Coordination Polymers in the Ce-Zn/Cd-dipic System T. K. Prasad and M. V. Rajasekharan* School of Chemistry, UniVersity of Hyderabad, Hyderabad-500 046, India

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 4 1346–1352

ReceiVed April 10, 2007; ReVised Manuscript ReceiVed December 3, 2007

ABSTRACT: Six linear chain coordination polymers containing alternating CeIV and ZnII/CdII coordination polyhedra are formed with dipicolinic acid (dipicH2) depending upon the crystallization conditions. They belong to three isomeric types that differ in the way the tricapped trigonal prismatic polyhedra of [Ce(dpic)3]2- are linked with the Zn/Cd octahedra. The different isomers differ in complexity as reflected in their angularity and repeat units. Highly angular chains lead to porous structures, one of which incorporates a 16-water cluster. Introduction Coordination polymers are of interest due to the diverse structural motifs they may present and the potentially useful properties they may possess.1 The role of multifunctional bridging ligands and the coordination preferences of metal ions in influencing the nature of coordination networks are wellknown. Somewhat less understood is the role of solvents and kinetic aspects of the crystallization process, often referred to as self-assembly.2 Solvent molecules, especially water, can sometimes occupy a substantial part of the unit cell volume of the crystals even without directly involving in the coordination, so much so that the coordination network and the lattice solvent may appear to have a symbiotic relationship with each other.3 We have previously reported hetero metallic coordination polymers of dipicolinic acid (dipicH2) containing Ce4+ and alkaline earth cations.4 In this paper, we report the Zn/Cd coordination counterparts of the alkaline earth networks obtained from different solvents: [Zn(H2O)4Ce(dipic)3] · 8H2O (1), [Zn(CH3OH)2(H2O)2Ce(dipic)3] · 3H2O (2), [Zn(CH3CH2OH)(H2O)3Ce(dipic)3] · CH3CH2OH · 2H2O (3), [Cd(H2O)4Ce(dipic)3][Ce(dipic)(dipicH)2] · 24H2O (4), [Cd(CH3OH)2(H2O)2Ce(dipic)3] · 3H2O (5), and [Cd(H2O)4Ce(dipic)3] · 3H2O (6). In the crystals of 1 there is a 16-water cluster which has been reported only once before.5 Besides crystal structure of all the compounds, dehydration–rehydration studies monitored by powder X-ray diffraction are also reported. Experimental Section All chemicals were reagent-grade commercial samples and were used without further purification. [Zn(H2O)4Ce(dipic)3] · 8H2O (1). To a 20 mL methanolic solution of dipicH2 (0.502 g, 3.00 mmol), a 5 mL aqueous solution containing (NH4)2Ce(NO3)6 (0.548 g, 0.999 mmol) and Zn(CH3COO)2 · 2H2O (0.221 g, 1.01 mmol) was added and stirred for 30 min. The pale yellow precipitate that formed was filtered and dried. It was recrystallized from hot water to get yellow blocks of 1. Yield: 0.482 g (0.526 mmol, 53%). Anal. Calcd. for C21H33CeN3O24Zn: C, 27.50; H, 3.63; N, 4.58% Found: C, 26.98; H, 3.26; N, 4.71%. IR (KBr disk, cm-1): 3526, 1639, 1429, 1367, 1269, 1180, 1078, 1024, 922, 761, 734, 692, 655. [Zn(CH3OH)2(H2O)2Ce(dipic)3] · 3H2O (2). The method is similar to that of 1 except that the solution was stirred very briefly just to mix the reactants and was filtered. The filtrate was kept for evaporation in * To whom correspondence should be addressed. Mailing address: Prof. M. V. Rajasekharan, School of Chemistry, University of Hyderabad, Hyderabad-500046, India. Phone: +91-40-2313-4857. Fax: +91-40-2301-2460. E-mail: mvrsc@ uohyd.ernet.in.

open air at room temperature. Crystals of 2 started forming within less than an hour and were removed after one day. Yield: 0.372 g (0.435 mmol, 44%). Anal. Calcd. for C23H27CeN3O19Zn: C, 32.31; H, 3.18; N, 4.91% Found: C, 31.51; H, 2.46; N, 4.51%. IR (KBr disk, cm-1): 3217, 1637, 1437, 1385, 1271, 1176, 1072, 1024, 920, 763, 731, 665. [Zn(CH3CH2OH)(H2O)3Ce(dipic)3] · CH3CH2OH · 2H2O (3). The method is similar to that of 2, except that ethanol is used in place of methanol. Yield: 0.392 g (0.444 mmol, 44%). Anal. Calcd. for C25H31CeN3O19Zn: C, 34.01; H, 3.54; N, 4.76% Found: C, 33.65; H, 2.82; N, 4.44%. IR (KBr disk, cm-1): 3500, 1641, 1429, 1363, 1271, 1176, 1074, 1024, 920, 733. [Cd(H2O)4Ce(dipic)3]2[Ce(dipic)(dipicH)2] · 24H2O (4). The method is similar to that of 1 except for the use of Cd(CH3COO)2 · 2H2O (0.266 g, 0.998 mmol). As before, the precipitate was recrystallized from hot water by slow evaporation over a period of several hours to get crystals of 4. Yield: 0.494 g (0.184 mmol, 37%). Anal. Calcd. for C63H93Cd2Ce3N9O67: C, 28.09; H, 3.48; N, 4.68% Found: C, 28.49; H, 2.49; N, 4.73%. IR (KBr disk, cm-1): 3530, 1645, 1425, 1369, 1178, 1072, 1024, 922, 733. [Cd(CH3OH)2(H2O)2Ce(dipic)3] · 3H2O (5). The method is similar to that of 2 except for the use of Cd(CH3COO)2 · 2H2O (0.266 g, 0.998 mmol). Yield: 0.382 g (0.424 mmol, 42%). Anal. Calcd. for C23H27CdCeN3O19: C, 30.63; H, 3.02; N, 4.65% Found: C, 29.32; H, 2.49; N, 4.57%. IR (KBr disk, cm-1): 3499, 1643, 1425, 1367, 1178, 1072, 1024, 922, 763, 690, 665. [Cd(H2O)4Ce(dipic)3] · 3H2O (6). The initial precipitate obtained in the preparation of 4 was found to be compound 6. Yield: 0.544 g (0.622 mmol, 62%). The crystals of 6 suitable for X-ray study were obtained by dissolving whole of the above precipitate in 15 mL boiling water and cooling to room temperature, whereupon the crystals were deposited in one lot within about 30 min. Anal. Calcd. for C21H23CdCeN3O19: C, 28.86; H,2.65; N, 4.81% Found: C, 29.44; H, 3.43; N, 5.18%. IR (KBr disk, cm-1): 3402, 1647, 1429, 1373, 1271, 1178, 1078, 1022, 916, 727. Physical Measurements. IR spectra were measured using a JASCO 5300 FT/IR infrared spectrometer. Elemental analysis was performed on a Perkin-Elmer 2400 CHNS/O analyzer. Thermo-gravimetric analysis was performed by using a NETZSCH STA 409 PC/PG instrument coupled with mass analyzer. X-ray Crystallography. X-ray data were collected for compounds on a Bruker SMART APEX CCD X-ray diffractometer, using graphitemonochromated Mo KR radiation (λ ) 0.71073 Å). The data were reduced using SAINTPLUS,6 and a multiscan absorption correction using SADABS7 was performed. The structures were solved using SHELXS-97 and full matrix least-squares refinement against F2 was carried out using SHELXL-97.8 All ring hydrogen atoms were assigned on the basis of geometrical considerations and were allowed to ride upon the respective carbon atoms. Drawings were made using Mercury.9 In compound 2 one of the dipic ions was disordered over two symmetry related positions and the coordinated methanol was also disordered in two positions. All the water molecules and methanol molecules were refined isotropically. In compound 3, the lattice ethanol molecule was refined isotropically. In compound 4, 6 out of the 12 lattice–water

10.1021/cg070349z CCC: $40.75  2008 American Chemical Society Published on Web 03/18/2008

Crystallization of Isomeric Chain Coordination Polymers

Crystal Growth & Design, Vol. 8, No. 4, 2008 1347

Table 1. Crystallographic Parameters of Compounds 1-6

formula F.W. crystal system space group T (K) a (Å) b (Å) c (Å) R ( °) β (°) γ (°) V (Å3) Z Dcalcd (g cm-3) µ (cm-1) R (Fo2) [I > 2σ(I)] Rw(Fo2) [I > 2σ(I)]

1

2

3

4

5

6

C21H33CeN3O24Zn 916.99 triclinic P1j 100(2) 10.2464(12) 12.5259(14) 13.6442(16) 95.145(2) 90.464(2) 111.482(2) 1621.4(3) 2 1.878 22.28 0.0344 0.0925

C23H27CeN3O19Zn 854.97 monoclinic C2/c 100(2) 10.0594(6) 21.8172(12) 14.6047(8) 90 110.1090(10) 90 3009.9(3) 4 1.887 23.81 0.0575 0.1481

C25 H31CeN3O19Zn 883.02 monoclinic P21/c 298(2) 10.3824(13) 22.829(3) 14.3127(18) 90 110.667(2) 90 3174.1(7) 4 1.848 22.61 0.0345 0.0873

C63H93Cd2Ce3N9O67 2691.61 monoclinic P2/n 298(2) 13.4100(9) 10.5921(7) 34.730(2) 90 94.3640(10) 90 4918.7(6) 2 1.817 19.03 0.0477 0.1327

C23H27CdCeN3O19 902.00 monoclinic P21/c 100(2) 10.2289(11) 22.036(2) 14.6701(13) 90 111.926(6) 90 3067.5(5) 4 1.953 22.48 0.0368 0.0866

C21H23CdCeN3O19 873.94 monoclinic Cc 298(2) 9.6829(17) 25.235(4) 11.947(2) 90 94.250(3) 90 2911.2(8) 4 1.994 23.65 0.0206 0.0500

Figure 1. One dimensional polymeric structure of compound 1; Ce, purple; Zn, green; O, red; N, pale blue; C, gray; H, omitted. molecules were highly disordered in 9 positions. The crystal data for the six compounds are presented in Table 1. X-ray powder diffractograms were measured (Cu KR) using a PW3710 model Philips Analytical X-ray diffractometer. Simulated powder patterns were obtained using Mercury.

Results and Discussion Synthesis. The six compounds were obtained by selfassembly from aqueous methanol or ethanol solutions under controlled conditions. Continued stirring of the solution always leads to the formation of a microcrystalline precipitate in every case, which, based on powder X-ray diffractograms, is structurally similar to compound 6. In fact, the cadmium compound 6 is obtained by rapid cooling of a hot supersaturated solution of the corresponding precipitate in water. Attempts to crystallize the analogous zinc compound were not successful. Slow crystallization from solutions of the initial precipitate in water led to compounds 1 and 4. Crystals of 2, 3, and 5, which contain coordinated methanol/ethanol molecules, were obtained from unstirred aqueous alcoholic solutions after removing the small amounts of initial precipitate. It would appear that 6 and its zinc equivalent are products formed under kinetic control, while 1 and 4, obtained by slow crystallization, are thermodynamically stable products. In the case of zinc, it was observed that the initial precipitate (structure type 6) was converted to small crystals of 1 just by wetting with a few drops of water and allowing it to stand for about an hour. Finally, it may be noted

that the structure of a cadmium complex obtained under conditions analogous to that of 3 could not be solved due to serious disorder in Cd position. Crystal structure of [Zn(H2O)4Ce(dipic)3] · 8H2O (1). The structure consists of a neutral zigzag one-dimensional (1D) coordination polymer and a 16-water cluster. The polymeric chain is made of alternating units of [Ce(dipic)3]2- and two crystallographically unique [Zn(H2O)4]2+ groups, situated on inversion centers and linked on either side by coordination through carbonyl oxygen atoms of two different dipic ligands (Figure 1). Ce4+ is nine coordinate with a distorted tricapped trigonal prismatic geometry and Zn2+ has a distorted octahedral geometry. Because of the unusual zigzag arrangement in the triclinic lattice a large empty space having a volume of 394 Å3 exists between the alternating polymeric chains (Figure 2). A total of 16 lattice–water molecules occupy this space, which amounts to 24% of the unit cell volume. The water molecules form a cluster as shown in Figures 3 and 4, which can be viewed as being made up of one planar tetramer, two approximately pyramidal tetramers, and two dimers. To the best of our knowledge, a 16-water cluster has been reported only once before,5 in which the H-bond O---O distances of the water molecules vary in the range 2.44–2.85 Å, compared to 2.69–2.94 Å, in the present cluster (Table 2). One hydrogen atom of each water molecule in the cluster is H-bonded to an adjacent water molecule while the other hydrogen atom bonds to acceptors in the coordination polymer. Two of the four coordinated water

1348 Crystal Growth & Design, Vol. 8, No. 4, 2008

Prasad and Rajasekharan

Figure 2. A view along c-axis of compound 1 by excluding lattice–water molecules; Ce, purple; Zn, green; O, red; N, pale blue; C, gray; H, dark gray.

Figure 3. Structure of the (H2O)16 cluster, H-bonding between water molecules and oxygen atoms in the coordination polymer are shown by doted lines.

H-bonding. Interaction of the 2D network with the water cluster results in a complex H-bonded 3D network.

Figure 4. Structure of the (H2O)16 cluster, showing only the oxygen atoms.

molecules on each Zn atom is H-bonded to carboxylate oxygen atoms of the adjacent 1D-coordination chain to form a 2D network, while the other two take part in intramolecular

Crystal structure of [Zn(CH3OH)2(H2O)2Ce(dipic)3] · 3H2O (2). The structure, shown in Figure 5, though grossly similar to that of 1, differs from it in the following aspects. There is only one unique Zn atom situated on an inversion center with two of the four coordinated water molecules replaced by methanol molecules. The angularity in the zigzag chain is greatly reduced leading to a more compact arrangement and there are now only three lattice–water molecules which form a water triangle with O---O distances 2.699(1), 2.78(2), and 2.78(2) Å. The coordinated water molecules take part in the intramolecular H-bonding with the carboxylate oxygen atoms, while the coordinated methanol molecules link adjacent 1D-chains to form a two-dimensional (2D) network. The solvent–water molecules further support the 2D network and no three-dimensional (3D) network is formed.



Table 2. Geometrical Parameters of Hydrogen Bonds for (H2O)16 Clustera O-H · · · O

O-H (Å) H · · · O (Å) O · · · O (Å) O-H · · · O (°)

O1W-H1A · · · O6W O1W-H1B · · · O44#1 O2W-H2A · · · O34#2 O2W-H2B · · · O7W O3W-H3A · · · O21#3 O3W-H3B · · · O7W O4W-H4A · · · O24 O4W-H4B · · · O5W O5W-H5A · · · O4W#4 O5W-H5B · · · O12#1 O6W-H6A · · · O4W#4 O6W-H6B · · · O34#3 O7W-H7A · · · O23 O7W-H7B · · · O8W O8W-H8A · · · O34#3 O8W-H8B · · · O5W#4

0.816 0.849 0.803 0.810 0.867 0.828 0.821 0.820 0.855 0.820 0.890 0.830 0.910 0.793 0.861 0.834

1.93 2.07 2.10 1.99 2.09 2.01 1.97 2.02 1.97 2.55 1.93 2.08 2.01 1.96 2.02 2.13

2.747(4) 2.898(3) 2.898(4) 2.791(5) 2.939(4) 2.824(5) 2.773(4) 2.799(4) 2.799(4) 3.295(4) 2.792(4) 2.903(4) 2.911(5) 2.694(8) 2.880(7) 2.937(7)

174 164 178 170 167 166 168 159 164 152 161 171 173 153 178 163

O · · · O · · · O (°)

O · · · O · · · O (°)

O3W-O7W-O8W 81.9(2) O5W-O8W-O7W 111.4(2) O8W-O7W-O2W#4 104.19(17) O4W-O5W-O4W #4 86.29(11) O5W-O4W-O5W#4 93.71(11)

O4W-O5W-O8W 77.12(17) O1W-O6W-O4W#4 122.51(13) O3W-O7W-O2W#4 86.19(12) O5W-O4W-O6W#4 93.28(14)

a Symmetry: #1 x, -1 + y, z; #2 - x, 1 - y, -z; #3 1 + x, y, z; #4 1 - x, 1 - y, 1 - z.

Crystal Structure of [(Zn(CH3CH2OH)(H2O)3Ce(dipic)3] · CH3CH2OH · 2H2O (3). The compound 3differs from 1 and 2 in having two different zinc coordination polyhedra, one having four water molecules as terminal ligands and the other with two water molecules and two ethanol molecules. These polyhedra alternate with Ce polyhedra leading to a 1D zigzag chain as before (Figure 6), having a similar angularity as that of 2. Two of the coordinated water molecules on the first type of zinc and the coordinated ethanol molecules on the other zinc are H-bonded to adjacent chains to form a 2D network. This network is further supported by the solvent ethanol molecules and one of the solvent–water molecules. The remaining solvent–water molecule acts as a bridge between the 2D planes to form a 3D network. There is no direct H-bonding interaction between the solvent molecules. Crystal Structure of [Cd(H2O)4Ce(dipic)3]2[Ce(dipic)(dipicH)2] · 24H2O (4). The polymeric part, [Cd(H2O)4Ce(dipic)3] (Figure 7) is exactly similar to the chain in compound 1 except Cd replaces Zn. The main difference is the presence of a free [Ce(dipic)2(dipicH2)] molecule and 24 lattice–water molecules. The zigzag chain is very angular like in 1. But, unlike in 1, the water molecules are arranged randomly in the lattice occupying 28% of the unit cell volume, and they along with [Ce(dipic)(dipicH2)] act as space fillers in the otherwise porous

lattice depicted in Figure 8. The H-bonds observed with the coordinated water molecules are similar to that of 1. The resulting 2D network is expanded to form a 3D network through the participation of the randomly arranged solvent–water molecules. There is no direct interaction between the coordination chain and the [Ce(dipic)(dipicH)2] molecules; instead these are H-bonded to the solvent–water molecules. Crystal Structure of [Cd(CH3OH)2(H2O)2Ce(dipic)3] · 3H2O (5). The structure of 5 is very similar to that of 2 with Zn replaced by Cd. The zigzag chain (Figure 9) is also similar to that of 2 and 3. The coordinated methanol molecules are H-bonded to carboxylate groups of the adjacent polymeric chains to form a 2D network. However, the coordinated water molecules do not participate in intermolecular H-bonding. While all the three solvent–water molecules further support the 2D network, two of them are also H-bonded to each other to form a water dimer. The O · · · O distance in the water dimer was 2.714(4) Å. Crystal Structure of [Cd(H2O)4Ce(dipic)3] · 3H2O (6). The coordination environment in compound 6 is similar to that of 1 and 4. The difference is only in the number of lattice–water molecules and the angularity of the zigzag chain. In compound 6 the polymeric chain is almost linear (Figure 10). Two of the coordinated water molecules are H-bonded to adjacent polymeric units, but unlike in the earlier structures, this interaction leads to a 3D network. This might be due to more linear nature of the coordination polymer leading to the close proximity of the H-bonds donors and acceptors. While all the three solvent–water molecules further support the 3D network, two of them are also H-bonded to each other to form a water dimer, as in 5. The O · · · O distance in the water dimer was 2.737(7) Å. Comparison of Structures. All the six structures described above consist of 1D coordination polymers made up of alternating tricapped trigonal prismatic Ce(IV) coordination polyhedra and octahedral Zn(II) or Cd(II) polyhedra. The Ce polyhedra are dianionic while Zn/Cd polyhedra are dicationic. Varying amounts of lattice solvent molecules are also present in the crystals. In one case (compound 4), there is also present a neutral nine-coordinate Ce(IV) complex molecule. In the polymeric chains, two out of the three dipic2- ligands on cerium act as bridging ligands. The carboxylate bridges are all (antisyn) type. As shown in Figure 11, there are three ways in which a tricapped trigonal prism (TCTP) can link to the Zn/Cd ions on either side. In this figure each thick line represents a dipic2ligand. Carboxylate oxygen atoms occupy the vertices of the prism while the pyridine-N atoms occupy the capping positions above the rectangular faces. The polymer chains in compounds 1 and 4 correspond to type A bridging, those in 2, 3, and 5 correspond to type B and the polymer in compound 6 has

Figure 5. One dimensional polymeric structure of compound 2; Ce, purple; Zn, green; O, red; N, pale blue; C, gray; H, omitted.



Figure 6. One dimensional polymeric structure of compound 3; Ce, purple; Zn, green; O, red; N, pale blue; C, gray; H, omitted.

Figure 7. One dimensional polymeric structure of compound 4; Ce, purple; Cd, blue; O, red; N, pale blue; C, gray; H, omitted.

Figure 8. A view along b-axis of compound 4, excluding lattice–water molecules; Ce, purple; Cd, blue; O, red; N, pale blue; C, gray; H, dark gray.

type C bridging mode. The three modes can in principle give rise to network isomers; however, for each compound reported here only one structure is actually formed. The angularity of the zigzag chains and the resultant free space

available for solvents are different for the different bridging modes. Coming to symmetry aspects of the polymer chains, compound 6 has the smallest repeat unit consisting of just one Ce and one Cd polyhedron. The remaining five com-





Figure 11. The three bridging modes of the TCTP polyhedra of [Ce(dipic)3]2-. The thick lines represent dipic2-, the circles represent the vertices (O atoms), with the capping vertices (N atoms) omitted. The vertices that are linked to adjacent Zn/Cd polyhedra are shown as filled circles.

Figure 12. Thermal decomposition of compounds 1-6. Heating rate 5 °C/min.

pounds have repeat units which are twice as long, viz., [CeMCeM] in which the cerium units are inversion related while M (Zn or Cd) may be crystallographically equivalent (2, 5) or independent (1, 3, 4). Thermal Analysis. The thermogravimetric analysis of all samples was done between 25 and 500 °C at a heating rate of 5 °C min-1 in a nitrogen atmosphere (Figure 12). For the compound 1, weight loss begins at the start of the experiment

and all lattice–water is lost at 127 °C (calculated for eight lattice–water molecules 15.7%, found 16.3%). The coordinated water is lost between 127 and 233 °C (calculated 7.9%, found 6.9%). Compound 2 behaves similarly except that there is no perfect boundary for the loss of lattice and coordinated solvents, which are all lost below 235 °C (calculated 18.0%, found 16.9%). Mass analysis of the gas evolved during the experiment showed that the methanol and water are leaving all along in the above temperature range. Weight loss of the compound 3 starts at 30 °C and all water molecules and ethanol molecules are lost below 235 °C (calculated 20.6%, found 18.3%). For compound 4 the weight loss starts at 25 °C and all the lattice–water and coordinated water molecules leave below 204 °C, in a single step (calculated 18.9%, found 19.7%). For compound 5 a gradual weight loss takes place between 25 and 235 °C (calculated 17.1%, found 16.7%). For the compound 6, the three lattice–water molecules leave in the temperature range 25–108 °C (calculated 6.2%, found 6.1%) and the four coordinated water molecules leave in the range 108–226 °C (calculated 8.2%, found 8.2%). For all compounds the solvents leave below 235 °C and the chain may be stable up to 400 °C. The discrepancies between some of the calculated and observed percentages are due to the instability of the compounds toward solvent loss. The transformations associated with thermal desolvation and resolvation were probed using powder X-ray diffractograms. On heating the powdered crystals of compound 1 at 120 °C for 30 min, the crystalline phase changes to that of the initial precipitate, which is structurally similar to compound 6. Upon adding drops of water, it is readily converted back to compound 1, interestingly without much loss of crystallanity (Figure 13). A similar result is obtained also with the initial precipitate when treated with drops of water or kept in a moist environment. Since 6 is a nearly linear chain with the smallest repeat unit while 1 is a highly angular zigzag chain with a repeat unit that is double in size, the conformational change is brought about by the requirements of the large 16-water cluster. The situation is different in the case of compound 4, which also has porosity caused by an angular polymer chain, but with the cavities occupied by a disordered set of water molecules and a free Ce(dipic)(dipicH)2 molecule. Upon heating, it is converted to a structure similar to 6 but the reverse conversion is not observed.



are obtained by the condensation of neutral building blocks, the so-called point zero charge species. In the present cases, the true building blocks are most probably the oppositely charged species, [Ce(dipic)3]2- and [Zn/Cd(H2O)6-nSn]2- with n ) 0 or 2 and S ) ethanol or methanol. The formation of different isomers under solvent control is more difficult to visualize. It is reasonable to propose that the H-bonded aggregates of solvent molecules as well as the nascent polymeric chains have a mutual templating effect on each other to drive the formation of a single final product in all cases reported here. In thermodynamic terms this would mean that, for example, the entropically demanding conversion of structure type 6 to structure type 1 is made facile by the reduction in enthalpy due to formation of numerous H-bonds. Acknowledgment. The X-ray data were collected at the diffractometer facility at University of Hyderabad, established by the Department of Science and Technology, Government of India. Infrastructure support from UGC (UPE program) is also acknowledged. This work was supported by CSIR, India. T.K.P. thanks the CSIR for award of a senior research fellowship. Supporting Information Available: X-ray crystallographic information files (CIF), thermal ellipsoid plots, figures showing H-bonds and selected bond lengths and angles. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 13. X-ray powder diffractograms showing the crystalline transformations on dehydration–rehydration in compound 1. The lowest trace corresponds to simulated powder pattern of compound 6.

Finally, the compounds 2, 3, and 5 tend to become amorphous when desolvated. Conclusions The six 1D coordination polymers reported here are Zn/Cd analogues of the previously reported Ca/Sr/Ba chains4b–d containing alternating CeIV and MII coordination polyhedra with the important difference that there are no chelating dipic on Zn/Cd in the present complexes. They belong to three classes of network isomers (A, B, C, Figure 11) which are essentially linkage isomers differing in the way the tricapped trigonal prismatic polyhedron of each Ce(dipic)32- ions is linked to its adjacent M2+ ions in the ..-Ce-M-Ce-.. infinite chain. The different isomers differ in their level of complexity as reflected in the angularity (A > B > C) and repeat unit (A ∼ B > C) of the polymeric chains. The simplest chain (C) appears to be a metastable isomer which is difficult to crystallize. Crystallization from methanol or ethanol leads to form B while crystallization from water leads to form A, which due to a high level of angularity in the chain generates a nanoporous structure incorporating several water molecules or water clusters in the voids. Recently, in an interesting perspective10 on the formation of metal organic frameworks, it was argued that neutral frameworks

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CG070349Z

Solvent Dependent Crystallization of Isomeric Chain Coordination

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