Structural Variability of Cobalt(II) Coordination Polymers: Three Polymorphs of Co3(TMA)2 [TMA ) Trimesate, C6H3(COO)33-] Deping Cheng, Masood A. Khan, and Robert P. Houser*
CRYSTAL GROWTH & DESIGN 2004 VOL. 4, NO. 3 599-604
Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman, Oklahoma 73019 Received December 2, 2003;
Revised Manuscript Received March 12, 2004
ABSTRACT: The aqueous reaction of cobalt(II) sulfate with trimesic acid (H3TMA ) 1,3,5-benzenetricarboxylic acid) in the presence of a base yields complexes with the general formula of Co3(TMA)2. The identity of the base determines the specific structural characteristics of the complexes formed. When sodium hydroxide is used, a novel cobalt(II) coordination polymer, {[NaCo3(TMA)2(µ3-OH)](µ2-H2O)4(H2O)7‚1.5H2O}n (1), is produced. Compound 1 contains NaCo3(OH) clusters connected via TMA ligands into 1D chains, and 1 crystallizes in the orthorhombic space group Pbca with a ) 18.907(2) Å, b ) 14.565(3) Å, c ) 21.414(4) Å, and Z ) 8. When triethylamine is used, the previously reported 1D zigzag chain coordination polymer, [Co3(TMA)2(H2O)12]n (2), is produced. When tetraethylammonium hydroxide is used, a trinuclear cobalt(II) complex, [Co3(TMA)2(H2O)14]‚4H2O (3), is produced that has a structure identical to a segment of the zigzag chain of 2. Compound 3 crystallizes in the triclinic space group P1 h with a ) 6.6542(5) Å, b ) 10.7114(7) Å, c ) 12.3567(7) Å, R ) 99.008(4)°, β ) 102.553(6)°, γ ) 107.344(7)°, and Z ) 1. Introduction The design and syntheses of new materials with beneficial properties remains an area of intense interest, with particular focus on technologies such as microelectronics,1 magnetism,2 catalysis and separations technologies,3 and nonlinear optics.4 Coordination polymers, which are one-, two-, and three-dimensional structures made up of metal-ligand and hydrogen bonds, continue to generate substantial interest.5 Although numerous compounds with unusual structures and often interesting properties have been reported, it is still difficult, if not impossible, to predict structure on the basis of the ligands and metals used.6 It is therefore a desirable goal to gain an appreciation of the underlying factors that contribute to the structures of coordination polymers. In our laboratory, we have been exploring the chemistry of coordination polymers made with transition metals and aromatic carboxylic acids, including trimesic acid (benzene-1,3,5-tricarboxylic acid, H3TMA),7 pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid, H4BTEC),8 and, most recently, 2,4,6-pyridinetricarboxylic acid (H3PTC).9 One of our principal goals is to study the influence of secondary ligands and pH on the structures of coordination polymers with divalent transition metal ions. Here we report three cobalt(II) complexes with the same general formula of Co3(TMA)2 whose structures depend on minor differences in reaction conditions. Experimental Procedures All reagents were obtained from commercial sources and used without further purification. All reactions and recrystallizations were performed under aerobic conditions. Elemental analyses were carried out by Atlantic Microlabs, Norcross, GA. Thermal gravimetric analysis (TGA) was performed on a * To whom correspondence should be addressed. E-mail: houser@ ou.edu.
Thermal Analyst 2000 TGA instrument under a helium atmosphere (25 mL/min) and 10 °C/min. IR spectra were recorded on a Nicolet Nexus 470 FTIR at room temperature using the KBr pellet technique. Powder XRD data were collected on an AEL Scintag XTA diffractometer operated at 40 kV and 30 mA, using Cu KR (λ ) 1.5406 Å) with a scan speed of 2°/min. {[NaCo3(TMA)2(µ3-OH)(µ2-H2O)4(H2O)7]‚1.5H2O}n (1). An H-tube equipped with a glass wool plug at the bridge was charged on one side with an aqueous solution of CoSO4 (0.2 M, 25 mL) and on the other side with an aqueous solution of H3TMA (0.13 M, 25 mL) with the pH adjusted to 6.9 using 0.5 M NaOH. The solutions slowly diffused together, producing pink single crystals of 1 after 6 days (0.31 g, 22%). Anal. Calcd for C18H32Co3NaO25.5: C, 25.25; H, 3.77. Found: C, 24.04; H, 4.40. FTIR (KBr): 3358(s), 2341(w), 1620(s), 1570(m), 1437(s), 1364(s), 1008(m), 761(w), 714(s) cm-1. {Co3(TMA)2(H2O)12}n (2). An H-tube equipped with a glass wool plug at the bridge was charged on one side with an aqueous solution of CoSO4 (0.2 M, 25 mL) and on the other side with an aqueous solution of H3TMA (0.13 M, 25 mL) and imidazole (5.0 mmol, 0.34 g) with the pH adjusted to 7.6 using triethylamine. The solutions slowly diffused together, producing pink single crystals of 2 after two weeks (0.49 g, 24%). Anal. Calcd for C18H30Co3O24: C, 26.78; H, 3.75. Found: C, 27.10; H, 3.80. FTIR (KBr): 3456(s), 1615(s), 1522(s), 1472(w), 1435(s), 1371s), 1108(m), 756(w), 719(s) cm-1. [Co3(TMA)2(H2O)14]‚4H2O (3). An H-tube equipped with a glass wool plug at the bridge was charged on one side with an aqueous solution of CoSO4 (0.2 M, 25 mL) and on the other side with an aqueous solution of H3TMA (0.13 M, 25 mL) with the pH adjusted to 6.9 using 20% tetraethylammonium hydroxide. The solutions slowly diffused together, producing pink crystals of 3 after 4 days (0.28 g, 18%). Anal. Calcd for C18H42Co3O30: C, 23.62; H, 4.62. Found: C, 24.06; H, 4.33. FTIR (KBr): 3421(s), 1615(s), 1540(s), 1436(s), 1364(s), 1107(m), 756(w), 721(m) cm-1. Single-Crystal X-ray Structure Determination. Crystals of 1-3 were mounted on glass fibers for data collection. X-ray data for 1 and 3 were collected on a Siemens P4 diffractometer using Mo KR radiation (λ ) 0.71073 Å). X-ray data for 2 was collected on a Bruker Apex CCD diffractometer using Mo KR radiation (λ ) 0.71073 Å). All structures were
10.1021/cg0342415 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/01/2004
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Crystal Growth & Design, Vol. 4, No. 3, 2004 Table 1. Crystallographic Data for 1 and 3
formula fw cryst size (mm3) cryst syst space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z Fcalcd (g/cm3) µ (mm-1) T (K) F(000) θ range for data reflns collected indep reflns (Rint) R indices [I > 2σ(I)] R1 wR2 R indices (all data) R1 wR2 GOF on F2 largest diff peak and hole (e/Å3)
1
3
C18H32Co3NaO25.5 856.22 0.42 × 0.22 × 0.16 orthorhombic Pbca 18.907(2) 14.565(3) 21.414(4) 90 90 90 5897.0(18) 8 1. 929 1.791 173(2) 3488 1.90-27.00 7114 6437 (0.0243)
C18H42Co3O30 915.31 0.32 × 0.16 × 0.12 triclinic P1 h 6.6542(5) 10.7114(7) 12.3567(7) 99.008(4) 102.553(6) 107.344(7) 797.08(9) 1 1.907 1.661 173(2) 471 2.30-27.50 3947 3617(0.0227)
0.0402 0.1017
0.0300 0.0752
0.0562 0.1103 1.034 1.313, -0.584
0.0386 0.0794 1.024 0.419, -0.419
Table 2. Selected Bond Lengths (Å) and Angles (°) for 1 Co1-O1 Co1-O7 Co1-O13 Co1-O18 Co1-O19 Co1-O20 Co2-O8 Co2-O11A Co2-O14 Co2-O15 Co2-O18 Co2-O20 Co1-O20-Co2 Co1-O20-Co3 Co2-O20-Co3 Co2-O15-Na1
2.110(2) 2.110(2) 2.022(2) 2.218(2) 2.229(2) 2.022(2) 2.047(2) 2.026(2) 2.074(3) 2.168(2) 2.243(2) 2.037(2) 99.32(9) 101.34(9) 125.43(11) 116.72(10)
Co3-O2 Co3-O12A Co3-O16 Co3-O17 Co3-O19 Co3-O20 Na1-O15 Na1-O16 Na1-O21 Na1-O22 Na1-O23 Na1-O24 Co3-O16-Na1 Co1-O18-Co2 Co1-O19-Co3
2.040(2) 2.019(2) 2.179(2) 2.101(3) 2.240(2) 2.043(2) 2.598(3) 2.692(3) 2.359(3) 2.424(4) 2.317(3) 2.375(3) 115.42(10) 87.79(8) 89.42(8)
Table 3. Selected Bond Lengths (Å) and Angles (°) for 3 Co1-O1 Co1-O7 Co1-O8 Co2-O5 Co2-O9 O1-Co1-O7 O1-Co1-O8 O1-Co1-O1A
2.0291(14) 2.1598(15) 2.0899(16) 2.0666(14) 2.1105(16) 87.04(6) 90.80(6) 180.0
Co2-O10 Co2-O11 Co2-O12 Co2-O13
2.0445(16) 2.1505(17) 2.0656(16) 2.1385(16)
O5-Co2-O13 O9-Co2-O11 O10-Co2-O12
176.73(6) 168.62(6) 177.22(6)
solved by direct methods using the SHELXTL system,10 and refined by full-matrix least squares on F2 using all reflections. All non-hydrogen atoms were refined anisotropically. Crystallographic data for 1 and 3 are summarized in Table 1. Selected bond lengths and angles for 1 and 3 are summarized in Tables 2 and 3. Intensity data for 1 were corrected for Lorentz and polarization effects and an absorption correction based in ψ-scans was applied. All of the hydrogen atoms in 1 were included with idealized parameters except the hydrogen atoms on the O13, O14, O17, O20, O21, O22, O23, O24, O25, O26A, and O26B atoms, which were located and refined isotropically in the initial cycles of refinement and held with fixed parameters in the final cycles of refinement. All of the water molecules in 1 (O13-O19 and O21-O26B) showed two peaks for the hydrogen atoms in the difference map in essentially correct geom-
Cheng et al. etry. The O20 atom showed only one peak for the hydrogen atom and was confirmed to be the correct choice for the hydroxide ion. The asymmetric unit of 1 contains one [NaCo3(TMA)2(OH)(H2O)11] moiety and 1.5 solvent water molecules. The 1/2 water molecule is disordered at two sites and refined with 25% occupancy for each component. The final R1 value of 0.040 is based on 5144 observed reflections [I > 2σ (I)], and wR2 ) 0.110 is based on all reflections (6437 unique data). Intensity data for 2, which approximately covered the full sphere of reciprocal space, were measured as a series of ω oscillation frames each 0.3° for 25 s/frame. The detector was operated in 512 × 512 mode and was positioned 6.00 cm from the crystal. Coverage of unique data was 99.5% complete to 54° (2θ). Cell parameters were determined from a nonlinear least-squares fit of 5434 reflections in the range of 4.6 < θ < 23.3°. The data were corrected for absorption by the multiscan method from equivalent reflections giving minimum and maximum transmissions of 0.5453 and 0.9273, respectively. All of the crystals of 2 were merohedral and racemic twins. Data were corrected for the twinning and made a very significant improvement in the refinement. Before applying the correction for twinning the R-factor could not be reduced below 13%, and several atoms could not be refined anisotropically. After the correction, refinement proceeded smoothly and gave an acceptable final R-factor. All the hydrogen atoms in 2 were included with idealized parameters except the hydrogen atoms on O1, O2, O7, O8, O11, and O12 atoms, which were refined isotropically. The asymmetric unit of 2 contains half of the trinuclear Co3(TMA)2(H2O)12 metalorganic framework. The Co3(TMA)2(H2O)12 unit has a crystallographic 2-fold symmetry with the Co2, C12, C11, C7, C1, C5, and C6 atoms lying along the 2-fold axis. The [Co3(TMA)2(H2O)12]n complex forms zigzag polymeric chains along the crystallographic c-axis, and form hydrogen bonds between each such chains. Final R1 ) 0.020 is based on 2960 “observed reflections” [I > 2σ (I)], and wR2 ) 0.051 is based on all reflections (2969 unique data). Intensity data for 3 were corrected for Lorentz and polarization effects and an absorption correction based on ψ-scans was applied. All of the hydrogen atoms were included with idealized parameters except the hydrogen atoms on the water molecules (O7-O15), which were refined isotropically. The asymmetric unit of 3, [Co1.5(TMA)(H2O)7]‚2H2O, contains one TMA ligand, 1.5 Co atoms, seven coordinated aqua ligands, and two water solvent molecules. One of the Co atoms (Co1) sits at the inversion center with 0.5 occupancy, which results in trimeric formula of [Co3(TMA)2(H2O)14]‚4H2O for 3. The final R1 value of 0.030 is based on 3098 observed reflections [I > 2σ (I)], and wR2 ) 0.079 is based on all reflections (3617 unique data).
Results and Discussion Coordination polymers 1-3 all have the basic cobalt(II)-TMA stoichiometry of Co3(TMA)2. Each species was synthesized under slightly different conditions, giving rise to a different structure in each case. Complexes 1 and 2 are both coordination polymers comprised of 1D polymeric chains, while complex 3 is comprised of discrete trinuclear molecules. The syntheses and structures of transition metal complexes with TMA have been thoroughly reported in the literature, with a wide variety of structures depending on numerous factors.7,11 In fact, coordination polymer 2 was synthesized by Yaghi and co-workers under hydrothermal conditions, and its structure was reported.11i We provide a brief discussion of the room temperature aqueous synthesis and characterization of 2 here to illustrate the importance of reaction conditions with respect to the final structure. Complexes 1-3 were synthesized under aqueous conditions at room temperature as illustrated in Scheme 1. In each case, trimesic acid is deprotonated
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Figure 1. (a) TGA of 1 under helium atmosphere and 10 °C/ min. (b) Solid 1 in the as-crystallized form (pink, left), and after dehydration under vacuum for 12 h (blue, right).
Scheme 1
by a base and diffused into an aqueous solution of cobalt(II) sulfate at a controlled pH, usually 7.5 ( 0.6. Attempts to synthesize 1-3 at pH > 8.1 resulted in the precipitation of cobalt hydroxide, while conditions where pH < 6.9 never produced crystalline product. The identity of the base used is critical to the structure of the complex formed. Sodium cation-containing chains are produced when sodium hydroxide is used as the base (coordination polymer 1), zigzag chains are produced when a triethylamine/imidazole solution is used as the base (coordination polymer 2), and a trinuclear molecular complex is formed when tetraethylammonium hydroxide is used as the base (complex 3). All reactions were conducted at nearly the same pH, so the subtle differences in base prove to have a significant effect on the structure of the complex formed. Coordination polymer 1 was synthesized by diffusing an aqueous cobalt(II) sulfate solution into an aqueous TMA/sodium hydroxide solution, with pink needleshaped crystals of 1 forming after 6 days. Under these conditions at close to neutral pH, sodium hydroxide deprotonates TMA to form the trianion (TMA3-), which coordinates to cobalt to form the coordination polymer. Sodium cations are also involved in the structure of 1, giving rise to an unusual and unprecedented structure. The formula unit of 1 was revealed by elemental analysis and X-ray crystallography to contain a cobaltsodium cluster possessing three cobalt(II) atoms, one sodium(I) atom, and two TMA ligands, and having the formula [NaCo3(TMA)2(OH)(H2O)11]‚1.5H2O. 1 is insoluble in water and common organic solvents. The TGA of 1 (Figure 1a) has two zones of mass loss. The first is between 50 and 150 °C, and is likely due to loss of water. The second mass loss occurs above 400 °C, presumably due to ligand decomposition. The loss of water above 100 °C leads to a color change in 1, going from pink in the fully hydrated as-crystallized form, to blue in the dehydrated form (Figure 1b). This color change is presumably due to changes in the coordination environment around the cobalt ions, giving rise to changes in the electronic transitions for the complex. This phenom-
Figure 2. Powder XRD of (a) hydrated 1, and (b) dehydrated 1. Curvature in the baseline is due to absorbance from cobalt.
enon has been reported for similar cobalt-TMA coordination polymers.11i Powder XRD of 1 before and after dehydration are shown in Figure 2. The powder XRD for hydrated 1 contains distinct, sharp peaks that almost totally disappear after dehydration. The loss of water in 1 appears to lead to the disappearance of crystallinity. In light of the structure of 1 (vide infra), the loss of crystallinity in the dehydrate is not surprising, since water plays an important structural role. X-ray crystal data was collected for 1, and the structure was solved and determined to be an unprecedented 1D chain comprised of TMA ligands and sodium-containing cobalt clusters. The asymmetric unit of 1 (Figure 3) contains three cobalt(II) ions, one sodium ion, two TMA ligands, 11 aqua ligands, and one µ3hydroxo ligand. Of the 11 aqua ligands, four are µ2-H2O bridging ligands and the remaining seven coordinate to metal ions as terminal aqua ligands. The cobalt ions sit in a triangular arrangement, with the hydroxo ligand bridging between them, and carboxylato and aqua ligands bridging around the periphery. The cobalt(II) and sodium atoms are six-coordinate octahedral with typical Co(II)-O and Na-O bond lengths and angles (see Table 2). Each cobalt is coordinated by two carboxylate oxygen atoms, two bridging aqua ligands, two
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Figure 3. Ball-and-stick representation of the asymmetric unit of {[NaCo3(TMA)2(µ3-OH)(µ2-H2O)4(H2O)7]‚1.5H2O}n (1): Co, blue; Na, purple; O, red; C, gray; H, white. The Na cation, uncoordinated water molecules and TMA hydrogen atoms are omitted for clarity. Inset: portion of the asymmetric unit of 1 illustrating coordination of Na cation.
terminal aqua ligands, and the µ3-hydroxo ligand. Co2 and Co3 have the same arrangement of ligands, with the carboxylato oxygen atoms in positions trans to each other, the µ2-H2O ligands also trans to each other, and the µ3-hydroxo ligand trans to the terminal aqua ligand. In the case of Co1, the carboxylato oxygens are cis to each other, the µ2-H2O ligands are also cis to each other, and the µ3-hydroxo ligand is trans to the terminal aqua ligand. The carboxylato groups coordinate in a bidentate, syn-syn bridging mode, creating a trinuclear cluster similar to the “triangular” carboxylates that have been known for many years.12 Oxo-centered trinuclear cobalt(III) carboxylates of this type have been reported,13 but to our knowledge 1 is the first example of a hydroxo-bridged tricobalt(II) carboxylate cluster. The Co‚‚‚Co distances are 3.093, 3.144, and 3.626 Å for Co1‚‚‚Co3, Co1‚‚‚Co2, and Co2‚‚‚Co3, respectively. The trinuclear cluster is capped by the sodium ion, which bridges to the cluster via aqua ligands (O15 and O16). The coordination sphere of the sodium is occupied solely by six aqua ligands, specifically two µ2-H2O bridging aqua ligands and four terminal aqua ligands. The extra positive charge of the sodium ion is balanced by the µ3hydroxo ligand, which plays a key role in the structure of the cluster, bridging between the three cobalt atoms. The heteronuclear NaCo3 clusters can be viewed as nodes in the polymeric structure of 1. The heteronuclear nodes are connected via the TMA ligands to create infinite 1D zigzag chains. The repeating unit of these chains can be represented by simple triangles for the TMA ligands and a vertex for the sodium/cobalt node (Figure 4a). When the repeating units are connected into the zigzag chain, the different TMA coordinating modes can be seen. One of the TMA ligands bridges between the cluster nodes to create the 1D chains, while the other TMA ligand coordinates in a nonbridging fashion to essentially cap the sides of the chain (Figure 4c). This structure lies in contrast to the typical µ3- or higher bridging mode of the TMA ligand in many copper(II) coordination polymers.7,11a,11b Viewed end-on in Figure 4b, the nesting of adjacent chains
Figure 4. Views of {[NaCo3(TMA)2(µ3-OH)(µ2-H2O)4(H2O)7]‚ 1.5H2O}n (1). (a) Repeating unit of 1 where the NaCo3 cluster is represented as the blue vertex, and the TMA ligands are red triangles. (b) 1 viewed down the chains that run perpendicular to the bc plane. (c) One chain of 1 that runs parallel to the a axis, and (d) multiple chains in one layer of 1 viewed perpendicular to the crystallographic ac plane. (e) Layers of chains in 1 viewed perpendicular to the ab plane.
Figure 5. Stick representation of two neighboring cluster nodes of {[NaCo3(TMA)2(µ3-OH)(µ2-H2O)4(H2O)7]‚1.5H2O}n (1) illustrating the hydrogen-bonding and π-stacking interactions (color scheme same as Figure 3).
becomes apparent. From this perspective, the chains have a concave shape that allows close hydrogen bonding and π-π interactions between chains. A closer view of these interchain contacts is illustrated in Figure 5. The closest centroid-centroid distance between aromatic rings in two chains is approximately 3.5 Å. A portion of one layer of parallel chains is shown in Figure
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Figure 6. Ball-and-stick representation of part of the polymeric zigzag chains of [Co3(TMA)2(H2O)12]n (2) (color scheme same as Figure 3).
4d. Here the significant π-π interactions can be seen from above in the overlapping triangles. The chains in 1 are therefore held together by hydrogen bonding and π-π interactions to create sheets that lie in the ac plane. The uncoordinated water molecules reside in the space between sheets. Coordination polymer 2 was originally synthesized by Yaghi and co-workers using cobalt(II) acetate and TMA under hydrothermal conditions.11i The same structure was obtained in our hands by diffusing an aqueous solution of cobalt(II) sulfate with an aqueous solution of TMA, imidazole, and triethylamine (Et3N) under ambient conditions. Despite our efforts to incorporate imidazole into the product of this reaction, only TMA, water, and cobalt are present. However, imidazole was required to crystallize 2, since reactions with Et3N alone never produced crystalline product. The formula unit of 2 was confirmed by elemental analysis and X-ray crystallography to be [Co3(TMA)2(H2O)12]n. We briefly describe the structure here to highlight the structural similarities and differences with 1. Similar to 1, coordination polymer 2 is also comprised of 1D zigzag chains, albeit of a different form. A portion of the chain is shown in Figure 6, and two views of the coordination polymer are shown in Figure 7. Two different cobalt sites are present in 2, both having a coordination number of six. Octahedral Co1 is coordinated by two O atoms from carboxylato groups in the axial positions, and four aqua ligands in the equatorial plane. Co2 is coordinated by a bidentate carboxylato group and four aqua ligands in the remaining four sites of the distorted octahedral coordination sphere. The TMA ligands in 2 have two different binding modes. One TMA ligand bridges between cobalt atoms via two of the carboxylate groups with the third carboxylate moiety not coordinated (top TMA ligands in Figure 6), while the other TMA ligand bridges with two carboxylates and the third carboxylate coordinates to Co2 (bottom TMA ligand in Figure 6). The zigzag chains in 2, represented in Figure 7a graphically as red triangles (TMA ligands) and blue vertexes (cobalt atoms), are different than the chains in 1 in that the cobalt nodes are two-coordinate, whereas in 1 they are three-coordinate.
Figure 7. Repeating chains of 2 where (a) the cobalt sites are represented as blue vertexes and the TMA ligands are represented as red triangles. (b) Multiple layers of 2 where alternating layers are green and yellow.
Simply by changing the base from a triethylamine/ imidazole solution to tetraethylammonium hydroxide, the polymeric structure of 2 gives way to the trinuclear complex 3. Using the same diffusion technique utilized for 1 and 2, complex 3 was crystallized from cobalt(II) sulfate, TMA, and Et4NOH. The formula for 3, [Co3(TMA)2(H2O)14]‚4H2O, is quite similar to the formula for 2. There are six more water molecules per formula unit in 3, two of which are aqua ligands and four of which are uncoordinated water of crystallization. A view of the structure of 3 (Figure 8) clearly shows structural analogy with 2. However, whereas the 1D chains in 2 would be continued through the coordination position trans to O5 and O5A, in 3 these sites are occupied by aqua ligands. The trinuclear molecules stack so that all of the TMA ligands lie in the ac crystallographic plane. Furthermore, the molecules are aligned along the a axis forming a thick plane. Terminal cobalt ions are involved in a hydrogen-bonding network to form a 2D network. Crystallization water molecules are localized in the interplane space and are involved in the hydrogenbonding network. An isostructural nickel(II) complex with the formula Ni3(TMA)2(H2O)14‚4H2O was synthesized from nickel(II) chloride using a similar diffusion technique.14 Summary and Conclusions The stoichiometry of cobalt and TMA ligands is the same for 1-3. In each case, minor changes in the reaction conditions (i.e., different bases) lead to different formulas and structures. Sodium hydroxide was used for the synthesis of 1, leading to an unprecedented NaCo3(OH) cluster coordination polymer. The formation of the cluster relies on the bridging hydroxo ligand, so the sodium cation is required to balance the charge. The
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Figure 8. View of an extended ball-and-stick asymmetric unit of [Co3(TMA)2(H2O)14]‚4H2O (3) along with the atomic numbering scheme. Uncoordinated water molecules and TMA hydrogen atoms are omitted for clarity (color scheme same as Figure 3).
structure of coordination polymer 1 contains overlapping chains with a large degree of hydrogen-bonding between chains. Using a triethylamine/imidazole solution instead of sodium hydroxide leads to the zigzag chain structure of 2. Surprisingly, using tetraethylammonium hydroxide produces the trinuclear structure of 3 that can be viewed as a truncated portion of the chain structure of 2. Complexes 1-3 are three polymorphs of Co3(TMA)2. Each polymorph contains a different number of aqua ligands, uncoordinated waters of crystallization, and in the case of 1, NaOH. These structures reinforce the difficulties faced in the field of coordination polymer synthesis and crystal engineering. Simply by changing the base used to deprotonate the trimesic acid, we generated three different structural polymorphs of Co3(TMA)2. Acknowledgment. Financial support for this research was provided by the Petroleum Research Fund, administered by the American Chemical Society and the University of Oklahoma. We also thank the NSF for the purchase of a CCD equipped X-ray diffractometer at the University of Oklahoma (CHE-013085). Supporting Information Available: X-ray structural information for 1 and 3 in CIF format is available free of charge via the Internet at http://pubs.acs.org.
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CG0342415
