Novel Sandwich Coordination Polymers Composed of Cobalt(II), 1,2,4

Deping Cheng, Masood A. Khan, and Robert P. Houser*. Department of Chemistry and Biochemistry, University of Oklahoma,. 620 Parrington Oval, Norman, ...
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Novel Sandwich Coordination Polymers Composed of Cobalt(II), 1,2,4,5-Benzenetetracarboxylato Ligands, and Homopiperazonium Cations Deping Cheng, Masood A. Khan, and Robert P. Houser*

CRYSTAL GROWTH & DESIGN 2002 VOL. 2, NO. 5 415-420

Department of Chemistry and Biochemistry, University of Oklahoma, 620 Parrington Oval, Norman, Oklahoma 73019 Received June 24, 2002

ABSTRACT: Two novel cobalt(II) coordination polymers, (H4hpz)[Co3(BTEC)2(H2O)12]‚11H2O (1) and (H4hpz)[Co(BTEC)(H2O)3]‚3H2O (2) (H4hpz ) homopiperazonium; BTEC ) 1,2,4,5-benzenetetracarboxylate), have been synthesized under aqueous conditions at room temperature and structurally characterized by single-crystal X-ray diffraction. Compound 1 crystallizes in the triclinic space group P1 h with a ) 9.823(4) Å, b ) 11.587(4) Å, c ) 20.385(7) Å, R ) 95.10(2)°, β ) 92.423(19)°, γ ) 98.90(2)°, and Z ) 2. Compound 2 also crystallizes in the triclinic space group P1 h with a ) 9.7755(17) Å, b ) 11.208(3) Å, c ) 11.224(2) Å, R ) 60.304(12)°, β ) 73.687(12)°, γ ) 84.311(14)°, and Z ) 2. Both compounds have layered structures. In the case of 1, homopiperazonium cations and 1D anionic chains of [Co(BTEC)(H2O)4]2-n are sandwiched between 2D sheets of [Co2(BTEC)(H2O)8]n. In the case of 2, homopiperazonium cations alone are sandwiched between 2D sheets of [Co(BTEC)(H2O)3]2-n. Introduction Research on the design, synthesis, and characterization of coordination polymers continues to expand as more research groups join in efforts to create synthetic materials with desirable physical and chemical properties.1 Most are motivated by the potential use of coordination polymers as adsorbents, catalysts, and materials with unique magnetic or optical properties. One particularly fruitful family of coordination polymers utilizes carboxylate ligands to bridge between two or more metal ions, creating 1D, 2D, and 3D networks. In particular, coordination polymers with ligands containing two,2 three,3 or four4 carboxylate groups connected by rigid backbones have been reported. In our laboratory we have focused our efforts on combining various aromatic carboxylates with transition metals and coligands to create coordination polymers with modified structures. Previously we reported that the addition of imidazole (Him) or 1-methylimidazole (Meim) influences the structure of the coordination polymer formed with copper ions and trimesic acid (1,3,5-benzenetricarboxylic acid, H3TMA), producing sheets of [Cu3(TMA)2(Him)6(H2O)]n.5 The sheets are held together via hydrogen bonding and π-π interactions of the interdigitated Him ligands to form stable 3D structures with large solvent-accessible channels. Here we report on our efforts to expand on the coligands at our disposal via the synthesis and polymeric structures of two closely related coordination polymers comprised of cobalt(II), 1,2,4,5-benzenetetracarboxylate (BTEC), and the coligand homopiperazine. This combination of components gives rise to unusual layered coordination polymers where, contrary to our expectations, the homopiperazine is protonated and noncoordinating. The coordination chemistry of BTEC is well represented, although there are not as many structures as * To whom correspondence should be addressed. Phone: (405) 3253551. Fax: (405) 325-6111. E-mail: [email protected]. URL: cheminfo. chem.ou.edu/faculty/rph.html.

have been reported for TMA.6 The structures of complexes and coordination polymers have been reported for the following metals: calcium,7 manganese,8 iron,9 cobalt,4,8a,10 nickel,4a,8a,11 copper,12 silver,13 and zinc.8a,14 Experimental Section General Methods. All reagents were obtained from commercial sources and used without further purification. Unless otherwise indicated all reactions and purification steps were performed under aerobic conditions. Elemental analyses were carried out by Atlantic Microlabs, Norcross, GA. Thermal gravimetric analysis was performed on a Thermal Analyst 2000 TGA instrument under a helium atmosphere. The IR spectra were recorded on a Nicolet Nexus 470 FTIR at room temperature using the KBr pellet technique. (H4hpz)[Co3(BTEC)2(H2O)12]‚11H2O (1). An aqueous solution of 1,2,4,5-benzenetetracarboxylic acid (1.27 g, 5 mmol) was slowly added to a 20 mL aqueous solution of homopiperazine (1.09 g, 10 mmol) with stirring. The pH was adjusted with aqueous homopiperazine until pH 7.5. Cobalt(II) sulfate heptahydrate (2.81 g, 10 mmol) was dissolved in 20 mL of water and mixed with the above solution with stirring for about 30 min. A small amount of precipitate was removed by filtration, and the filtrate was allowed to stand at room temperature. Pink block-shaped crystals of 1 were obtained after 2 days. Yield: 1.47 g (49.32%). Anal. Calcd for C25H64Co3N2O39: C, 25.13; H, 5.36; N, 2.35. Found: C, 25.45; H, 5.26; N, 2.55. FTIR (KBr, cm-1): 3367 (s), 1576 (s), 1483 (m), 1420 (w), 1376 (s), 1322 (m), 1140 (s), 1076 (w), 916 (m), 798 (s), 678 (s), 538(s), 396 (w), 306 (w). (H4hpz)[Co(BTEC)(H2O)3]‚3H2O (2). Crystals of 1 were allowed to soak in their mother liquor for 1 week. After this period, all of the pink crystals of 1 had transformed into dark pink needle-shaped single crystals of 2. Yield: 1.21 g (46.54%). Anal. Calcd for C15H28CoN2O14: C, 34.69; H, 5.43; N, 5.39. Found: C, 34.65; H, 5.48; N, 5.34. FTIR (KBr, cm-1): 3365 (s), 1585 (s), 1488 (m), 1426 (w), 1379 (s), 1322 (m), 1141 (m), 1129(s), 1078 (w), 1026(w), 992(w), 920 (m), 851(w), 804(w), 768 (w), 683 (w), 593(w), 531 (w), 306 (w). Single-Crystal X-ray Structure Determination. A light pink block-shaped crystal of 1 and a dark pink needle-shaped crystal of 2 were mounted on glass fibers for data collection. The data were collected at 173(2) K on a Bruker P4 diffractometer using Mo KR (λ ) 0.710 73 Å) radiation. The data were

10.1021/cg025547z CCC: $22.00 © 2002 American Chemical Society Published on Web 08/16/2002

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Table 1. Crystallographic Data for Compounds 1 and 2 empirical formula fw cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g/cm3) µ (mm-1) F(000) cryst size (mm) 2θ range (deg) index range (h, k, l) no. of rflns collected no. of indep rflns (Rint) transmission no. of data/restraints/ params GOF on F2 final R indices (I > 2σ(I)) R1 wR2 R indices (all data) R1 wR2 largest diff peak/ hole (e/Å3)

1

2

C25H64Co3N2O39 1193.57 triclinic P1 h 9.823(4) 11.587(4) 20.385(7) 95.10(2) 92.423(19) 98.90(2) 2279.5(14) 2 1. 739 1.199 1242 0.48 × 0.42 × 0.28 4-50 +11, (13, (24 8547 7994 (0.0511) 0.7301-0.5969 7994/18/620

C15H28CoN2O14 519.32 triclinic P1 h 9.7755(17) 11.208(3) 11.224(2) 60.304(12) 73.687(12) 84.311(14) 1024.1(4) 2 1.684 0.917 542 0.42 × 0.26 × 0.04 4-50 -11, (11, (12 3842 3360 (0.0994) 0.9643-0.6994 2360/72/356

1.104

1.019

0.0713 0.2110

0.0473 0.1199

0.0870 0.2273 1.496/-1.201

0.0586 0.1280 0.927/-1.447

corrected for Lorentz and polarization effects; an absorption correction based on ψ-scans was applied. The structures were solved by direct methods using SHELXTL15 and refined by fullmatrix least squares on F2 using all reflections. All nonhydrogen atoms were refined anisotropically, and all hydrogen atoms on the water molecules and on the N atoms were located. For 1 these hydrogen atoms were initially refined isotropically. However, many hydrogen atoms could not be refined with reasonable thermal and positional parameters, and in the final cycles of refinement all these hydrogen atoms were included with fixed positional and thermal parameters and given reasonable H-bond geometry. For 2 the hydrogen atoms on the water molecules and on the N atoms were refined isotropically. SHELXTL restraints (SADI) were applied to maintain uniform O-H and N-H distances. The asymmetric unit of 1 contains one [Co(BTEC)(H2O)4]2- unit which makes up the 1D chains, one neutral [Co2(BTEC)(H2O)8] unit comprised of one unique cobalt atom and two cobalt atoms sitting at inversion centers with 0.5 occupancy which makes up the 2D sheets, one [H4hpz]2+ cation, and 11 molecules of solvent water. The final R1 value of 0.071 is based on 6612 observed reflections (I > 2σ(I)), and wR2 ) 0.227 is based on all reflections (7994 unique data). The asymmetric unit of 2 contains one BTEC ligand, two Co atoms that sit at inversion centers with 0.5 occupancy, three aqua ligands, one [H4hpz]2+ dication, and three molecules of solvent water. The final R1 value of 0.047 is based on 2822 observed reflections (I > 2σ(I)), and wR2 ) 0.128 is based on all reflections (3360 unique data). Details of the crystal data are given in Table 1.

Results and Discussion The new coordination polymers described here were synthesized with the original goal of introducing homopiperazine (H2hpz) as a coligand that could coordinate to transition metal ions via the amine groups and disrupt the expected polymeric structure of the coordination polymers formed between transition metals and 1,2,4,5-benzenetetracarboxylate (BTEC). To our sur-

Cheng et al. Chart 1.

1,2,4,5-Benzenetetracarboxylate Anion and Homopiperazonium Cation

prise, in this case H2hpz does not coordinate to the metal ions but resides between sheets as the protonated dication homopiperazonium, H4hpz2+ (Chart 1). Synthesis. The coordination polymer 1 was synthesized by combining cobalt(II) sulfate with BTEC and H2hpz in aqueous solution at room temperature according to eq 1. Under these reaction conditions at pH 7.5,

3Co2+(aq) + 2 H4BTEC(aq) + H2hpz(aq) f (H4hpz)[Co3(BTEC)2(H2O)12](s) + 6H+(aq) (1) 1 homopiperazine acts as a Brønsted base and is protonated to form homopiperazonium cations (H4hpz2+), while BTEC4- anions coordinate to cobalt(II) ions to form complex negatively charged coordination polymers (vide infra). Pink block-shaped crystals of 1 formed upon evaporation of the mother liquor after 2 days. The formula unit of 1, (H4hpz)[Co3(BTEC)2(H2O)12]‚11H2O, was shown by elemental analysis and X-ray crystallography to contain one homopiperazonium cation (2+) and one complex anion (2-) containing three cobalt(II) atoms and two BTEC ligands. 1 is insoluble in water and common organic solvents. Thermal gravimetric analysis (TGA) revealed that uncoordinated water contained in the crystals is lost above 100 °C, and 1 undergoes decomposition above 300 °C (see Supporting Information). Upon standing in the mother liquor for an additional week, the color and crystal morphology of 1 changed from light pink blocks to dark pink needles. These crystals were formulated as (H4hpz)[Co(BTEC)(H2O)3]‚ 3H2O (2) on the basis of elemental analysis and X-ray crystallography. 2 is also insoluble in water and common organic solvents. Although the anionic charge per formula unit is still 2-, the ratio H4hpz2+:Co2+:BTEC4is now 1:1:1, whereas for 1 the ratio is 1:3:2. The change in ratios necessitates the removal of two cobalt(II) cations and one BTEC4- anion, presumably into the mother liquor (see eq 2). The reason for the conversion

(H4hpz)[Co3(BTEC)2(H2O)12](s) f 1 (H4hpz)[Co(BTEC)(H2O)3](s) + 2 2Co2+(aq) + BTEC4-(aq) (2) of 1 to 2 is not clear, but structural information from crystallography provides some insight. Crystal Structures. The structure of 1 is comprised of alternating layers of sheets and chains. The sheets are made up of neutral [Co2(BTEC)(H2O)8]n, and the anionic chains are comprised of [Co(BTEC)(H2O)4]2-n. The homopiperazonium cations reside in the space flanked by adjacent sheets, packed between chains (see

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Figure 1. 3D structure of 1 viewed down the ac diagonal, illustrating the alternating layers of 2D sheets and 1D chains: Co, light blue; O, red; C, gray; N, dark blue. Homopiperazonium cations are nested between chains in the inter-sheet space. Hydrogen atoms and uncoordinated water molecules are omitted for clarity.

Figure 1). The sheets run parallel to the plane created by the crystallographic b axis and the ac diagonal, while the chains run parallel to the ac diagonal and are perpendicular to the b axis. The [Co2(BTEC)(H2O)8]n sheets are separated by approximately 7 Å, as measured from coordinated water oxygen atoms on neighboring sheets, while the sheet-chain separation is about 3 Å. There is significant hydrogen bonding in 1, involving the O atoms of BTEC ligands, the N atom protons of the H4hpz cations, aqua ligands, and uncoordinated water molecules. The neutral sheets in 1 are made up of a square grid of BTEC ligands in a bridging µ4-trans-syn mode, where the outer oxygen atoms form coordinate bonds. Here we define outer oxygen atoms as the four closest to the BTEC ring carbon atoms in the 3- and 6-positions and inner oxygen atoms as the remaining four oxygen atoms. The BTEC ligands are at the vertexes of the square, and the cobalt atoms are on the sides (Figures 2 and 3). The BTEC aromatic rings are approximately coplanar with the sheets, while the carboxylato groups are twisted slightly out of the plane of the aromatic rings. The carboxylate-ring dihedral angles are 30, 36, 51, and 54° for the carboxylato groups containing O15, O25, O17, and O23, respectively. The resulting sheet topology is essentially flat, with the trans carboxylato O-Co-O vectors slightly out of the plane of the sheets. The cobalt atoms in the sheets possess a typical sixcoordinate, octahedral geometry with the oxygen atoms from the BTEC ligands in axial positions and four aqua ligands in the equatorial positions. The Co-O(carboxylato) bond lengths in 1 are all between 2.11 and 2.15 Å, while the Co-O(aqua) bond lengths are 2.05-2.12 Å, all of which are typical for these types of bonds.4,16 The bond angles involving the cobalt atoms in the sheets are all close to the ideal 90 or 180° for octahedral geometry (see Figure 2). Sandwiched between the sheets of 1 are the [Co(BTEC)(H2O)4]2-n 1D chains of alternating cobalt and BTEC ligands. The BTEC ligands are coordinated via two p-carboxylato groups in a µ2-trans-syn manner. As in the case of the sheets in 1, the outer oxygen atoms form the coordinate bonds with the cobalt ions. The two

Figure 2. Molecular structures represented as 50% thermal ellipsoids of portions of the polymeric 1D chains (top) and 2D sheets (bottom) in (H4hpz)[Co3(BTEC)2(H2O)12]‚11H2O (1): Co, light blue; O, red; C, gray. Hydrogen atoms and uncoordinated water molecules are omitted for clarity. Selected bond distances (Å) and angles (deg): Co1-O1 ) 2.108(4); Co1-O2 ) 2.126(3); Co1-O3 ) 2.066(4); Co1-O4 ) 2.062(4); Co1-O5 ) 2.150(4); Co1-O9 ) 2.084(4); Co2-O13 ) 2.054(4); Co2-O14 ) 2.116(4); Co2-O15 ) 2.144(3); Co3-O17 ) 2.119(4); Co3O19 ) 2.071(4); Co3-O20 ) 2.102(4); Co3-O21 ) 2.060(4); Co3-O22 ) 2.124(4); Co3-O23 ) 2.130(4); Co4-O25 ) 2.149(3); Co4-O27 ) 2.050(4); Co4-O28 ) 2.089(4); O1-Co1O2 ) 177.86(15); O1-Co1-O3 ) 91.49(15); O1-Co1-O4 ) 88.28(15); O1-Co1-O5 ) 85.00(14); O1-Co1-O9 ) 92.70(14); O2-Co1-O3 ) 90.59(14); O2-Co1-O4 ) 89.62(14); O2-Co1O5 ) 94.48(13); O2-Co1-O9 ) 87.69(14); O3-Co1-O4 ) 177.27(15); O3-Co1-O5 ) 90.25(14); O3-Co1-O9 ) 93.11(14); O4-Co1-O5 ) 87.02(14); O4-Co1-O9 ) 89.62(14); O5-Co1-O9 ) 175.98(13); O13-Co2-O13A ) 180.000(1); O13-Co2-O14 ) 92.02(15); O13-Co2-O14A ) 87.98(15); O13-Co2-O15 ) 86.96(14); O13-Co2-O15A ) 93.04(14); O14-Co2-O15 ) 91.37(13); O14-Co2-O15A ) 88.63(13); O15-Co2-O15A ) 180.000(1); O17-Co3-O19 ) 93.44(14); O17-Co3-O20 ) 86.09(14); O17-Co3-O21 ) 90.17(14); O17-Co3-O22 ) 90.56(13); O17-Co3-O23 ) 179.66(13); O19-Co3-O20 ) 179.37(14); O19-Co3-O21 ) 90.03(14); O19-Co3-O22 ) 91.99(14); O19-Co3-O23 ) 86.36(14); O20-Co3-O21 ) 90.39(14); O20-Co3-O22 ) 87.61(14); O20-Co3-O23 ) 94.12(14); O21-Co3-O22 ) 177.81(14); O21-Co3-O23 ) 89.56(14); O22-Co3-O23 ) 89.71(14); O25-Co4-O25A ) 180.000(1); O25-Co4-O27 ) 91.73(14); O25-Co4-O27A ) 88.27(14); O25-Co4-O28 ) 91.98(13); O25-Co4-O28A ) 88.02(13); O27-Co4-O27A ) 180.0(2); O27-Co4-O28 ) 88.05(16); O27-Co4-O28A ) 91.95(16); O28-Co4-O28A ) 180.000(1). See the Supporting Information for fully labeled ORTEP drawings and complete tables of bond lengths and angles.

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Figure 3. View of one layer of the sheet-chain structure in 1 from a position normal to the plane of the 2D sheets: One 2D sheet (represented by sticks with Co atoms in light blue, O atoms in red, and C atoms in gray) is shown above the chains (represented by gold CPK spheres). Homopiperazonium cations, solvent water molecules, and H atoms are omitted for clarity.

noncoordinating carboxylate groups participate in hydrogen bonding with the coordinated water molecules on adjacent sheets. This hydrogen bonding presumably contributes to the stabilization of the sheet-sheet interactions. The coordination geometry of the cobalt atoms in the chains is also octahedral with typical cobalt-carboxylato bonds (2.150 and 2.084 Å) and cobalt-aqua bonds (2.06-2.13 Å). The chains, which are parallel to the ac diagonal and perpendicular to the b axis, are aligned with the sheets such that they overlap with the grid (Figure 3). The aromatic rings of the BTEC ligands in the chains are askew with respect to the plane of the 2D sheets by about 30°. This, combined with twisted noncoordinating carboxylate groups (dihedral angles of 54 and 61°), better places the noncoordinating carboxylate groups in position for hydrogen bonding. Eleven uncoordinated water molecules and one H4hpz cation per formula unit occupy the space between the chains in 1, held by considerable hydrogen bonding. The structure of 1 is similar to those of coordination polymers synthesized with BTEC, piperazine, and divalent transition metals, M2+ (M ) Co, Ni, Zn).4a These complexes are isomorphous and contain anionic chains of [M(BTEC)(H2O)4]2-n interspersed with piperazonium cations (H4piperazine2+). The critical difference between these coordination polymers and 1 is the presence of the neutral 2D sheets of [Co2(BTEC)(H2O)8]n that alternate with the layers containing chains and H4hpz cations that are found in 1. The structure of 2 is comprised of anionic 2D sheets of [Co(BTEC)(H2O)3]2-n with H4hpz cations sandwiched between the sheets (Figure 4). In contrast to 1, the structure of 2 does not contain the chains that lie between sheets in 1. Only H4hpz2+ cations and uncoordinated water molecules reside in the inter-sheet space. The sheets in 2 lie in the bc plane and are made up of a rhombic grid with BTEC ligands on the corners, cobalt atoms on the edges, and one extra BTEC ligand that bridges between two of the cobalt ions parallel to the b axis (Figure 5). Two BTEC ligands and four aqua

Figure 4. View of the 3D structure of 2 looking down the c axis parallel to the sheets: Co, light blue; O, red; C, gray; N, dark blue. Hydrogen atoms and unccordinated water molecules are omitted for clarity.

ligands coordinate to Co1, while four BTEC ligands and two aqua ligands coordinate to Co2. Similarly, the BTEC ligands on the corners of the rhombic grid have a µ4 bridging mode, while the BTEC ligands that bridge between cobalt ions in the b direction are only µ2. There are therefore two chemically distinct cobalt atoms and two distinct BTEC bridging modes in 2 (Figure 5). This is somewhat reminiscent of the two environments found in the sheets and chains of 1. Unlike the binding mode of the BTEC ligands in 1, the structure of 2 contains BTEC ligands that coordinate via a combination of outer and inner carboxylate oxygen atoms. The µ4-BTEC anions on the corners of the rhombic grid coordinate to the cobalt atoms via both outer carboxylato oxygen atoms (O6-Co2) and inner carboxylato oxygen atoms (O3-Co1). These carboxylato groups are twisted from the plane of the benzene ring by dihedral angles of 54 and 45°, respectively. This bridging mode of the µ4-BTEC ligands creates a rhombic grid (as opposed to a square grid) that is aligned with the crystallographic axes, resulting in rhombi with angles that correspond closely to the R angle of 60.3°. The µ2-BTEC ligands coordinate to the Co2 atoms via the outer carboxylato oxygen atoms. The noncoordinating carboxylates of the µ2-BTEC ligands are involved in hydrogen bonding with the NH2 protons of the H4hpz cations, uncoordinated water molecules, and a coordinated aqua ligand from the nearby Co1 ion. This is possible due to the extreme twisting of the µ2-BTEC ligands out of the plane of the sheets. The benzene rings of the µ2-BTEC ligands are nearly coplanar with the crystallographic ab plane, twisting them out of the plane of the sheets by about 80°. This places the noncoordinating carboxylate groups less than 3 Å away from the noncoordinating carboxylates on the neighboring sheets. Other sheet-sheet distances, such as the distance between aqua ligands, are closer to 5 Å.

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case of 1 the chains are anionic while the sheets are neutral; however, for 2 the extra BTEC ligand in the sheets gives rise to anionic sheets and the absence of chains. It is tempting to view the transformation of 1 into 2 as a merging of the square grid sheets with the linear chains to form the modified rhombic sheet structure in 2. The conversion of 1 to 2 occurs gradually over the course of several days. It is not clear what the driving force behind this conversion is, but it is presumably closely related to inter-sheet interactions (including hydrogen bonding). Additionally, there are fewer water molecules per cobalt ion in 2 than there are in 1, raising the possibility that the conversion is also partially driven by entropy. Further studies of coordination polymers with the BTEC anion and transition metals with other coligands are currently underway. Acknowledgment. We thank the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the University of Oklahoma for financial support for this research. Supporting Information Available: Crystallographic data (in CIF format), fully labeled ORTEP drawings, and TGA data for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

References Figure 5. Molecular structure represented as 50% thermal ellipsoids of a portion of the 2D sheets in (H4hpz)[Co(BTEC)(H2O)3]‚3H2O (2) (top) and molecular structure of one 2D sheet of 2 (bottom): Co, light blue; O, red; C, gray. Homopiperazonium cations, solvent water molecules, and hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Co1-O1 ) 2.133(2); Co1-O2 ) 2.0951(18); Co1-O3 ) 2.0535(19); Co2-O6 ) 2.0771(17); Co2-O7 ) 2.1479(19); Co2O8 ) 2.112(2); O1-Co1-O1A ) 180.00(8); O1-Co1-O2 ) 88.37(8); O1-Co1-O2A ) 91.63(8); O1-Co1-O3 ) 88.45(7); O1-Co1-O3A ) 91.55(7); O2-Co1-O2A ) 180.000(1); O2Co1-O3 ) 92.60(8); O2A-Co1-O3 ) 87.40(8); O3-Co1-O3A ) 180.000(1); O6-Co2-O6C ) 180.00(15); O6-Co2-O7 ) 97.98(7); O6-Co2-O7A ) 82.02(7); O6-Co2-O8 ) 87.88(7); O6-Co2-O8B ) 92.12(7); O7-Co2-O7A ) 180.00(12); O7Co2-O8 ) 88.82(8); O7A-Co2-O8 ) 91.18(8); O8-Co2-O8B ) 180.00(7). See the Supporting Information for fully labeled ORTEP drawings and complete tables of bond lengths and angles.

The coordination geometry of the cobalt ions in 2 is octahedral, with two carboxylato ligands and four aqua ligands coordinated to Co1 and with four carboxylato ligands and two aqua ligands coordinated to Co2. The plane created by the carboxylato oxygens coordinating to Co2 is approximately in the plane of the sheet. The cobalt-carboxylato oxygen bond lengths and angles in 2 are slightly shorter than those in 1, while still well within the expected range (see Figure 5). Conclusions The structures of 1 and 2 are closely related by the fact that they are composed of sheets of cobalt-BTEC coordination polymers and homopiperazonium cations sandwiched between them. Although the stoichiometries of cobalt(II), BTEC, and H4hpz are different in 1 and 2, they both contain anionic coordination polymers. In the

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