Synthesis, X-ray structure and properties of a calcium(II) coordination polymer showing μ2-η1:η1 and μ3-η2:η1 coordination modes of 4-nitrobenzoate

The reaction of the monomeric compound [Ca(H2O)(4)(eta(1)-4-nba)(eta(2)-4-nba)] 1 (4-nba = 4-nitrobenzoate) with N-methylimidazole (N-Melm) or imidazole (Im) at 100 degrees C results in the formation of the anhydrous mixed ligand complexes of composition [Ca(L)(4-nba)(2)] (L = N-Melm 2: L = Im 3). The compounds were characterized by elemental analysis, IR, UV-Vis and H-1 NMR spectra, TC-DSC thermograms, weight loss studies and the structure of 2 was determined by single crystal X-ray crystallography. Compounds 2 and 3 can be transformed into 1 by heating with water. At elevated temperatures all compounds (1-3) are thermally decomposed to CaO. Compound 2 crystallizes in the centrosymmetric triclinic space group P (1) over bar with all atoms located in general positions. The structure reveals that 2 is a polymer and can be formulated as catena-poly[[(N-methylimidazole)calcium(II)](mu(2)-eta(1):eta(1)-4-nitrobenzoato)(mu(3)-eta(2):eta(1)-4-nitrobenzoato)]. The structure of [[Ca(N-Melm)](mu(2)-eta(1):eta(1)-4-nba)(mu(3)-eta(2):eta(1)-4-nba)](n) 2 consists of a central Ca(II) ion, a terminal N-Melm ligand and two crystallographically independent 4-nba ligands. In the crystal structure, each Ca(II) is bonded to a nitrogen atom of N-Melm and five oxygen atoms from five symmetry related 4-nba ligands resulting in a distorted {CaO5N} octahedron. One of the unique 4-nitrobenzoate in each formula unit functions as a bridging bidentate ligand (mu(2)-eta(1):eta(1)-4-nba) and is linked to two symmetry related Ca(II) ions leading to the formation of an infinite chain extending along a axis with a long Ca center dot center dot center dot Ca separation of 5.531 (1) angstrom. The second independent 4-nba ligand (mu(3)-eta(2):eta(1)-4-nba) is coordinated to three different Ca(II) ions with both the carboxylate oxygen atoms linked to the same infinite chain formed earlier and one of the carboxylate oxygen atoms functions as a monoatomic bridge between two symmetry related Ca(II) ions. This bridge links two chains with a shorter Ca center dot center dot center dot Ca separation of 3.8585(7) angstrom resulting in the formation of a one-dimensional ladder structure. The oxygen atoms of the nitro groups are involved in C-H center dot center dot center dot O interactions. A comparative study of 30 Ca(II) coordination polymers is described. (C) 2008 Elsevier Ltd. All rights reserved.