Synthesis, X-ray structure and properties of a calcium(II) coordination polymer showing μ 2-η 1:η 1 and μ 3-η 2:η 1 coordination modes of 4-nitrobenzoate

Abstract

The reaction of the monomeric compound [Ca(H 2O) 4(η 1-4-nba)(η 2-4-nba)] 1 (4-nba = 4-nitrobenzoate) with N-methylimidazole ( N-MeIm) or imidazole (Im) at 100 oC results in the formation of the anhydrous mixed ligand complexes of composition [Ca(L)(4-nba) 2] (L = N-MeIm 2; L = Im 3). The compounds were characterized by elemental analysis, IR, UV–Vis and 1H NMR spectra, TG-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 ¯ 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)](μ 2-η 1:η 1-4-nitrobenzoato)(μ 3-η 2:η 1-4-nitrobenzoato)] . The structure of [[Ca( N-MeIm)](μ 2-η 1:η 1-4-nba)(μ 3-η 2:η 1-4-nba)] n 2 consists of a central Ca(II) ion, a terminal N-MeIm ligand and two crystallographically independent 4-nba ligands. In the crystal structure, each Ca(II) is bonded to a nitrogen atom of N-MeIm and five oxygen atoms from five symmetry related 4-nba ligands resulting in a distorted {CaO 5N} octahedron. One of the unique 4-nitrobenzoate in each formula unit functions as a bridging bidentate ligand (μ 2-η 1:η 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⋯Ca separation of 5.531(1) Å. The second independent 4-nba ligand (μ 3-η 2:η 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⋯Ca separation of 3.8585(7) Å resulting in the formation of a one-dimensional ladder structure. The oxygen atoms of the nitro groups are involved in C–H⋯O interactions. A comparative study of 30 Ca(II) coordination polymers is described.