Dinuclear Calcium Complexes with Intramolecularly NH···O Hydrogen-Bonded Dicarboxylate Ligands

A novel dinuclear calcium complex, [Ca2{(2-OCO-3-CH3C6H3NHCO)2C(CH3)2}2(CH3OH)6] (1), was synthesized
as a structural model of 8-coordinated Ca(II) ions in the double calcium-binding site of thermolysin. The complex
has four NH···O hydrogen bonds between the amide NH and the carboxylate oxygen anion. Two types of bridging
coordination of the carboxylate ligand to Ca(II) were found in 1. The amide NH forms a strong NH···O hydrogen
bond with the anionic oxygen of the two carboxylate oxygens. A ligand-exchange reaction between the dinuclear
calcium complex and eight equimolar amounts of 2,4,6-trimethylbenzoic acid or 2-CH3-6-t-BuCONHC6H3COOH
indicates that the NH···O hydrogen bond prevents the dissociation of the Ca−O bond.