Structural diversities of cobalt(II) coordination polymers with citric acid

The reactions of cobalt(II) ion with citric acid (C6H8O7 = H4cit) have been studied in an acidic aqueous solution of pH 1−4, which result in the isolations of four new polymeric cobalt(II) citrate complexes:  [Co(H2cit)(H2O)]n (1), [Co(H2O)4]n[Co2(Hcit)2(H2O)4]n·6nH2O (2), [Co(H2O)6]n[Co(Hcit)(H2O)]2n·2nH2O (3), and (NH4)2n[Co2(Hcit)2(H2O)2]n (4). The complexes have been characterized by spectroscopic and single crystal X-ray diffraction studies. The Co(II) ion in each complex exists in an octahedral coordination environment. The citrate ligand binds the Co(II) ion tridentately via its α-hydroxyl, α-carboxylate, and one of the β-carboxylate groups as a basic feature. The polymeric structures are constructed by the further coordination of α-carboxylate or β-carboxylate groups. Complex 1 forms a chiral helical chain running along the a-axis of the crystal via the two bridged bonded oxygen atoms of the α-carboxylate group, leaving the β-carboxylic acid group free and forming strong hydrogen bond. The dimeric...