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 cobalt(II) citrate unit [Co2(Hcit)2(H2O)4]2- in the complex 2 forms a one-dimensional polymeric chain through the coupling
of long-arm β-carboxylate groups with the planar [Co(H2O)4] and [Co2(Hcit)2(H2O)4] units. The [Co(Hcit)(H2O)]-
unit in complex 3 forms an infinite chain along a 21 axis. Complex 4 forms a layered complex through the links of
the new dimeric unit [Co2(Hcit)2(H2O)2] by the oxygen atoms of the β-carboxylate groups. Interconversions between
1, 3, and 4 are found to be pH- and counterion-dependent. Heating of coordination polymer 2 results in the irreversible
formation of 3. The structural diversities of these cobalt(II) citrate polymers demonstrate that pH, counterions, and
reaction temperature play essential roles in the formations of such one-dimensional and two-dimensional frameworks.