Structural Diversities of Cobalt(II) Coordination Polymers with Citric Acid

The reactions of cobalt(II) ion with citric acid (C 6 H 8 O 7 = H 4 cit) 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(H 2 cit)(H 2 O)] n ( 1 ), [Co(H 2 O) 4 ] n [Co 2 (Hcit) 2 (H 2 O) 4 ] n ·6 n H 2 O ( 2 ), [Co(H 2 O) 6 ] n [Co(Hcit)(H 2 O)] 2 n ·2 n H 2 O ( 3 ), and (NH 4 ) 2 n [Co 2 (Hcit) 2 (H 2 O) 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 [Co 2 (Hcit) 2 (H 2 O) 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(H 2 O) 4 ] and [Co 2 (Hcit) 2 (H 2 O) 4 ] units. The [Co(Hcit)(H 2 O)] - unit in complex 3 forms an infinite chain along a 2 1 axis. Complex 4 forms a layered complex through the links of the new dimeric unit [Co 2 (Hcit) 2 (H 2 O) 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.