Two Co coordination polymers of biphenyl-2,2',5,5'-tetracarboxylic acid with flexible N-donor ligands: syntheses, structures and magnetic properties.

Two Co-based coordination polymers, namely poly[(μ-biphenyl-2,2',5,5'-tetracarboxylato){μ-1,3-bis[(1H-imidazol-1-yl)methyl]benzene}dicobalt(II)], [Co(CHO)(CHN)] or [Co(o,m-bpta)(1,3-bimb)] (I), and poly[[aqua(μ-biphenyl-2,2',5,5'-tetracarboxylato){1,4-bis[(1H-imidazol-1-yl)methyl]benzene}dicobalt(II)] dihydrate], {[Co(CHO)(CHN)(HO)]·4HO} or {[Co(o,m-bpta)(1,4-bimb)(HO)]·4HO} (II), were synthesized from a mixture of biphenyl-2,2',5,5'-tetracarboxylic acid, i.e. [H(o,m-bpta)], CoCl·6HO and N-donor ligands under solvothermal conditions. The complexes were characterized by IR spectroscopy, elemental analysis, single-crystal X-ray diffraction and powder X-ray diffraction analysis. The bridging (o,m-bpta) ligands combine with Co ions in different μ-coordination modes, leading to the formation of one-dimensional chains. The central Co atoms display tetrahedral [CoNO] and octahedral [CoNO] geometries in I and II, respectively. The bis[(1H-imidazol-1-yl)methyl]benzene (bimb) ligands adopt trans or cis conformations to connect Co ions, thus forming two three-dimensional (3D) networks. Complex I shows a (2,4)-connected 3D network with left- and right-handed helical chains constructed by (o,m-bpta) ligands. Complex II is a (4,4)-connected 3D novel network with ribbon-like chains formed by (o,m-bpta) linkers. Magnetic studies indicate an orbital contribution to the magnetic moment of I and II due to the longer Co...Co distances. An attempt has been made to fit the χT results to the magnetic formulae for mononuclear Co complexes, the fitting indicating the presence of weak antiferromagnetic interactions between the Co ions.