Three-dimensional iron-series coordination networks based on antiferromagnetic and ferromagnetic dinuclear motifs with mixed carboxylate and aqua bridges

Three coordination polymers of the iron-series transition metal ions with 4-(3-pyridyl)benzoic acid (4,3-Hpybz) ligand were synthesized and structurally and magnetically characterized. They have the formulas [M2(4,3-pybz)4(H2O)] (M = Fe, 1; Co, 2; and Ni, 3). In these isomorphous compounds, octahedrally coordinated metal ions are linked by the mixed (μ-syn, syn-COO)2(μ-H2O) triple bridges to give dinuclear motifs, which serves as tetrahedral building units and are further linked by 4,3-Hpybz to give rise to diamond networks. Magnetic studies demonstrated that the (COO)2(H2O) triple bridges induce antiferromagnetic coupling between metal ions in 1 and 2, but compound 3 with the same mixed triple bridges exhibits ferromagnetic coupling, which may be related to orbital countercomplementarity.