Heteroleptic Ni(II) complexes containing chelating benzoato ligand with sharp bite angle: syntheses, crystal structures, and magnetism

From the system Ni(OH)2 – 4,4́-dmbpy – HBz (HBz = benzoic acid; 4,4́-dmbpy = 4,4́-dimethyl-2,2́-bipyridine) under various experimental conditions three complexes were isolated, namely light blue [Ni(4,4́-dmbpy)(Bz)2(H2O)] (1), violet [Ni(4,4́-dmbpy)2(Bz)](Bz)·2HBz (2), and pink microcrystalline product [Ni(4,4́-dmbpy)3](Bz)2·3.5H2O (3). The crystal structures of 1 and 2 were determined, that of 2 at two temperatures (100 and 277 K). The crystal structure of 1 is formed of neutral molecules, in which the Ni(II) central atom is hexa-coordinated by one chelating diamine ligand, one chelating and one monodentate benzoato ligands, and one aqua ligand. The crystal structure of 2 is ionic and in the complex cation the Ni(II) atom is hexa-coordinated by two chelating diamine ligands and one chelating benzoato ligand. The positive charge of the complex cation is counterbalanced by a benzoate anion, and two additional molecules of benzoic acid are present in the asymmetric unit. Magnetic properties of 1 and 2 were studied in the temperature range 1.8 – 300 K and confirmed strong axial single-ion anisotropy (D/kB = -7.78 K for 1, D/kB = -6.52 K for 2) of the Ni(II) ions in line with the deformed coordination spheres of the central atoms. In addition, antiferromagnetic spin dimers of Ni(II) ions mediated by hydrogen bonds are present in 1 with exchange coupling J/kB = -2.19 K. The obtained experimental results were supported by ab initio and DFT calculations.