Synthesis, characterization and crystal structures of nickel complexes with dissymmetric tetradentate ligands containing a mixed-donor sphere

Four new nickel(II) complexes of dissymmetric tetradentate ligands, containing mixed-ligand donor sets of NSNS or NSNO, have been synthesized. These complexes were prepared by facile template reactions of the appropriate aldehyde and amine in the presence of [Ni(H₂O)₆](BF₄)₂, resulting directly in the desired nickel compounds. The nickel compounds were characterized by analytical, spectroscopic and electrochemical methods. The structures of [Ni(pyzs)]BF₄, [Ni(pyrs)]BF₄ and [Ni(pyzo)]BF₄ (see Scheme 1) have been determined by single-crystal X-ray crystallography, showing the geometry of the nickel ion to be square-planar. Vis–NIR spectra show that the phenolate-containing complexes [Ni(pyzo)]BF₄ and [Ni(pyro)]BF₄ (see Scheme 1) are essentially square-planar in nitromethane, but tetragonal octahedral in methanol, whereas the thiophenolate-containing compounds [Ni(pyzs)]BF₄ and [Ni(pyrs)]BF₄ remain square-planar in both solvents. Titration of the thiophenolate-containing complexes with 1-methylimidazole results in diamagnetic five-coordinated complexes. Electrochemistry shows quasi-reversible reductions to Ni(I) to occur for [Ni(pyzo)]BF₄, [Ni(pyrs)]BF₄ and [Ni(pyro)]BF₄.