Mononuclear versus dinuclear complex formation in nickel(II) sulfate/phenyl(2-pyridyl)ketone oxime chemistry depending on the ligand to metal reaction ratio: Synthetic, spectral and structural studies

The reactions of phenyl(2-pyridyl)ketone oxime (py)C(ph)NOH, with nickel(II) sulfate hexahydrate under reflux, in the absence of an external base, have been investigated. The reaction of NiSO4 center dot 6H(2)O with two equivalents of (py)C(ph)NOH in H2O/MeOH leads to the dinuclear complex [Ni-2(SO4)(2){(py)C(ph)NOH}(4)] (1), while an excess of the organic ligand affords the 1:3 cationic complex [Ni{(py)C(ph)NOH}(3)](SO4) (2). Compound 1 is transformed into 2 by a reaction with an excess of ligand in refluxing H2O/MeOH. Reactions of land 2 with a limited amount of LiOH give the known cluster [Ni-6(SO4)(4)(OH){(py)C(ph)NO}(3){(py)C(ph)NOH}(3)(H2O)(3)]. The structures of 1 and 2 have been determined by single-crystal X-ray crystallography. In both complexes the organic ligandchelates through its 2-pyridyl and oxime nitrogen atoms. The metal centers of 1 are bridged by two eta(1):eta(1):mu sulfato ligands; each metal ion has the cis-cis-trans deposition of the coordinated sulfato oxygen, pyridyl nitrogen and oxime nitrogen atoms, respectively. The cation of 2 is the fac isomer considering the positions of the coordinated pyridyl and oxime nitrogen atoms. The crystal structures of both complexes are stabilized by hydrogen bonds. Compounds I and 2 join a small family of structurally characterized metal complexes containing the neutral or anionic forms of phenyl(2-pyridyl)ketone oxime as ligands. The IR spectra of the two complexes are discussed in terms of the nature of bonding and their structures. From the vibrational spectroscopy viewpoint, the SO42- groups in 1 and 2 appear to have lower symmetries compared with those deduced from X-ray crystallography; this is attributed to the participation of sulfates in hydrogen bonding interactions. (C) 2007 Elsevier B.V. All rights reserved.