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 NiSO₄·6H₂O with two equivalents of (py)C(ph)NOH in H₂O/MeOH leads to the dinuclear complex [Ni₂(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 1 and 2 with a limited amount of LiOH give the known cluster [Ni₆(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 ligand chelates through its 2-pyridyl and oxime nitrogen atoms. The metal centers of 1 are bridged by two η¹:η¹:μ 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 1 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 SO₄^2- 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.