Nickel(II) complexes of tridentate N,N-,O-donor ligands: Syntheses, structures and redox properties

Abstract

Nickel(II) complexes ([NiL2) of tridentate Schiff bases (HL) containing amide functionality are described. The Schiff bases, Hpabh and Hpamh (H refers to the dissociable amide proton), are derived from 2-pyridinecarboxaldehyde and benzhydrazide, and 2-pyridinecarboxaldehyde and 4-methoxybenzhydrazide, respectively. The reaction of two equivalents of HL and one equivalent of Ni(O2CCH3)2·4H2O in methanol affords [NiL2] in high yield. The complexes are characterised by analytical, spectroscopic, magnetic and electrochemical techniques. The structures of both complexes have been determined by X-ray crystallography. The distorted octahedral NiN4O2 sphere in each complex is assembled by the two meridional N,N,O-donor ligands. Each ligand binds the metal ion via the pyridine-N, imine-N and deprotonated amide-O atoms. The solid state room temperature (298 K) magnetic moments are consistent with a d8 (S= 1) ground state electronic configuration. Electronic spectra of the complexes in CH3CN solutions display the ν1 band at ∼ 850nm followed by charge transfer bands in the range 381-241 nm. The [NiIIIL2]+-[NiIIL2] couple was observed in the cyclic voltammograms of both complexes. The potentials are 0.97 and 0.91 V (versus Ag-AgCl) for [Ni(pabh)2] and [Ni(pamh)2], respectively.