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 amine-O atoms. The solid state room temperature (298 K) magnetic moments are consistent with a d ⁸ (S = 1) ground state electronic configuration. Electronic spectra of the complexes in CH₃CN solutions display the v ₁ band at ∼ 850 nm followed by charge transfer bands in the range 381–241 nm. The [Ni^IIIL₂]⁺-[Ni^IIL₂] 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.

Key Words:

• Nickel(II) complex
• X-ray structure
• amide-O coordination
• redox