Cobalt(II) and cobalt(III) complexes with N-(aroyl)-N′ -(picolinylidene)hydrazines. Spin-crossover in the cobalt(II) complexes

Abstract

Complexes of cobalt(II) and cobalt(III) with tridentate N-(aroyl)-N′-(picolinylidene)-hydrazines (HL, H stands for the dissociable amide proton) are described. The Schiff bases (HL) have been obtained by condensation of 2-pyridine-carboxaldehyde with benzhydrazide or 4-substituted benzhydrazides. The reactions of Co(O2CCH 3)2·4H2O and HL at room temperature under aerobic condition afford the complexes with general formulae [Co IIL2] (-Cl and -NO2 as substituents) and [CoIIIL2]+ (-H, -CH3, -OCH 3 and -N(CH3)2 as substituents). The cationic complexes have been isolated as PF6- salts. Analytical, spectroscopic, magnetic and electrochemical techniques have been used for the characterization of these complexes. X-ray structure of [Co(pamh) 2]PF6 has been determined. In each complex, the metal centre is in N4O2 coordination sphere constituted by the meridionally spanning pyridine-N, imine-N and deprotonated amide-O donor ligands. Infrared spectra are consistent with the enolate form of the amide functionalities in both ligands. Cobalt(III) complexes, [CoL2]PF 6, are diamagnetic and NMR active. In acetonitrile solutions the complexes behave as 1:1 electrolyte. Electronic spectra of the complexes display charge transfer bands in the range 453-221 nm. All the complexes are redox active and display the Co(III)-Co(II) couple in the potential range -0.23 to +0.09 V (vs. Ag/AgCl). The trend in these potential values reflects the effect of the electronic nature of the substituents on the aroyl moiety of the ligands. Cobalt(II) complexes, [CoL2], are paramagnetic and electrically nonconducting in solutions. Variable temperature magnetic susceptibility measurements and EPR spectra reveal S = 3/2 ↔ S = 1/2 spin-crossover in both cobalt(II) complexes.