Transition Metal Complexes Containing Quinone Ligands: Studies on Intramolecular Metal-Ligand Electron Transfer

Abstract

Charge distribution in complexes containing quinone ligands coordinated with electroactive transition metal ions is determined by the relative energies of metal and quinone electronic levels. Intramolecular metal-quinone electron transfer may result from variations in the form of the complex which change the order of metal and quinone orbital energies. Iron complexes have been prepared with 3,5-di-t-butyl-l,2-benzoquinone (DBBQ). The structures of the complexes have been determined using crystallographic methods, the charge on the metal and the charge distribution in the molecule have been studied using spectroscopic techniques, and magnetic coupling between paramagnetic semiquinone ligands and the high spin ferric center has been investigated using variable temperature magnetic susceptibility measurements. Reactions carried out between 3,5-di-t-butylcatechol and ammonia in the presence of the divalent metal ions of Fe, Mn, and Co lead to neutral complexes of form ML 2, where L is a tridentate ligand resulting from Schiff base condensation of two catecholate ligands. This ligand may exist in charges ranging from +1 to -3, and spectroscopic, structural and magnetic methods of analysis have been used to determine charge distribution in the complexes. EPR spectra indicate that complexes containing the more oxidized form of the ligand have surprisingly localized electronic structures. © 1988 IUPAC