Investigations of the nature of dehydrogenation of the α-carbon atom in the oxidation of amines coordinated to ruthenium

Abstract

The oxidations of the amine complexes Ru(bpy)₂(A-B)²⁺ (where A-B = ampy [2-(aminomethyl)pyridine], d₂-ampy [2-(1,l-dideuterioaminomethyl)pyridine], and Meampy [2-( 1-aminoethy1)pyridinel and bpy = 2,2'-bipyridine) in aqueous solution to the corresponding imine species have been studied by using flash photolysis techniques. The rate-determining step of the reaction involves removal of a hydrogen from the a-carbon atom and the concomitant intramolecular transfer of two electrons from ligand to metal. Deuterium exchange and deuterium isotope effect studies, and stereochemical arguments, are consistent with a mechanism involving either (i) a hydride transfer from C to Ru or (ii) a base (solvent water)-assisted proton abstraction. Chirality studies indicate no loss of configurational integrity about the metal center during oxidation, which favors the second proposal. The X-ray crystal structure of the more rapidly oxidized diastereoisomer of [Ru(bpy)₂(Meampy)]S₂O₆.5H₂O it to be the λS(δR) form, in which the methyl substituent on the a-carbon atom is equatorial and the hydrogen substituent axial.