Understanding the mechanisms of transition metal catalysed redox reactions

Abstract

Transition metal complexes catalyse a number of important synthetic chemical reactions. Two such reactions which rely on copper and ruthenium complexes respectively are atom transfer radical polymerisation (ATRP) and the Ley-Griffith oxidation of alcohols.In ATRP a copper(I) complex bearing a chelate ligand 'L' homolytically cleaves the carbon-halogen bond of an organo halide initiator (R-X) to produce an alkyl radical and the corresponding copper(II)-halido complex (Cu I L + RX → R• + Cu II LX). The reverse reaction is coined 'deactivation' and provides control to the process by keeping the concentration of the radical low. It is experimentally difficult to determine the rate of this reaction because it is so fast. Herein, a new method for measuring the rate of deactivation is developed using cyclic voltammetry coupled to simulations. The mechanism of deactivation is also unknown and this aspect is investigated by a kinetic study of halide substitution reactions on three ATRP-relevant Cu ). EPR spectroscopy alsoshows that NMO forms an outer-sphere associated complex with perruthenate which may be important in facilitating this reaction. Finally, synergistic EPR and UV-vis spectroscopy demonstrate that even with a large excess of NMO, a small amount of ruthenium dioxide still forms during the Ley-Griffith oxidation and acts as an accelerant for the reaction -i.e. the reaction is auto-catalytic. Declaration by author