The role of substrate in unmasking oxyl character in oxomanganese complexes: the key to selectivity?

Abstract

The reaction profile for sulfide oxidation by formally Mn(V)=O species depends critically on the electronic structure of the isolated oxidant. In cases where the ground state has dominant oxyl radical character, the oxidation occurs in sequential one-electron steps, the first of which is barrierless. In contrast, if the oxyl radical character is 'masked' in the ground state by electron transfer from either the metal or the porphyrin co-ligand, the interaction between oxidant and substrate is repulsive at large separations, only becoming attractive when the incoming nucleophile approaches close enough to drive an electron out of oxide ppi manifold, thereby 'unmasking' the oxyl radical. The shape of the repulsive wall at long range will depend on the properties of both oxidant and substrate, offering the potential for substrate discrimination that is one of the most remarkable properties of the oxygen evolving complex. The electronic properties of the oxidant depend critically on the identity of the axial co-ligand, but also on the chosen density functional. As a result, hybrid and non-hybrid functionals give very different qualitative descriptions of the oxidation reaction.