Multifaceted Examination of Multielectron Transfer Reactions

Abstract

Multielectron transfer reactions, which are important in many biological and technological contexts, are paradoxical, because transfer of a second unit of charge should be energetically more difficult than the first. Straightforward changes in structure or composition often are offered as an explanation for the redox potential inversion that leads to multielectron behavior. We have explored the basis of this phenomenon by carrying out density functional theory studies of two-electron redox systems that have been carefully characterized by electrochemical methods. Principal findings include the fact that the energy of vertical electron attachment predominates that of structural or compositional change and thus controls the energetics of a given redox event. In addition, decisive energy changes in electron attachment processes often occur in lower-lying rather than frontier orbitals. Thus, simple tracking of LUMO/SOMO/HOMO energy changes (Walsh’s rule) is sometimes inadequate for complete understanding of redox-triggered events. These concepts are illustrated in our combined experimental/computational examination of the following two-electron redox systems: (i) reduction of binuclear, ligand-bridged complexes of Mo and W accompanied by metal−metal bond cleavage and structural rearrangement, (ii) reduction of bis(hexamethylbenzene) complexes of Fe, Ru, and Os accompanied by a change in ligand hapticity, and (iii) reduction of Pt(IV) antitumor prodrugs, whose reductive elimination of axial ligands generates the active Pt(II) form of the drug.