Molecular Constraints and Electronic Structure Direct Multi-Step PCET Mechanisms in the Electrochemical Oxidation of Ruthenium Complexes

Abstract

Extensive studies on the dehydrogenative oxidation of alkylamines on transition metals represent a generalized reactivity, with some reports on accessible intermediates and reactive species offering different interpretations. Here, the electrochemical oxidation of ruthenium amine complexes is examined to evaluate the reactivity of high-valent species. We report a combined spectroelectrochemical and DFT study of the complexes [RuII(bpy)2(LL)]2+ with LL = (NH3)2, en (ethylenediamine), and en* (2,3-dimethyl-2,3-diamino-butane), and bpy = 2,2′-bipyridine. Comparison among these three complexes revealed different electrochemical oxidation mechanistic pathways, yet all of them follow the same overall acid–base–dependent reactivity. In both protic and aprotic solvents, the first oxidation proceeds as a pure one-electron process under strongly acidic conditions. In the presence of a base, a multiproton-coupled electron transfer (PCET) process completes the net 2e–/2H+ reaction. The oxidation mechanism of [Ru(bpy)2(en*)]2+ is revisited to achieve improved consistency with long-standing experimental proposals. Electrochemical and spectral analysis, along with DFT calculations, support the assignment of the intermediates formed. Molecular orbital analysis reveals interactions between dπRu and pNsp2 orbitals, which stabilize specific reaction intermediates through geometric constraints. This report highlights the relevance of Ru-amine/amido complexes as multi-PCET redox models for bond-activation catalysis.