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

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.