As a contribution to understanding
catalysis by transition metal complexes with redox-active ligands
(here: catecholate – cat), we report a computational study
on the mechanism of a catalytic cycle where (i) O<sub>2</sub> is activated
at the metal center of the catecholate complex [Re<sup>V</sup>(O)(cat)<sub>2</sub>]<sup>−</sup> to yield [Re<sup>VII</sup>(O)<sub>2</sub>(cat)<sub>2</sub>]<sup>−</sup>, which (ii) subsequently is
applied to oxidize alcohols. We were able to identify the steps where
the redox-active ligands played a crucial role as e<sup>–</sup> buffer. For O<sub>2</sub> homolysis, a series of sequential 1e<sup>–</sup> steps leads to superoxo and bimetallic intermediates,
followed by facile cleavage of the bimetallic peroxo O–O linkage.
The <i>trans–cis</i> isomerization of <i>trans</i>-[Re<sup>V</sup>(O)(cat)<sub>2</sub>]<sup>−</sup> is the crucial
step of O<sub>2</sub> activation, with an absolute free energy barrier
of 16.8 kcal mol<sup>–1</sup> in methanol. Due to the ionic
character of intermediates, all reaction barriers of O<sub>2</sub> activation are significantly lowered in a polar solvent, thus rendering
O<sub>2</sub> homolysis kinetically accessible. With computational
results for the activation barriers of all elementary steps as well
as the calculated solvent effects, we are able to rationalize all
pertinent experimental findings. For catalytic alcohol oxidation,
we propose a novel cooperative mechanism that involves two units of
the metal complexes, ruling out the reaction via a seven-coordinated
active oxidant, as previously hypothesized. We present in detail calculated
energies and barriers for the reaction steps of the oxidation of methanol
as model alcohol as well as the energetics of crucial steps of the
experimentally studied oxidation of benzyl alcohol, both transformations
for methanol as solvent