Scheme 3.

Two possible mechanisms for the catalytic reduction of duroquinone by complex 1. a) Reduction through two sequential one-electron transfers. Figure S3 in the Supporting Information shows a plot of the singly occupied molecular orbital of the Ir^II d⁷ complex. b) Reduction through a single two-electron transfer. The pairs of numbers associated with each step are potential-energy/free-energy couples (in kcal mol⁻¹). The semiquinone has a pK_a of around 5 and is therefore negatively charged at pH 7.2, whereas the quinol is protonated (pK_a higher than 11).¹⁴