Modulation of Proton-Coupled Electron Transfer through Molybdenum–Quinonoid Interactions

An expanded series of π-bound molybdenum–quinonoid complexes supported by pendant phosphines has been synthesized. These compounds formally span three protonation–oxidation states of the quinonoid fragment (catechol, semiquinone, quinone) and two different oxidation states of the metal (Mo⁰, Mo^II), notably demonstrating a total of two protons and four electrons accessible in the system. Previously, the reduced Mo⁰–catechol complex 1 and its reaction with dioxygen to yield the two-proton/two-electron oxidized Mo⁰–quinone compound 4 was explored, while, herein, the expansion of the series to include the two-electron oxidized Mo^II–catechol complex 2, the one-proton/two-electron oxidized Mo–semiquinone complex 3, and the two-proton/four-electron oxidized Mo^II–quinone complexes 5 and 6 is reported. Transfer of multiple equivalents of protons and electrons from the Mo⁰ and Mo^II catechol complexes, 1 and 2, to H atom acceptor TEMPO suggests the presence of weak O–H bonds. Although thermochemical analyses are hindered by the irreversibility of the electrochemistry of the present compounds, the reactivity observed suggests weaker O–H bonds compared to the free catechol, indicating that proton-coupled electron transfer can be facilitated significantly by the π-bound metal center.