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, MoII), 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 MoII–catechol
complex 2, the one-proton/two-electron oxidized Mo–semiquinone
complex 3, and the two-proton/four-electron oxidized
MoII–quinone complexes 5 and 6 is reported. Transfer of multiple equivalents of protons
and electrons from the
Mo0 and MoII 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.