In this work, we report the first
example of the PCET
reactivity
for a boron cluster compound, the zwitterionic nido-carboranyl diphosphonium derivative 7-P(H)tBu2-10-P(H)iPr2-nido-C2B10H10. This main-group reagent efficiently
transfers two electrons and two protons to quinones to yield hydroquinones
and regenerate a neutral closo-carboranyl diphosphine,
1-PtBu2-2-PiPr2-closo-C2B10H10. As we have
previously reported the conversion of this closo-carboranyl
diphosphine into the zwitterionic nido- derivative
upon reaction with main group hydrides, the transformation reported
herein represents a complete synthetic cycle for the metal-free reduction
of quinones, with the redox-active carboranyl diphosphine scaffold
acting as a mediator. The proposed mechanism of this reduction, based
on pK
a determination, electrochemical
studies, and kinetic isotope effect determination, involves the electron
transfer from the nido- cluster to the quinone coupled
with the delivery of protons.