Redox-Active Carboranyl Diphosphine as an Electron and Proton Transfer Agent.

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) t Bu 2 -10-P(H) i Pr 2 - nido -C 2 B 10 H 10 . This main-group reagent efficiently transfers two electrons and two protons to quinones to yield hydroquinones and regenerate a neutral closo -carboranyl diphosphine, 1-P t Bu 2 -2-P i Pr 2 - closo -C 2 B 10 H 10 . 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 p K 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.