CO2 reduction with protons and electrons at a boron-based reaction center

Abstract

Borohydrides are widely used reducing agents in chemical synthesis and have emerging energy applications as hydrogen storage materials and reagents for the reduction of CO₂. Unfortunately, the high energy cost associated with the multistep preparation of borohydrides starting from alkali metals precludes large scale implementation of these latter uses. One potential solution to this issue is the direct synthesis of borohydrides from the protonation of reduced boron compounds. We herein report reactions of the redox series [Au(B₂P₂)]ⁿ (n = +1, 0, −1) (B₂P₂, 9,10-bis(2-(diisopropylphosphino)phenyl)-9,10-dihydroboranthrene) and their conversion into corresponding mono- and diborohydride complexes. Crucially, the monoborohydride can be accessed via protonation of [Au(B₂P₂)]⁻, a masked borane dianion equivalent accessible at relatively mild potentials (−2.05 V vs. Fc/Fc⁺). This species reduces CO₂ to produce the corresponding formate complex. Cleavage of the formate complex can be achieved by reduction (ca. −1.7 V vs. Fc/Fc⁺) or by the addition of electrophiles including H⁺. Additionally, direct reaction of [Au(B₂P₂)]⁻ with CO₂ results in reductive disproportion to release CO and generate a carbonate complex. Together, these reactions constitute a synthetic cycle for CO₂ reduction at a boron-based reaction center that proceeds through a B–H unit generated via protonation of a reduced borane with weak organic acids.