A Sterically Encumbered Triarylborane Enabling CO2 Reduction to Formate by Electrons and Protons

Abstract

Borohydrides are widely used sources of hydride ions (H^–), but their conventional synthesis from highly reducing metal hydrides is energetically demanding and undermines sustainable applications. Herein, we report a sterically encumbered thioether-substituted triaryl borane, 1,2-(^tBuS)C₆H₄(BMes₂) (3) (Mes = 2,4,6-Me₃C₆H₂), which is highly air- and moisture-stable. 3 exhibits a one-electron reduction at E⁰ = −2.62 V vs Fc^+|0 and a radical anion half-lifetime of ∼0.5 s in THF, enabling bimolecular follow-up reactivity. Indeed, two-electron reduction and protonation of 3 with KC₈/[DBUH]⁺ (DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) or hydride transfer from Li(AlH₄) affords the borohydride 4^–. From a hydricity–potential correlation, we estimate a hydricity of ∼36 kcal mol^–1 in MeCN, which allows 4^– to reduce H⁺ to H₂ and CO₂ to formate under ambient conditions in almost quantitative yields. The reaction of 3, [DBUH]⁺, KC₈, and CO₂ furnishes potassium formate in ∼90% spectroscopic yield, establishing a closed synthetic cycle for CO₂ reduction by protons and electrons at a boron center. These results highlight how tailor-made, redox-active boranes can complement transition-metal hydrides as modular, metal-free platforms for coupling renewable electrons and protons in CO₂ utilization chemistry.