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

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.