Benzimidazoles as Metal-Free and Recyclable Hydrides for CO₂ Reduction to Formate

Abstract

We report a novel metal-free chemical reduction of CO₂ by a recyclable benzimidazole-based organo-hydride, whose choice was guided by quantum chemical calculations. Notably, benzimidazole-based hydride donors rival the hydride-donating abilities of noble-metal-based hydrides such as [Ru(tpy)(bpy)H]⁺ and [Pt(depe)₂H]⁺. Chemical CO₂ reduction to the formate anion (HCOO^-) was carried out in the absence of biological enzymes, a sacrificial Lewis acid, or a base to activate the substrate or reductant. ¹³CO₂ experiments confirmed the formation of H¹³COO^- by CO₂ reduction with the formate product characterized by ¹H NMR and ¹³C NMR spectroscopy and ESI-MS. The highest formate yield of 66% was obtained in the presence of potassium tetrafluoroborate under mild conditions. The likely role of exogenous salt additives in this reaction is to stabilize and shift the equilibrium toward the ionic products. After CO₂ reduction, the benzimidazole-based hydride donor was quantitatively oxidized to its aromatic benzimidazolium cation, establishing its recyclability. In addition, we electrochemically reduced the benzimidazolium cation to its organo-hydride form in quantitative yield, demonstrating its potential for electrocatalytic CO₂ reduction. These results serve as a proof of concept for the electrocatalytic reduction of CO₂ by sustainable, recyclable, and metal-free organo-hydrides.