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.

Addressing the Reproducibility of Photocatalytic Carbon Dioxide Reduction

“…This correlates with the reported results for the electrocatalytic CO 2 reduction utilizing complex 1 , where HCO 2 H was the main product . However, we already observed a low reproducibility for the HCO 2 H TONs, which we at first attributed partially to direct CO 2 reduction by BIH . Interestingly, conducting the same reaction with the isolated heteroleptic copper complex CuPS 2 instead of the in situ prepared photosensitizer resulted in a total TON of 865 and a CO/H 2 /HCO 2 H ratio of 1/7/3 (average of 2 reactions, for single experiment results see Table S1).…”

Sequential Co-immobilization of Enzymes in Metal-Organic Frameworks for Efficient Biocatalytic Conversion of Adsorbed CO2 to Formate

“…To obtain a sustainable society, it is highly desirable to explore energy-efficient technologies for the conversion of CO 2 to useful chemicals or fuels (e.g., methanol). Up to date, considerable efforts have been made to catalyse the hydrogenation of CO 2 via chemical (Fogeron et al, 2018; Lim et al, 2019), electrochemical (Wang et al, 2016; Coskun et al, 2017; Miller et al, 2019), photochemical (Bachmeier et al, 2014; Tu et al, 2014; Wang D. et al, 2014; Wang and Wang, 2015; Xu et al, 2015; Sokol et al, 2018; Zhang L. et al, 2018), enzymatic conversions (Yang Z.-Y. et al, 2012; Fixen et al, 2016; Shah and Imae, 2017; Cai et al, 2018; Liu et al, 2018), or photocatalyst/biocatalyst integrated systems (Yadav et al, 2014).…”

An NADH-Inspired Redox Mediator Strategy to Promote Second-Sphere Electron and Proton Transfer for Cooperative Electrochemical CO2 Reduction Catalyzed by Iron Porphyrin

“…59,[82][83][84] Indeed, few organic hydrides are strong enough donors to reduce CO 2 directly, 72,75 and those that can be regenerated encounter slow rates of hydride transfer that limit integration into photo-or electrocatalytic cycles. 74,85 This cooperative, bioinspired approach to electrocatalysis, which leverages primary metal centers with secondary dual electron-proton reservoirs, should be applicable to a broader array of chemical transformations where bond activation and catalytic turnover relies on controlling both electron and proton inventories.…”