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

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journal contribution
posted on 26.03.2019, 00:00 by Chern-Hooi Lim, Stefan Ilic, Abdulaziz Alherz, Brady T. Worrell, Samuel S. Bacon, James T. Hynes, Ksenija D. Glusac, Charles B. Musgrave
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 spectroscopies, 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 towards 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.


This work is supported in part by NSF grants CHE-1214131 (CBM and CHL), CHE-1112564 (JTH), the ACS PRF grant 54436-ND4 (KDG) and a postdoctoral fellowship from the Arnold and Mabel Beckman Foundation (BTW). CHL is grateful for an NIH F32 Postdoctoral Fellowship (F32GM122392). We gratefully acknowledge use of XSEDE supercomputing resources (NSF ACI-1053575), Ohio Supercomputer Center PCS0201-5 (KDG) and the Janus supercomputer, which is supported by NSF (CNS-0821794) and the University of Colorado Boulder. We greatly appreciate the use of the laboratory facilities of Chris Bowman at the University of Colorado Boulder. We also thank CU Crowdfunding for financial support, especially the following donors: Lim Kee Moi, Lim Lynn Hwee, Cho Teng Kit and Foo Mei Khoon. We thank Marija R. Zoric for her guidance and assistance with the controlled potential electrolysis experiments.


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Lim, C. H., Ilic, S., Alherz, A., Worrell, B. T., Bacon, S. S., Hynes, J. T., . . . Musgrave, C. B. (2019). Benzimidazoles as Metal-Free and Recyclable Hydrides for CO2 Reduction to Formate. Journal of the American Chemical Society, 141(1), 272-280. doi:10.1021/jacs.8b09653


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