Elucidating the CO 2 RR mechanism of M(bpy)(CO) 3 X by TRIR (M = Mn, Re) Samir Chattopadhyay 1 , Sudip Barman 2 , Mun Hon Cheah 1 , Reiner Lomoth 1 , Leif Hammarström 1 1 Department of Chemistry - Ångström Laboratories, Uppsala University, Sweden, 2 School of Chemical Sciences, Indian Association for the Cultivation of Science, Kolkata, India Re complex can reduce CO 2 to CO without the need for a Brønsted acid, whereas the Mn counterpart requires an acid source to activate CO 2 . Current renewable energy research is focused on developing molecular CO 2 RR catalysts with 2 nd sphere residues around their active sites, residues that provide H + transfer, electrostatic interaction or H-bonding, and Lewis acid residues for intermediates stabilization. This approach enhances efficiency and product selectivity. To design efficient catalysts for producing formate, CO, CH 4 , and other products from CO 2 , understanding the mechanistic cycle is crucial. Mechanistic proposals based on combination of experimental and computational results have been advanced, 5 but direct experimental evidence for several critical reaction steps and intermediates have been lacking. Re and Mn bipyridine tricarbonyl complexes (M(bpy)(CO) 3 X, M= Re/Mn) are well-known CO 2 RR catalyst. 1-4 The This work has used synthesized intermediates of the catalytic cycle that were reduced by decamethyl cobaltocene after stopped-flow mixing, and studied with rapid-scan FTIR spectroscopy. We observed, for the first time, the reduced Re-tetracarbonyl species, [Re(bpy)(CO) 4 ] 0 , proposed to be common for electro- and photocatalytic cycles, and monitored its CO release (t~55 ms). 6 Additionally, we detected the accumulation of [Re(bpy) (CO) 3 (CH 3 CN)] + as a by-product following product release – a significant side reaction under conditions with a limited supply of reducing equivalents. The process could be unambiguously attributed to an electron transfer- catalyzed ligand substitution reaction (previously proposed 7 ) involving [Re(bpy)(CO) 4 ] 0 by simultaneous real-time detection of all involved species. We believe that this side reaction significantly impacts the CO 2 RR efficiency of this class of catalysts under photochemical conditions or during or electrocatalysis at mild overpotentials. To shift the selectivity of CO 2 RR by Mn(bpy)(CO) 3 X complexes towards formate formation, second-sphere hydroxyl or amine functional groups have been used. However, the direct spectroscopic evidence for the bifurcation pathways leading to CO and HCOOH remained elusive. We identified for the first time the key intermediates in this bifurcation pathway for a Mn complex with second-sphere hydroxyl groups in real time under catalytic conditions. 8 The rate constants align with reported TOF values from electrochemical studies, validating the relevance of the results to electrocatalyticconditions. We showed that HCOOH production involves proton transfer from hydroxyl groups to the doubly reduced Mn center, forming the Mn-hydride intermediate, followed by CO 2 insertion, leading to the Mn-formate intermediate. However, the inability of the resulting phenolate to rebind protons from weak acids like water leads to rapid catalyst degradation. References 1. Hawecker, J.; Lehn, J.-M.; Ziessel, R. J. Chem. Soc., Chem. Commun. 1984 , 6 , 328. 2. Bourrez, M.; Molton,F.; Chardon Noblat,S.; Deronzier, Angew. Chem. Int. Ed . 2011 , 50 , 9903 3. Zhang, S.; Fan, Q.; Xia, R.; Meyer, T. J. Acc. Chem. Res . 2020 , 53 , 255. 4. Amanullah, S.; Saha , P.; Nayek , A.; Ahmed, M. E.; Dey, A. Chem. Soc. Rev ., 2021 , 50 , 3755. 5. Riplinger, C.; Sampson, M. D.; Ritzmann, A. M.; Kubiak, C. P.; Carter, E. A. J. Am. Chem. Soc. 2014 , 136 , 16285
6. Chattopadhyay, S., Cheah, M.H.; Lomoth. R.; Hammarström, L. ACS Catal. 2024 , 14 , 16324. 7. Grice, K. A.; Gu, N. X.; Sampson, M. D.; Kubiak, C. P. Dalton Trans. 2013 , 42 , 8498-8503 8. Chattopadhyay, S., Barman, S.; Lomoth. R.; Hammarström, L. J. Am. Chem. Soc. 2025 , in press
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