Re(I)-based imidazole-pyridyl complexes as photocatalysts for CO 2 reduction Marcos E. G. Carmo 1 , Gabrielly Moreira 1 , Vanessa F. Silva 1 , Jaqueline C. Desordi 2,3 , Pablo J. Gonçalves 2,3 , Antonio E. H. Machado 1,4 and Antonio O. T. Patrocinio 1, 2 1 Laboratory of Photochemistry and Materials Science, Institute of Chemistry, Federal University of Uberlândia, Uberlândia, MG, Brazil, 2 Centro de Excelência em Hidrogênio e Tecnologias Energéticas Sustentáveis (CEHTES), Goiânia, GO, Brazil, 3 Instituto de Física, Universidade Federal de Goiás, Goiânia, Brazil, 4 Programa de Doutorado em Ciências Exatas e Tecnológicas, Universidade Federal de Catalão – UFCat, Catalão 75704-020, Goiás, Brazil E-mail: otaviopatrocinio@ufu.br Re(I) tricarbonyl complexes have been extensively investigated as electron/photocatalysts for CO 2 reduction [1-3] . Since the first report by Lehn et. al . [1] , different studies have been carried out aiming at detailed investigation on the role of the polypyridyl ligand on the catalytic processes. Following CO 2 coordination to the Re center after reduction, proton transfer yields a Re-CO 2 H intermediate. The presence of a proton relay close to the metal center could then improve the reaction rates as observed for other homogeneous catalysts [4] . Re(I)-based imidazole-pyridyl ligands can combine interesting excited state and redox properties along with the ability to work as proton relay [5,6] .In this work, two Re(I) tricarbonyl complexes with imidazole-pyridine ligands, fac-[Re(CO) 3 (pbiH)Cl], (Re-pbiH), and fac-[Re(CO) 3 (bbzp)Cl] (Re-bbzp), where pbiH= 2-(2-pyridy)benzimidazole, bbzp = 2,6-bis(2-benzimidazolyl)pyridine, were synthesized and their photophysical, electrochemical and photochemical properties investigated aiming at CO 2 photoreduction. The complexes exhibit similar absorption features with intense bands on the UV region due to intraligand and low-lying metal-to-ligand charge transfer transitions, Fig. 1a. The additional benzimidazole group in Re-bbzp stablizes the lowest-lying triplet state, as evidenced by emission measurements at 298 K and 77K, Fig. 1b, Irradiation of Re-bbzp in CO 2 -saturated CH 3 CN with 1,3-dimethyl-2-phenyl-2,3-dihydro1H-benzo[d]imidazole (BIH) as a sacrificial electron donor leads to CO evolution with a turnover number (TON CO ) of 45 ±1, while Re-pbiH shows lower activity (TON CO = 8 ± 1), Fig. 1c. The superior performance of Re-bbzp was investigated in detail using IR-spectroelectrochemistry (IR-SEC) and in-situ IR experiments of the photocatalytic reactions. The IR-SEC and in-situ IR studies evidenced the formation of both one-electron and two-electron reduced species during CO 2 reduction. However, the superior photocatalyst activity of Re-bbzp can be rationalized by its ability to function as both two-electron and one-proton reservoir, stabilizing key reaction intermediates in the photocatalytic cycle as shown in the in-situ IR studies and corroborated by quenching experiments between Re-bbzp and BIH monitored by steady-state emission and emission lifetime measurements. The results provide new insights into the development of imidazole-pyridine- based Re(I) complexes for CO 2 reduction, with the protonation of the pendant benzimidazole group in Re-bbzp as a key property for its improved performance.
Figure 1. (A)Absorption spectra of the RepbiH (orange) and Rebbzp (blue) in CH 3 CN at 298 K (B) emission spectra at 298 K ( ___ ) and Prop/But (5:4) at 77 K (---); l exc = 365 nm. (C) CO evolution following irradiation of the different photocatalysts in CH 3 CN(BIH concentration = 6.7 mmol L −1 ; l > 370 nm). References 1. Hawecker, J et al. Chem. Comm. 1983, 9 , 536-538. 2. Kuramochi, Y.; Ishitani, O. I., Hitoshi. Coord. Chem. Rev. 2018, 373 , 333-356. 3. Müller, A. V. et al. ACS Catal. 2022, 13 (1), 633-646.
4. Qiu, L. Q. et al. Chem. Eur. J. 2021, 27 (62), 15536-15544. 5. Wilting, A. et al. Inorg. Chem. 2017, 56 (7), 4176-4185. 6. Wilting, A.; Siewert, I. Chemistry Select 2018, 3 (17), 4593-4597.
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