Mechanistic investigations of a hydrogen-evolving cobalt diimine-dioxime complex in an oxygen environment: secondary coordination sphere and Bro̷nsted acid Tsai Yu-Syuan 1 , Yu-Wei Chen 1 , Charasee Laddika Dayawansa 1 , Hsuan Chang 1 , Wen-Ching Chen 2 , Jiun-Shian Shen 2 , Tiow-Gan Ong 2 , Glenn P. A. Yap 3 and Vincent C.-C. Wang 1* 1 Department of Chemistry, National Sun Yat-sen University, Kaohsiung, 80424, Taiwan, R.O.C., 2 Institute of Chemistry, Academia Sinica, Taipei, 11529 Taiwan, R.O.C., 3 Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware, 19716 USA Metal complexes are promising electrocatalysts for the hydrogen evolution reaction (HER), but many degrade in the presence of molecular oxygen due to the formation of reactive oxygen species during HER electrocatalysis. Prior studies by Artero’s 1 and Reisner’s group 2 showed that the cobalt diimine-dioxime and related cobaloxime complexes can mediate the HER under ambient condtion. However, its oxygen tolerance remains poorly understood, prompting us to investigate the underlying mechanism of cobalt diimine-dioxime complex. 3 In our research, we found the turnover frequency of the oxygen reduction reaction (ORR) was at least two orders of magnitude higher than that of HER. As a result, HER is almost suppressed by ORR in the presence of molecular oxygen. Further mechanistic investigations suggested that the strength of Brønsted acids affect the proton transfer pathway (i.e. intramolecular proton transfer vs intermolecular proton transfer), thereby controlling the product distribution of ORR (i.e. water vs hydrogen peroxide). Finally, UV–vis spectroscopy studies revealed that hydrogen peroxide induces degradation of the cobalt diimine-dioxime complex, whereas water does not. These findings provide a new strategy to develop oxygen-tolerant electrocatalysts. References 1. N. Kaeffer, A. Morozan and V. Artero, J. Phys. Chem. B. 2015, 119 , 13707–13713. 2. F. Lakadamyali, M. Kato, N. M. Muresan and E. Reisner, Angew. Chem. Int. Ed. 2012, 51 , 1–5. 3. Y.-S. Tsai, Y.-W. Chen, C.L. Dayawansa, H.-Chang, W.-C. Chen, J.-S. Shen, T.-G. Ong, G. P. A. Yap and V.-C.-C. Wang, Inorg. Chem. 2025, 64 , 4213–4222.
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