Structure–Function Relationship within Cu-Peptoid Electrocatalysts for Water Oxidation

Abstract

Water oxidation (WO) is the first step in the water-splitting process aiming at the production of hydrogen as a green renewable fuel. To successfully perform WO, potent strategies for overcoming the high energetic barrier and slow kinetics of this reaction are urgently required. One such strategy is the use of molecular catalysis. Specifically, Cu-based catalysts have been highlighted over the last decade due to their stability and fast kinetics. Among them, Cu-peptoids, where peptoids are peptidomimetics akin to peptides and are N-substituted glycine oligomers, can act as stable and active catalysts for oxidation transformations including electrocatalytic WO. Previously, we suggested that a benzyl group incorporated as a side chain near the catalytic site within a Cu-peptoid electrocatalyst for WO has a structural role in the activity of the electrocatalyst in phosphate buffer (PBS). Herein, we aimed to test this hypothesis and understand how an incorporated structural element side chain affects WO. To this aim, we prepared a set of peptoid trimers each with a different structural element replacing the benzyl group by either naphthyl, cyclohexyl, benzyl, propyl chloride, or propyl side chains as well as a peptoid lacking a structural element. We studied the structure of their Cu complexes and tested these complexes as electrocatalysts for WO. We discovered that while all the peptoids self-assemble to form dinuclear Cu-peptoid complexes, the duplex that has no structural side chain, Cu2(BE)2, is structurally different from the others in the solid state. Moreover, Cu2(BE)2 remains dinuclear in a PBS at pH 11, while all the other duplexes are mononuclear in the PBS. Finally, though most of the complexes showed low electrocatalytic activity for WO, the dinuclear complex Cu2(BE)2 performed with the highest turnover frequency of 484 s−1. Nevertheless, this dinuclear complex slowly decomposes to the corresponding mononuclear complex as a more stable species during WO, while the other mononuclear complexes retain their structure in solution but display much slower kinetics (ca. 5 to 8 s−1) under the same conditions. Overall, our results demonstrate that bulkier side chains hamper the stability of dinuclear Cu-peptoids in a PBS, and hence, their efficiency as WO electrocatalysts is also hampered.