Bioinspired Metallopeptoids as Electrocatalysts for Homogeneous Water Oxidation

Bioinspired Metallopeptoids as Electrocatalysts for Homogeneous Water Oxidation

26/06/2023

Seminar room

11:30

Mr. Guilin Ruan Maayan Group

The Laboratory for Organic and Inorganic Chemistry
Final PhD Seminar

Water electrolysis is by far the most promising method for hydrogen evolution due to its precise control, low cost, mild reactive conditions, high efficiency, and safety. However, the high overpotential and slow kinetics of the anodic reaction, specifically water oxidation, have limited its applicability. To overcome these limitations, researchers have turned to nature and drawn inspiration from the oxygen evolution complex (OEC) found in the enzyme photosystem II (PS-II). This has led to the development of enormous molecular catalysts for water oxidation in the last two decades.
Peptoids, N-substituted glycine oligomers, are excellent biomimetic scaffolds for the mimicry of biocatalysts. Peptoids can be efficiently generated by a solid-phase method that employs primary amines and enables the incorporation of various side chains including catalytic groups and metal binding ligands. Combining Co-polypyridine scaffold and the advantages of using peptoids as ancillary ligands, a peptoid trimer incorporating terpyridine and ethanol forms an intermolecular Co(III) complex, Co(TPT)2 is designed and synthesized, which performs as a soluble electrocatalyst for water oxidation with a high turnover frequency of 108 s-1. The ethanolic sidechain facilitates water binding with hydrogen bonding, thus mimicking an enzymatic second coordination sphere. This secondary sphere effect enhances the stability of Co(TPT)2 and decreases the minimum overpotential for water oxidation to only 350 mV. Furthermore, copper (Cu) enzymes, such as monooxygenase and hemocyanin, are known to contain oxygen active species, which give Cu great potential for O-O bond formation, which is a crucial transformation in water oxidation. Recently, several studies suggested that either dinuclear copper (Cu) centers or the use of borate buffer can lead to efficient catalysis. Our group previously demonstrated another remarkable ability of peptoids: the capacity to form self-assembled dicopper complexes. Capitalizing on these features herein a unique Cu-peptoid duplex, Cu2(BEE)2, is designed and is a fast and stable homogeneous electrocatalyst for water oxidation in borate buffer at pH 9.35. Cu2(BEE)2 performs in controlled potential electrolysis with high Faradaic efficiency of 95% and with the highest TOF as reported for Cu-based electrocatalysts, 5503 s-1. Extensive mechanistic studies suggest multiple proton-coupled electron transfer (PCET) steps and the O-O bond formation during catalysis are facilitated by the ethanolic sidechains within the peptoid scaffold, forming intramolecular hydrogen bonds; and by an original cooperativity between the two copper centers and borate as a non-innocent buffer, which acts as an oxygen-atom donor.
These important insights contribute to the understanding of the biomimetic structural-activity relationship of metallopeptoids and the catalyst-buffer cooperativity for electrocatalytic water oxidation. They hold a great potential for the development of futural advanced materials using metallopeptoids with selective buffer as an intact system.