A Copper-Peptoid as a Highly Stable, Efficient, and Reusable Homogeneous Water Oxidation Electrocatalyst

Abstract

Water electrolysis is among the simplest methods to generate hydrogen, which can be used as a clean and renewable energy source. Within this process, the oxidation of water into molecular oxygen is considered as the bottleneck reaction because it involves the transfer of four electrons toward the oxidation of a highly stable small molecule. Challenges in this area include the development of stable and effective electro-and photocatalysts that utilize readily available metal ions. Herein we report a copper− peptidomimetic complex as an electrocatalyst for water oxidation, which is both highly stable and efficient. Inspired by enzymatic catalysis, which is largely based on intramolecular cooperativity between a metal center and functional organic molecules located on one scaffold, we have designed and synthesized a peptoid trimer bearing a 2,2′-bipyridine (bipy) ligand, an −OH group, and a benzyl group. Both experimental and computational data reveal that binding of Cu II to this peptoid in aqueous medium occurs via the bipy group and two hydroxyl moieties from the solution. Based on a systematic electrochemical study, we show that this complex is an active electrocatalyst for water oxidation in aqueous phosphate buffer solution enabling oxygen evolution at pH 11.5 with a turnover frequency of 5.8 s −1 and a Faradaic efficiency of up to 91%. Importantly, this catalyst is highly stable over at least 15 h of electrolysis. Thus, we could reuse it for at least 9 times in 40 min electrolysis experiments, demonstrate that it retains its activity in every experiment, and obtain oxygen evolution with an overall turnover number record (based on moles oxygen to moles catalyst) of >56 in 6 h. Moreover, based on electrochemical experiments, spectroscopic data, and density functional theory-D3 calculations, we identified a key peroxo intermediate and propose an intramolecular cooperative catalytic path for this reaction, which suggests that the −OH group has a major role in the high stability of the complex.

Peptoids as tools and sensors

“…B, H‐Nphen‐NTEMPO‐Npm‐NH 2 peptoid trimer used for copper‐mediated alcohol oxidations and imine synthesis . C, H‐Npm‐Nhe‐Nhe(bipy)‐NH 2 peptoid trimer employed as a copper‐centred catalyst for water oxidation…”

“…Ghosh et al . used a similar peptoid trimer format, H‐Npm‐Nhe‐Nhe(bipy)‐NH 2 (Figure C), to reveal a copper(II/III) homogeneous electro‐catalyst for water oxidation.…”

A Resin‐Bound Peptoid as a Recyclable Heterogeneous Catalyst for Oxidation Reactions

“…Peptoids, N‐substituted glycine oligomers, are an important class of peptidomimetic foldamers, with demonstrated utilities such as metal binding and solution catalysis . Peptoids can be efficiently generated on resin by a solid‐phase method (the “submonomer method”, Figure ) from various primary amines, with high sequence specificity, and are chemically inert towards many catalytic transformations.…”

“…Based on these features, we have recently designed and developed a unique bio‐inspired oxidation catalyst, BT , which incorporates two catalytic side‐chains, phenanthroline‐copper and 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO), and one non‐catalytic benzyl group. [7b], [7c] We showed that BT , combined with Cu I and N ‐methyl imidazole (NMI) in an acetonitrile solution, performs in the oxidation of benzylic, allylic and aliphatic primary alcohols with turn‐over‐frequency (TON) of up to 490 (at 0.1 mol‐% catalyst). This TON is 16 times higher than the TON obtained when a mixture of the two catalytic groups, or the peptoid dimer ( DI ) that is lacking the non‐catalytic group were used in the same reaction conditions.…”

“…Our suggested explanation for the differences in reactivity is that the non‐catalytic group constricts the spatial distance between the two catalytic groups leading to the creation of a catalytic pocket that performs via intramolecular cooperativity. [7c] We also demonstrated the higher catalytic activity of BT in the oxidative coupling of alcohols and amines for the production of various imines (TON up to 980 at 0.1 mol‐% catalyst),[7b] compared with the reported catalytic system, in which the two catalytic groups were mixed in solution (TON up to 50) . Following this initial success we thought to further develop BT as a recyclable catalyst capable of performing on‐resin in a heterogeneous system, and test whether it can perform with much higher TON in the oxidation of primary alcohols and the oxidative coupling of alcohols and amines.…”