Redox-inactive metal single-site molecular complexes: a new generation of electrocatalysts for oxygen evolution?. Issue 19 (19th August 2021)

• Record Type: Journal Article
• Authors: Benkó, Tímea; Lukács, Dávid; Frey, Krisztina; Németh, Miklós; Móricz, Márta M.; Liu, Dongyu; Kováts, Éva; May, Nóra V.; Vayssieres, Lionel; Li, Mingtao; Pap, József S.
• Is Part Of: Catalysis science & technology. Volume 11:Issue 19(2021)
• Page Start: 6411
• Page End: 6424
• Publication Date: 2021-08-19
• Subjects: Catalysis -- Periodicals; 541.395
• DOI: 10.1039/d1cy01087e
• Languages: English
• ISSNs: 2044-4753

Abstract

Abstract : Bypassing the metal-based oxidation in a Cu-containing water oxidation catalytic system. Abstract : A molecular pre-catalyst complex, [Cu II (indH)(OClO3 )(NCCH3 )](ClO4 )·CH3 CN (1 ·CH3 CN) with the 3N pincer ligand 1, 3- bis (2′-pyridyl)iminoisoindoline (indH) was immobilized on indium tin oxide (ITO) transparent conducting substrate to generate O2 electrocatalytically for over 20 hours at pH 10 in a carbonated buffer, reaching a turnover of 139 with no signs of CuO x /Cu(OH)2 formation at the surface. Further electrolysis experiments revealed that the catalyst was present in the aqueous phase, despite the poor initial solubility of the pre-catalyst (1 ). In order to identify the actual form responsible for this important catalytic reaction, the aquo complex [Cu II (ind)(OClO3 )(OH2 )]·CH3 OH (2 ·CH3 OH) was structurally characterized. Spectroscopic investigations of a solid isolated from the buffer used in the electrolysis reaction and solution equilibrium studies using 2 indicated that the [Cu II (ind)(OH)] form occurs at pH 10. Electron paramagnetic resonance (EPR) spectroscopy and DFT calculations confirmed a distorted {3N, O}eq coordination plane in solution, as found in 2 . The buffer ( i.e. bicarbonate/carbonate) may affect reactivity in two ways: as an external base facilitating the proton-coupled electron transfer steps; and/or displacing the inner-sphere solvent molecules from the favourable quasi-equatorial position, thus inhibiting the catalysis. Structural features of a tri-nuclear cluster [Cu II 3 (ind)3 (μ3 -CO3 )(CH3 OH)(OClO3 )] (3 ) isolated under basic conditions confirmed that beside acting as an external base, the inhibiting effect of carbonate anions may also play a role. In acetonitrile-water solutions, where both 1 and 2 exhibit reasonable solubility, experimental findings supported by DFT calculations suggest that it is the ind − ligand which is being oxidized while the cupric ion remains redox-inactive which is very unusual yet of great significance for the creation of a new generation of low-cost Cu-based water oxidation catalysts as well as potentially other 1st row transition metals.