Redox-inactive metal single-site molecular complexes: a new generation of electrocatalysts for oxygen evolution?

• Published in: Catalysis science & technology 2021-10, Vol.11 (19), p.6411-6424
• Main 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
• Format: Article
• Language: English
• Subjects: Acetonitrile; Bicarbonates; Buffers; Carbonation; Catalysis; Catalysts; Chemical evolution; Coordination compounds; Cupric ions; Electrocatalysts; Electrolysis; Electron paramagnetic resonance; Electron transfer; Indium tin oxides; Ligands; Mathematical analysis; Oxidation; Solubility; Substrates; Transition metals
• ISSN: 2044-4753; 2044-4761
• DOI: 10.1039/d1cy01087e

A molecular pre-catalyst complex, [Cu II (indH)(OClO 3 )(NCCH 3 )](ClO 4 )·CH 3 CN ( 1 ·CH 3 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 O 2 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)(OClO 3 )(OH 2 )]·CH 3 OH ( 2 ·CH 3 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 (μ 3 -CO 3 )(CH 3 OH)(OClO 3 )] ( 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. Bypassing the metal-based oxidation in a Cu-containing water oxidation catalytic system.