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Electrocatalytic proton reduction by a cobalt(III) hydride complex with phosphinopyridine PN ligands

DOI: 10.1021/acs.inorgchem.0c02505 DOI Help

Authors: Khanittha Walaijai (University of York) , Stuart A. Cavill (University of York; Diamond Light Source) , Adrian C. Whitwood (University of York) , Richard E. Douthwaite (University of York) , Robin N. Perutz (University of York)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Inorganic Chemistry

State: Published (Approved)
Published: December 2020

Abstract: Cobalt complexes with 2-(diisopropylphosphinomethyl)pyridine (PN) ligands have been synthesized with the aim of demonstrating electrocatalytic proton reduction to dihydrogen with a well-defined hydride complex of an Earth-abundant metal. Reactions of simple cobalt precursors with 2-(diisopropylphosphino-methyl)pyridine (PN) yield [CoII(PN)2(MeCN)][BF4]21, [CoIII(PN)2(H)(MeCN)][PF6]22, and [CoIII(PN)2(H)(Cl)][PF6] 3. Complexes 1 and 3 have been characterized crystallographically. Unusually for a bidentate PN ligand, all three exhibit geometries with mutually trans phosphorus and nitrogen ligands. Complex 1 exhibits a distorted square-pyramidal geometry with an axial MeCN ligand in a low-spin electronic state. In complexes 2 and 3, the PN ligands lie in a plane leaving the hydride trans to MeCN or chloride, respectively. The redox behavior of the three complexes has been studied by cyclic voltammetry at variable scan rates and by spectroelectrochemistry. A catalytic wave is observed in the presence of trifluoroacetic acid (TFA) at an applied potential close to the Co(II/I) couple of 1. Bulk electrolysis of 1, 2, or 3 at a potential of ca. −1.4 V vs E(Fc+/Fc) in the presence of TFA yields H2 with Faradaic yields close to 100%. A catalytic mechanism is proposed in which the pyridine moiety of a PN ligand acts as a pendant proton donor following opening of the chelate ring. Additional mechanisms may also operate, especially in the presence of high acid concentration where speciation changes.

Journal Keywords: Anions; Ligands; Crystal structure; Electrodes; Cobalt

Subject Areas: Chemistry

Instruments: I10-Beamline for Advanced Dichroism