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

TY - JOUR

T1 - Electrocatalytic proton reduction by a cobalt(III) hydride complex with phosphinopyridine PN ligands

AU - Walaijai, Khanittha

AU - Cavill, Stuart Alan

AU - Whitwood, Adrian C.

AU - Douthwaite, Richard E

AU - Perutz, Robin Noel

N1 - © 2020 American Chemical Society. This is an author-produced version of the published paper. Uploaded in accordance with the publisher’s self-archiving policy. Further copying may not be permitted; contact the publisher for details.

PY - 2020/12/4

Y1 - 2020/12/4

N2 - 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]2 1, [CoIII(PN)2(H)(MeCN)][PF6]2 2 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 –1.4 V in the presence of TFA yields H2 with Faradaic yields close to 100%. Foot-of-the-wave analysis allows calculation of kcat values of 860 and 1560 M-1 s-1 for 1 and 2 measured at 0.025 and 0.030 M TFA, respectively. The catalytic performance of 1 and 2 is compared to those of related catalysts by means of Tafel plots. 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.

AB - 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]2 1, [CoIII(PN)2(H)(MeCN)][PF6]2 2 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 –1.4 V in the presence of TFA yields H2 with Faradaic yields close to 100%. Foot-of-the-wave analysis allows calculation of kcat values of 860 and 1560 M-1 s-1 for 1 and 2 measured at 0.025 and 0.030 M TFA, respectively. The catalytic performance of 1 and 2 is compared to those of related catalysts by means of Tafel plots. 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.

U2 - 10.1021/acs.inorgchem.0c02505

DO - 10.1021/acs.inorgchem.0c02505

M3 - Article

SN - 0020-1669

JO - Inorganic Chemistry

JF - Inorganic Chemistry

ER -