Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen

Abstract

Oxidation of H2 (1 atm) is catalyzed by the manganese electrocatalysts [(P2N2)MnI(CO)(bppm)]+ and [(PNP)MnI(CO)(bppm)]+ (P2N2 = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane; PNP = (Ph2PCH2)2NMe); bppm = (PArF2)2CH2; ArF = 3,5-(CF3)2C6H3). In fluorobenzene solvent using 2,6-lutidine as the exogeneous base, the turnover frequency for [(P2N2)MnI(CO)(bppm)]+ is 3.5 s–1, with an estimated overpotential of 700 mV. For [(PNP)MnI(CO)(bppm)]+ in fluorobenzene solvent using N-methylpyrrolidine as the exogeneous base, the turnover frequency is 1.4 s–1, with an estimated overpotential of 880 mV. Density functional theory calculations suggest that the slow step in the catalytic cycle is proton transfer from the oxidized 17-electron manganese hydride [(P2N2)MnIIH(CO)(bppm)]+ to the pendant amine. The computed activation barrier for intramolecular proton transfer from the metal to the pendant amine is 20.4 kcal/mol for [(P2N2)MnIIH(CO)(bppm)]+ and 21.3 kcal/mol for [(PNP)MnIIH(CO)(bppm)]+. The high barrier appears to result from both the unfavorability of the metal to nitrogen proton transfer (thermodynamically uphill by 9 kcal/mol for [(P2N2)MnIIH(CO)(bppm)]+ due to a mismatch of 6.6 pKa units) and the relatively long manganese–nitrogen separation in the MnIIH complexes.