Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen

Oxidation of H 2 (1 atm) is catalyzed by the manganese electrocatalysts [(P 2 N 2 )Mn I (CO)(bppm)] + and [(PNP)Mn I (CO)(bppm)] + (P 2 N 2 = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane; PNP = (Ph 2 PCH 2 ) 2 NMe); bppm = (PAr F 2 ) 2 CH 2; Ar F = 3,5-(CF 3 ) 2 C 6 H 3 ). In fluorobenzene solvent using 2,6-lutidine as the exogeneous base, the turnover frequency for [(P 2 N 2 )Mn I (CO)(bppm)] + is 3.5 s –1, with an estimated overpotential of 700 mV. For [(PNP)Mn I (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 [(P 2 N 2 )Mn II H(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 [(P 2 N 2 )Mn II H(CO)(bppm)] + and 21.3 kcal/mol for [(PNP)Mn II H(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 [(P 2 N 2 )Mn II H(CO)(bppm)] + due to a mismatch of 6.6 p K a units) and the relatively long manganese–nitrogen separation in the Mn II H complexes.