Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen

Oxidation of H₂ (1 atm) is catalyzed by the manganese electrocatalysts [(P₂N₂)Mn^I(CO)(bppm)]⁺ and [(PNP)Mn^I(CO)(bppm)]⁺ (P₂N₂ = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane; PNP = (Ph₂PCH₂)₂NMe); bppm = (PAr^F ₂)₂CH₂; Ar^F = 3,5-(CF₃)₂C₆H₃). In fluorobenzene solvent using 2,6-lutidine as the exogeneous base, the turnover frequency for [(P₂N₂)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 [(P₂N₂)Mn^IIH(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)Mn^IIH(CO)(bppm)]⁺ and 21.3 kcal/mol for [(PNP)Mn^IIH(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)Mn^IIH(CO)(bppm)]⁺ due to a mismatch of 6.6 pK_a units) and the relatively long manganese-nitrogen separation in the Mn^IIH complexes.