Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen

Abstract

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, and 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 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 [(P2N2)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 [(P2N2)Mn II H(CO)(bppm)] + due to a mismatch of 6.6 pKa units), as well as the relatively long manganese-nitrogen separation in the Mn II H complexes.

Catalytic Activity of Thiolate‐Bridged Diruthenium Complexes Bearing Pendent Ether Moieties in the Oxidation of Molecular Dihydrogen

“…Highly localized deposition and limited reserve of platinum metal have prompted us to develop alternative catalysts. A number of transition metal complexes have been found to work as effective catalysts in the transformation of protons/electrons to molecular dihydrogen,, however, successful examples of the catalytic oxidation of molecular dihydrogen into protons/electrons are still limited …”

“…Highly localized deposition and limitedr eserve of platinum metal have prompted us to develop alternative catalysts.Anumber of transition metal complexes have been found to work as effective catalysts in the transformation of protons/electronst o molecular dihydrogen, [1,2] however,s uccessful examples of the catalytic oxidation of molecular dihydrogen into protons/electrons are stilllimited. [3][4][5][6][7][8][9][10][11] It is well known that hydrogenase enzymes in nature perform the interconversion between molecular dihydrogen and protons/electrons quite rapidly ande fficiently. [12] As ar esult, biomimetic systems of hydrogenase enzymes have been investigated energetically for the last decade.…”

Energy Production and Storage Promoted by Organometallic Complexes

“…[(PNP)Mn I (CO)(bppm)] + requires a higher overpotential of 880 mV for HOR in presence of N ‐methylpyrrolidine as the exogeneous base. The turnover frequency of this reaction is 1.4 s –1 …”

“…Hydrogenases are enzymes that can reversibly oxidize hydrogen with high efficiencies in nature. Inspired by these enzymes Bullock, DuBois and collaborators, developed complexes based on Ni, Fe, and Mn for the electrocatalytic oxidation of molecular hydrogen. All complexes are based on a P R 2 N R′ 2 cyclic 1,5‐diaza‐3,7‐diphosphacyclooctane ligand (R, R′ represent alkyl or aryl groups), that has pendent amines in close proximity to a vacant coordination site on the metal, leading to bifunctional activation of H 2 during the heterolytic cleavage of the H–H bond, mimicking the structure of the [Fe‐Fe] hydrogenase enzymes.…”

“…Structure of the complexes [(P 2 N 2 )Mn I (CO)(bppm)] + ( 1 ) and [(PNP)Mn I (CO)(bppm)] + ( 2 ). Reproduced from ref . with permission (Copyright 2015 American Chemical Society).…”