Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen

Elliott B. Hulley, Neeraj Kumar, Simone Raugei, R Morris Bullock

Research output: Contribution to journalArticle

23 Citations (Scopus)

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 = (PArF 2)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.

Original languageEnglish
Pages (from-to)6838-6847
Number of pages10
JournalACS Catalysis
Volume5
Issue number11
DOIs
Publication statusPublished - Nov 6 2015

Fingerprint

Proton transfer
Electrocatalysts
Carbon Monoxide
Manganese
Hydrogen
Oxidation
Amines
Nitrogen
Fluorobenzenes
Metals
Hydrides
Density functional theory
Chemical activation
Electrons

Keywords

  • electrocatalysis
  • hydrogen
  • manganese
  • oxidation
  • proton transfer
  • quantum chemistry

ASJC Scopus subject areas

  • Catalysis

Cite this

Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen. / Hulley, Elliott B.; Kumar, Neeraj; Raugei, Simone; Bullock, R Morris.

In: ACS Catalysis, Vol. 5, No. 11, 06.11.2015, p. 6838-6847.

Research output: Contribution to journalArticle

Hulley, Elliott B. ; Kumar, Neeraj ; Raugei, Simone ; Bullock, R Morris. / Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen. In: ACS Catalysis. 2015 ; Vol. 5, No. 11. pp. 6838-6847.
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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 = (PArF 2)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.",
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T1 - Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen

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N2 - 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 = (PArF 2)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.

AB - 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 = (PArF 2)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.

KW - electrocatalysis

KW - hydrogen

KW - manganese

KW - oxidation

KW - proton transfer

KW - quantum chemistry

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