Manganese-Based Molecular Electrocatalysts for Oxidation of Hydrogen

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• 作者：Elliott B. Hulley ; Neeraj Kumar ; Simone Raugei ; R. Morris Bullock
• 刊名：ACS Catalysis
• 出版年：2015
• 出版时间：November 6, 2015
• 年：2015
• 卷：5
• 期：11
• 页码：6838-6847
• 全文大小：571K
• ISSN：2155-5435

文摘

Oxidation of Hb>2b> (1 atm) is catalyzed by the manganese electrocatalysts [(Pb>2b>Nb>2b>)Mn^I(CO)(bppm)]⁺ and [(PNP)Mn^I(CO)(bppm)]⁺ (Pb>2b>Nb>2b> = 1,5-dibenzyl-3,7-diphenyl-1,5-diaza-3,7-diphosphacyclooctane; PNP = (Phb>2b>PCHb>2b>)b>2b>NMe); bppm = (PAr^Fb>2b>)b>2b>CHb>2b>; Ar^F = 3,5-(CFb>3b>)b>2b>Cb>6b>Hb>3b>). In fluorobenzene solvent using 2,6-lutidine as the exogeneous base, the turnover frequency for [(Pb>2b>Nb>2b>)Mn^I(CO)(bppm)]⁺ is 3.5 s^鈥?, 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^鈥?, 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 [(Pb>2b>Nb>2b>)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 [(Pb>2b>Nb>2b>)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 [(Pb>2b>Nb>2b>)Mn^IIH(CO)(bppm)]⁺ due to a mismatch of 6.6 pKb>ab> units) and the relatively long manganese鈥搉itrogen separation in the Mn^IIH complexes.