Two pathways for electrocatalytic oxidation of hydrogen by a nickel bis(diphosphine) complex with pendant amines in the second coordination sphere

Jenny Y. Yang, Stuart E. Smith, Tianbiao Liu, William G. Dougherty, Wesley A. Hoffert, W. Scott Kassel, M. Rakowski Dubois, Daniel L. Dubois, R. Morris Bullock

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81 Citations (Scopus)

Abstract

A nickel bis(diphosphine) complex containing pendant amines in the second coordination sphere, [Ni(PCy2Nt-Bu 2)2](BF4)2 (PCy 2Nt-Bu2 = 1,5-di(tert-butyl)-3,7-dicyclohexyl- 1,5-diaza-3,7-diphosphacyclooctane), is an electrocatalyst for hydrogen oxidation. The addition of hydrogen to the NiII complex gives three isomers of the doubly protonated Ni0 complex [Ni(PCy 2Nt-Bu2H)2](BF4) 2. Using the pKa values and NiII/I and Ni I/0 redox potentials in a thermochemical cycle, the free energy of hydrogen addition to [Ni(PCy2Nt-Bu 2)2]2+ was determined to be -7.9 kcal mol -1. The catalytic rate observed in dry acetonitrile for the oxidation of H2 depends on base size, with larger bases (NEt3, t-BuNH2) resulting in much slower catalysis than n-BuNH2. The addition of water accelerates the rate of catalysis by facilitating deprotonation of the hydrogen addition product before oxidation, especially for the larger bases NEt3 and t-BuNH2. This catalytic pathway, where deprotonation occurs prior to oxidation, leads to an overpotential that is 0.38 V lower compared to the pathway where oxidation precedes proton movement. Under the optimal conditions of 1.0 atm H2 using n-BuNH2 as a base and with added water, a turnover frequency of 58 s-1 is observed at 23 C.

Original languageEnglish
Pages (from-to)9700-9712
Number of pages13
JournalJournal of the American Chemical Society
Volume135
Issue number26
DOIs
Publication statusPublished - Jul 3 2013

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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