[Ni(PPh 2NC6H4X 2) 2]2+ Complexes as electrocatalysts for H2 production: Effect of substituents, acids, and water on catalytic rates

Uriah J. Kilgore, John A.S. Roberts, Douglas H. Pool, Aaron M. Appel, Michael P. Stewart, M. Rakowski Dubois, William G. Dougherty, W. Scott Kassel, R. Morris Bullock, Daniel L. Dubois

Research output: Contribution to journalArticle

271 Citations (Scopus)

Abstract

A series of mononuclear nickel(II) bis(diphosphine) complexes [Ni(P Ph2NC6H4X2)2](BF 4)2 (PPh2NC6H4X 2 = 1,5-di(para-X-phenyl)-3,7-diphenyl-1,5-diaza-3,7- diphosphacyclooctane; X = OMe, Me, CH2P(O)(OEt)2, Br, and CF3) have been synthesized and characterized. X-ray diffraction studies reveal that [Ni(PPh2NC6H4Me 2)2](BF4)2 and [Ni(P Ph2NC6H4OMe2)2](BF 4)2 are tetracoordinate with distorted square planar geometries. The Ni(II/I) and Ni(I/0) redox couples of each complex are electrochemically reversible in acetonitrile with potentials that are increasingly cathodic as the electron-donating character of X is increased. Each of these complexes is an efficient electrocatalyst for hydrogen production at the potential of the Ni(II/I) couple. The catalytic rates generally increase as the electron-donating character of X is decreased, and this electronic effect results in the favorable but unusual situation of obtaining higher catalytic rates as overpotentials are decreased. Catalytic studies using acids with a range of pKa values reveal that turnover frequencies do not correlate with substrate acid pKa values but are highly dependent on the acid structure, with this effect being related to substrate size. Addition of water is shown to dramatically increase catalytic rates for all catalysts. With [Ni(PPh2NC6H4CH2P(O)(OEt)22) 2](BF4)2 using [(DMF)H]+OTf - as the acid and with added water, a turnover frequency of 1850 s-1 was obtained.

Original languageEnglish
Pages (from-to)5861-5872
Number of pages12
JournalJournal of the American Chemical Society
Volume133
Issue number15
DOIs
Publication statusPublished - Apr 20 2011

ASJC Scopus subject areas

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

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