Cobalt complexes containing pendant amines in the second coordination sphere as electrocatalysts for H2 production

Ming Fang, Eric S. Wiedner, William G. Dougherty, W. Scott Kassel, Tianbiao Liu, Daniel L. Dubois, R. Morris Bullock

Research output: Contribution to journalArticlepeer-review

43 Citations (Scopus)

Abstract

A series of heteroleptic 17e cobalt complexes, [CpCoII(PtBu2NPh2)](BF4), [CpC6F5CoII(PtBu2NPh2)](BF4), and [CpC5F4NCoII(PtBu2NPh2)](BF4) (where PtBu2NPh2 = 1,5-diphenyl-3,7-di-tert-butyl-1,5-diaza-3,7-diphosphacyclooctane, CpC6F5 = C5H4(C6F5), and CpC5F4N = C5H4(C5F4N)) were synthesized, and the structures of all three were determined by X-ray crystallography. Electrochemical studies showed that the CoIII/II couple of [CpC5F4NCoII(PtBu2NPh2)]+ appears 250 mV positive of the CoIII/II couple of [CpCoII(PtBu2NPh2)]+ as a result of the strongly electron withdrawing perfluoropyridyl substituent on the Cp ring. Reduction of these paramagnetic CoII complexes by KC8 led to the diamagnetic 18e complexes CpCoI(PtBu2NPh2), CpC6F5CoI(PtBu2NPh2), and CpC5F4NCoI(PtBu2NPh2), which were also characterized by crystallography. Protonation of these neutral CoI complexes led to the CoIII hydrides [CpCoIII(PtBu2NPh2)H](BF4), [CpC6F5CoIII(PtBu2NPh2)H](BF4), and [CpC5F4NCoIII(PtBu2NPh2)H](BF4), and crystal structures of each of these cobalt hydrides were determined. The cobalt complex with the most electron withdrawing Cp ligand, [CpC5F4NCoII(PtBu2NPh2)]+, is an electrocatalyst for production of H2 using [p-MeOC6H4NH3][BF4] (pKaMeCN = 11.86), with a turnover frequency of 350 s-1 and an overpotential of 0.86 V at Ecat/2. A pKa value of 15.6 was measured in CH3CN for [CpC5F4NCoIII(PtBu2NPh2)H], which was used in conjunction with electrochemical measurements to obtain thermodynamic data for cleavage of the Co-H bond.

Original languageEnglish
Pages (from-to)5820-5833
Number of pages14
JournalOrganometallics
Volume33
Issue number20
DOIs
Publication statusPublished - Oct 27 2014

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

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