Water Oxidation by the [Co4O4(OAc)4(py)4]+ Cubium is Initiated by OH- Addition

Paul F. Smith, Liam Hunt, Anders B. Laursen, Viral Sagar, Shivam Kaushik, Karin U D Calvinho, Gabriele Marotta, Edoardo Mosconi, Filippo De Angelis, G Charles Dismukes

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Abstract

The cobalt cubium Co4O4(OAc)4(py)4(ClO4) (1A+) containing the mixed valence [Co4O4]5+ core is shown by multiple spectroscopic methods to react with hydroxide (OH-) but not with water molecules to produce O2. The yield of reaction products is stoichiometric (>99.5%): 41A+ + 4OH- O2 + 2H2O + 41A. By contrast, the structurally homologous cubium Co4O4(trans-OAc)2(bpy)4(ClO4)3, 1B(ClO4)3, produces no O2. EPR/NMR spectroscopies show clean conversion to cubane 1A during O2 evolution with no Co2+ or Co3O4 side products. Mass spectrometry of the reaction between isotopically labeled 16O(bridging-oxo) 1A+ and 18O-bicarbonate/water shows (1) no exchange of 18O into the bridging oxos of 1A+, and (2) 36O2 is the major product, thus requiring two OH- in the reactive intermediate. DFT calculations of solvated intermediates suggest that addition of two OH- to 1A+ via OH- insertion into Co-OAc bonds is energetically favored, followed by outer-sphere oxidation to intermediate [1A(OH)2]0. The absence of O2 production by cubium 1B3+ indicates the reactive intermediate derived from 1A+ requires gem-1,1-dihydoxo stereochemistry to perform O-O bond formation. Outer-sphere oxidation of this intermediate by 2 equiv of 1A+ accounts for the final stoichiometry. Collectively, these results and recent literature (Faraday Discuss., doi:10.1039/C5FD00076A and J. Am. Chem. Soc. 2015, 137, 12865-12872) validate the [Co4O4]4+/5+ cubane core as an intrinsic catalyst for oxidation of hydroxide by an inner-sphere mechanism.

Original languageEnglish
Pages (from-to)15460-15468
Number of pages9
JournalJournal of the American Chemical Society
Volume137
Issue number49
DOIs
Publication statusPublished - Dec 16 2015

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ASJC Scopus subject areas

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

Cite this

Smith, P. F., Hunt, L., Laursen, A. B., Sagar, V., Kaushik, S., Calvinho, K. U. D., Marotta, G., Mosconi, E., De Angelis, F., & Dismukes, G. C. (2015). Water Oxidation by the [Co4O4(OAc)4(py)4]+ Cubium is Initiated by OH- Addition. Journal of the American Chemical Society, 137(49), 15460-15468. https://doi.org/10.1021/jacs.5b09152