Autocatalytic O2 cleavage by an OCO3- trianionic pincer CrIII complex: Isolation and characterization of the autocatalytic intermediate [CrIV]2(μ-O) dimer

Matthew E. O'Reilly, Trevor J. Del Castillo, Joseph M. Falkowski, Vasanth Ramachandran, Mekhala Pati, Marie C. Correia, Khalil A. Abboud, Naresh S. Dalal, David E. Richardson, Adam S. Veige

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33 Scopus citations


Synthetic and kinetic experiments designed to probe the mechanism of O 2 activation by the trianionic pincer chromium(III) complex [ tBuOCO]CrIII(THF)3 (1) (where tBuOCO = [2,6-(tBuC6H3O)2C 6H3]3-, THF = tetrahydrofuran) are described. Whereas analogous porphyrin and corrole oxidation catalysts can become inactive toward O2 activation upon dimerization (forming a μ-oxo species) or product inhibition, complex 1 becomes more active toward O2 activation when dimerized. The product from O2 activation, [ tBuOCO]CrV(O)(THF) (2), catalyzes the oxidation of 1 via formation of the μ-O dimer {[tBuOCO]CrIV(THF)} 2(μ-O) (3). Complex 3 exists in equilibrium with 1 and 2 and thus could not be isolated in pure form. However, single crystals of 3 and 1 co-deposit, and the molecular stucture of 3 was determined using single-crystal X-ray crystallography methods. Variable (9.5, 35, and 240 GHz) frequency electron paramagnetic resonance spectroscopy supports the assignment of complex 3 as a CrIV-O-CrIV dimer, with a high (S = 2) spin ground state, based on detailed computer simulations. Complex 3 is the first conclusively assigned example of a complex containing a Cr(IV) dimer; its spin Hamiltonian parameters are giso = 1.976, D = 2400 G, and E = 750 G. The reaction of 1 with O2 was monitored by UV-visible spectrophotometry, and the kinetic orders of the reagents were determined. The reaction does not exhibit first-order behavior with respect to the concentrations of complex 1 and O2. Altering the THF concentration reveals an inverse order behavior in THF. A proposed autocatalytic mechanism, with 3 as the key intermediate, was employed in numerical simulations of concentration versus time decay plots, and the individual rate constants were calculated. The simulations agree well with the experimental observations. The acceleration is not unique to 2; for example, the presence of OPPh3 accelerates O2 activation by forming the five-coordinate complex trans-[tBuOCO]CrIII(OPPh3)2 (4).

Original languageEnglish (US)
Pages (from-to)13661-13673
Number of pages13
JournalJournal of the American Chemical Society
Issue number34
StatePublished - Aug 31 2011
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Catalysis
  • General Chemistry
  • Biochemistry
  • Colloid and Surface Chemistry


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