Autocatalytic O₂ cleavage by an OCO^3- trianionic pincer Cr^III complex: Isolation and characterization of the autocatalytic intermediate [Cr^IV]₂(μ-O) dimer

Synthetic and kinetic experiments designed to probe the mechanism of O ₂ activation by the trianionic pincer chromium(III) complex [ ^tBuOCO]Cr^III(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]Cr^V(O)(THF) (2), catalyzes the oxidation of 1 via formation of the μ-O dimer {[^tBuOCO]Cr^IV(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 Cr^IV-O-Cr^IV 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 g_iso = 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]Cr^III(OPPh₃)₂ (4).