Autocatalytic O2 Cleavage by an OCO3– Trianionic Pincer CrIII Complex: Isolation and Characterization of the Autocatalytic Intermediate [CrIV]2(μ-O) Dimer

Synthetic and kinetic experiments designed to probe the mechanism of O2 activation by the trianionic pincer chromium(III) complex [tBuOCO]CrIII(THF)3 (1) (where tBuOCO = [2,6-(tBuC6H3O)2C6H3]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)...