Toluene and Ethylbenzene Aliphatic C-H Bond Oxidations Initiated by a Dicopper(II)-μ-1,2-Peroxo Complex

With an anisole-containing polypyridylamine potential tetradentate ligand L-O, a mu-1,2-peroxo-dicopper(II) complex [{(LCuII)-L-O)(2)(O-2(2-))](2+) forms from the reaction of the mononuclear compound [Cu-1(L-O)(MeCN)]B(C6F5)(4) ((LCu1)-L-O) with O-2 in noncoordinating solvents at -80 degrees C. Thermal decay of this peroxo complex in the presence of toluene or ethylbenzene leads to rarely seen C-H activation chemistry; benzaldehyde and acetophenone/1-phenylethanol mixtures, respectively, are formed. Experiments with O-18(2) confirm that the oxygen source in the products is molecular O-2 and deuterium labeling experiments indicate k(H)/k(D) = 7.5 +/- 1 for the toluene oxygenation. The O-2-reaction of [Cu-1(L-Bz)(CH3CN)](+) ((LCu1)-L-Bz) leads to a dicopper(III)-bis-mu-oxo species [{(LCuIII)-L-Bz}(2)(mu-O2-)(2)](2+) at -8.0 degrees C, and from such solutions, very similar toluene oxygenation chemistry occurs, Ligand L-Bz is a tridentate chelate, possessing the same moiety found in L-O, but without the anisole O-atom donor. In these contexts, the nature of the oxidant species in or derived from [{(LCuII)-L-O}(2)(O-2(2-))](2+) is discussed and likely mechanisms of reaction initiated by toluene H-atom abstraction chemistry are detailed. To confirm the structural formulations of the dioxygen-adducts, UV-vis and resonance Raman spectroscopic studies have been carried out and these results are reported and compared to previously described systems including [{Cu-II(L-Py)}(2)(O-2)(2+) (L-Py = TMPA = tris(2-methylpyridyl)amine). Using (L)Cu-1, CO-binding properties (i.e., upsilon(C-O) values) along with electrochemical property comparisons, the relative donor abilities of L-O, L-Bz, and L-Py are assessed.