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

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Abstract
With an anisole-containing polypyridylamine potential tetradentate ligand OL, a μ-1,2-peroxo-dicopper(II) complex [{OLCuII}2(O22−)]2+ forms from the reaction of the mononuclear compound [CuI(OL)(MeCN)]B(C6F5)4(OLCuI) with O2 in noncoordinating solvents at −80 °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 18O2 confirm that the oxygen source in the products is molecular O2 and deuterium labeling experiments indicate kH/kD = 7.5 ± 1 for the toluene oxygenation. The O2-reaction of [CuI(BzL)(CH3CN)]+ (BzLCuI) leads to a dicopper(III)-bis-μ-oxo species [{BzLCuIII}2(μ-O2−)2]2+ at −80 °C, and from such solutions, very similar toluene oxygenation chemistry occurs. Ligand BzL is a tridentate chelate, possessing the same moiety found in OL, but without the anisole O-atom donor. In these contexts, the nature of the oxidant species in or derived from [{OLCuII}2(O22−)]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 [{CuII(PyL)}2(O2)]2+ (PyL = TMPA = tris(2-methylpyridyl)amine). Using (L)CuI, CO-binding properties (i.e., νC−O values) along with electrochemical property comparisons, the relative donor abilities of OL, BzL, and PyL are assessed.

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