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

Abstract

With an anisole-containing polypyridylamine potential tetradentate ligand ^OL, a μ-1,2-peroxo-dicopper(II) complex [{^OLCu^II}₂(O₂²⁻)]²⁺ forms from the reaction of the mononuclear compound [Cu^I(^OL)(MeCN)]B(C₆F₅)₄(^OLCu^I) 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 k_H/k_D = 7.5 ± 1 for the toluene oxygenation. The O₂-reaction of [Cu^I(^BzL)(CH₃CN)]⁺ (^BzLCu^I) leads to a dicopper(III)-bis-μ-oxo species [{^BzLCu^III}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 [{^OLCu^II}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 [{Cu^II(^PyL)}₂(O₂)]²⁺ (^PyL = TMPA = tris(2-methylpyridyl)amine). Using (L)Cu^I, CO-binding properties (i.e., ν_C−O values) along with electrochemical property comparisons, the relative donor abilities of ^OL, ^BzL, and ^PyL are assessed.