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 (O)L, a mu-1,2-peroxo-dicopper(II) complex [{(O)LCu(II)}(2)(O(2)(2-))](2+) forms from the reaction of the mononuclear compound [Cu(I)((O)L)(MeCN)]B(C(6)F(5))(4) ((O)LCu(I)) 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 (18)O(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(I)((Bz)L)(CH(3)CN)](+) ((Bz)LCu(I)) leads to a dicopper(III)-bis-mu-oxo species [{(Bz)LCu(III)}(2)(mu-O(2-))(2)](2+) at -80 degrees C, and from such solutions, very similar toluene oxygenation chemistry occurs. Ligand (Bz)L is a tridentate chelate, possessing the same moiety found in (O)L, but without the anisole O-atom donor. In these contexts, the nature of the oxidant species in or derived from [{(O)LCu(II)}(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)((Py)L)}(2)(O(2))](2+) ((Py)L = TMPA = tris(2-methylpyridyl)amine). Using (L)Cu(I), CO-binding properties (i.e., nu(C-O) values) along with electrochemical property comparisons, the relative donor abilities of (O)L, (Bz)L, and (Py)L are assessed.