Binucleating ligand structural effects on (μ-peroxo)- and bis(μ-oxo)dicopper complex formation and decay: Competition between arene hydroxylation and aliphatic C-H bond activation

The reactivity of dicopper(I) complexes of the ligands α,α'-bis(4,7-diisopropyl-l,4,7-triazacyclononan-l-yl)-/?and m-xylene (p- and m-XYL^iPr4) with dioxygen was examined by spectroscopic and rapid stopped-flow kinetics methods. Only bis(μ-oxo)dicopper(III) core formation was observed with p-XYL^iPr4, but both (μ-η²:η²-peroxo)-dicopper(II) and bis(μ-oxo)dicopper(III) species were generated in the m-XYL^lPr4 case, their relative proportions being dependent on the solvent, concentration of the dicopper(I) precursor, and temperature. Subsequent decomposition under conditions that favored bis(μ-oxo) core formation resulted in oxidative N-dealkylation of isopropyl groups, whereas μ-η²:η²-peroxo decay led to the product resulting from hydroxylation of the bridging arene, [(m-XYL^iPr4-O)Cu₂(μ-OH)](SbF₆)₂. Evidence from kinetics studies, decomposition product analyses, and comparison to the chemistry exhibited by complexes of other substituted 1,4,7-triazacyclonane ligands support a model for the oxygenation of the m-XYL^iPr4 compound involving initial, essentially rate-limiting 1:1 Cu:O₂ adduct formation followed by partitioning between intra- and intermolecular pathways. At low temperature and high starting material concentrations, the latter route that yields tetranuclear "dimer-of-dimer" species and/or higher order oligomers with bis(μ-oxo) cores is favored, while at higher temperatures and dilution, intramolecular reaction predominates to afford a (peroxo)dicopper(II) species. The course of the subsequent decompositions of these oxygenated products correlates with their proposed formulations. Thus, analysis of final products and kinetics data, including with selectively deuterated compounds, showed that N-dealkylation arises from the high-nuclearity bis(μ-oxo) species and arene hydroxylation occurs upon decay of the intramolecular peroxo complex. Geometric rationales for the divergent oxygenation and decomposition reactions supported by p- and m-XYL^iPr4 are proposed.