Reversible dioxygen binding and arene hydroxylation reactions: Kinetic and thermodynamic studies involving ligand electronic and structural variations.

Copper-dioxygen interactions are of intrinsic importance in a wide range of biological and industrial processes. Here, we present detailed kinetic/thermodynamic studies on the O(2)-binding and arene hydroxylation reactions of a series of xylyl-bridged binuclear copper(I) complexes, where the effects of ligand electronic and structural elements on these reactions are investigated. Ligand 4-pyridyl substituents influence the reversible formation of side-on bound μ-η(2):η(2)-peroxodicopper(II) complexes, with stronger donors leading to more rapid formation and greater thermodynamic stability of product complexes [Cu(II) (2)((R)XYL)(O(2) (2-))](2+). An interaction of the latter with the xylyl π-system is indicated. Subsequent peroxo electrophilic attack on the arene leads to C-H activation and oxygenation with hydroxylated products [Cu(II) (2)((R)XYLO(2-))((-)OH)](2+) being formed. A related unsymmetrical binucleating ligand was also employed. Its corresponding O(2)-adduct [Cu(II) (2)(UN)(O(2) (2-))](2+) is more stable, but primarily because the subsequent decay by hydroxylation is in a relative sense slower. The study emphasizes how ligand electronic effects can and do influence and tune copper(I)-dioxygen complex formation and subsequent reactivity.