The design, synthesis, and characterization of binuclear copper(I) complexes and investigations of their dioxygen reactivities are of interest in understanding fundamental aspects of copper/O-2 reactivity and in modeling copper enzyme active-site chemistry. in the latter regard, unsymmetrical binuclear systems so of interest. Here, we describe the chemistry of new unsymmetrical binuclear copper complexes, starting with the binucleating ligand UN2-H, possessing a m-xylyl moiety linking a bis[2-(2-pyridyl)ethyl]amine (PY2) tridentate chelator and a 2-[2-(methylamino)ethyl]pyridine bidentate group. Dicopper(Ij complexes of UN2-H, [Cu-2(UN2-H)(2+) (1), as PF6- and ClO4- salts, are synthesized. These react with O-2 (Cu:O-2 = 2:1, manometry) resulting in the hydroxylation of the xylyl moiety, producing the phenoxohydroxodicopper(II) complex [Cu-2(UN2-O-)(OH-)(CH3CN)](2+) (2). Compound 2(PF6)(2) is characterized by X-ray crystallography, which reveals features similar to those of a structure described previously (Karlin, K. D.; et al. J. Am Chem. Sec. 1984, 106, 2121-2128) for a symmetrical binucleating analogue having two tridentate PY2 moieties; here a CH3CN ligand replaces one pyridylethyl arm. isotope labeling from a reaction of 1 using O-18(2) shows that the ligand UN2-OH, extracted from 2, possesses an O-18-labeled phenol oxygen atom. Thus, the transformation 1 + O-2 --> 2 represents a monooxygenase model system. [Cu-2(I)(UN2-OH)(CH3CN)](2+) (3), a new binuclear dicopper(I) complex with an unsymmetrical coordination environment is generated either by reduction of 2 with diphenylhydrazine or in reactions of cuprous salts with UN2-OH. Complex 3 reacts with O-2 at -80 degreesC, producing the (mu -1,1-hydroperoxo)dicopper(II) complex [Cu-2(II)(UN2-O-)(OOH-)](2+) (4) (lambda (max) 390 nm (epsilon 4200 M-1 cm(-1)), formulated on the basis of the stoichiometry of O-2 uptake by 3 (Cu:O-2 = 2:1, manometry), its reaction with PPh3 giving O=PPh3 (85%), and comparison to previously studied close analogues. Discussions include the relevance and comparison to ether copper bioinorganic chemistry.