A Peroxynitrite Dicopper Complex: Formation via Cu-NO and Cu-O2 Intermediates and Reactivity via O-O Cleavage Chemistry

A mixed-valent Cu(I)Cu(II) complex, [Cu-2(I,II)-(UN-O-)](2+) (1), reacts with NO(g) at -80 degrees C to form [Cu-2(I,II)(UN-O-)(NO)](2+) (2), best described as a mixed-valent nitrosyl complex that has a nu(N-O) band at 1670 cm(-1) in its infrared (IR) spectrum. Complex 2 undertakes a one-electron oxidation via the addition of O-2(g) to generate a new intermediate, best described as a superoxide and nitrosyl adduct, [Cu-2(II)(UN-O-)(NO)(O-2(-))](2+) (3), based on its distinctively blue-shifted nu(N-O) band at 1853 cm(-1). Over the course of 20 min at -80 degrees C, 3 is converted to the peroxynitrite (PN) complex [Cu-2(II)(UN-O-)(-OON=O)](2+) (4), which was characterized by low-temperature electrospray ionization mass spectrometry (ESI-MS) and IR spectroscopy; nu(N-O) absorptions at 1520 and 1640 cm(-1) have been assigned as cis- and trans-conformers of the PN ligand in 4. Alternatively, the superoxide complex [Cu-2(II)(UN-O-)(O-2(center dot-))](2+) (5) is found to react with NO(g) to generate the same intermediate superoxide and nitrosyl adduct 3 (based on IR criteria), which likewise converts to the same PN complex 4. The O-O bond in 4 undergoes heterolysis in dichloromethane solvent and is postulated to produce nitronium ion, leading to ortho-nitration of 2,4-di-tert-butylphenol (DTBP). However, in 2-methyltetrahydrofuran as solvent, the O-O bond undergoes homolysis to generate center dot NO2 (detected spectrophotometrically) and a putative higher-valent complex, [Cu-2(II,III)(UN-O-)(O2-)](2+), that abstracts a H-atom from DTBP to give [Cu-2(II)(UN-O-)(OH)](2+) and a phenoxyl radical. The latter may dimerize to form the bis-phenol observed experimentally or couple with the center dot NO2 present, leading to o-phenol nitration.