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

A mixed-valent Cu(I)Cu(II) complex, [Cu^I,II₂(UN-O^-)]²⁺ (1), reacts with NO_(g) at -80 °C to form [Cu^I,II₂(UN-O^-)(NO)]²⁺ (2), best described as a mixed-valent nitrosyl complex that has a ν(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^II₂(UN-O^-)(NO)(O₂^-)]²⁺ (3), based on its distinctively blue-shifted ν(N-O) band at 1853 cm^-1. Over the course of 20 min at -80 °C, 3 is converted to the peroxynitrite (PN) complex [Cu^II₂(UN-O^-)(^-OON═O)]²⁺ (4), which was characterized by low-temperature electrospray ionization mass spectrometry (ESI-MS) and IR spectroscopy; ν(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^II₂(UN-O^-)(O₂^•-)]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 ^•NO₂ (detected spectrophotometrically) and a putative higher-valent complex, [Cu^II,III₂(UN-O^-)(O^2-)]²⁺, that abstracts a H-atom from DTBP to give [Cu^II₂(UN-O^-)(OH)]²⁺ and a phenoxyl radical. The latter may dimerize to form the bis-phenol observed experimentally or couple with the ^•NO₂ present, leading to o-phenol nitration.