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^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.