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A Peroxynitrite Dicopper Complex: Formation via Cu–NO and Cu–O2 Intermediates and Reactivity via O–O Cleavage Chemistry
journal contribution
posted on 2016-12-02, 19:36 authored by Rui Cao, Lee Taylor Elrod, Ryan L. Lehane, Eunsuk Kim, Kenneth D. KarlinA mixed-valent
Cu(I)Cu(II) complex, [CuI,II2(UN-O–)]2+ (1), reacts
with NO(g) at −80 °C to form [CuI,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 O2(g) to generate a new intermediate,
best described as a superoxide and nitrosyl adduct, [CuII2(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 [CuII2(UN-O–)(−OONO)]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 [CuII2(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, [CuII,III2(UN-O–)(O2–)]2+, that abstracts a H-atom from DTBP to give [CuII2(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.