Search for Blues Brothers: X-ray Crystallographic/Spectroscopic Characterization of Tetraarylbenzidine Cation Radical as a Product of Aging of Solid Magic Blue

摘要

Magic blue (>MB^+• SbCl6⁻ salt), i.e. tris-4-bromophenylamminium cation radical, is a routinely employed one-electron oxidant that slowly decomposes in solid state upon storage to form so called ‘blues brothers’, which often complicate the quantitative analyses of the oxidation processes. Herein, we disclose the identity of main ‘blues brother’ as the cation radical and dication of tetrakis-(4-bromophenyl)benzidine (>TAB) by a combined DFT and experimental approach, including isolation of >TAB^+• SbCl6⁻ and its X-ray crystallography characterization. The formation of >TAB in aged magic blue samples occurs by a Scholl-type coupling of a pair of >MB followed by a loss of molecular bromine. This recognition led us to rational design and synthesis of tris(2-bromo-4-tert-butylphenyl)amine, referred to as ‘blues cousin’, (>BC: Eox1 = 0.78 V vs Fc/Fc⁺, λmax(>BC^+•) = 805 nm, εmax = 9930 cm⁻¹ M⁻¹), whose oxidative dimerization is significantly hampered by positioning the sterically demanding tert-butyl groups at the para-positions of aryl rings. A ready two-step synthesis of >BC from triphenylamine and the high stability of its cation radical (>BC^+•) promises that >BC will serve as a ready replacement for >MB and oxidant of choice for mechanistic investigations of one-electron transfer processes in organic, inorganic, and organometallic transformations.