A search for blues brothers: X-ray crystallographic/spectroscopic characterization of the tetraarylbenzidine cation radical as a product of aging of solid magic blue

Abstract

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.

A Spherically Shielded Triphenylamine and Its Persistent Radical Cation

“…For detailed synthetic protocols we refer the reader to the Supporting Information. Starting compound 5 is available on a multigram scale by Friedel–Crafts alkylation of triphenylamine and subsequent bromination with N ‐bromosuccinimide . Exhaustive lithium/bromine exchange of 5 with n BuLi at −78 °C and subsequent addition of 9‐fluorenone afforded compound 6 (58 % yield per functional unit).…”

An Electron‐Rich Calix[4]arene‐Based Receptor with Unprecedented Binding Affinity for Nitric Oxide

“…The cation radical of 3 was generated in solution by means of quantitative redox titrations using [ THEO +. SbCl 6 − ] ( THEO =1,2,3,4,5,6,7,8‐octahydro‐9,10‐dimethoxy‐1,4:5,8‐dimethano‐anthracene, E red =0.67 V vs. Fc/Fc + , λ max =518 nm, ϵ max =7300 cm −1 m −1 ) as oxidant.…”

“…A complete consumption of the oxidant (i.e., THEO +. ) after the addition of 1 equiv of neutral 3 , was determined by a quantitative deconvolution of the component spectra of various species present at each titration point. A plot of the mole fractions of THEO +.…”

“…The redox potential of 3 was evaluated by electrochemical oxidation at ap latinume lectrode as a2m m solutioni nd ichloromethane containing 0.1 m nBu 4 N + PF 6 À as the supporting electrolyte, with ferrocene as added internal standard at À30 8C. Figure 3c The cation radicalo f3 was generated in solution by means of quantitative [24,25] redox titrationsu sing [THEO + CSbCl 6 À ] (THEO = 1,2,3,4,5,6,7,8-octahydro-9,10-dimethoxy-1,4:5,8-dimethano-anthracene, E red = 0.67 Vv s. Fc/Fc + , l max = 518 nm, e max = 7300 cm À1 m À1 ) [26,27] as oxidant. For example, Figure 4A shows the electronic spectra obtainedu pon an incremental (sub-stoichiometric) addition of aC H 2 Cl 2 solution of 3 to as olution of [THEO + CSbCl 6 À ]a tÀ30 8C.…”

“…For example, Figure 4A shows the electronic spectra obtainedu pon an incremental (sub-stoichiometric) addition of aC H 2 Cl 2 solution of 3 to as olution of [THEO + CSbCl 6 À ]a tÀ30 8C. Ac omplete consumption of the oxidant (i.e., THEO + C)a fter the addition of 1equiv of neutral 3, was determined by aq uantitative deconvolution [24,25] of the component spectra of various speciesp resent at each titration point.Ap lot of the mole fractionso fTHEO + C and 3 + C against the added equivalents of 3 established a1 :1 stoichiometry of the redox reaction, that is, 3 + THEO + C!3 + C + THEO (Figure 4B).…”