Electron transfer reactions in SbC1₃-rich media

The role of SbCl₃-rich solvents with the components SbCl₃s, AICI₃ and N-(l-butyl)pyridinium chloride (BPCl) in the electron transfer reactions of the solutes anthracene (Ant), 10H-phenoxazine (POZ) and 10H-phenothiazine (PTZ) was investigated by using electrochemical, spectroscopic and product recovery and analysis techniques. Mixtures of these ternary solvents were restricted to the composition range (m/o = mol %) 60 m/o SbCl₃, 19-22 m/o AICI₃ and 21-18 m/o BPC1, within which the oxidizing power of Sb(III) and the activities of the Lewis acid SbC1⁺₂(solvated) and Lewis base C1^-(solvated) are strongly composition dependent. Examples are provided of the direct participation of the redox, Lewis acid and Lewis base solvent functionalities in the electron transfer reactions of the solutes.

The ability of Sb(III) to act as an oxidant, i.e., its oxidizing power, was measured as a function of composition by potentiometric titration. The potential of the Sb(III)/Sb couple increased by 0.43 V at 27°C between the compositions 60-19-21 m/o (basic) and 60-22-18 m/o (acidic) SbCl₃-AICl₃-BPC1. Basic melts were not oxidizing with respect to P0Z and PTZ, while in acidic melts these solutes underwent spontaneous and nearly complete oxidations by Sb(III) to radical cations. The reversibility of the Sb(III)/Sb couple was probed by means of current-overvoltage curves, with results which indicated that the oxidation of Sb and the reduction of Sb(III) were kinetically slow processes. The ternary mixtures were exploited as electrochemical solvents with an overall window of 1.8 V for the compositions 60-19-21 m/o and 60-21-19 m/o SbCl₃-AICl₃-BPC1. Density and viscosity measurements for these compositions over the temperature range 24-100°C are also reported.

The involvement of the Lewis base functionality, C1^--(solvated), in reactions initiated by electron transfer was observed for Ant in 60-19-21 m/o SbCl₃-AICl₃-BPC1, SbCl₃-10 m/o BPC1 and SbCl₃-6 m/o KCl. In the ternary liquids, oxidation of Ant electrochemically or by Sb(V) led to the unusually selective formation of 9-chloroanthracene over the range 25-100°C. The reaction involved the nucleophilic attack of Cl^- on Ant radical cations, and the mechanism was deduced to be ECEC where the initial electron transfer was plausibly the rate limiting step. Differences in the rates and product distributions of the Ant chlorination reaction in the binary and ternary SbCl₃-rich media were attributed to differences in chloride activity for the solvents. In 60-21-19 m/o , SbCl₃-AICl₃-BPC1, where the chloride activity is very low. Ant radical cations underwent intermolecular aryl-aryl coupling reactions.

In acidic SbCl₃-AICl₃-BPC1, the electron transfer reactions between the reductants P0Z and PTZ and the oxidants Sb(III) and tris(2,2'-bipyridine-N,N')ruthenium(III) resulted in the formation of POZ and PTZ radical cations. Reactions between the heteroaromatic solutes and Sb(V) in this solvent afforded P0Z and PTZ dications. A technique was developed to monitor these redox reactions in situ by optical spectroscopy. The reactions in acidic SbCl₃-AICl₃-BPC1 were very slow, and proceeded via bonded mechanisms as opposed to a nonbonded mechanism (as defined by the Marcus theory of electron transfer). The evidence strongly suggested that the key kinetic intermediates were moderately stable Lewis acid-base complexes between SbC1⁺₂ (solvated) and the nitrogen atoms of the neutral heteroaromatic solutes. As a preliminary to this study, electrochemistry, spectroelectrochemistry and esr spectroscopy demonstrated that the one electron oxidation products of POZ and PTZ in SbCl₃-AICl₃-BPC1 were stable radical cations. Esr spectroscopy yielded evidence for Lewis acid-base interactions between the solvent and the chalcogen atoms of the heteroaromatic radical cations. Electrochemistry, spectroelectrochemistry and nmr spectroscopy demonstrated that the one electron oxidation products of POZ and PTZ radical cations in acidic SbCl₃-AICl₃-BPC1 were unusually stable dications.