Exploring the fate of the tris(pentafluorophenyl)borane radical anion in weakly coordinating solvents

Abstract

We report a kinetic and mechanistic study into the one-electron reduction of the archetypal Lewis acid tris(pentafluorophenyl)borane, B(C₆F₅)₃, in dichloromethane and 1,2-difluorobenzene. Electrochemical experiments, combined with digital simulations, DFT computational studies and multinuclear NMR analysis allow us to obtain thermodynamic, kinetic and mechanistic information relating to the redox activity of B(C₆F₅)₃. We show that tris(pentafluorophenyl)borane undergoes a quasi-reversible one-electron reduction followed by rapid chemical decomposition of the B(C₆F₅)₃˙⁻ radical anion intermediate via a solvolytic radical pathway. The reaction products form various four-coordinate borates of which [B(C₆F₅)₄]⁻ is a very minor product. The rate of the follow-up chemical step has a pseudo-first order rate constant of the order of 6 s⁻¹. This value is three orders of magnitude larger than that found in previous studies performed in the donor solvent, tetrahydrofuran. The standard reduction potential of B(C₆F₅)₃ is reported for the first time as −1.79 ± 0.1 V and −1.65 ± 0.1 V vs. ferrocene/ferrocenium in dichloromethane and 1,2-difluorobenzene respectively.