Minisci Substitution with C–C Bond Formation in Reactions of Trityl Radicals with a Ruthenium-Bound Pyridyl-Imidazole Ligand

Abstract

Carbon radicals are key to many biological and organic reactions, and stable triphenylmethyl (trityl) radicals are often used to model these processes. Here, we report reactions of a RuIII-pyridine-imidazolate complex (RuIII(pyIm), where pyIm = pyridine-imidazolate) with a series of para-substituted trityls (R = MeO, tBu, Ph, and CF3) that surprisingly proceed via C−C bond formation. Reactions of metal complexes with carbon radicals more commonly form C−X bonds (X = O, N, or halide), as in the ubiquitous rebound mechanism. RuIII(pyIm), however, reacts by a Minisci-type substitution to form RuII(5-trityl-imidazole) complexes. The reduced imidazole complexes can be oxidized to the corresponding RuIII(5-trityl-imidazolate) by abstraction of the imidazole NH hydrogen by an additional trityl. These reactions form triphenylmethanes and a mixture of tritylated RuII and RuIII complexes. The imidazolate is rendered electrophilic, as required for Minisci reactions, by the RuIII center that accepts 1e− in the reaction. Alternatively, the reaction can be viewed as a trityl radical coupling with the RuIII(pyIm) SOMO that has contributions from the imidazolate π-HOMO and a half-filled t2g orbital on RuIII. Because of kinetic competition between the C−C bond formation and H-abstraction steps, more electron-rich trityls form mostly RuII-trityl-imidazoles, while electron-poor trityls form RuIII-trityl-imidazolates.