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

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.