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 Ru^III-pyridine-imidazolate complex (Ru^III(pyIm), where pyIm = pyridine-imidazolate) with a series of para-substituted trityls (R = MeO, <i>t</i>Bu, 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. Ru^III(pyIm), however, reacts by a Minisci-type substitution to form Ru^II(5-trityl-imidazole) complexes. The reduced imidazole complexes can be oxidized to the corresponding Ru^III(5-trityl-imidazolate) by abstraction of the imidazole NH hydrogen by an additional trityl. These reactions form triphenylmethanes and a mixture of tritylated Ru^II and Ru^III complexes. The imidazolate is rendered electrophilic, as required for Minisci reactions, by the Ru^III center that accepts 1<i>e</i>^- in the reaction. Alternatively, the reaction can be viewed as a trityl radical coupling with the Ru^III(pyIm) SOMO that has contributions from the imidazolate π-HOMO and a half-filled t_2g orbital on Ru^III. Because of kinetic competition between the C-C bond formation and H-abstraction steps, more electron-rich trityls form mostly Ru^II-trityl-imidazoles, while electron-poor trityls form Ru^III-trityl-imidazolates.