Ruthenium(II) Porphyrin Quinoid Carbene Complexes: Synthesis, Crystal Structure, and Reactivity toward Carbene Transfer and Hydrogen Atom Transfer Reactions

Reactivity study of novel metal carbene complexes can offer new opportunities in catalytic carbene transfer reactions as well as in other synthetic protocols. Metal complexes with quinoid carbene (QC) ligands are assumed to be key intermediates in a variety of metal-catalyzed QC transfer reactions using diazo quinones, which demands development of the chemistry of QC transfer of well characterized metal-QC complexes. Herein we report the isolation and QC transfer of ruthenium porphyrins [Ru(Por)-(QC)] which contribute the first examples of (i) structurally characterized metal-QC complex (by X-ray crystallography) and (ii) isolated metal-QC complex that undergoes QC transfer reaction. The complexes [Ru(Por)(QC)] were prepared from reaction of [Ru(Por)(CO)] with diazo quinones and exhibited dual reactivity, i.e., hydrogen atom transfer (HAT) as well as QC transfer. The stoichiometric QC transfer reactions from these Ru-QC complexes to nitrosoarenes (ArNO) afforded nitrones in up to 90% yield, and the corresponding catalytic reactions were also developed. Both the stoichiometric and catalytic reactions for a series of QC ligands bearing electron-donating and-withdrawing substituents showed a reverse substituent effect on the QC transfer reactivity. Complexes [Ru(Por)(QC)] are also reactive toward C-H and X-H (X = N, 5) bonds and can catalyze aerobic oxidation of 1,4-cyclohexadiene; their stoichiometric HAT reactions with unsaturated hydrocarbons gave product yields of up to 88%. The unique dual reactivity and electronic feature of [Ru(Por)(QC)] were studied by spectroscopic means and density functional theory (DFT) calculations.