Cationic Ir(III) alkylidenes are key intermediates in C-H bond activation and C-C bond-forming reactions.

This work describes the chemical reactivity of a cationic (η(5)-C(5)Me(5))Ir(III) complex that contains a bis(aryl) phosphine ligand, whose metalation determines its unusual coordination in a κ(4)-P,C,C',C" fashion. The complex (1(+) in this paper) undergoes very facile intramolecular C-H bond activation of all benzylic sites, in all likelihood through an Ir(V) hydride intermediate. But most importantly, it transforms into a hydride phosphepine species 4(+) by means of an also facile, base-catalyzed, intramolecular dehydrogenative C-C coupling reaction. Mechanistic studies demonstrate the participation as a key intermediate of an electrophilic cationic Ir(III) alkylidene, which has been characterized by low-temperature NMR spectroscopy and by isolation of its trimethylphosphonium ylide. DFT calculations provide theoretical support for these results.