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
(η5-C5Me5)­Ir­(III) complex that
contains
a bis­(aryl) phosphine ligand, whose metalation determines its unusual
coordination in a κ4-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.