Reactivity of a Cationic (C5Me5)IrIII-Cyclometalated Phosphine Complex with Alkynes

The electrophilic cationic complex [(η5-C5Me5)­Ir­(C∧P)]+, which contains a metalated phosphine derived from PMe­(Xyl)2 (Xyl = 2,6-Me2C6H3), reacts at −60 °C with the alkynes HCCH, PhCCH, and PhCCMe, with formation of the corresponding π adducts [(η5-C5Me5)­Ir­(C∧P)­(η2-alkyne)]+. Thermal activation of these complexes leads to products that result from the C–C coupling of the alkyne and the σ Ir–C bond of the metalated phosphine, whose nature depends markedly upon the alkyne involved. Thus, for HCCH the carbon–carbon bond -forming reaction leads to an iridium-bound alkene moiety as the thermodynamic product, whereas the analogous complexes derived from the bulkier PhCCH and PhCCMe alkynes undergo further transformation into allylic structures. Mechanistic studies supported by the use of PhCCD demonstrate the implication of an undetected vinylidene structure, IrCC­(H)­Ph, in the key carbon–carbon bond-forming step of the PhCCH reaction system, whereas for the internal alkyne PhCCMe a migratory insertion mechanism is operative. However, no clear distinction between these two routes can be made for the C–C bond-forming reaction for which HCCH is responsible.