Reactivity of a Cationic (C₅Me₅)Ir^III-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