Formation of Iridium(III) Allene Complexes via Isomerization of Internal Alkynes

Syntheses of the alkyne complexes (POCOP)­Ir­(η2-RCCR) (1-DPA, R = Ph; 1-Oct, R = C3H7; 1-Hex, R = C2H5; POCOP = 2,6-bis­(di-tert-butylphosphonito)­benzene) are reported. 1-DPA is stable in both the solid state and as a benzene solution; however, both 1-Oct and 1-Hex slowly isomerize to afford the corresponding allene complexes 2-Oct and 2-Hex, respectively. A single-crystal X-ray structure determination of 2-Oct confirmed the assignment of an iridium-bound allene isomerization product. The rates of isomerization were measured using NMR techniques over a range of temperatures to allow determination of thermodynamic parameters. Finally, in contrast to prior reports, initial mechanistic studies have indicated the isomerization process is not catalyzed by the presence of an acid.