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.