It is well-known that the liquid-phase homogeneous unmodified rhodium-catalyzed
hydroformylation of alkenes is irreversibly poisoned by the presence of trace quantities of
alkynes. In the present contribution, we examined the reaction of four series of monosubstituted and disubstituted alkynes (20 compounds) with Rh4(CO)12 in n-hexane solvent at
293 K under both (A) 2.0 MPa CO and (B) 2.0 MPa CO and 2.0 MPa H2. The analytic method
used was in-situ high-pressure infrared spectroscopy. It was observed that (I) all alkynes
used in this study reacted quantitative with Rh4(CO)12 in a matter of hours, (II) the final
spectra were not influenced by the presence of hydrogen, (III) the monosubstituted alkynes
consistently gave a final product involving six terminal νCO vibrations in the region 2036−2121 cm-1 and two vibrations at ca. 1668 and 1689 cm-1, and (IV) the disubstituted alkynes
consistently gave a final spectrum consistent with the superposition of the spectra obtained
in III plus a spectrum involving five terminal νCO vibrations in the region 2030−2100 cm-1
and one vibration at ca. 1630 cm-1. These results are consistent with the existence of two
primary types of observable species in the final products. Due to the band positions and
absorptivities, we tentatively propose that these species are substituted dirhodium carbonyl
species, specifically Rh2(CO)6{μ-η1-(CO−HC2R)} for terminal alkynes and Rh2(CO)6{μ-η1-(CO−R1C2R2)} and Rh2(CO)6{μ-η2-(R1C2R2) }in the case of disubstituted alkynes. These
complexes are the rhodium analogues of well-known dicobalt carbonyl alkyne complexes. It
appears that, in the case of terminal alkynes, the dirhodium−alkyne complexes undergo
rapid CO insertion under 2.0 MPa CO. In the case of disubstituted alkynes, CO insertion
seems more difficult to obtain, and an observable equilibrium is established between the
bridged alkyne species and the insertion product. In both cases the final alkyne complexes
are stable under CO even in the presence of molecular hydrogen. This is probably the primary
reason that trace quantities of alkynes are able to poison the catalytic alkene hydroformylation reaction.