Liquid-Phase Reaction of Monosubstituted and Disubstituted Alkynes with Tetrarhodium Dodecacarbonyl under CO and CO/H2 Mixtures. In-Situ IR Spectroscopic Characterization of 20 New Alkyne−Rhodium Complexes

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 Rh 4 (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 H 2 . 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 Rh 4 (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 ν CO 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 ν CO 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 Rh 2 (CO) 6 {μ-η 1 -(CO−HC 2 R)} for terminal alkynes and Rh 2 (CO) 6 {μ-η 1 -(CO−R 1 C 2 R 2 )} and Rh 2 (CO) 6 {μ-η 2 -(R 1 C 2 R 2 ) }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.