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

Abstract

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.

Semi-Batch Homogeneous Catalytic In-Situ Spectroscopic Data. FTIR Spectral Reconstructions Using Band-Target Entropy Minimization (BTEM) without Spectral Preconditioning

“…We have undertaken the present experimental and computational study using FTIR data, since it compliments our previous work on the rhodium-catalyzed hydroformylation reaction and further illustrates the potential of spectroscopic system identification. Previously, the application of preconditioned BTEM to a multiple-experiment stoichiometric organometallic system was shown, and now the application of non-preconditioned BTEM to a semi-batch homogeneous catalytic system is demonstrated.…”

Operandomonitoring of mechanisms and deactivation of molecular catalysts

“…Section 3) and also the provision of illustrative application examples thereof. 105,134,135 With regard to catalyst deactivation, the group investigated the influence of alkynes 136 and dienes 137 on the unmodified Rh 4 (CO) 12 hydroformylation catalyst. Being more reactive towards the catalyst, these typical alkene feedstock impurities block the catalytic activity for alkene hydroformylation.…”

“…Being more reactive towards the catalyst, these typical alkene feedstock impurities block the catalytic activity for alkene hydroformylation. In a first study, 136 Rh 4 (CO) 12 was reacted with 20 structurally varying terminal and internal alkynes under CO pressure. The forming catalyst species were monitored by connecting a high-pressure infrared transmission cell to the autoclave.…”

“…Both complexes showed high stability in the presence of H 2 gas, so that the catalyst deactivation can be interpreted as irreversible poisoning. 136 In a similar work, Rh 4 (CO) 12 was exposed to a series of conjugated dienes both in non-competitive situations and as impurities under cis-oct-4-ene hydroformylation conditions. Spectral deconvolution was carried out using a precursor version of BTEM, suggesting the presence of at least three mononuclear species which do not occur in the hydroformylation of uncontaminated alkenes.…”

20 Organometallic chemistry of bi- and poly-nuclear complexes

“…In the case of disubstituted alkynes, CO insertion is less facile and an observable equilibrium is established between the bridged alkyne species and the insertion product. 110 118 The crystal structure of [Co 4 (CO) 12 ] has been determined at 120 K and redetermined at 292 K. 119…”