A Comparative In Situ HP‐FTIR Spectroscopic Study of Bi‐ and Monodentate Phosphite‐Modified Hydroformylation

Abstract

The rhodium‐catalyzed phosphite‐modified hydroformylation of 3,3‐dimethyl‐1‐butene is comparatively studied for a bidentate and a monodentate phosphite using in situ high‐pressure (HP) FTIR spectroscopy and GC analysis. With the bidentate ligand at 70 °C, a pseudo‐first‐order reaction with respect to the olefin takes place, with the pentacoordinate hydrido complex being the only detectable intermediate during the reaction. In contrast, for the monodentate ligand, a zeroth‐ to pseudo‐first‐order shift is characteristic with the major intermediate for this system subsequently changing from the coordinatively saturated acyl complex to the respective hydrido complex already at low conversions. Application of the PCD (pure component decomposition) algorithm to the reaction spectra affords the concentration versus time profiles of these intermediates, providing proof that the reaction rate remains controlled by rhodium acyl hydrogenolysis even at medium to high olefin conversions when the corresponding hydrido complex is the major organometallic component. If the reaction is carried out at a temperature of 30 °C in neat olefin, results from both GC and HP‐FTIR verify an intermediate regime of saturation kinetics and also the presence of an acyl complex at low olefin conversions for the diphosphite. Initial turnover frequencies of 237 h−1 and 1040 h−1 are obtained for the mono‐ and the diphosphite, respectively, at 30 °C, which implies an intrinsically faster hydrogenolysis of the diphosphite‐derived acyl rhodium complex at this low temperature.

Exploring Between the Extremes: Conversion‐Dependent Kinetics of Phosphite‐Modified Hydroformylation Catalysis

“…Improved spectroscopic analysis and concentration profiles of organometallic intermediates : In a preceding paper we showed that the hydrido complex (Cat) does coexist with the acyl complex (CatS) over a wide conversion range. However, it has not been detected at a maximum substrate concentration 11. By changing the solvent from cyclohexane to n ‐hexane and advanced background treatment, now better resolved FTIR spectra were obtained.…”

“…As a phosphite stabilizer bis(2,2,6,6‐tetramethyl‐4‐piperidyl)‐sebacate (Tinuvin 770 DF, 100 %, Ciba) was added in equimolar amounts to the ligand 11…”

“…With decreasing substrate concentration, there is a shift to a first‐order reaction. We showed in a recent study that this is coupled with a subsequent decrease of the concentration of complex 7 from the beginning of the reaction, and the regeneration of the hydrido complex 1 11. Thus, with such catalysts only the initial rates obtained at high substrate concentrations are correlated in a linear fashion with the hydrogen concentration 10e,f.…”

“…In this paper, we present an extended study on the monophosphite‐modified rhodium‐catalyzed hydroformylation as a continuation of earlier work 11. In situ FTIR spectroscopy has been combined with DFT calculations to assign the structures of the rhodium complexes observed during the hydroformylation.…”

Supramolecular Control of Ligand Coordination and Implications in Hydroformylation Reactions

“…[9] Using high-pressure IR spectroscopy, we studied the formation of complexes cis-[Rh(H)(CO) 3 1 a] and trans-[Rh(H)(CO) 3 1 a(2) 2 ] under actual catalytic conditions, using 20 bar of syngas (H 2 /CO 1:1) and concentrations identical to those in catalysis experiments. [12] In the presence of [Rh-(acac)CO 2 ] and ligand (S)-1 a, the tris(carbonyl) rhodium hydride complex cis-[Rh(H)(CO) 3 1 a] was obtained, as was evident from the three peaks in the carbonyl region, namely 2054, 2000, and 1982 cm À1 . The rhodium complex formed in the presence of (S)-1 a and two equivalents of porphyrin 2 shows three absorption bands that are shifted to higher wavenumbers (2055,2022,1998 cm À1 ), [9] in line what we expected as the CO is a stronger p-accepting ligand than the phosphoroamidite.…”