A series of hydrido rhodium complexes containing mono-, bi- and hemilabile tridendate phosphite ligands has been investigated by in situ HP NMR as well as by in situ HP IR spectroscopy, DFT and isotopic labeling. Spectroscopic measurements were performed at conditions typical for hydroformylation catalysis, thus pressures up to 60 bar CO/H2 and temperatures of 30…120°C have been applied. A NMR gas flow cell and an online coupled transmission IR cell were used. With the monodentate ligand applied, the permanent coexistence of both the hydrido and the acyl rhodium complex during olefin conversion was proven. A special algorithm has been applied to the experimental IR spectroscopic data sets to get access to the IR spectra of the pure components and to construct precisely the time dependent concentration profiles at a low total [Rh]&lt; 0.3 mM. The molar ratio of rhodium complexes shifts with olefin conversion, however, the reaction rate remains correlated to the concentration of the acyl intermediate even at low olefin concentrations where the hydride is the dominating species. Evidence is given from spectroscopy that the monophosphite flips from an equatorial coordination site in [HRh(CO)3L], to an axial one in the acyl complex [RC(O)Rh(CO)3L]. The Rh(I) acyl is usually not seen for active rhodium catalysts formed from bidentate ligands. We have found conditions, where such complexes dominate in solution and show a higher hydroformylation activity compared to the catalysts formed with a bulky monophosphite. For the respective hydrides, a rare species of the type [HRh(CO)2(bisphosphite)] could be isolated and studied by X-ray diffraction. Furthermore, a new catalyst lead structure has been found with distinct hemilabile trisphosphites. An unexpected high activity and n-regioselectivity, respectively, in the isomerizing hydroformylation of internal olefins was measured. The monocarbonyl hydrido complex was isolated and characterized by X-ray analysis. Spectroscopy reveals an equilibrium between the mono- and dicarbonyl hydrido complex which is not affected by the olefin concentration. HP NMR as well as HP IR spectroscopic studies, combined with deuteration experiments and DFT calculations were performed to elucidate the individual solution structures of the rhodium hydrides involved.

Insights into catalytically active rhodium complexes

255th National Meeting (2018)

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