Interplay between Catalyst Complexes and Dormant States: In Situ Spectroscopic Investigations on a Catalyst System for Alkene Hydroformylation

Results from a detailed in situ IR and NMR spectroscopic study on a monophosphite-modified rhodium catalyst system are presented. Equilibria that occur between the catalytically active hydrido rhodium(I) complexes and formally inactive dinuclear rhodium(0) complexes as well as mononuclear ortho-metalated rhodium(I) complexes have been investigated. The latter can be considered as “dormant states”, which are formed at conditions with lower hydrogen pressures or [H2]/[Rh] ratios. It was found that dimer formation is dominant at lower temperatures and higher rhodium concentrations, whereas ligand metalation is effective at higher temperatures. At the appropriate partial pressure of hydrogen, the regeneration of hydrido rhodium(I) complexes takes place on a short time scale from both states. Especially, the temporary stabilization of the catalyst via ligand metalation during a catalyst recycling procedure represents a feasible approach.