Impact of the P-Ligand Concentration on the Formation of Hydroformylation Catalysts: An in situ FTIR Spectroscopic Study

The formation of hydrido rhodium(I) complexes of the type [HRh(CO)₃L] and [HRh(CO)₂L₂] from [Rh(acac)(CO)₂]/L (L: bulky monophosphite ligand) requires a ligand-to-rhodium ratio [L]/[Rh] > 10 to prevent the possible partial generation of [Rh₆(CO)₁₆] under typical hydroformylation conditions. Significant fractions of formed [Rh₆(CO)₁₆] at lower ratios of [L]/[Rh] can be reactivated towards the hydrido complexes after the addition of the monophosphite ligand in the presence of synthesis gas. It was also found that [Rh₆(CO)₁₆] can be transformed by treatment with an excess of monophosphite ligand under inert gas to form a mononuclear complex of the type [L`Rh(CO)L] with an ortho-metallated ligand.