In situ FTIR spectroscopic investigations on rhodium carbonyl complexes in the absence of phosphorus ligands under hydroformylation conditions

Abstract

In situ FTIR spectroscopy was combined with the peak group analysis (PGA) for investigations on the composition of ‘unmodified’ rhodium carbonyl complexes in the absence of phosphorus ligands at conditions relevant for alkene hydroformylation. As a precursor complex [Rh(acac)(CO)₂] was selected since it is commonly used for rhodium catalyzed hydroformylations. At higher pressures of synthesis gas (CO/H₂) and elevated temperatures, [Rh(acac)(CO)₂] is decomposed to [Rh₄(CO)₁₂] and [Rh₆(CO)₁₆] as spectroscopically observable components. [Rh₄(CO)₁₂] represents an intermediate that readily reacts towards [Rh₆(CO)₁₆] at higher temperatures and lower partial pressures of carbon monoxide. The degradation of [Rh(acac)(CO)₂] is hampered significantly at higher concentrations of acetylacetone. The involvement of a hydrido species in the reaction sequence from [Rh(acac)(CO)₂] to polynuclear rhodium complexes under H₂/CO is in aggreement with the detection of the infrared spectrum of [HRh(CO)₄] and the considerable decrease of the decomposition rate in the presence of pure CO and D₂/CO. With ethene as the alkene substrate, acyl complexes of the type [CH₃CH₂C(O)Rh(CO)₃(π-C₂H₄)] and [CH₃CH₂C(O)Rh(CO)₄] have been observed, whereas only the tetra carbonyl complex [(CH₃)₃CCH₂CH₂C(O)Rh(CO)₄] was detected for neohexene.