Composition of catalyst resting states of hydroformylation catalysts derived from bulky mono-phosphorus ligands, rhodium dicarbonyl acetylacetonate and syngas

Abstract

The paper describes the composition of the resting states of several catalysts for alkene hydroformylation derived from bulky monophosphorus ligands. The results presented assess how bulky ligands compete with CO for the rhodium, and hence the role of ‘unmodified’ catalysts in alkene hydroformylation in the presence of these ligands. High Pressure Infra-Red (HPIR) spectroscopy was carried out at the rhodium and syngas concentrations typically used during catalysis experiments. These HPIR studies revealed that two ligands previously studied in Rh-catalysed hydroformylation react with [Rh(acac)(CO)2] and H2/CO to give the unmodified rhodium cluster, [Rh6(CO)16], as the only detectable species. Both less bulky phosphoramidites, and 1,3,5,7-Tetramethyl-6-phenyl-2,4,8-trioxa-6-phosphaadamantane, on the other hand, do not show the presence of [Rh6(CO)16], and hence catalysis proceeds by purely ligand modified species under normal conditions. In the case of the Rh/ phosphaadamantane catalysts, anecdotal evidence that this only forms a particularly useful catalyst above a certain pressure threshold can be understood in terms of how the catalyst composition varies with pressure. The ligands discussed have all been assessed in the hydroformylation of propene to separate their innate branched selectivity from their ability to isomerise higher alkenes to internal isomers.