Ionic Phosphine Ligands with Cobaltocenium Backbone:  Novel Ligands for the Highly Selective, Biphasic, Rhodium-Catalyzed Hydroformylation of 1-Octene in Ionic Liquids^†

The use of electron-poor phosphine-substituted cobaltocenium salts as ligands for the biphasic hydroformylation in ionic liquids has been investigated. Using improved oxidation methods, 1,1‘-bis(diphenylphosphino)cobaltocenium nitrate, 1,1‘-bis(diphenylphosphino)cobaltocenium hexafluorophosphate, and 1,1‘-bis[1-methyl(1-diphenylphosphino)ethyl]cobaltocenium hexafluorophosphate have been synthesized. 1,1‘-Bis(diphenylphosphinocobaltocenium hexafluorophosphate in particular proved to be a very suitable ligand for the biphasic hydroformylation of 1-octene in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM PF₆), enabling high catalyst activity, high selectivity to the n-product, and no detectable catalyst leaching. In contrast to aqueous biphasic systems, the ionic liquid BMIM PF₆ provides for the rhodium catalyst a low-coordinating medium with limited but sufficient solubility for 1-octene to allow high reaction rates.