Rhodium Complexes Based on Phosphorus Diamide Ligands:  Catalyst Structure versus Activity and Selectivity in the Hydroformylation of Alkenes

The rhodium-catalyzed hydroformylation reaction of 1-octene with phosphorus diamide ligands has been investigated. Four monodentate phosphorus diamide ligands and six bidentate phosphorus diamide ligands derived from a 1,3,5-trisubstituted biuret structure have been synthesized. These types of ligands combine steric bulk with π-acidity. The rhodium complexes formed under CO/H 2 have been characterized by high-pressure spectroscopic techniques. Spectroscopic experiments revealed that the monodentate ligands form mixtures of HRhL 2 (CO) 2 and HRhL(CO) 3 . The ratio HRhL 2 (CO) 2:HRhL(CO) 3 depends on the ligand concentration and its bulkiness. The bidentate ligands form stable, well-defined catalysts with the structure HRhL∩L(CO) 2 under hydroformylation conditions. Both monodentate and bidentate ligands have been tested in the hydroformylation reaction of 1-octene. The monodentate ligands form very active catalysts, but the linear-to-branched ratio of the product is low. The bidentate ligands show improved selectivity compared to the monodentate ligands and an activity that is only slightly lower.