Novel Amphiphilic Diphosphines: Synthesis, X-ray Structure, Rhodium Complexes, Use in Hydroformylation, and Rhodium Recycling

Three novel amphiphilic diphosphines have been synthesized: bis[2-(phenyl(3-pyridyl)phosphino)ethyl] ether (POPpy) and bis[2-((4-((diethylamino)methyl)phenyl)phenylphosphino)ethyl] ether (POPam), based on bis[2-(diphenylphosphino)ethyl] ether (POP), and 4,6-bis[bis(4-((diethylamino)methyl)phenyl)phosphino]-10,10-dimethylxanthene (xantham), based on 4,6-bis(diphenylphosphino)-10,10-dimethylxanthene (Xantphos). The crystal structure of xantham has been determined. Solution structures of rhodium xantham complexes have been studied using NMR and IR spectroscopy. When POPam is used in the hydroformylation of oct-1-ene, a linear/branched ratio (l/b) of 7.3 is achieved (88% nonanal) without isomerization to oct-2-ene. The use of POPpy results in an activity that is twice as high, a higher l/b-ratio of 9, and 0.7% isomerization. When xantham is used in the hydroformylation of hex-1-ene, oct-1-ene, and dodec-1-ene (80 °C, 20 bar of syngas, toluene), l/b-ratios of 50 have been achieved (94% nonanal) together with 4% of isomerized octenes. The pH-dependent distribution characteristics of the free ligands have been determined, and it is shown that POPpy, POPam, and xantham can be extracted for 40% from a Et2O solution into an aqueous solution of H2SO4 at pH values of, respectively, 2.5, 5, and 5. Rhodium recycling experiments using POPam and xantham show that rhodium can be extracted into an aqueous layer at pH 5−5.5 for more than 99.95%, as determined by ICP-AES analysis, which allows complete separation of the product aldehydes and catalyst. After neutralization of the aqueous phase containing protonated POPam with an aqueous solution of NaHCO3, only 65% of the rhodium can be re-extracted into fresh toluene. In the case of xantham, 98% of the rhodium is re-extracted. Pressurizing the recovered rhodium and excess xantham to 20 bar at 80 °C resulted in regeneration of the original active hydride complex, as judged by the retention of activity of up to 86%. POPpy has proved inappropriate for the recycling of rhodium.