Hydroformylation and Aldehyde-Water Shift Catalysis by Dirhodium Tetraphosphine Complexes

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The use of a unique tetraphosphine ligand (et,ph-P4) allows for the chelating of two rhodium centers to perform bimetallic coopertivity in hydroformylation reactions to better the yields the linear aldehyde product. The proposed catalyst is [Rh2(μ-H)2(CO)4(rac-et,ph-P4)]2+ and has been studied by in situ FT-IR and NMR. When tested in a polar phasic acetone/water (30 % by vol.), the catalyst forms a monocationic system [Rh2(μ-H)2(CO)4(rac-et,ph-P4)]+ that has an initial TOF of 26 min-1, selectivity of 27:1 L:B, and low byproducts of 4.6 % alkene isomerization and 1.3 % hydrogenation for the conversion of 1-hexene to heptanal. Previous in situ FT-IR and NMR research as well as DFT calculations have determined the active catalyst and mechanism for both the dicationic and monocationic systems. A new stronger chelating tetraphosphine ligand (et,ph-P4-Ph) as been built to function similarly to the et,ph-P4 ligand. After a preliminary synthetic scheme for the production of et,ph-P4-Ph, optimization of the key synthetic steps has been accomplished. This optimization allows for the increase in yields of those steps and the separation of the rac- and meso-diastereomers of the ligand. Use of this ligand to chelate two rhodium centers has occurred and a crystal structure of [Rh2(nbd)2(rac-et,ph-P4-Ph)](BF4)2 has been obtained. This complex has been studied by in situ FT-IR and NMR experiments to determine the nature of the catalytic system. Out of these studies the highly active complex [Rh2(μ-CO)(CO)3(rac-et,ph-P4)](BF4)2 has been synthesized and characterized by X-ray diffraction. When used in hydroformylation in a polar phasic DMF/water (25 % by vol.), the new catalyst has initial TOF of 35 min-1, an aldehyde L:B selectivity of 18:1, 1.9 % alkene isomerization, and < 1 % alkene hydrogenation. Attempts have also been made to use this complex for aldehyde-water shift reactions. Further work is underway to further these studies for publication.