We have designed and synthesized a new class of binucleating polydentate phosphine ligands for preparing novel bimetallic transition metal complexes. Considerable interest has been shown in showing that multiple metal centers can exhibit cooperative behavior in catalytic reactions and show superior activities and selectivities relative to monometallic complexes. This goal has been dramatically achieved through the design and synthesis of a new binucleating tetraphosphine ligand that can bridge and chelate two metal centers. This ligand, called et,ph-P4, exists in the two diastereomeric forms shown to the left. [Rh₂(nbd)₂(et,ph-P4)]²⁺ (nbd = norbornadiene) is a precursor for a remarkable bimetallic hydroformylation catalyst. Hydroformylation is the largest homogeneous industrial catalytic process for converting alkenes, H₂ and CO, into aldehyde products.

Our bimetallic catalyst has higher activity and selectivity for linear aldehyde products compared to current commercial Rh/PPh3 catalysts. The activity and selectivity of our bimetallic catalyst is extremely unusual because electron-rich chelating phosphines like et,ph-P4 are well known to generally deactivate rhodium toward hydroformylation and lower the selectivity to linear aldehyde. We have demonstrated (Science, 1993) that this catalyst represents one of the most dramatic examples of bimetallic cooperativity in homogeneous catalysis.

We have performed extensive in situ FT-IR and NMR studies on our catalyst system and have strong to tentative structural assignments for all complexes observed in the NMR. This work has been featured on the cover of Angewandte Chemie (1996). We have proposed that the active bimetallic catalyst has the highly unusual dicationic dihydride Rh(+2) metal-metal bonded structure shown to the left. The localized cationic charge on each rhodium is critically important for the activity of this catalyst, as is the Rh-Rh bond for creating an extremely well defined catalyst binding site.

We have also discovered that the addition of water to the acetone solvent used for our hydroformylation catalytic studies causes a dramatic increase in the rate of catalysis and far better selectivity with virtually no alkene isomerization and hydrogenation side reactions. For example, with 30% water in acetone we see an initial turnover frequency of over 4000 hr-1, an aldehyde linear to branched regioselectivity of 33:1, and alkene isomerization and hydrogenation well under 0.2%. This makes our system the most active and selective hydroformylation catalyst known.

Figure:Docking of vinyl acetate onto the catalyst showing the two pro-chiral coordination modes and the energy differences between them as calculated by molecular modeling. Experimentally we find an enantioselectivity of 85% for this reaction with the same chirality predicted by the modeling study.

We are actively continuing our studies into this exciting new bimetallic catalyst system, with emphasis on asymmetric catalytic reactions and extensions of bimetallic cooperativity into other new types of catalytic reactions such as hydrocarboxylation, for which we have exciting preliminary results.

Our research laboratory has state-of-the-art computerized autoclave facilities and a high pressure IR cell for in situ catalytic studies. There is considerable industrial interest in this radically new catalyst, and we are working closely with several companies on this project.