Kinetic and Mechanistic Studies of a Bimetallic Hydroformylation Catalyst.

Abstract

Dicationic bimetallic rhodium complexes based on the novel binucleating tetraphosphine ligand system racemic-Et2PCH2CH 2P(Ph)CH2P(Ph) CH2CH2PEt2, et,ph-P4, are highly active and selective hydroformylation catalysts that make use of bimetallic cooperativity to operate. In situ FT-IR and NMR studies indicate that the most active catalyst is the unique dinuclear dicationic hydrido-carbonyl Rh(+2) oxidation state complex, [racemic-Rh 2H2(mu-CO)2(CO)2(et,ph-P4) 2+]. A kinetic study was performed to determine the rate orders of the dinuclear dicationic catalyst, and the substrate (1-hexene). The rate orders were found to be one for both components. The study was also performed to test the hypothesis of a bis-acyl species, which was not observed. The addition of 25--50% water to the acetone solvent caused a dramatic improvement in the hydroformylation catalysis with a 40% increase in the initial turnover frequency and a reduction in alkene isomerization and hydrogenation side reactions to less than 1%. The presence of water coupled with H 2 poor reaction conditions initiates a new catalytic reaction---an aldehyde-water shift process that takes aldehyde and water and produces carboxylic acid and H2. Too much H2 strongly inhibits this aldehyde-water shift reaction, which is why it is not observed under normal hydroformylation catalysis conditions. The carboxylic acid products have very high linear/branched selectivities. These pieces of evidence continue to support our theory of bimetallic cooperativity.