Bimetallic Homogeneous Hydroformylation

A dirhodium hydrido-carbonyl catalyst system based on a binuclating tetraphosphine ligand is discussed. Spectroscopic and DFT computational studies support the formulation of the key catalyst complex in acetone solvent as [Rh₂(μ-H)₂(CO)₂(rac-P4)]²⁺, which is highly active and regioselective for producing linear aldehydes under mild conditions. This dicationic catalyst suffers from facile fragmentation reactions in acetone that lead to inactive monometallic and bimetallic complexes. The addition of water to the acetone solvent leads to deprotonation from the dicationic catalyst to form monocationic dirhodium catalyst species that are far less susceptible to deactivation. Spectroscopic and DFT computational studies indicate that the key monocationic catalyst is [Rh₂(μ-H)(CO)₃(rac-P4)]⁺. Although the monocationic bimetallic catalyst is less active on a per molecule basis relative to the dicationic catalyst, there is a higher concentration present producing better overall catalyst rates and selectivity.