Dirhodium catalyst, dicationic monocationic

Although we initially proposed that the water was inhibiting the phosphine ligand dissociation and bimetallic fragmentatiOTi from generating inactive 12r and 13rr [44], the actual situation is quite different. The dicationic dihydride catalyst llr/llr can easily deprotonate to form a new monocationic monohydride dirhodium catalyst. This is supported by in situ FT-IR, NMR, the acidity of the catalyst solution, and DPT computational studies. A 1 mM catalyst solution in 30% water/acetone after exposure to H2/CO has a pH of 3.1, while a 10 mM solution has a pH of 2.2 - consistent with a strong monoprotic acidic species. 

The DFT calculated energetics for the main hydroformylation reaction steps based on 15r starting with the 15r-alkene complex are shown in Fig. 14. The two largest activation barriers are for the initial alkene-hydride migratory insertion step (16.8 kcal/mol) and for the final reductive elimination of the acyl and hydride (21.6 kcal/mol). The computational prediction, therefore, is that the final aldehyde reductive elimination is the rate determining step for the monocationic catalyst 15r. The largest activation barrier for the dicationic dirhodium catalyst (Fig. 8) is only 13 kcal/mol, indicating that the monocationic dirhodium catalyst should be less active on a per molecule basis, which is completely consistent with the impact of... 

The other significant difference between the two bimetallic catalysts is that the monocationic monohydride dirhodium catalyst needs to oxidatively add Hj in order to gain the hydride(s) to allow the reductive elimination of aldehyde. The dicationic dihydride system has the second hydride already present and ready to go for the acyl reductive elimination step. H2 then oxidatively adds to the dicationic catalyst to regenerate the dihydride llr/llr. But since the monocationic catalyst system has a low activation barrier for H2 oxidative addition (8.7 kcal), this is not a bottleneck in the catalysis cycle. 

We believe that the presence of free H in the acetone/water solvent system plays a role in the monocationic system. The rate determining step, once again, is the reductive elimination of aldehyde with a calculated barrier of 21.6 kcal (Fig. 14). Protonation of the monocationic dirhodium acyl is an alternate and likely pathway for eliminating aldehyde and forming the dicationic dirhodium catalyst Hr. Due to the very low activation barrier for the monocationic aUcyl-CO migratory insertion step, protonation of Rh-alkyl species to produce alkane is far less likely and consistent with the much lower alkane side reactions for 15r. 

---