IR studies on Ligand-free Rhodium Carbonyl Catalysts

IR studies on Ligand-free Rhodium Carbonyl Catalysts 

The rate of hydroformylation was proportional to the concentration of the acyl complex. The apparent activation parameters were Ai-T = 49.3 kj mol and AS = 121 J moT K . Both the activation parameters and the reaction order are consistent with the hydrogenolysis reaction being rate determining. The low order of 0.1 in alkene suggests that the rate-determining step is not purely the reaction with hydrogen and that either a pre-equilibrium also contributes or one of the earlier steps in the cycle is also somewhat slower. 

Assuming that all the rhodium occurs as rhodium acyl species, the ideal rate equation, applying the steady state approximation, is shown in Eq. (2)  

Garland et al. have developed a powerful method for the reconstruction of individual pure component spectra from complex catalytic mixtures [20]. Using this band-target entropy minimization (BTEM) protocol, he was able to identify the mononuclear rhodium acyl intermediate in the hydroformylation reaction of 3,3-dimethylbut-l-ene starting from Rh4(a-CO)9(p-CO)3 as catalyst precursor [21]. In addition to the catalyst precursor and the more stable decomposition product 

The first term represents the classic unicyclic rhodium catalysis, while the second indicates a hydride attack on an acyl species. These spectroscopic and kinetic results strongly suggested the presence of bimetallic catalytic binuclear elimination as the origin of synergism of both metals rather than cluster catalysis. This detailed evidence for such a catalytic mechanism, and its implications for selectivity and nonlinear catalytic activity illustrate the important mechanistic knowledge that can be revealed by this powerful in situ spectroscopic technique. 

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