Mechanistic Studies and Differences

Extensive spectroscopic and other evidences are available for all the three catalytic cycles. For the Co-based catalytic cycle, good kinetic, spectroscopic, and structural data on model complexes exist. For rhodium-catalyzed carbonylation, oxidative addition is found to be the rate-determining step. In contrast, for iridium-catalyzed carbonylation, insertion of CO is the rate-determining step. Thus kinetic measurements show that for 4.13 the insertion reaction is about 700 times faster than that for 4.11. Computational studies, as mentioned earlier (see 3.5), are also in agreement with the kinetic data. 

In situ infrared (IR) spectroscopy of the reaction mixture, at a pressure and temperature close to that of the actual catalytic process, indicates [MCCO) ] (M = Rh or Ir) to be the resting state of the catalyst. However, by varying the conditions under which IR and multinuclear nuclear magnetic resonance (NMR) spectra are recorded, most of the catalj4ic intermediates can be observed. 

The spectral data in solution for some of the catalytic intermediates of rhodium and iridium are indicative of the presence of a mixture of isomers. This is expected as fluxionaUty for five coordinate complexes such as 4.7 and 4.14 is common. 

Evidences for the rhodium- and iridium-catalyzed carbonylation cycles also come from isolated and model complexes. From the reaction of CHjI with 4.5, under noncatalytic conditions, a solid has been isolated. 

Organometallic complexes of the 5d elements are generally more stable than their Ad analogues. Because of this, isolation and full structural characterization of two catalytic intermediates of the Cativa process, 4.11 and 4.15, are possible under optimized conditions. In the absence of additional CO, two isomers of the neutral complex 4.17, which is a dimer of 4.12, can also be isolated and characterized. 

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