Oxidation organometallic systems

In this chapter we report on the gas-phase preparation of metal-supported catalysts, that is on the deposition of dispersed metallic nanoparhcles onto a surface. Taking most of the examples from the thoroughly studied chemistry of the [Mo(CO)is]/oxide support system, we successively consider (i) surface organometallic chemistry issues, (ii) the methods used to avoid chemical contaminahon of the deposit and (iii) the competition between nucleation and growth. 

One possible solution to this problem is a combination of the MO and the empirical molecular mechanics (MM), treating the active and important part of the molecule with the MO method and the remainder, such as bulky substituents or other chemical environments, with the MM method. Such treatments have been made for some organometallic systems [83-86]. However, in these treatments, only the geometry of the MM part is optimized under the assumption that the MO part is frozen at the optimized geometry of the small model system. This frozen assumption can result in a substantial overestimation of the MM energy. Recently, Maseras and Morokuma have proposed a new integrated MO + MM scheme, called IMOMM, in which both the MO part and the MM part of geometry are simultaneously optimized [87]. This method can combine any MO approximation with any molecular mechanics force field. The application of this method at the IMOMM(HF MM3) and IMOMM(MP2 MM3) levels to the oxidation addition reaction of H2 to Pt(PRa)2, where R = H, Me, r-Bu, and Ph [88], has shown a promise that more realistic models of elementary reactions and catalytic cycles may be studied in the near future with this method. 

The two processes are common for transition metals and have been studied in considerable depth for many organometallic systems. They are also important for p-block elements. Most p-block elements exhibit multiple valence states, with the valence differing by two units, which makes them suitable candidates for oxidative addition and reductive elimination. 

Barteauwas able to demonstrate for well-defined Ti02 and ZnO surfaces the activity and selectivity for C2 oxygenate (carboxylates and aldehydes) and hydrocarbon (alkynes) coupling reactions over these model metal oxide surfaces under UHV conditions that is typically only seen in organometallic systems in solution. 

The mechanism of olefin metathesis does not involve the classic reactions we have covered—namely, oxidative addition, reductive elimination, (3-hydride elimination, etc. Instead, it simply involves a [2+2] cycloaddition and a [2+2] retrocycloaddition. The [2+2] terminology derives from pericyclic reaction theory, and we will analyze this theory and the orbitals involved in this reaction in Chapter 15. In an organometallic [2+2] cycloaddition, a metal alkylidene (M=CR2) and an olefin react to create a metal lacyclobutane. The metalla-cyclobutane then splits apart in a reverse of the first step, but in a manner that places the alkylidene carbon into the newly formed olefin (Eq. 12.83). Depending upon the organometallic system used, either the alkylidene or the metallacycle can be the resting state of the... 

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