Surface organometallic route

Figure 2.16 Characterization by high-resolution electron microscopy of a bimetallic nanoparticle of a Pt-Sn alloy obtained via the surface organometallic route.

The reaction of organometallics with metal surfaces is a very promising aspect of surface organometallic chemistry related to fine chemicals. The precise understanding of the reactions between organometallics and metal surfaces is essential to obtain highly selective catalysts by this route.292,293... 

J. R Candy, B. Didillon, E. L. Smith, T. M. Shay, and J. M. Basset, Surface organometallic chemistry on metals A novel and effective route to custom-designed bimetallic catalysts, J. Mol. Catal. 86, 179-204 (1994). 

Several synthetic methods for the preparation of semiconductor nanoparticles have been reported. Colloidal and organometallic routes have probably been identified as the two major methods in use [11-16], although nano dimensional particles have been also synthesized in confined matrices such as zeolites [17], layered solids [18], molecular sieves [19,20], vesicles/micelles [21,22], gels [23,24], and polymers [25]. An ideal synthetic route should produce nanoparticles which are pure, crystalline, reasonably monodisperse and have a surface which is independently derivatized. 

J2.4 Surface organometallic chemistry on metals a novel and effective route to custom-designed bimetallic catalysts... 

All these facts clearly indicate the importance of preparing well defined PtSn catalysts. Among the routes proposed to produce such bimetallic catalysts, the Surface Organometallic Chemistry on Metals (SOMC/1 techniques is the one most often employed [7]. SOMC/M techniques consist of the reaction between alkyltin compounds and a supported transition metal, providing adequate methodologies that avoid the waste of metal promoter by direct interaction with the supported material. 

Various routes can be used to prepare such compounds but it appears progressively clear that surface organometallic chemistry on metals is a new way to obtain and characterize such well controlled bimetalHc catalysts [65], First, by careful control of the reaction parameters, the organometallic complex may react selectively with the host metal and not with the chemical functionalities present at the surface of the support, e.g. silica. Even if there is a possibihty of reaction between the organometallic complex and the oxide, such reaction usually proceeds... 

EVANS provides a summary of the reactions of organometallics with oxide surfaces that lead to well-defined surface species including mononuclear and polynuclear complexes and monometallic and bimetallic particles. These surface reactions are described by the same principles encountered in molecular chemistry the reaction classes include nucleophilic attack at the ligands, electrophilic attack at the metal-carbon bond, oxidative addition, Lewis base adduct formation, redox reactions, etc. The synthesis of well-defined reactive sites on surfaces by these organometallic routes will facilitate the study of elementary steps in surface chemistry. 

Chemical routes involving the use of tungsten salts have been reported. W2C was claimed to be formed by the pyrolysis of an organometallic complex containing cyclopentadiene and carbonyl groups.8 The W2C formed was useful for ceramic applications such as wear resistant surfaces and cutting tools. The same phase was also prepared by the reduction of WC14 with sodium triethylborohydride.9 The material was formed as 1-5 nm-sized crystallites as shown by SEM and TEM. 

One major reason for the great interest in the processes of thin metal-containing films is that reactions on the surface of small metal clusters can be studied. Indeed, prior to the development of thin-film chemistry, reactions of similar particles were studied only in the gas phase at rather high temperatures. Under these conditions, most of the primary products are unstable and decompose in the course of further reaction, which is non-selective. As a result, the information obtained on the routes and mechanisms of reactions of disperse metals appears to be scarce, while the use of such reactions in synthesis is inexpedient. Conversely, low-temperature reactions in the films of co-condensates are very promising from the standpoint of determining the detailed reaction mechanism, as well as for synthesis of previously unknown complexes and organometallic compounds. It is important that atoms of only a few metals react with organic compounds immediately at the instant of their contact on the cooled substrate. Rather often, atoms and/or small (molecular) clusters are first stabilized in the film, and then their transformations are observed. 

Two-electron redox processes are facile in metal sulfide surfaces, and thus M-S bond breaking and re-making in such extended arrays may provide an energetically favorable pathway to modify the electron count at a particular metal center. Although Tl -bonded thiophenes have not been invoked in heterogeneous HDS mechanisms, T T transformations such as the ones defined in organometallic complexes could be envisaged to take place at the active sites of HDS catalysts, as possible routes for... 

---