Organometallic catalysis defined

Ionic liquids, which can be defined as salts that do not crystallize at room temperature [46], have been intensively investigated as environmentally friendly solvents because they have no vapor pressure and, in principle, can be reused more efficiently than conventional solvents. Ionic liquids have found wide application in organometallic catalysis as they facilitate the separation between the charged catalysts and the products. 

Our understanding of organic reactions catalyzed by soluble metal complexes ( homogeneous catalysis ) is based on the properties and stoichiometric reactions of organometallic complexes, defined as molecules containing metal-carbon bonds. Significant aspects are summarized below, but for details the reader is recommended to one of the excellent texts cited at the end of this Appendix. 

Probably the most interesting aspect of catalysis via surface organometallic chemistry is the fact that if the system is well defined it may be possible to follow the various steps of the catalytic cycle, understand deactivation, increase activity and/or selectivity by changing the ligand environment of the active site. We review here some of our recent catalytic results obtained on oxides and on metals. 

Whether or not such electrophilic organometallic species can be identified, or indeed isolated, depends primarily on the stability of the counteranion. The per-fluorophenyl boron compounds B(C,sF5)3 and [B(C6F5)4] , first prepared by Stone and co-workers in 1963 [33], proved particularly useful in this respect. Their use in metallocene polymerisation catalysis [34, 35] led to significantly more active catalysts and well-defined catalyst systems that proved mechanistically informative. These results have then enabled similar species to be detected in the more complex MAO-activated catalyst systems (vide infra). 

This reaction was one of the first examples of catalysis by a supported organometallic compound. In 1964 it was observed that Mo (CO) 6/ A1203, after activation by heating in vacuo at 120°C, catalyzed the conversion of propylene into ethylene and 2-butene (82). The nature of the active site in this catalyst system is still not fully defined (83). Since the initial discovery many heterogeneous and homogeneous catalyst systems have been reported (84, 85), the latter being more amenable to kinetic and mechanistic studies. 

Dendritic polymers can be covalently functionalized with organometallic complexes to obtain a dendritic catalyst with molecularly defined catalytic sites [5-7]. Moreover, a considerable number of reports on the applicability of functionalized dendrimers in catalysis have led to the idea of a dendritic effect on the catalyst activity/selectivity, which can either be positive or... 

C. Coperet, and J.-M. Basset, Strategies to Immobilize Well-Defined Olefin Metathesis Catalysts Supported Homogeneous Catalysis vs. Surface Organometallic Chemistry, Adv. Synth. Catal. 349, 78-92 (2007). 

---