Bridging cyclopentadienyl derivative

The broader subject of the interaction of stable carbenes with main-group compounds has recently been reviewed. Accordingly, the following discussion focuses on metallic elements of the s and p blocks. Dimeric NHC-alkali adducts have been characterized for lithium, sodium, and potassium. For imidazolin-2-ylidenes, alkoxy-bridged lithium dimer 20 and a lithium-cyclopentadienyl derivative 21 have been reported. For tetrahydropyrimid-2-ylidenes, amido-bridged dimers 22 have been characterized for lithium, sodium, and potassium. Since one of the synthetic approaches to stable NHCs involves the deprotonation of imidazolium cations with alkali metal bases, the interactions of alkali metal cations with NHCs are considered to be important for understanding the solution behavior of NHCs. 

Monomeric, paramagnetic chlorohydrides snch as MH2CI2 (PR3)4 derive from facile oxidative addition of H2 to MCl2(PR3)4 complexes. Of these 17e dodecahedral complexes, (32) is the most interesting (eqnation 17). Its thermolysis in solution offers a qnadruply bridged hydride (33) and it can also be rednced to a monomeric Ta 16e species (34). Ta hydrides display a hydridic character (see Hydride Complexes of the Transition Metals), as shown by the reduction of CO by cyclopentadienyl derivatives. (see Niobium Tantalum Organometallic Chemistry)... 

The organometalhc chemistry of the rare earths deals mainly with bis(cyclopentadienyl) derivatives due to the easy available bis(cyclopentadienyl) rare earth chlorides and other halides via reaction of the rare earth trichlorides with two equivalents of a cyclopentadienyl alkali salt. Bis(cyclopentadienyl)lanthanide chlorides are formed as chloride-bridged dimers (Figure 28a), as monomers stabihzed by a donor molecule like THF (Figure 28b) or as ate complexes with alkali hahdes (Figure 28c). 

One of the earliest examples of the use of zirconium in catalysis is the area of Ziegler-Natta polymerisation, the catalysts employed are typically based on a family of Cp2ZrCl2 metallocene complexes." With judicious choice of the cyclopentadienyl ligand either atactic (bridged Cp-derived systems),... 

Catalysts that yield highly syndiotactic polypropylene (86% racemic pentads) were also developed. One of them is /-propylene(ri -cyclopentadienyl-r fluorenyl)zirconium dichloride [291]. Initial disclosures of metallocene catalysts were followed by numerous publications in the literature that described similar materials for the polymerizations of either ethylene or propylene, or both, and for formation of various copolymers. Thus, for instance, Kaminsky et al. [292], reported preparaticm of a zirconium dichloride-type catalyst for copolymerization of cyclic olefins with ethylene. These cyclic olefins are cyclopentene, norbomene, and the hindered cyclopentadiene adducts of norbomene [292]. The catalytic system consists of a bridged indene derivative that is combined with methylaluminoxane ... 

Annulated derivatives of cyclopentadienyl, such as indenyl and fluorenyP (Figure 3.15), as well as acenaphthyl-substituted cyclopentadienyl, and pentalenyl, are some of the most important cyclopentadienyl derivatives. They, and their derivatives with interarmular bridges, play a major role in controlling stereochemistry during the metal-catalyzed polymerization of olefins, as is described in Chapter 21. A number of permethylindenyl complexes have been reported.- ... 

---