Biscyclopentadienyl complexes

Many crystal structures of the biscyclopentadienyl rare-earth compounds have been determined four examples are shown in Fig. 18.4.3. 

The CpjYb unit can be coordinated by saturated hydrocarbons, as in MeBeCp. The positions of the hydrogens on the bridging methyl group show that this is not a methyl bridge having a 3c-2e bond between Yb-C-Be, as shown in Fig. 18.4.3(b). 

This adduct is monomeric with one coordinated THF, forming a pseudo-tetrahedral arrangement (each Cp- ligand is considered to be tridentate), as shown in Fig. 18.4.3(c). 

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The classical route to many l,3-dichalcogenole-2-thiones starts from a vicinal dihalogenoethene followed by lithiation and reaction with selenium the intermediate diselenate may be isolated as a titanium-biscyclopentadienyl complex (137) as shown in the example in Scheme 31 <93CL1851>. 

Tabelle 1. Geometrical parameters for phosphido bridged biscyclopentadienyl complexes. 

Different polystyrene-bound biscyclopentadienyl complexes have been prepared. One example is shown as 69 in Eq. 5-9 [180]. Treatment of soluble copolymers of styrene and methacrylic acid with dichlorotitanocene resulted in soluble titanium-containing copolymers 70 (Eq. 5-10] [181]. Finally polymeric organometallic mesogens 71 with [l,3-(diethynyl)cyclobutadiene]cyclopentadi-enyl Co(II) moieties were prepared by Heck coupling of a corresponding cyclobutadiene monomer with 2,5-diiodothiophene [182],... 

Important contributions to biscyclopentadienyl scandium and yttrium chemistry have been made by Lappert and also by Evans. They have reported a variety of biscyclopentadienyl complexes, normally together with analogous lanthanide compounds. As illustrative examples, we mention here CP2MR2AIR2 (M = Sc, R = CH , C H. M = Y, R = CH3) [17], (Cp YCH )... 

Biscyclopentadienyl complexes and bridged metallocene complexes of titanium show lower polymerization activities than the monocyclopentadienyl complexes (39). The catalytic activity and S5nidiospecificity for bridged metallocenes does increase by decreasing the bite-angle (51). Although ansa-monocyclopentadienyl-amido titanium complexes are essentially inactive for homopolymerization of styrene, efficient sPS formation with nonbridged amido cylcopentadienyl complexes of titanium has been reported (52). 

Early transition-metal complexes have been some of the first well-defined catalyst precursors used in the homogeneous hydrogenation of alkenes. Of the various systems developed, the biscyclopentadienyl Group IV metal complexes are probably the most effective, especially those based on Ti. The most recent development in this field has shown that enantiomerically pure ansa zirconene and titanocene derivatives are highly effective enantioselective hydrogenation catalysts for alkenes, imines, and enamines (up to 99% ee in all cases), whilst in some cases TON of up to 1000 have been achieved. 

The expected intermediate for the metathesis reaction of a metal alkylidene complex and an alkene is a metallacyclobutane complex. Grubbs studied titanium complexes and he found that biscyclopentadienyl-titanium complexes are active as metathesis catalysts, the stable resting state of the catalyst is a titanacyclobutane, rather than a titanium alkylidene complex [15], A variety of metathesis reactions are catalysed by the complex shown in Figure 16.8, although the activity is moderate. Kinetic and labelling studies were used to demonstrate that this reaction proceeds through the carbene intermediate. 

Tricyclopentadienide complexes of many of the actinides are known (Ac = Th, U, Pu, Am, Cm, Bk, Cf). Indeed, these are the only cyclopentadienide complexes known for the transplutonium elements, where -(-3 is the most stable oxidation state. The transplutonium elements were all prepared by a microchemical procedure which utilized a melt of biscyclopentadienyl beryllium (6) according to ... 

Reaction of achiral biscyclopentadienyl zirconium-( /2-acyl-C,0) complexes, such as la and lb, with sodium hexamethyldisilazide generates the di-hapto enolates 2a and 2b91. Enolate 2b is believed to adopt a Z geomeLry, only one isomer was observable by H-NMR spectroscopy (d.r. >98 2). 

Both enolate species 2a and 2b are readily alkylated by iodomethane to provide the corresponding acyl complexes 3a and 3b, respectively (yield of 3b is not reported)91. No examples of the generation of zirconium-acyl enolates from chiral biscyclopentadienyl zirconium complexes has yet been reported. The reported preparation of the enantiomerically pure complex 4 indicates that chiral zirconium-acyl enolate species may be accessible92. 

A rather special case of bimolecular termination was described recently in the literature by Chien (7). It concerns the polymerization of ethylene initiated by a soluble biscyclopentadienyl titanium dichloride-dimethyl aluminium chloride complex. Such a polymerization should be classified as a coordination polymerization and not as an ionic polymerization. Nevertheless, some similarity to anionic polymerization justifies its discussion at this place. It was shown that the termination is kineti-cally bimolecular, and it is postulated that it involves the reduction of two TiIV+ to TiIII+ complexes, proceeding simultaneously with the disproportion of 2 polymeric chains,... 

Structures of some biscyclopentadienyl rare-earth complexes (a) (Cp2ScCl)2, (b) Cp Yb(MeBeCp ), (c)... 

For comparative purposes the syntheses and properties of formally seven-coordinate d4 biscyclopentadienyl alkyne complexes will be presented. The absence of a vacant dir orbital places these d4 complexes unambiguously in the two-electron alkyne category, i.e., N = 2 applies where N is defined as the formal electron donor number for the alkyne ligand. As in the porphyrin case a reductive ligation route provided access to these molecules. Sodium amalgam reduction of Cp2MoQ2 in the presence of alkyne yields Cp2Mo(RC=CR) [Eq. (28)] (82). Note that, in... 

Reactions of Lu(03SCF3)3 with Na(C5H5) in THF generate readily the title monocy-clopentadienyl- and biscyclopentadienyl-lutetium complexes when the appropriate stoichiometric amount of Na(C5H5) is added. The highly reactive monocyclopentadi-enyl-lutetium unit seems to be stabilized by the bulky triflate anions. 

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