Metallocenes compounds

Appllca.tlons. The first widely appHcable Ic separation of enantiomeric metallocene compounds was demonstrated on P-CD bonded-phase columns. Thirteen enantiomeric derivatives of ferrocene, mthenocene, and osmocene were resolved (7). Retention data for several of these compounds are listed in Table 2, and Figure 2a shows the Ic separation of three metallocene enantiomeric pairs. P-Cyclodextrin bonded phases were used to resolve several racemic and diastereomeric 2,2-binaphthyldiyl crown ethers (9). These compounds do not contain a chiral carbon but stiU exist as enantiomers because of the staggered position of adjacent naphthyl rings, and a high degree of chiral recognition was attained for most of these compounds (9). 

More recently, it was the seminal work of Aitken, Harrod and Samuel which showed that Group IV metallocene compounds catalyzed the dehydrogenative coupling of primary silanes, RSiH3, to give polysilanes of relatively low molecular weight, [RSiH] [8], that attracted many others into the silicon/transition metal chemistry field [9],... 

There have been several efforts geared toward reducing the oligomeric nature of the group 2 metallocene compounds. Reduction of the nuclearity has been accomplished via the use of sterically demanding cyclopentadienides and intermolecular Lewis bases (38-40). Hanusa and Williams have used bidentate amines to coordinatively saturate... 

Cyclopentadienyl compounds (i.e. metallocenes) (Fig. 5), which have at least one direct metal-carbon bond to the C5H5 ligand, were first synthesised in the 1950s [79,80]. Since then, reactions of cyclopentadienyl reagents have been applied for almost every element [123]. The main application of metallocenes is their use as catalysts in the polymerisation of olefins by Ziegler-Natta polymerisation processes. As many metallocene compounds are volatile and thermally stable, they are also suitable for use as precursors in MOCVD [124-127]. Although cyclopentadienyl compounds have attracted considerable interest as precursors in CVD depositions they are sometimes too reactive [128]. However, high reactivity and thermal stability make cy-... 

Certain types of metallocene compounds based on indenyl moieties are known as catalyst components for the polymerization of olefins (25,26). 

In typical metallocene compounds. Cp2M, all of the C—C bonds are of the same length and the rings are parallel. However, there are several cyclopeniadienyl com-... 

Fig. 16.53 Sirueturex of some carbollyl metallocene compounds. in) Dicarbollyl species ...

Many of the metallocene compounds display unusual reactivities and reactions, of which none is more startling than the discovery by the Russian chemist, M. E. Vol pin, of absorption of dinitrogen, N2, by titanocene, (C5H3)2Ti, to form a complex or complexes that can be reduced easily to form ammonia. The nature of these complexes is in doubt, but very clear evidence has been obtained by J. E. Bercaw for the structure of the complex 9 formed from decamethylzirconocene and dinitrogen ... 

Lanthanide metallocene compounds are also active catalysts for the dimerization of terminal alkynes, giving predominantly the linear head-to-head enyne dimer with a double bond of E configuration [17]. In recent years, however, novel organo-lanthanide [13] and organoactinide [18, 19] systems have shown their ability to produce, with high selectivity, Z and geminal enyne products, respectively. 

On the other hand, as regards the bridge connecting the ligands of metallocene compounds, a one-membered silicon bridge and two-membered ethylene... 

Fig. 14. ORTEP view of CaCp) showing the bent structure found in the Ca, Sr, and Ba metallocene compounds. The molecule has an average Ca—C bond distance of 2.64(1) A and a Cp entroid-Ca-Cpjcntroicl bond angle of 147.7°. (Redrawn from Ref. 78.)...

The main method of preparation of the alkaline-earth poly(pyrazolyl) borates is by halide metathesis. Belderrain et al. have reported the synthesis of the Ca, Sr, and Ba poly(pyrazolyl)borates MTp by reaction of the metal iodides, MI2, with KTp (Tp = HB(3,5-Me2pz)3) in THF, similar to the method described for the metallocene compounds.111... 

Bonding of ligands to alkali, alkaline earth and lanthanoid metal ions is mainly electrostatic. Consequently, these bonds can be described as a combination of electrostatic and van der Waals terms (see Chapter 15). Similar approaches have also been used for metallocene compounds (see Chapter 14). 

Metallocene catalysts are bicomponents consisting of group four transition metal compounds and cocatalysts. Metallocene compounds have long been known and were used as a compound for Ziegler-Natta catalysts (Table 1) [11],... 

A number of early transition metal compounds, e.g., Ti111 hydrides, have long been known as hydrogenation catalysts. Similarly, metallocene compounds of lanthanides and actinides can be extremely active, as a comparison of the turnover numbers of 1-hexene hydrogenations at 25°C (1 bar H2) show CpfLuH, 120,000 [Ir(COD)-(py)(PCy3)]PF6, 6400 [Rh(COD)(PPh3)2]PF6, 4000 RuHCl(PPh3)3, 3000 ... 

Treatment of the di-lithium salt of 1, Li2[CpCMe2CBioHioC], with MCI (M = Ti, Zr, Hf) in a 2 1 ratio in toluene affords new group 4 metallocene compounds rac- 5 1-CpCMe2-CBioHioC)2M (3a Ti, 3b Zr, 3c Hf). Compounds 3, which are stable in air for an extended period of time, can be also prepared in a stepwise fashion via (t)5-CpCMe2CBioHioCH)2MCl2 as outlined in Scheme 3. 

It is well known that ferrocene and related metallocene compounds undergo reversible oxidation by a variety of chemical oxidants. The particular ease with which ferrocene undergoes oxidation to its ferricinium cation allows one to determine the effect of a series of substituents on the oxidation potential of the ferrocene system. Thus a sensitive probe is available for determining the electronic effect a substituent has on the iron heteroatom and a device is made available by which one might elucidate the mechanism by which such an effect is transmitted from the substituent to the iron atom in the ferrocene system. It is unfortunate that lack of suitable derivatiyes has precluded similar studies in systems containing other transition metal atoms. 

Fig. 16.53 Siruciuros of some carbollyl metallocene compounds, (a) Dicarbollyl species M = Fc, II = 2, and M = Co. ii - I are isociectronic with ferrocene and cobaltocenium ion , M = Fe, a = I is isoelecironic with ferrocenium ion (b) mixed carbollyl-cydopenliidienyl amduguc of ferrocene, ii = I (c) mixed carbollyl-carbonyl compound. M = Mn. Re. [ a) and (b from Hawthorne. M, F. Andrews. T. D. J. Am. Chem. SiK 1965. iV7. 2496 (c) from Adler. R. G. Hawthorne, M. F. J. Am. Ctwm. Soc. 1970. 92, 6174. Reproduced with permission.]...

---