Cyclopentadienyl Complexes Metallocenes

CyclopentadienylthaHium and its alkylated derivatives are used in the synthesis of metallocenes (qv) and other transition-metal cyclopentadienyl complexes (29). 

Metallocenes with substituted cyclopentadienyl rings. Metallocenes with methylated rings were among the first heavy alkaline earth metallocenes to be structurally characterized, but many other substituents have been incorporated into bis(cyclopentadienyl) complexes. Under this classification are included compounds with indenyl ligands, which in... 

The polymerization of ethylene was also qualitahvely inveshgated by pulse injec-hons of ethylene into helium flowing over thorium (67) and uranium (86) metallocene hydrocarbyl complexes supported on 7-AI2O3.950 at 25 °C, both revealing similar achvihes [171, 173]. Supported thorium half-sandwich complexes 65 exhibited higher achvity than surface species, resulhng from coordinatively more saturated tris(cyclopentadienyl) and metallocene U/Th-alkyl/hydride complexes, that is, 77, 79, 82, 90 and 91 [171]. C CP MAS NMR spectra revealed no clear evidence of ethylene insertion into [Th-CHs] or [AL5-CH3] moiehes of material... 

A number of molecular mechanics studies of metal-cyclopentadienyl complexes have been reported recently. The systems studied include linear metallocenes (in particular ferrocene), ferrocene derivatives (such as complexes with substituted cy-clopentadienyl ligands, bis(fulvalene)diiron complexes, ferrocenophanes and mixed-ligand complexes with carbonyls and phosphines), and nonlinear cyclopentadienyl complexes 8,153,221 231]. 

Two communications on propene polymerization by non-metallocene catalysts that include DFT/MM calculations have been recently published [60, 61]. They deal with group 4 bidentate non-cyclopentadienyl complexes. In the first communication [60], the topic addressed is the fact that a C2-symmetric precatalyst of titanium leads to a syndiotactic polymer, contrary to observations of metallocene catalysts. The chirality at the metal center is found to play a key role in the stereocontrol of the process. The second communication [61] addresses the fact that a C2-symmetric precatalyst of zirconium very similar to the previous one produces an isotactic polymer, finds out that it is due to a complicated concourse of synergic steric and electronic effects, and emphasizes the key role that serendipity still plays in the design of new catalysts. 

Germyl metallocene derivatives of the early transition metals, Ti, Zr and Hf (M) have been synthesized by the combination of metallocene halides and germyl anions (equations 4 and 5)38-40. The cyclopentadienyl complexes Cp2M(GePh3)Cl39 are difficult to purify due to their instability in solution however, use of the pentamethylcyclopen-tadienyl group(Cp ) enhances both the solubility and stability of these complexes40,41. 

The stereocontrol in the polymerization of MMA mediated by chiral organolanthanide metallocenes has been investigated. Flydride, hydrocarbyl, and amido bis(cyclopentadienyl) complexes (Scheme 268) were used as precatalysts for the processes. The achiral hydride precatalysts yielded syndiotactic PMMA with narrow polydispersities (Mw/Mn = 1.02-1.05). The polydispersities of PMMA from the chiral and achiral hydrocarbyl precatalysts are larger then those from the hydride precatalysts, and amide precatalysts gave the largest polydispersities.982... 

Broad or bimodal molecular mass distributions have been obtained using mixtures of metallocenes,452 487-491 linked homo- and heterobimetallic bis(cyclopentadienyl) complexes for broad molecular mass distribution,492 or amino-functionalized bis-Cp complexes.476... 

Although a large share of the EP co-polymers and EPD terpolymers is still manufactured with vanadium-based catalysts at low temperature, metallocene catalysts have added a whole new dimension to EP co-polymerization, and to the range of material properties that can be achieved, as has been the case for the ethylene-based low-density copolymers discussed in the previous section. The subject of EP co-polymerization with group 4 bis(cyclopentadienyl) complexes has been reviewed in detail up to 1998.59... 

The closest analogues of the d-block metallocenes, for the f-block elements that have been examined by P.E.S., are the tris- and tetra-cyclopentadienyl complexes, M(r7-C5H5)3 and M(t -C5H5)4. For uranium the tris cyclopentadienyl retains a donor ligand in the vapour phase and has been measured as U(i7-C5H5)3 THF. 

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