Metallocenes complexes

EinaHy, in 1976, Kaminsky and Sinn in Germany discovered a new family of catalysts for ethylene polymerization. These catalysts (ie, Kaminsky catalysts) contain two components a metallocene complex, usually a zkconocene, and an organoaluminum compound, methylaluminoxane (8,9). These catalysts and thek various later modifications enable the synthesis of ethylene copolymers with a high degree of branching uniformity. Formally classified as MDPE, LLDPE, or VLDPE, the resins thus produced have a number of properties that set them apart from common PE resins in terms of performance... 

Metallocene Catalysts. Polymerization of cycloolefins with Kaminsky catalysts (combinations of metallocenes and methylaluminoxane) produces polymers with a completely different stmcture. The reactions proceeds via the double-bond opening in cycloolefins and the formation of C—C bonds between adjacent rings (31,32). If the metallocene complexes contain bridged and substituted cyclopentadienyl rings, such as ethylene(hisindenyl)zirconium dichloride, the polymers are stereoregular and have the i j -diisotactic stmcture. 

The metallocene complexes of M = Ti, Zr, and Hf are most stable when the two Cp groups are not parallel, in contrast to most other transition metal—Cp complexes. The most stable angle for the zirconium metallocenes is ca 40°, which partially accounts for the more interesting chemistry of these compounds compared to other transition metallocenes. 

Sumi K, Kumobayashi H (2004) Rhodium/Ruthenium Applications. 6 63-96 Suzuki N (2005) Stereospecific Olefin Polymerization Catalyzed by Metallocene Complexes. 8 177-215... 

The proposed mechanism for the metallocene complexes of Group 5 metals and lanthanides involves the coordination of alkene to the metal hydride (51), followed... 

Explain how a metallocene complex reduces the activation energy of the polymerization reaction. 

Cationic Metallocene Complexes as Active Species for a Metallocene/MAO System... 

Table 3. Catalytic activity of selected metallocene complexes activated with MAO for the polymerization of propylene... 

Cationic alkyl metallocene complexes are now considered the catalytically active species in metallocene/MAO systems. Spectroscopic observation has confirmed the presence of cationic catalytic centers. X-ray photoelectron spectroscopy (XPS) on the binding energy of Zr(3d5/2) has suggested the presence of cationic species, and cationic hydride species such as ZrHCp2 that are generated by /1-hydride elimination of the propagating chain end... 

The active species of the metallocene/MAO catalyst system have now been established as being three-coordinated cationic alkyl complexes [Cp2MR] + (14-electron species). A number of cationic alkyl metallocene complexes have been synthesized with various anionic components. Some structurally characterized complexes are presented in Table 4 [75,76], These cationic Group 4 complexes are coordinatively unsaturated and often stabilized by weak interactions, such as agostic interactions, as well as by cation-anion interactions. Under polymerization conditions such weak interactions smoothly provide the metal sites for monomers. 

Table 4. Bond lengths for cationic metallocene complexes... 

Eisch s work promoted investigation into the preparation of cationic metallocene complexes of Group 4 metals. Several preparative routes to cationic group 4 metallocene complexes are illustrated in Scheme II. Catalytic activities of some selected cationic metallocene complexes for the polymerization of a-olefins are summarized in Tables 5 and 6. The catalyst systems based on these cationic complexes are just as active as M AO-activated metallocene catalysts for the polymerization of a-olefins. 

Table 5. Catalytic activity of selected cationic metallocene complexes of Group 4 metals for the polymerization of ethylene... 

Neutral Group 3 Metallocene Complexes as Catalysts of Polymerization... 

Table 7. Catalytic activity of metallocene complexes of Group 3 metals... 

A scandium complex, Cp ScH, also polymerizes ethylene, but does not polymerize propylene and isobutene [125]. On the other hand, a linked amidocyclo-pentadienyl complex [ Me2Si( / 5-C5 Me4)( /1 -NCMe3) Sc(H)(PMe3)] 2 slowly polymerizes propylene, 1-butene, and 1-pentene to yield atactic polymers with low molecular weight (Mn = 3000-7000) [126, 115]. A chiral, C2-symmetric ansa-metallocene complex of yttrium, [rac-Me2Si(C5H2SiMe3-2-Buf-4)2YH]2, polymerizes propylene, 1-butene, 1-pentene, and 1-hexene slowly over a period of several days at 25°C to afford isotactic polymers with modest molecular weight [114]. 

Marks et al. reported the co-polymerization of ethylene and 1-hexene by using ansa-type complexes of lanthanide metals [127]. Recently, bulky alkyl substituted ansa-type metallocene complexes of yttrium have been reported to exhibit high activity for the polymerization of 1-hexane. [114, 119, 128]... 

Some non-metallocene complexes of lanthanide metals such as Sml2... 

Non-metallocene complexes, such as aryloxide 31 and amide 138, have also been utilized as catalyst systems for the polymerization of a-olefins. Moreover, the homogeneous olefin polymerization catalysts have been extended to metals other than those in Group 4, as described in Sect. 7. Complexes such as mono(cyclopentadienyl)mono(diene) are in isoelectronic relationship with Group 4 metallocenes and they have been found to initiate the living polymerization of ethylene. These studies will being further progress to the chemistry of homogeneous polymerization catalysts. 

Similar to the reaction of zirconacyclopropene 1, titanacyclopropene 14 reacted with C02 to give titanacycle 15 (Scheme 5) I0,i0a-i0c j-[owever5 the reaction of Cp TiC /Mg with l,4-bis(trimethylsilyl)-l,3-butadiyne did not afford a titanacyclocumulene species, but yielded titanacyclopropene instead 16, which on reaction with C02 gave the titanacycle complex 17.7 In the case of the titanium half-metallocene complex 18, the five-membered titanacyclocumulene 19 was obtained but the insertion of C02 took place only at one of the two Ti-carbon bonds, leading to the formation of 20 (Scheme 5),11 which is in contrast with what was observed in the case of the Zr analog 3. The... 

In summary, characterizations of stereosequences in polymers obtained by catalytic systems based on well-characterized metallocene complexes have produced a general acceptance of the chain migratory insertion mechanism and of models described in i-iii. 

In hydrogenation, early transition-metal catalysts are mainly based on metallocene complexes, and particularly the Group IV metallocenes. Nonetheless, Group III, lanthanide and even actinide complexes as well as later metals (Groups V-VII) have also been used. The active species can be stabilized by other bulky ligands such as those derived from 2,6-disubstituted phenols (aryl-oxy) or silica (siloxy) (vide infra). Moreover, the catalytic activity of these systems is not limited to the hydrogenation of alkenes, but can be used for the hydrogenation of aromatics, alkynes and imines. These systems have also been developed very successfully into their enantioselective versions. 

In Group IV metal complexes, metallocene complexes are the main catalyst precursors for hydrogenation. Two major catalytic systems have been used 1) Cp2MR2 (R=H, Alkyl, Aryl) and 2) Cp2MX2 in combination with alkylating agent or an hydride (Table 6.1). The catalytic tests are typically run with 50 equiv. of substrate per metal, but in some cases turnover numbers (TONs) exceeding 1000 can be achieved [35]. 

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