Metallocene/borate complexes

Another group of compounds that were recently reported as capable of initiating cationic polymerizations are metallocene/borate complexes. Such material can, for instance, be generated from zirconocene dimethyl compounds (Cp2ZrMe2) and anUinium borate. Thus, [HNMe2Ph] -1- [B ( 5115)4] will polymerize amine-functionalized a-olefins [51] as well as isobutylene homo and copolymerizations [52]. Also, when compounds, like Cp MMe3 (M = Ti, Zr, and Hf where... 

The reactions of bis(alkynyl)metallocenes Cp2M(C=CR)2 (M = Zr, Hf R = Me, Pr11, Bu11, Gy) with B(C6F5)3 afford the corresponding metallocene borate betaines of structure type 760.586 An assumed intramolecular alkyne insertion reaction leads to their less stable methylenecyclopropene-derived isomers, which are effectively trapped by the added / /7-butylisocyanide to yield complexes 764 which contain a methylenecyclopropene derived [Pg.906]

Hi) Borate, N-heterocyclic carbene, and carborane Atrispyrazolyl borate-based complex (Hgure 2, Ru-23), which is isoelectronic to the 18-elertron half-metallocene-type complexes, also induced living radical polymerization of MMA either with or without additives, in which the rates and molecular weights were not changed upon the addition of Al(Oi-Pr)3. ... 

The exchange of zirconium in isostructural complexes leads to a new family of asymmetric metallocenes (Fig. 1) bearing a 2-methyl substituent and varied substituents in positions 5, 6, and 7 of the indenyl moiety. After borate activation all catalysts show an unexpected high and constant activity toward the polymerization of propylene and lead to significantly increased molecular weight products compared to the zirconocene species [9-11],... 

Most of the spectroscopic investigations discussed above were carried out on well-defined metallocene systems, either isolated species or those generated from a well-defined metallocene alkyl precursor activated with one equivalent of a borane or borate activator. Most practical polymerisation catalysts, on the other hand, include a scavenger, usually an aluminum alkyl, and may contain ill-defined activators such as methylaluminoxane (MAO), usually at high MAO/Zr ratios. Such systems are less amenable to quantitative studies nevertheless, the identifications of species such as those depicted in Schemes 8.5-8.8 has enabled similar compounds to be identified in more complex mixtures. An idea of the possible mode of action... 

Keys to the high polymerization activities of single-site catalysts are the cocatalysts. MAO is most commonly used and is synthesized by controlled hydrolysis of trimethyl aluminum. Other bulky anionic complexes which show a weak coordination, such as borates, also play an increasingly important role. One function of the cocatalysts is to form a cationic metallocene and an anionic cocatalyst species. Another function of MAO is the alkylation of halogenated metallocene complexes. In the first step, the monomethyl compound is formed within seconds, even at -60°C (69). Excess MAO leads to the dialkylated species, as shown by NMR measurements. For the active site to form, it is necessary that at least one alkyl group be bonded to the metallocene (70). 

Cationic or dicationic metallocene complexes can also be formed by reactions of perfluorinated triphenylborane or trityltetrakis(pentafluorophenyl)-borate (86). 

The cocatalyst has various functions. The primary role of MAO as a cocatalyst for olefin polymerization with metallocenes is alkylation of the transition metal and the production of cation-like alkyl complexes of the type Cp2MR+ as catalytically active species (91). Indirect evidence that MAO generates metallocene cations has been furnished by the described perfluorophenyl-borates and by model systems (92, 93). Only a few direct spectroscopic studies of the reactions in the system CP2MCI2/MAO have been reported (94). The direct elucidation of the structure and of the function of MAO is hindered by the presence of multiple equilibria such as disproportionation reactions between oligomeric MAO chains. Moreover, some unreacted trimethylaluminum always remains bound to the MAO and markedly influences the catalyst performance (77, 95, 96). The reactions between MAO and zirconocenes are summarized in Fig. 8. 

Dialkyl metallocenes and other dialkyl Group 4 transition metal complexes are useful as precatalysts in combination with co-catalysts such as tris(perfluoro-aryl)boranes or tetrakis(periluoroaryl)borate salts [18]. Recently, an expedient procedure for the production of dimethyl metallocenes and Cp-amido dimethyl metal complexes in high yields and purity has been reported. The direct synthesis of Group 4 dimethylmetallocenes [19] consists of the one-pot reaction between the r-ligand, a 2-fold excess of MeLi, and MtCU. This simple method produces the dimethylated complexes in higher overall yield, and saves on reaction time and solvents, compared to the classic two-step route, which consists in the synthesis of the metallocene dichloride followed by its methylation with 2 equiv. MeLi. 

The base-free dimethyl Sc complex 121 was a highly active catalyst precursor for ethylene polymerization under B(C6F5)3, trityl borate, or methylaluminoxane (MAO)-type activation. The catalytic activity of 121 was similar to those observed of Group 4 metallocene complexes [81]. Generally, cationic scandium complexes are believed to be the active species. Activation of the catalyst was studied by reacting 120 and 121 with various equivalences of B(C6F5)3. The monomeric bulky rBu-substituted dimethyl complex 121 reacted with 1 equiv of B(C6F5)3... 

The B shifts of a series of metal complexes of poly(l-pyrazolyl)-borates, M[H2- B(pz)2+ ]2> where pz = 1-pyrazolyl group, have been discussed.A rather different type of complex of iron with the 1,3,4-triaza-2,5-diborine system has been described by Noth and Regnet. Two B resonance signals were observed at —29 7 and —26-8 p.p.m. (from Et20,BF3) and the complex was considered to have a metallocene structure. 

A recent exhaustive review [1] presents the known and well characterised lanthanide hydrides. Recently, a few additional original complexes have been synthesised. These include phospholyl [7] and indenyl derivatives [8], a bimetallic metallocenic structure (Scheme 1) [4], a samarium hydride supported by a calix-tetrapyrrolyl ring, with a triply bonded hydride ligand (Scheme 2) [9], metallocenes with chiral groups which has been used for the asymmetric hydrogenation [10], a tris(pyrazolyl)borate ytterbium(II) hydride [11], Cp amido-[12] and bis(amido)yttrium hydrides [13]. The latter was not isolated but obtained in situ from the methyl derivative (Scheme 2). 

Evidence has now been presented that indicates that the above compound behaves as a carbocationic polymerization initiator for styrene, W-vinylcarbazole, vinyl ethers, and isobutylene. The mechanism of initiation and polymerization of these monomers by such metallocene complexes is still being investigated. It was suggested by Wang et al. [53], that the mechanism of carbocationic polymerization of such olefins by the above complex would involve coordination of the olefins, as shown below, in a nonclassical p -fashion, with the metal-olefin. This interaction is stabilized by a complementary borate-olefin interaction. The next step in the polymerization process by this mechanism, then involves attack on the carbocationic centers of the metal ions-activated olefin molecules by secondary olefin monomers, followed by chain growth [53] ... 

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