Metallocenes cationic complex

The structure of the metallocene cation energy minimised with the Car-Parrinello method agrees well with the experimentally obtained crystal structures of related complexes. Typical features of the structure as obtained from X-ray diffraction on crystals of very similar neutral complexes (e.g., the dichlorides), such as small differences in distances between C atoms within a cyclopentadienyl (Cp) ring, as well as differences in distances between the C atoms of the Cp ring and the Zr atom, were revealed from the simulations. 

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. 

The use of weakly coordinating and fluorinated anions such as B(C6H4F-4)4, B(C6F5)4, and MeB(C6F5)3 further enhanced the activities of Group 4 cationic complexes for the polymerization of olefins and thereby their activity reached a level comparable to those of MAO-activated metallocene catalysts. Base-free cationic metal alkyl complexes and catalytic studies on them had mainly been concerned with cationic methyl complexes, [Cp2M-Me] +. However, their thermal instability restricts the use of such systems at technically useful temperatures. The corresponding thermally more stable benzyl complexes,... 

Collins and co-workers have performed studies in the area of catalytic enantioselective Diels—Alder reactions, in which ansa-metallocenes (107, Eq. 6.17) were utilized as chiral catalysts [100], The cycloadditions were typically efficient (-90% yield), but proceeded with modest stereoselectivities (26—52% ee). The group IV metal catalyst used in the asymmetric Diels—Alder reaction was the cationic zirconocene complex (ebthi)Zr(OtBu)-THF (106, Eq. 6.17). Treatment of the dimethylzirconocene [101] 106 with one equivalent of t-butanol, followed by protonation with one equivalent of HEt3N -BPh4, resulted in the formation of the requisite chiral cationic complex (107),... 

It is now well recognised that the active species is a cationic complex, or more precisely a solvent-separated or tight ion pair, the structure of which depends on the mode of catalyst activation. Early spectroscopic and synthetic studies on metallocene dimethyl precursors helped to outline the principal reaction pathways, these have been reviewed [16, 21, 23]. Some of this chemistry is briefly summarised here since it presents the background for the understanding of later studies on methyl-aluminoxane (MAO) systems. 

The data discussed in Sections 8.5 and 8.6 make it clear that in the low-dielectric media typically employed for polymerisation reactions, the counteranions in metallocene ion pair catalysts are closely associated with the cationic complex as either inner-sphere or outer-sphere ligands. If anions are coordinated in the transition state, they must be expected to exert a significant influence on the stereochemistry of alkene polymerisation, even though the formation of syndiotactic and isotactic 1-alkenes have been readily explained by considering only the cationic metallo-cenium species and their ligand structure [21, 23, 122, 132, 133]. 

Reaction of the anion 21 with Cp or Cp metal fragments provides further metallocene-type complexes with a pendant phosphaferrocene side-chain. For example, the reaction of the thallium derivative T1 21 with [Cp RhCl2]2 yields the cationic pentamethylrhodocenium 24 as its chloride (Scheme 1.5.10). This is an interesting species because it is a chiral water-soluble P ligand. The chloride anion can be exchanged by PF,s to make the compound more soluble in organic solvents. 

The pentazolate anion. Ns (11.2), is estimated to have a half-life of 2.2 days, whereas that of the parent pentazole HN5 is predicted to be only ca 10 min in methanol at 0 Although HN5 is unknown, the cyclic anion N5 has been detected by tandem mass spectrometric studies of 4-hydroxyphenylpentazole. Similarly to its congener P5 (Section 11.2), N5 (isoelectronic with cyclopenta-dienide [C5H5] ) has the potential to form metallocene-like complexes. The acyclic (V-shaped) cation Ns has been isolated as a hexafluoroantimonate salt, which decomposes at ca 70 °C. The estimated energy density of [N5] [N5] is approximately twice that of hydrazine, a well-known rocket propellant, suggesting that this ionic polynitrogen allotrope would be an excellent monopropellant... 

Homogeneous alkylaluminium-free olefin polymerisation catalysts also comprise non-metallocene cationic group 4 metal complexes such as those with benzyl ligands [162]. A distinct group of alkylaluminium-free homogeneous olefin polymerisation catalysts consists of nickel complexes [181-183],... 

It must be emphasised that the relatively low activation energy of the insertion reaction in this complex is characteristic of complexes having formally a d° 16-electron configuration, which is just adopted by the cationic group 4 metallocene species complexed with a coordinating olefin as the two-electron donor [136]. 

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. 

Even at -60° C, the metallocene and MAO form a complex, as shown by a new absorption maximum in the infrared spectrum (97). Following complexation, rapid alkylations and dissociation into an ion pair occur. An equilibrium is set up between the ion pair of the cationic metallocene and the anionic MAO and the resulting complexes (98). Both systems showpoly-merization activity, but the cationic complex is significantly more active. The two active centers show differences in the molecular weights of the resulting polymers (99). 

Reaction of bromopentafluorobenzene (20) with butyllithium leads to the biphenyl 22 via per-fluorobenzyne (21). This dimerization can be conducted to give a 91 % yield and the bro-mononafluorobiphenyl 22 obtained can be converted to tris(2.2, 2"-nonafluorobiphciiyl)-borane, which is used in preparing cationic complexes of metallocenes. ... 

The active species in B(C6F5)3-activated metallocene catalysts is an ion pair, consisting of an electron-deficient cation, such as [Cp2ZrMe]+, stabilised by a weakly coordinating anion, here [MeB(C6F5)3]". One aspect of our research recently has been the attempt of building the Lewis acidic activator function into the metallocene precursor complex, in an effort to synthesise self-activating systems. The principle is illustrated... 

The active species in MAO cocatalyzed metallocene polymerization is likely a coordinatively unsaturated cationic complex, as in 9-24 (for a zirconium-based metallocene), where substituents on the metal atoms have been omitted for clarity. The Zr—O—A1 bond withdraws electron density from the Zr atom. The mechanism for ethylene polymerization is proposed to be as follows, where the monomer forms a t-complex with the transition metal [ 14] ... 

Resurgent interest in Ziegler-Natta polymerization was triggered by Kaminsky s report of the high catalytic activity of Cp2ZrX2 and, to a lesser extent, the Hf counterpart coupled with methylaluminoxane (MAO) in a-olefin polymerizations [4]. There is evidence that the cationic complexes play an active role stereoregulation, isotactic or syndiotactic, became possible with the advent of structurally tailored metallocenes (Fig. 12) [27]. 

The alumoxane cocatalysts have at least two functions alkylation of the metallocene component, which takes place within seconds even at -60 °C (eq. (2)) and formation of the active species by abstraction of Me" (eq. (3)). The resulting active species is discussed as being a 14e (= 14 valence electron) cationic alkylmetallocenium ion formed by dissociation of the metallocene alumoxane complex [26,27]. The [alumoxane-Me]" anion is regarded as weakly or non-co-ordinating. Nearly every zirconocene atom is active, forming a single-site catalyst [28, 29]. 

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