Cp2M and Metallocenes

Among polymerization catalysts, Ziegler catalysts (EtsAl and TiCLt), Natta catalysts (EtsAl and TiCT), and metallocene catalysts (Cp2M(Ti, Zr, Hf, Fe) and -(O-Al(R))n-(R=Me, Ft)) are used as vinyl polymerization catalysts. Ca, Hg, Zn, and Cd compounds and Ti, Ge, Sn, Pb, and Sb compounds are used for ester exchange and their polyester polymerization catalysts, respectively. 

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,... 

FIGURE 14.1 (See color insert following page 302.) Frontier molecular orbitals of (a) bent metallocenes (Cp2M M = Ti, Zr) and (b) carbene. 

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-... 

The 14-electron species denoted here as Cp2M (M = Ti, Zr), generated from Group 4 metallocenes, mediate useful oxidative cyclizations of dienes 231 [103,104], enynes 233 [105] and diynes 235 [105,106] via the metallacycles 232, 234 and 236. These metallacycles are useful intermediates, which are hydrogenolysed, carbonylated, halogenated, attacked by electrophiles and converted to functionalized cyclic compounds such as 238 and 239 [107],... 

One of the most important carbon 7i-donors are the cyclopentadienes and their heteroanalogues, for instance 15 and 16. In general, cyclopentadiene itself forms three general types of mononuclear Cp transition metal complexes Cp2M 3 (symmetric molecules with mutually parallel Cp rings examples M = Fe, Cr, Ni), Cp2ML 124 [bent metallocenes, L = H, R, CO, etc. (n=l-3)], and half-sandwich compounds CpML 125 (n — 1 4) [21b], In the bent sandwich complexes... 

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. 

FIGURE 27. General molecular orbital diagrams showing the construction of the orbitals of 14 electron Cp2M metallocenes, in parallel (D d) and bent (C2V) conformations, from the orbitals of the Cp2 and M fragments (only the occupied orbitals are shown). Adapted from Reference 389... 

The equilibrium geometries of Cp2M (M = Yb, Eu, Sm) were studied by ab initio pseudopotential calculations at the Hartree-Fock (HF), MP2 and CSID levels. In the Hartree-Fock calculations [118] all the metallocenes favoured regular sandwich-type equilibrium structures with increasingly shallow potential energy surfaces for the bending motions along the series, M = Ca, Yb, Sr, Eu, Sm, and Ba. 

Metallocene complexes of Ti, Zr, and Hf have attracted considerable attention in recent years because of their high activity and since their ligand framework can be tailored to a wide variety of polymerization requirements.102 The active species is the 14-electron cationic alkyl [Cp2M—R]+, with a pseudo-tetrahedral structure and a vacant site suitable for forming a weak complex with ethylene. Calculations show that the alkyl transfer to ethylene is stabilized by an a-agostic interaction with the metal, with a very low (ca. 2 kJ mol-1) activation barrier 103... 

A number of metallocene cyanate complexes have been reported for these metals. Samuel (648) and Coutts and Wailes (219) reported cp2M(CNO)2 (M = Ti, Zr) independently in 1966. Samuel made no definite assignments of the structure, but Coutts and Wailes suggested the compounds were JV-cyanato complexes. Later, Burmeister et al. (145, 146) reexamined the compounds in more detail and argued that they, and the hafnium analog, were 0-cyanato complexes. This was... 

Metallocene-thiocyanate complexes have been reported similar to the cyanate complexes described previously. There is general agreement that these are N-thiocyanato complexes, cp2M(NCS)2 (M = Ti, Zr, Hf) 145, 146, 219, 320, 648). The compound cpTi(NCS)3 has been prepared, and infrared measurements show it to be as Avritten 458) cp2Ti(NCS) is trimeric but the mode of bridging is not known 220). 

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