Metallocene hydride

Further studies quickly revealed that the rapid dehydrogenative coupling of primary organosilanes to oligomers and the slower coupling of secondary silanes to dimers can be effected under ambient conditions with compounds of the type CP2MR2 (M = Ti, R = alkyl M = Zr, R = alkyl or H)(11,12,13). None of the other metallocenes, metallocene alkyls, or metallocene hydrides of groups 4, 5 or 6 have shown any measurable activity for polymerization... 

Woo s combinative catalyst system of CpjMCE/Hydride is different from the catalyst systems using Cp2MCl2/2 alkyllithiums of Corey, Tanaka, and Harrod. Real catalytic species in the dehydrocoupling of hydrosilanes could be a metallocene hydride based on a sigma-bond metathesis mechanism.12,1315 Inorganic hydrides effectively produce a metallocene hydride whereas alkyllithium can produce a metallocene hydride via... 

Recently, trans insertion of hexafluorobutyne into one of the M—H bonds in some metallocene hydrides, Cp2MH , was studied in some detail (47). Experiments carried out in the presence of various radical-sensitive reagents such as TV-phenyl-a-naphthylamine suggested that a free radical mechanism was unlikely. A stepwise ionic mechanism, involving a zwitter-ionic intermediate, Cp2(H2)M+—C(CF3)==CCF3, is improbable, since (i) the stereochemistry and the apparent rate are not influenced by the polarity of the solvents, (ii) no deuterium is incorporated in the reaction in EtOD, and (iii) the trend in reactivity (Mo > W) does not reflect the trend in v-basicity or M—C bond stability (W > Mo). An essentially concerted trans-insertion mechanism is inferred, which is supported inter alia by the low kinetic deuterium isotope effect (kH/k0 = 1). 

Manganese complex, 46 Metallocene derivatives, Group 4, 18 Metallocene hydride, 14 Metallocenes, Group 4, mechanism proposed by Tilley, 14... 

The bis(i7-pentamethylcyclopentadienyl)zirconium dinitrogen complex [tj-C5(CH3)5kZrj(N2)3 (22), in solution reversibly releases its dinitrogen in vacuo over several hours (Section III,E). The solutions remaining are said to consist mainly of [T7-C5(CH3)5][T7-C5(CH3)4CH2]ZrH (23) (33). Permethylzirconocene [tj-C5(CH3 )5l,Zr (24) may exist in these solutions in equilibrium with this hydride tautomer (as in the case of titanium). However, thus far permethylzirconocene has not been isolated as a discrete compound or otherwise characterized. It is very likely that permethylzirconocene is less stable and more prone to form complex metallocene hydrides than permethyltitanocene (16). 

The dual function of the precatalysts 4 opened the way to well-controlled block polymerization of ethylene and MMA (eq. (5)) [89, 90]. Homopolymerization of ethylene (Mn = 10000) and subsequent copolymerization with MAA (Mn 20000) yielded the desired linear AB block copolymers. Mono and bis(alkyl/silyl)-substituted flyover metallocene hydride complexes of type 8 gave the first well-controlled block copoymerization of higher a-olefins with polar monomers such as MMA or CL [91]. In contast to the rapid formation of polyethylene [92], the polymerization of 1-pentene and 1-hexene proceeded rather slowly. For example, AB block copolymers featuring poly( 1-pentene) blocks (M 14000, PDI = 1.41) and polar PMMA blocks (M 34000, PDI = 1.77) were obtained. Due to the bis-initiating action of samarocene(II) complexes (Scheme 4), type 13-15 precatalysts are capable of producing ABA block copolymers of type poly(MMA-co-ethylene-co-MMA), poly(CL-co-ethylene-co-CL), and poly(DTC-co-ethylene-co-DTC DTC = 2,2-dimethyltrimethylene carbonate) [90]. 

One of the keys for understanding the reactivity of transient species is the study of their behaviour in solution. As an example, during the attempts to synthesise non hindered metallocenic hydrides, the synthesis of (C5H4 Bu)2SmH... 

Allylic activation represents a possible evolution path for the intermediate which is obtained after unimolecular /3-hydrogen transfer to the metal, that is, the olefinic chain end coordinated to the metallocene hydride complex. The DPT calculations of Ziegler and co-workers on the geometrically costrained H2Si(Cp)(NH)Ti-R+ system and of Brintz-inger and Prosenc on the Cp2Zr(methallyl)+ system ... 

The first mechanism to be widely accepted was proposed by TiUey et al., which is called a o-bond metathesis (Fig. 17) [75]. In this model a metallocene precursor is first transformed to a metallocene hydride that enters the hypothetic catalytic cycle, forming a metal silyl species via an initial a-bond metathetic step with the simultaneous production of hydrogen. The second o-bond metathetic step forms the Si-Si bond and is supposed to regenerate the catalytically active metal hydride. 

Describe a method for the preparation of the following metallocene hydrides ... 

Group 3 metallocene hydrides, which usually form dimers, are known to be single-component catalysts for ethylene polymerization [50]. The olefin inserted species, Cp 2Ln-R, is isoelectronic to active cationic species of group 4 metallocenes, and it can catalyze polymerization without such co-catalysts as MAO and borane compounds. Examples of isospecific a-olefin polymerization catalysts are, however, rather rare. 

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