Metallocene dichlorides

The metallocene dichloride of zirconium and hafnium 20b and 20c were also prepared and underwent reduction with potassium to give monomeric metallocene monochloride complexes 21b and 21c (Eq. 8) [39b]. The structure of the zirconocene complex 21 b in the crystal showed a conformation which suggests a less steric strain as compared to 21a due to zirconium s larger atomic size. As a consequence of the coordinative unsaturation an unusually short Zr —Cl bond length was found. 

In metallocene-catalyzed polymerization reactions the active species is built through the reaction of a metallocene dichloride with the Lewis acid MAO. In the first step, a monomethyl-monochloro complex is formed and with an excess of MAO a dimethyl complex is created. In a fast equilibrium reaction through an... 

These conditions were optimized for Cp2HfCl2 activation, but are also applicable to the zirconocene version of the reaction. In the original procedure, a large excess of the metallocene dichloride and silver salt was employed to enable the rapid glycosidation of sensitive substrates, but this is not usually necessary. 

Although metallocene silyl and hydride complexes are the active species, many researchers have sought to develop more convenient precatalysts. The original dimethyltitanocene system reported by Harrod is reasonably easy to prepare, but Corey140 and others have shown that in situ catalyst generation from metallocene dichlorides (Ti, Zr, and Hf) and "BuLi is both simpler and equally effective. However, Harrod and Dioumaev have shown... 

In most cases that were reported in the literature so far, the (s-cis-ri4-diene)metallocene complexes (and their s-trans- counterparts), were synthesized by treatment of the respective bent metallocene dichloride precursor with a suitable conjugated-diene-magnesium reagent. However, in a few cases alternative entries and procedures have been developed and described. None of these has found such a widespread application as the original procedure, but some interesting chemistry has evolved among those methods. 

Most of the Group 4 dehydrocoupling catalysts just listed are photochemi-cally and/or thermally unstable with short shelf lives at room temperature. They are also generally not commercially available and have to be synthesized from a metallocene dichloride. There are, therefore, obvious advantages to catalysts that can be generated in situ. Corey and co-workers49,50... 

Toney JH, Marks TJ (1985) Hydrolysis chemistry of the metallocene dichlorides M(r)5-CsHs CL, M = titanium, vanadium, or zirconium. Aqueous kinetics, equilibria, and mechanistic implications for a new class of antitumor agents. J Am Chem Soc 107 947-953... 

Kopf-Maier P, Krahl D (1981) In vitro treatment with metallocene dichlorides determination of the intracellular distribution of the metal atoms by use of the electron energy loss spectroscopy. Cult Tech, Symp Prenatal Dev, 5th 509-517... 

McLaughlin ML, Cronan JM Jr, Schaller TR, Snelling RD (1990) DNA-metal binding by antitumor-active metallocene dichlorides from inductively coupled plasma spectroscopy analysis titanocene dichloride forms DNA-Cp2Ti or DNA-CpTi adducts depending on pH. J Am Chem Soc 112 8949-8952... 

Metallocenes may also be fixed on silica or alumina by direct reaction of the two components. Marks has shown that reaction of dimethyl metallocenes with alumina results in the formation of an active catalyst [174]. Others have investigated the direct reaction of metallocene dichlorides with silica (and alumina) and faced the problem of metallocene decomposition [175-177]. Nevertheless an active species is formed which produces isotactic polypropene but with rather low activities. 

These important compounds with regards to their reactivity are obtained by transmetalation see Transmetalation) between the appropriate metallocene dichloride (see Metallocene Complexes) and an alkyl- or aryl-lithium reagent. Dialkyl and diaryl zirconium complexes are very moisture-sensitive, degrading to form /r-oxo species (see Section 5). 

Other complexes of great interest are the / -conjugated diene metallocenes. They are conveniently prepared by the one-pot reaction of the photochemically induced metallocene with the diene at low temperature. Alternative routes include the treatment of the metallocene dichloride with a conjugated diene dianion equivalent or the coupling of alkenyl ligands in the coordination sphere of the metal. The resulting systems show unusual behavior. For example, buta-1,3-diene directly leads to the s-trans complex (8), which can be thermally equilibrated into the s-cis isomer (9). This form exhibits a substantial metallacyclopentene character and is best described as a d metal (cr, 7r)-type complex (10), as shown by X-ray crystal analysis. On the other hand, the s-trans isomer should be formally regarded as a real d metal -alkene 7T-complex. 

Treatment of Cp2ZrCl2 with LiAlH4 and then HsGaNMes or reaction of the preformed hydride Cp2ZrH2 with H3 AINMes affords the heterobimetallic complex (15). Boron-containing heterobimetallic complexes (16) can be prepared by reaction of metallocene dichlorides with the cyclic hydroborate anion [H2BC5H10] then reduction of MX l bond with LiH. 

It is generally assumed that MAO firstly undergoes a fast ligand exchange reaction with the metallocene dichloride ( - Me). Either CH or CH3 is further abstracted from the complex by an Al-center from MAO, thus forming a cationic species (step 1). In the step 2, the monomer (alkene) coordinates to the empty orbital of the complex. Step 3 is related to the insertion see Insertion) of the alkene into the zirconium-alkyl bond which generates a new free... 

Polyoximes (77) with the molecular weight of 10 -10 were obtained by interfacial synthesis of different dioximes (solved in NaOH) and metallocene dichlorides (solved in chloroform) (Eq. 35) The solubility of polymers depends on chain length and asymmetry of R. ... 

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 reduction potential of a metallocene dichloride complex is one indicator of the electron density at the metal center. A systematic study of the one-electron reduction potential for the series (C5H5 Me )2ZrCl2 ( = 0 — 4) revealed a shift to more negative potential by 0.071 V per methyl group. The deviation from this trend for n = 5 was... 

Disubstituted fulvenes are a common source for alkyl-substituted Cp ligand. The reaction of the in-situ generated Bu ZrCV species with 2equiv. of substituted fulvenes in hydrocarbons produces alkyl-substituted zirconocene dichlorides 575 in high yields (Scheme 130).403 6,6-Disubstituted fulvenes were also used to synthesize sterically hindered metallocene dichlorides 576 and 577 via the salt metathesis route.404... 

Other ligand variations have resulted in a large number of metallocene dichloride complexes these include cc-phenylalkyl-substituted Cp zirconocenes,417 mixed zirconocenes of the type (L)(Ind)ZrCl2 (L = o -phenylalkyl-substituted Cp, 1-phenylsilyl-substituted Ind) and (L)(Flu)ZrCl2 (L = 1 -benzyl-substituted Ind or 2-benzyl... 

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