Zirconium bimetallics

Bimetallic Transition Metal-Zirconocene Complexes from Zirconium Hydrides... 

Interestingly, Hoveyda and coworkers observed a second-order dependence of the reaction rate on the concentration of zirconium in these reactions, suggesting that the zirconacyclopentane is formed from a bimetallic alkene-zirconate complex such as A in Fig. 1 [21]. This finding suggests that olefin alkylations and substitutions occur via reaction of a nucleophilic alkene unit [23]. 

Figure 4 shows the remarkable structural similarity between the bimetallic carbene (1 2) and alkoxy complexes formed from diverse paths 1,2 addition of Zr-H to a carbonyl bound to tungsten (eq. 1) and 1,1 addition of Re-H to a zirconium-bound acetyl (eq. 2). 

It has recently been established that carbozirconation with organylzirconocene derivatives evidently requires dipolar (and mostly bimetallic) activation and/or small-ring zirconacycles, especially three-membered ones, that can undergo cyclic carbozirconation (Generalization 6 ) [149]. One of the recent examples has been shown to involve both bimetallic and cyclic organo-zirconium species [150], These reactions can be either stoichiometric or catalytic in Zr. 

The impetus for the development of gem-bimetallics was initially to discover alkylidene-transfer reagents akin to Tebbe s reagent [14]. Schwartz prepared bimetallic aluminum—zirconocene derivatives by the hydrometallation of various vinyl metallic compounds [15—17]. Knochel has developed zinc—zirconium gem-bimetallics by hydrozircona-tion of vinylzincs and has used them as alkylidene-transfer reagents [18], More recently, other gem-bimetallics have been developed that exhibit different reactivities of the two carbon—metal bonds. Thus, Normant and Marek have reported the allylmetallation of vinyl metals to afford zinc—magnesium and zinc—lithium gem-bimetallics, which react selectively with various electrophiles such as ClSnBu3, H20, etc. [19, and references cited therein]. However, selective and sequential cleavage of the two carbon—metal bonds... 

This chapter is intended to highlight the synthesis and use of geminal bimetallic compounds such as gem-metallozirconocenes, but we will concentrate on the chemistry of aluminum, boron, lithium, gallium, germanium, tin, zinc, and zirconium. 

Figure 7.4. Crystal structure of the 1,1-bimetallic complex of boron and zirconium 22. Adapted by the authors.

Treatment of alkylidene-bridged zirconium—aluminum species with HMPA activates the C—A1 bond of the alkylidene unit, making it susceptible to electrophilic attack [146]. Ligand-based activation of the C—A1 bond can also be used to convert alkylidene-bridged zirconium-aluminum reagents to other bimetallic species. Thus, treatment of 3 with HMPA followed by addition of a weakly electrophilic metal salt can give rise to a new heterome-tallic species. Slow addition of a solution of R3SnCl in toluene to a solution of 3 and 1... 

Despite the formal similarity of the reaction, the mechanism of Cp2ZrCl2-catalyzed ethylalumination [64] with AlEt3 is different from that of either methylalumination with AlMe3 or ethylalumination with Et2AlCl [62]. The involvement of dimetallic species was confirmed by NMR spectroscopy as well as deuterolysis (Scheme 8.31). The proposed mechanism features an interesting zwitterionic bimetallic species, in which the zirconium center is cationic. A highly instructive treatise on the mechanistic pathways of carbometalation is presented in [65],... 

The same reason was given in the literature for the formation of a number of other bimetallic oxocomplexes such as Cd4Sn4(p4-O)(OCH2Bu,)10(OAc)10 [325] and [PbTi2(p4-0)(0Et)7(0Ac)]2 [320]. When the alkoxides of zirconium, hafnium, or cerium are used for this reaction, the formation of the oxocomplexes can be due to the elimination of an unsaturated hydrocarbon from the initial alkoxide [1565], leading to the formation of monometallic oxoalkox-ides as intermediates (the latter are formed already on desolvation of [M(OR)4(ROH)]2 and are always present as admixtures in the samples of des-olvated M(OR)4, where R is a primary or secondary radical [1612] (see also Section 12.12). In this case the possible sequence of transformations (taking place usually on reflux in toluene) can look as follows ... 

No formation of bimetallic complexes is observed on MoO(OPr )4 dissolution in the alcohol solutions of zirconium isopropoxide, due presumably to the high stability of the structure of the latter. A very unusual complex of Zr3Mo,024(OPri)12( PrOH)4 composition precipitates slowly from solutions of isopropoxides in hexane subjected in advance to evacuation to dryness and redissolution repeated three times. The structure of the complex obtained is very close to that of the zirconium methoxide hydrolysis product, Zr O OMe) (Fig. 5.1 c) [901]. The formation of a complex very rich in oxoligands is presumably due to the trend of ZrCOPhV PrOH to form oxocomplexes on desolvation (see Section 12.12). 

The application of achiral cationic zirconocene compounds for methyl methacrylate polymerisation, e.g. a mixture of [Cp 2ZrMe(THF)]+[BPh4] and Cp 2ZrMe2 in methylene chloride solution, leads to the formation of syndio-tactic poly(methyl methacrylate). The species responsible for propagation are believed to be the bimetallic ones, involving cationic zirconium enolate and neutral zirconocene, which facilitates the process. Propagation is postulated to occur via the Michael reaction between the coordinating monomer and the cationic enolate [537] ... 

The ability to modify the backbones of LMP structures with phosphonates allows for wide flexibility in the design of new materials containing photoactive binuclear metal cores. The goal of our initial studies has been to demonstrate that a ligating functionality within the gallery is accessible for reaction with a bimetallic core. To demonstrate these initial objectives, we have chosen to study zirconium phosphate modified with alkyl carboxylate, which is a good ligand of bimetallic cores. 

Stable zirconium, platinum, molybdenum, and tungsten complexes of cyclooctyne, a zirconium complex of cydoocta-5-enyne, and a bimetallic molybdenum complex of cyclocta-3,7-dienyne have been discussed in earlier reviews.28 More recently, two stable zirconocene complexes of cycloocta-trienyne (275 and 276) have been prepared101 by /3-hydride elimination from 274 in the presence of PMe2R [Eq. (45)]. 

With increasing zirconium concentration the molecular weight decreases nearly linearly. This leads to the conclusion that chain transfer occurs via a bimetallic mechanism. 

Zinc and zirconium 1,1-bimetallic reagents 4.104, prepared by the hydrozirconation of alkenylzinc halides 4.103, react with carbonyl compounds to produce alkenes with high R-stereoselectivity (Scheme 4.51). Ketones give an E/Z mixture of stereoisomers . 

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