Zirconocene complexes structures

Fig. 1. The structure of the ethylene-zirconocene complex (SiH2Cp2)ZrCHj-C2H4. The corresponding titanocene has basically the same structure, except that the Ti-C distances are obviously different from the Zr-C distances.

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. 

Spectroscopic and structural data for some of the hydrido zirconocene complexes described above have been reported and discussed in references cited in this section (see also [75, 76]). 

C02-Bridged bimetallic zirconocene complexes have been formed from 1 and metallocarboxylic acids [229]. Reachon of 1 with metal enolates Cp(CO)3WCHR COX (X = OEt, Me, Ph) gives Cp(CO)3WCH(R )CH(R)OZrCp2(Cl). The structure for R = H and R = Me was solved by an X-ray analysis and the chemical reactivity of these organometallic products have been studied [230]. 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

Diphenyl zirconocene complex 102 is unstable at room temperature and undergoes electrophilic substitution at the alkylated cyclopentadienyl ligand to produce the boratacycle 44 in high yield (Equation 10). The reaction involves loss of 1 equiv of benzene from the central zirconium, and the X-ray crystal structure of 44 (detailed in Section 7.14.3) clearly shows interaction between the zirconium and one of the perfluorophenyl ligands attached at boron <2004CC1020>. 

Protonation of (/u.-alkenyl)bis(zirconocene) complexes under nonnucleophilic conditions takes a different regio-chemical course than the B(C6p5)3 addition (equation 27). The X-ray crystal structure of (60) is consistent with distorted square-pyramidal pentacoordinated geometry at the central atom Cl. 

Cationic ansa metallocenes can be utilized as chiral catalysts in Diels-Alder reactions. For example, in the presence of the cationic zirconocene complex [(ebthi)Zr(Ot-Bu) thf]+, the [4 + 2] cycloaddition of acrolein and cyclopentadiene proceeds efficiently to afford endo and exo cycloadducts (equation 71). In reactions in which methyl acrylate is used as the dienophile, cycloadditions occur with lower levels of enan-tioselection (23% ee), but with significantly higher degrees of diastereoselectivity (17 1 endo, exo). In these processes, recent studies demonstrated the great influence of chiral metallocene structure and the dramatic solvent effect. ... 

The activation barrier of this characteristic automerization process of (5-c -conjugated diene)zirconocene and -hafnocene complexes has proved to be very dependent on structure and substituents of the diene ligand (22, 45). The highest known activation energy was observed for compound 51, the zirconocene complex of Hoogeveen s diene ... 

Selected Structural Data for (s-cis-DiENE)ziRcoNOCENE Complexes (5) and for Bis(cyclopentadienyl)zirconaindan (16a) - ... 

By the nature of its molecular mechanism, the carbonyl-insertion reaction represents a typical reaction mode of o alkyltransition metal complexes. Formation of the new C—C cr-bond takes place during a 1,2-alkyl-migration step, transforming an alkylmetal carbonyl moiety [cts-M(CO)R] into an acylmetal unit (M—COR) (89). In general, (s-cir-diene)-zirconocene complexes 5 appear to exhibit a substantial alkylmetal character (90). Therefore, it is not too surprising that some members of this class of compounds [in contrast to most other dienetransition metal complexes (97)] react with carbon monoxide with C—C bond formation (45). However, as demonstrated by X-ray structural data for 5 (Tables V... 

The structural characterization of (775-C5H5)2Zr(772-C6H4)(PMe3) 255 in 1986 established the utility of formally divalent zirconocene complexes to stabilize otherwise reactive and transient organic molecules.131 The synthesis and reactivity of these species has been the subject of two recent reviews.74,132 Building on these seminal discoveries, formally low-valent zirconocene fragments have been used to stabilize other aryne species. Typically, these syntheses are achieved by... 

Gyclooctatrienyne zirconocene complexes of type 704 can be prepared by /3-hydrogen elimination from zirconocene biscyclooctatetraenyl complexes (Scheme 171).528 These complexes are fluxional by a ring inversion process, with activation barriers in the range typical of cyclooctatetraenes. An X-ray crystal structure (R = Ph) reveals a boat conformation of the cyclooctatrienyne ring with no significant flattening when compared with cyclooctatetraene, thus the structure more closely resembles substituted cyclooctatetraenes than that expected of a cyclooctatrienyne. 

Zirconocene complexes 705 that contain an acetylide ligand bridging between a main group metal (aluminum) and a transition metal (zirconium) are obtained by treatment of dimethyl zirconocene with (alkynyl)dimethylaluminum, (Equation (43)).529 In this reaction, an ( 72-alkyne)zirconocene complex is presumably formed in situ, and it is then trapped by the excess (alkynyl)dimethylaluminum to yield the final product. The molecular structures of the complexes 705 (R = SiMe3, Cy) contain a dimetallabicyclic framework, and one of the bridgehead positions is a planar tetracoordinate carbon center. In these complexes, the -C=CR bridge between zirconium and aluminum can be described as being mainly of /x-(cr-acetylide) character. 

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