Cyclopentadienyl group

Some simple zirconium organometaUic compounds, such as tetramethylzirconium [6727-89-5] are known. In general, these compounds are very unstable. It appears that zirconium must be TT-bonded to at least one moderately large ligand, such as a cyclopentadienyl group, for the compound to be stable. The abbreviation Cp is used here for the cyclopentadienyl group and Cp for [0 (0112)5]. 

Beryllium forms a series of cyclopentadienyl complexes [Beftj -CsHiY] with Y = H, Cl, Br, Me, —C=CH and BH4, all of which show the expected C5, symmetry (Fig. 5.10a). If the pe/ifo/topfo-cyclopentadienyl group (p. 937) contributes 5 electrons to the bonding, then these are all 8-electron Be complexes consistent with the octet rule for elements of the first short... 

Dicyclopentadienyltin(II) is readily prepared from cyclopentadienyl-lithium or -sodium and tin(II) chloride, and is the best known, monomeric, RtSn compound. The planes of the 7)-cyclopentadienyl groups subtend an angle of 55°, and the unshared pair of electrons can act as a ligand towards such Lewis acids as BF3 and AICI3 i319). 

The cyclopentadienyl groups are readily displaced by protic acids HX (e.g., alcohols, phenols, thiols, and oximes), providing a convenient route to other Sn(II) compounds (320-323). 

Noteworthy NMR studies involving nuclei other than phosphorus have been carried out for some P-chloro-NHPs where the possible occurrence of spontaneous P-Cl bond dissociation was probed by II NMR titrations and 35C1 NMR [20], and for P-cyclopentadienyl derivatives where measurement of solid-state 13C CP-MAS NMR spectra allowed one to substantiate the preservation of the circumambulatory ring migration of cyclopentadienyl groups in the solid state [47], Several neutral and cationic derivatives have also been studied by 15N NMR [20, 53],... 

Two equiv. of 6,6-di(cyclopropyl)fulvene react at 60 °C over a period of a week with Ca[N(SiMe3)2]2-(THF)2 bis in THF to yield the metallocene 170. The heteroleptic amido complex 171 is detected as an intermediate with 111 and 13C 1H NMR spectroscopy. A 1 1 reaction of the calcium amide and 170 also produces 171 in solution, an equilibrium involving these three derivatives exists (Equation (30)). The calcocene 170 crystallizes at — 20 °C from THF as colorless cuboids. The metal center is surrounded by the four ligands in a distorted tetrahedral manner, and the cyclopentadienyl group and the propylidene fragment are coplanar with each other.393... 

The reaction of 2equiv. of a pyrrole-substituted cyclopentadiene with Zn[N(SiMe3)2]2 (Scheme 19) afforded the dicyclopentadienylzinc complex 23.52 A solid-state structure of this compound was not obtained, but room-temperature 1H NMR spectroscopic studies showed two equivalent cyclopentadienyl groups, whose signals broadened on cooling. 

At present, the difficulties described in 7-9 can be solved. The use of MAO can be avoided by the use of non-MAO catalysts such as cationic Group 4 metallocenes or neutral Group 3 metallocenes. It has been found that the molecular weight of the resulting polymer can be increased by introducing substituents at the 2,5-position in the cyclopentadienyl group of the... 

Cp SnCp, Cp SnCp 13C and 119Sn NMR, CP-MAS-NMR, Moss-bauer, IR, Raman, MS and powder XRD. A Sn(II) compound. Parallel staggered conformation proposed for decaphenylstannocene while the cyclopentadienyl groups are not parallel in pentaphenylstannocene0. 151, 152... 

The cyclopentadienyl group is another interesting ligand for immobilization. Its titanium complexes can be transformed by reduction with butyl lithium into highly active alkene hydrogenation catalysts having a TOF of about 7000 h 1 at 60 °C [85]. Similar metallocene catalysts have also been extensively studied on polymer supports, as shown in the following section. 

Candida cyclindracea lipase circular dichroism capillary electrophoresis Cahn-Ingold-Prelog 1,5-cyclooctadiene cyclopentadienyl group m - ch I o r o pc r be n z o i c acid circularly polarized light camphorsulfonic acid chemical shift reagent... 

Brintzinger s complex 2 [42] (10 mol%) performed poorly in terms of both the enantio-selectivity of the epoxide opening (56 %) and the product yield (55 %). The titanocene complex 3 [42] obtained from (1R,2S,5R) menthol gave variable results due to its sensitivity to traces of moisture (ee = 20—52 %). Clearly, the axially positioned cyclopentadienyl group is not ideal. 

A satisfactory result was obtained with the ligand 4 [43], which was synthesized from neo-menthol and contains an equatorial cyclopentadienyl group. The enantioselectivity of the opening attained synthetically useful levels (97 3) and the isolated yields were reasonable. Complex 5 [44], incorporating a ligand derived from phenylmenthone, also performed well. An enantioselectivity of 96.5 3.5 was observed. Phenylmenthol has already been extensively and successfully used as a chiral auxiliary [45]. 

In particular the last observation is easily understood by the molecular orbital picture given above. The 7z>orbitals of the monomers are required for a stabilization of the otherwise anti-bonding cluster orbitals of t2 symmetry which must accept six electrons. If this mixing is prevented because these orbitals adopt the electron density of the ligands, e.g., 7z>electrons of the side-on coordinated cyclopentadienyl groups, their contribution to the cluster stability is minimized or in particular cases the formation of clusters does not occur at all. Thus, the substituents attached terminally to the clusters strongly influence their stability by the different donor or acceptor capabilities. A further effect may result from the different steric demand of the substituents which will be discussed below. 

Alkoxides and imido are used as anionic ligands in zirconium and titanium catalysts for the polymerisation of alkenes, sometimes as the only anions, but often in combination with cyclopentadienyl ligands. Imides linked to cyclopentadienyl groups form part of the single-site catalyst developed by Dow (Chapter 10) (Figure 1.9, 1). In very different titanium catalysts, namely those used for epoxidation of alkenes, also alkoxide ligands are used (Chapter 14). 

Another illustration of the stabilizing effect of phosphines is supplied by cyclo-pentadienylcopper triphenylphosphine, one of the very few examples in which a cyclopentadienyl group is t -bonded to copper (see Fig. 1.6) [52]. 

Somewhat similar diagrams can be constructed for the interaction between a cyclopentadienyl group and ligated main-group moieties, MLjj. That for a CsHs - -MH interaction is Illustrated in Figure lb. 

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