Cyclopentadienyl-metal compounds from cyclopentadiene

Syntheses of cyclopentadienyl metal compounds from cyclopentadiene are carried out under widely varying conditions depending on the properties of the metallic reactants. These reactions may be carried out in the vapor phase at temperatures as high as 600° C or they may be carried out in the liquid phase in the presence of a solvent at 25° C. The cyclopentadienyl metal compounds produced are isolated either by sublimation, if the reaction has been run in the vapor phase, or by crystallization, if the compound has been formed in solution. If the compound is to be used as an intermediate in a subsequent synthesis, then it is not usually isolated in pure form, but is used in solution in a suitable solvent of high dielectric constant or as a slurry in a hydrocarbon solvent. 

Synthesis of cyclopentadienyl metal compounds from basic metal salts derived from a base and a neutral or acidic metal salt is limited primarily to cyclopentadiene and in a few instances methylcyclopentadiene (37). The use of other substituted cyclopentadienes has not been reported and it is probable that only the very strongest bases would be effective in promoting their reaction. 

Nearly all synthetic methods for cyclopentadienyl metal compounds require first the preparation of cyclopentadiene from dicyclopentadiene. This is accomplished by thermal depolymerization of dicyclopentadiene at temperatures above 180° C. Because cyclopentadiene is unstable at room temperature it must be stored at low temperatures to prevent dimerization to dicyclopentadiene. The rate of dimerization is about 0.05% per hour at — 20° C and 1% per hour at 10° C (5). 

While this is the simplest method for preparing metal cyclopentadienyl compounds, it is one of the least general. It is limited to the elements Li, Na, K, Ca, and Sr and under rather vigorous conditions of temperature to Mg, In, Tl, and Fe. The reaction is usually carried out in the liquid phase at 25° to 100° C in the presence of a solvent for lithium, sodium, and potassium, or in the vapor phase at 400°- 600°C for the less reactive elements. Usually the cyclopentadienides of lithium, sodium, and potassium are not isolated but are used in solution as intermediates for the preparation of other cyclopentadienyl metal compounds. Compounds produced by reaction of cyclopentadiene vapor with metal are usually sufficiently volatile to sublime away from the reaction zone and condense in the cooler portion of the apparatus. 

In the usual experimental procedure, an acidic salt is first complexed with base and then treated with cyclopentadiene which reacts to form the cyclo-pentadienyl metal compound. The by-product salt is separated from the cyclopentadienyl metal compound by extraction with water. 

Unlike the compounds obtained with the heavier halogens, polyfluori-nated metallocene derivatives cannot be prepared from the permercurated metallocenes. Thermally stable pentafluorinated ruthenocenes have been made from the (oxocyclohexadienyl)(cyclopentadienyl)metal complexes by flash vacuum pyrolysis (Scheme 6).53 54 Decafluorometallocenes, [M(C5F5)2], are still unknown potential routes to these compounds starting from fluorinated cyclopentadiene (C5F5H)55 or from the decachlorometal-locenes48 have been unsuccessful. However, there seems to be no fundamental steric or electronic barrier to their eventual preparation. 

In a first approximation supra-Cp metal complexes can be prepared the same way as normal or other Cp-metal and organo-metal bonds in general. The methods used most often (see Appendix) are the metathesis reaction [Eq. (1)] followed in number by oxidative additions (Eq. (2)] and metallation/deprotonation reactions [Eq. (3)]. The latter is especially important for the cyclpentadienyl alkali metal compounds. A useful variation of reaction (3) is the formation of CpTl in an acid/base reaction from cyclopentadiene and thallium ethoxide [Eq. (3b)]. This represents a convenient route to cyclopentadienylthallium compounds, which are also valued (in place of Cp alkalis) as mild Cp-transfer reagents for the synthesis of difficultly isolable cyclopentadienyl derivatives (77). 

P. L. Pauson Compounds derived from cyclopentadiene, pp. 107-140a (197) an account of metal cyclopentadienyls, and the organic chemistry of ferrocene. A50. G. A. Olah, ed., Friedel-Crafts and Related Reactions. Wiley (Interscience), New York, 1965. 

Monocyclopentadienylzirconium trichloride has been prepared from zirconium tetrachloride by reaction with cyclopentadienylmagnesium chloride in toluene/diethyl ether solution 240, 241). Both the chloride and bromide have been prepared from the corresponding tetrahalides and magnesium cyclopentadienide in xylene at 100°-110°C 451), or by continuous recirculation of cyclopentadiene vapor upward through a bed of zirconium trihalide (250°-300°C) resting on a glass sinter. The products were purified by sublimation. Yields were only about 15% compared to the 60-70% obtained from syntheses carried out in solution. The melting points and colors for the monocyclopentadienyl metal trihalides and for other cyclopentadienyl metal halide compounds are tabulated in Table I. 

Synthesis of cyclopentadienyl compounds of metals from cyclopentadiene depends on the strongly acidic properties of cyclopentadiene. This acidity may be attributed to the ready formation of the highly resonance-stabilized cyclopentadienide anion, CsHs , in which there are six delocalized ir electrons. This method is limited primarily to the active metals of Groups lA and IIA and to their strongly basic compounds. The compounds produced are principally the ionically bonded metal cyclopentadienides. 

These results clearly underline that the antitumor activity of cyclopentadienyl metal complexes cannot simply be ascribed to the isolated action of free cyclopentadiene molecules released from a decomposing polymer [(C5H5)Ti0]402, as was postulated recently The hypothesis that the cyclopentadienyl ring ligands in metallocene diacido and fenicenium complexes may play an important, perhaps crucial, role for the cytostatic action of these compounds would certainly explain why structurally quite different cyclopentadienyl metal complexes like tetrahedral metallocene (Fig. 4) and linear metal-licenium (Fig. 5) complexes are equally effective and characterized by a similar spectrum of activity, especially as cyclopentadiene can be released from the complexes by hydrolytic cleavage of the cyclopentadienyl-metal bond On the other hand, the complexed metal atoms also seem to play an important role whether as a kind of carrier of one or both cyclopentadienyl rings to the site of action, or an anchor capable of... 

Cyclopentadiene, CsHg, is a colourless liquid which readily forms a monosodium derivative. This in turn reacts with anhydrous transition-metal halides to form derivatives M(CsHs) , some of which may be made direct from the hydrocarbon and the metal carbonyls at about 300°C. Some of these compounds, including Fe(CsHj)2 ( ferrocene ) can be oxidized to cations. The following are examples of cyclopentadienyl compounds ... 

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