Cyclopentadienides, with metal compounds

B. Reaction of Ionic Cyclopentadienides with Metal Compounds. . 375... 

The reaction of the ionic cyclopentadienides with metal compounds is by far the most general method for synthesis of cyclopentadienyl metal compounds. This method is applicable both to transition metals, which form 7T-bonded cyclopentadienyl compounds principally, and to main group metals which form a-bonded cyclopentadienyl compounds. 

The reaction of ionically bonded metal cyclopentadienides with metal compounds to form cyclopentadienyl metal compounds may be represented by the general equation... 

Cyclopentadienyl compounds synthesized by the reaction of ionic cyclopentadienides with metal compounds are usually isolated by evaporating the solvent under vacuum and, if the product is unstable, extracting the dry residue with a hydrocarbon solvent, such as benzene or toluene. Air-sensitive products of sufficient volatility may be sublimed away from nonvolatile residues. When more than gram quantities are required the inconvenience of evaporating to dryness and transferring a pyrophoric residue renders these techniques rather impractical. It is then more convenient to add a hydrocarbon solvent which is less volatile than the reaction solvent and... 

Cyclopentadienyl—metal bonds may be synthesized by direct reaction of cyclopentadiene with certain metals and their compounds or by reaction of ionic metal cyclopentadienides with compounds of metals wherein cyclo-pentadienide anion displaces a ligand bonded to the metal. These bonds may also be formed in a limited number of instances by reaction of alkenes or alkynes with metal compounds. 

As we saw with metal complexed vinyl carbons, or-ganometallic complexes of aromatic compounds exhibit l3C chemical shifts upfield of the uncomplexed compound. For example, metal-coordinated benzene carbons are found in the range 8 74-111, depending on the exact structure of the complex. Similarly, metal-coordinated cyclopentadienide (C5H5-, abbreviated Cp ) appears in the range 8 75-123. 

Methyl lithium and butyl lithium are widely used for the synthesis of other organolithium compounds. For example, lithium cyclopentadienides are generally prepared by the reaction of the cyclopentadiene with butyl lithium. In contrast, the amido-alkali metal compounds are becoming increasingly important in the synthesis of organoalkali metal compounds with the heavier alkali metals. For example, l,2,4-tris(trimethylsilyl)-l,3-cyclopentadiene reacts with potassium bis(trimethylsilyl)amide to form potassium l,2,4-tris(trimethylsilyl)cyclopentadienide. ... 

Cyclopentadienyl complexes, as previously discussed, are exceptionally numerous and have been the subject of extensive study. Several routes are available for introducing this ligand into a metal complex. One approach is to react a metal compound with the cyclopentadienide ion, C5H5. This ion can be purchased as the sodium salt in solution it can also be prepared by the following two-step process. 

The carbon—M bonds also form by cocondensation of acidic hydrocarbons with alkaline-earth metal vapor. In this way the cyclopentadienides and indenyl compounds of Ca, Sr and Ba that are difficult to obtain by other methods for Sr and not at all for Ba can be prepared nearly quantitatively. Because of the required vacuum techniques, this method is suitable for the preparation of small amounts only. 

A general method for the preparation of all / e ta/za/Uocyclopentadienyl compounds is the reaction of sodium cyclopentadienide with a metal halide or complex halide in tetrahydrofuran, ethylene glycol dimethyl ether, di-methylformamide or similar solvent. A solution of the sodium salt is obtained by treating dispersed sodium with the hydrocarbon in tetrahydrofuran ... 

Recently, efforts have been made to expand the number of homogeneous vanadium catalysts using ancillary, nonreactive ligands. These catalysts may be in the -e5, +4 or q-3 oxidation states. The dianionic imido ligand is isolobal with cyclopentadienide and has been used to prepare diamagnetic, formally d° metal compounds like (10) (Figure 13) which, when activated by MAO, display modest activity for ethylene polymerization. VCI3 complexes of Q -diimines... 

In 1951, Michael J. S. Dewar (1918-97) published a molecular orbital (MO) theory of bonding between unsaturated compounds and transition metals later augmented by Joseph Chatt (1914-94) and Leonard A. Duncanson. It recognized o-type overlap of an occupied 7t-orbital on the ligand (e.g., ethylene, benzene, cyclopentadienide) with a vacant d-orbital of appropriate symmetry on the transition metal, coupled with back-donation through 7t-type overlap of an occupied d-orbital on the metal with a vacant (antibonding) 7t -type orbital on the ligand. 

Cyclopentadienyl-carbonyl compounds are conveniently prepared by the reaction of metal carbonyl halides with sodium cyclopentadienide, or by the reaction of Nacp with metal halides in the atmosphere of carbon monoxide (sometimes under pressure) ... 

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. 

Wilkinson et al. 12, IS) found tetrahydrofiiran and 1,2-dimethoxyethane to be most convenient and versatile solvents for the reaction of alkali metals with cyclopentadienes. Sodium cyclopentadienide is generally formed in greater than 95% yields in tetrahydrofuran (THF) and only a few per cent of by-product cyclopentene is formed. Other solvents which have been used are diethyl ether 14), xylene, /crf-butanol 15), and diethyl ether-xylene mixtures 16). A systematic study of the effect of solvent on yield in the reaction of Li, Na, and K with fluorene indicated that yield decreased in the order 1,2-dimethoxyethane > diethyl ether, % THF > dioxane > diethyl ether (77). In general, however, sodium cyclopentadienide in tetrahydrofuran has been the most widely used reagent for the preparation of other cyclopentadienyl metal compounds. (See Section II,A,la representative experimental methods are collected at the end of each section.)... 

The ionic cyclopentadienide is customarily prepared in advance in the same solvent to be used in the preparation of the cyclopentadienyl metal compound. Stock solutions of reactive cyclopentadienides may be prepared and used as needed. A solution of 2.5 M sodium cyclopentadienide in tetra-hydrofuran may be conveniently prepared and stored for this purpose. Since ionic cyclopentadienides are air-sensitive, all reactions must be carried out in an inert atmosphere, usually nitrogen. It is important that the metal compound, usually a chloride or bromide, be anhydrous because the ionic cyclopentadienide reacts preferentially with water. 

Co (CO) 2, and (n -C5H5)(n -Ci4Phtj)Co. 35 As a consequence, alternate methods for the preparation of functionally substituted cyclo-pentadienyl metal compounds became an important goal of our research. Sodium cyclopentadienide containing aldehyde, ketone, and ester substituents can be made by the reactions of C5H5Na with ethyl formate, methyl acetate, methyl chloroformate, and dimethyl carbonate (Scheme l),36-39... 

The interaction of equimolar amounts of lithium or sodium alkylcyclopentadienides with LnClj in THF leads to the formation of adducts [(R, C5H5.JLnCl3]M [215, 234, 275, 277, 285]. Evidently, the same compounds are the first products in all reactions of alkali metals cyclopentadienides with salts of REM ... 

Bis[(tris(isopropyl)cyclopentadienyl)]zinc (Zn C5(Pr1)3H2 2, 21) and bis[(tetrakis(isopropyl)cyclopentadienyl)]zinc (Zn C5(Pr1)4H 2, 22) were synthesized from the respective potassium cyclopentadienides and zinc iodide as shown in Scheme 18.50 The same slipped sandwich compounds were also isolated from zinc-reduced VC13 solutions when they were treated with these alkali metal cyclopentadienides at room temperature.51 The outcomes of these reactions suggest that zincocenes are likely intermediates in the syntheses of transition metal metallocenes, in which the metal halides have been pre-reduced with zinc. The solid-state structure of Zn G5(Pr1)4H 2 is shown in Figure 10. The sole... 

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