Metal atoms ethers

Uranium tetrachloride [10026-10-5], UCl, has been prepared by several methods. The first method, which is probably the best, involves the reduction/chlorination of UO [1344-58-7] with boiling hexachloropropene. The second consists of heating UO2 [1344-57-6] under flowing CCl or SOCI2. The stmcture of the dark green tetrachloride is identical to that of Th, Pa, and Np, which all show a dodecahedral geometry of the chlorine atoms about a central actinide metal atom. The tetrachloride is soluble in H2O, alcohol, and acetic acid, but insoluble in ether, and chloroform. Industrially the tetrachloride has been used as a charge for calutrons. 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

Each of the sandwich compounds forms two isomers, described as clockwise and counterclockwise, respectively. Clockwise means that the atomic sequence in both rings is the same, counterclockwise that the atomic sequence is opposite. The syntheses occur best in THE at -78°C. After warming, the solvent is removed. Purification can be carried out by crystallization from petroleum, ether or better by sublimation at 60-70°C and 10 torr. The yields vary between 25 and 85%. The 17-and 18-electron complexes with V and Fe atoms show the metal atoms to be fixed above and below the ring centers. In contrast, the 19- and 20-electron complexes of Co and Ni possess slipped rings. 

In the first half of the nineties, there has been a continuing trend from synthetic studies of classical crown ethers towards the polyazamacromolecules and the introduction of multiple heteroatoms, including most recently the metal atom centers. 

The aluminium-solvent slurry produced by metal atom/solvent co-condensation at — 196°C is so reactive that oxygen is abstracted from the solvent ether as the mixture is allowed to melt. Hydrocarbon solvents are more suitable (but halocarbon solvents would react explosively). 

Solvated electrons were first produced in liquid ammonia when Weyl (1864) dissolved sodium and potassium in it the solution has an intense blue color. Cady (1897) found the solution conducts electricity, attributed by Kraus (1908) to an electron in a solvent atmosphere. Other workers discovered solvated electrons in such polar liquids as methylamine, alcohols, and ethers (Moissan, 1889 Scott et al, 1936). Finally, Freed and Sugarman (1943) showed that in a dilute metal—ammonia solution, the magnetic susceptibility corresponds to one unpaired spin per dissolved metal atom. 

Background alkali metal chemistry. The alkali metals have the lowest ionization potentials of any group in the periodic table and hence their chemistry is dominated by the M+ oxidation state. However, it has been known for some time that a solution of an alkali metal (except lithium) in an amine or ether forms not only M+ ions and solvated electrons but also alkali anions of type M (Matalon, Golden Ottolenghi, 1969 Lok, Tehan Dye, 1972). That is, although an alkali metal atom very readily loses its single s-shell electron ... 

The 1,3-dipolar cycloaddition of nitrones to vinyl ethers is accelerated by Ti(IV) species. The efficiency of the catalyst depends on its complexation capacity. The use of Ti( PrO)2Cl2 favors the formation of trans cycloadducts, presumably, via an endo bidentate complex, in which the metal atom is simultaneously coordinated to the vinyl ether and to the cyclic nitrone or to the Z-isomer of the acyclic nitrones (800a). Highly diastereo- and enantioselective 1,3-dipolar cycloaddition reactions of nitrones with alkenes, catalyzed by chiral polybi-naphtyl Lewis acids, have been developed. Isoxazolidines with up to 99% ee were obtained. The chiral polymer ligand influences the stereoselectivity to the same extent as its monomeric version, but has the advantage of easy recovery and reuse (800b). 

Heterocyclic compounds are frequently used as hydrogen donors in the reduction of C-C double and triple bonds catalyzed by complexes of transition metals. Cyclic ethers such as [l,4]dioxane (39) and 2,3-dihydrofuran are known to donate a pair of hydrogen atoms to this type of compound. 2,3-Dihydro-[l,4]di-oxine (41), the product of dioxane (39), is not able to donate another pair of hydrogen atoms [46, 60, 73, 74]. These heterocyclic compounds are in general also very good solvents for both the catalyst and the substrates. 

Adapted from Sasidharan and Kumar (257). Reaction conditions catalyst, 150 mg methyl trimethyl-silyl dimethylketene acetal (silyl enol ether), 10 mmol benzaldehyde, 10 mmol dry THF as dispersion medium, 10 mL temperature, 333 K reaction time, 18 h. Yield refers to the isolated product yield. Moles of product per mole of metal per hour. b The metal atom is substituted in the tetrahedral position. 

More than 50 macrocyclic crown ethers were synthesised by Pedersen, and many were found to solublise alkali metal salts in non-polar solvents. He isolated 1 1 complexes with metal salts (87) and also showed that if the cation was too large to fit in the central hole, complexes with ratios of 1 2 or 2 3 (metal ether) could be obtained (88). Some of the larger ethers have been shown to complex two metal atoms simultaneously (89). Stability constants in solution are affected by the nature of the anion and the solvent. Both are also important in obtaining crystalline products. 

In a recent example metalloporphyrins are used as the sensing dyes for a wide variety of odorants, including alcohols, amines, arenes, ethers, halocarbons, ketones, phosphines, thioethers and thiols. An array of four different metalloporphyrins are placed on a substrate and exposed to the vapours for 30 s. The various vapour molecules coordinate onto the central metal atoms of the porphyrins, causing them to change colour and producing a unique four-colour array. The resulting colour array is compared with a library of known chemicals or mixtures and identification is achieved. 

Further metallation of lithiated propargylic ethers LiC=CCH2OR, has to be carried out with the super-basic reagent BuLU-BuOK in THF the introduction of the second metal atom using this base proceeds at a sufficient rate at temperatures in the region of -50 C. This low temperature is necessary in view of the limited stability of MG=CCH(M)OR. 

The reaction between metal atoms and ethers is varied. Deoxygenation occurs with the more electropositive transition metals, e.g., Ti, V, Cr, and probably Fe, but Klabunde has been able to use tetrahydrofuran as a medium to form active nickel slurries by condensing in nickel vapor (56, 60). 

A recent development of work with metal vapors, which lies between atom chemistry and conventional synthetic chemistry, is the preparation of reactive metal slurries. When a metal vapor is condensed with an inert organic compound, e.g., an alkane or sometimes an ether, and the condensate is allowed to warm to room temperature, the resultant slurry contains metal in a reactive form. It is less reactive than the metal atoms because aggregation of the atoms has occurred and is comparable in reactivity to active forms of metal produced by other methods, e.g., Raney nickel. The catalytic and synthetic potential of these metal slurries is being explored (55, 60). 

Alkulidc anions. M, discussed in Chapter 12, may be stabilized by various macrocytic ligands.-8 The dissolution of sodium and heavier alkali metals in ethers gives not only solvated M+ and e, but also solvated M, which results from disproportionation of the metal atom.39... 

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