Alkali metal complexes 1,2-dimethoxyethane

It is worthwhile mentioning that there are some solvents that combine good solvency power with coordinating properties. The most salient example is 1,2-dimethoxyethane (DME), which can form chelate complexes with alkali cations. This makes easier one-electron reduction of organic substances by means of alkali metals, with the formation of anion radicals and alkali cations. 

A motif found in the majority of alkali metal stabilized carbanion crystal structures is a nearly planar four-membered ring (13) with two metal atoms (M ) and two anions (A ), i.e. dimer. This simple pattern is rarely observed unadorned as in (13), yet almost every alkali metal and alkaline earth carbanion aggregate can be built up from this basic unit The simplest possible embellishment to (13) is addition of two substituents (S) which produces a planar aggregate (14). Typically the substituents (S) in (14) are solvent molecules with heteroatoms that serve to donate a lone pair of electrons to the metal (M). Only slightly more complex than (14) is the four coordinate metal dimer (15). Often the substiments (S) in (15) are joined by a linear chain. The most common of these chains are tetramethylethylenediamine (TMEDA) or dimethoxyethane (DME) so that the spirocyclic structure (16) ensues. Alternatively the donors (S) in (16) have been observed as halide anions (X ) when the metal (M ) is a divalent cation, e.g. (17) or (18). Obviously, the chelate rings found in (16) are entropically favorable relative to monodentate donors (S) in (14), (15), (17) or (18) (Scheme 2). 

The reduction of [Cr(>f6-naphthalene)2]+ by lithium naphthalide was thought to give [Cr(>f6-naphthalene)2] (410), but this is doubtful since the formal potential of the naphthalene-naphthalide couple is at least several hundred millivolts positive of that of the complex (416). A similar reaction between [Cr(f/-C6H6)2]+ and alkali metals in thf or 1,2-dimethoxyethane yielded the radical [Cr(f/-C6H6)(f/-C6H5)] (36) ESR spectroscopy showed an intramolecular, interannular hydrogen exchange reaction (Scheme 32) with a rate of 107 sec"1 (430). 

The TCID method was systematically used to examine crown ether-alkali-metal-ion (Li+-Cs+) complexation energies (Table For comparison with the tetra- to hexadentate crown ethers, binding data were also determined for smaller ethers, that is, monodentate dimethylether (DME) and bidentate dimethoxyethane (DXE). In solution, each crown ether binds most strongly to that alkali cation which fits best into the crown s cavity. For example, K+ is most strongly bound to 18-crown-6, while Na prefers 15-crown-5. With the gas-phase data in hand, the vahdity of this best fit model can be tested under environment-free conditions. 

HUl, S.E., Feller, D. and Glendening, E.D. (1998) Theoretical study of cation/ether complexes alkali metal cations with 1,2-dimethoxyethane and 12-crown-4. J. Phys. Chem. A, 102, 3813-3819. 

The simplest chelates of this type, ethyleneglycol (eg) and 1,2-dimethoxyethane (glyme-2), give complexes with alkali and alkaline earth metal cations. The complexes MX2-2eg = Mg, Ca X = NOi-), MXz-Seg (MJ = Mg, Ca X = Cr, Br-), CaX2-4eg (X = Cr, Br ) and SrBr2 2eg have been isolated and IR studies suggest that the majority are likely to contain monodentate The Sr complex is, however, believed to have bidentate eg present. 

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