THF molecules

The first three equations illustrate exchange reactions in which an alkyl group is exchanged from the initiator to the THF molecule and the last equation illustrates the addition mechanism. 

In the monomeric structure of 10.21 the solvation of both Li" ions by two THF molecules prevents further aggregation. The four S-N bond lengths are equal at ca. 1.60 A indicating that the negative charge is delocalized over the S(N Bu)4 unit. In the dimer 10.22 one [McS(N Bu)3] anion is coordinated to both Li" ions, one of which is bis-solvated by THF, while the other is chelated only to the unsolvated Li" ion. 

Fig. 1.3. Potassium enolate of methyl t-butyl ketone open circles are oxygen and small circles are potassium, (a) left panel shows only the enolate structures (b) right panel shows only the solvating THF molecules. The actual structure is the superposition of both panels. Reproduced from J. Am. Chem. Soc., 108, 462 (1986), by permission of the American Chemical Society.

In the l3C-NMR spectrum of the iminosilane, the C20 signal of the THF molecule appears at 73.78 ppm—a downfield shift of nearly 6 ppm compared with that for free THF. This reflects the strong Lewis acid character of three-coordinated silicon in the iminosilane. 

The trigonal planar zinc phenoxide complex [K(THF)6][Zn(0-2,6-tBu2C6H3)3] is formed by the reaction of a zinc amide complex, via a bis phenoxide, which is then further reacted with potassium phenoxide. TheoX-ray structure shows a nearly perfect planar arrangement of the three ligands with zinc only 0.04 A out of the least squares plane defined by the three oxygen atoms.15 Unlike the bisphenoxide complexes of zinc with coordinated THF molecules, these complexes are not cataly-tically active in the copolymerization of epoxides with C02. The bisphenoxide complex has also been structurally characterized and shown to be an effective polymerization catalyst. 43... 

There are a reasonable number of structurally characterized zinc compounds with bound THF molecules. For example, a six-coordinate zinc porphyrin complex with axial THF donors and a four-coordinate zinc center with two THF ligands and two phenolate ligands.341,357 Although less common there are other structural examples of ether solvents, such as diethyl ether, coordinated.358 The X-ray structure of zinc chloride with 1,4-dioxane ligands shows a monomeric four-coordinate zinc center with two 1,4-dioxane ligands.359... 

Fig. 14. Molecular structure of Ali2(AlBr2)io 12(THF) 6. Only the O atoms of the THF molecules are shown.

In 62, two zincate ions of different composition, namely [Zn(CCPh)3] 62a and its THF adduct [Zn(CCPh)3(THF)] 62b, co-crystallized (Figure 31). Both, the coordination of only one zincate ion with THF and the trigonal-pyramidal structure of 62b, show that the interaction between zinc and the THF molecule in 62b must be weak. The zinc-carbon bonds in 62a and 62b are 1.967(7) and 2.003(5) A long, respectively. 

It is also possible to isolate bis(carbene) complexes involving the heavier alkaline earth metals. Thus, the reaction of two equivalents of 4 (R = Me or (Bu, R = H) with calcium, strontium and barium bis(trimethylsilyl)amides [M N(SiMe3)2 2(thf)2] (M = Ca, Sr, Ba) resulted in the displacement of two thf molecules to afford the corresponding biscarbene species, 19 (19). The solubilities and stabilities of these complexes were found to decrease from calcium to barium. 

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