Solvated alkali metal hydrocarbyls

1 [10], Finally, a monomeric molecule, LiPh(pmdta), 71, is obtained when the tri-dentate amine ligand pmdta is employed [4], 

An interesting series of lithiated cyclopropenes was structurally investigated by von Schleyer et al [8]. Such lithiocyclopropenes are useful intermediates in organic syntheses [59], but structural information was lacking until recently. Mono- and dilithiated species were prepared according to Eqs (2-4). 

Crystallization of the crude products from hexane-THF or hexane-tmeda afforded the well-defined crystalline solvates 3-6. Although NMR data revealed the presence of monomers in THF solution, all four compounds are dimeric in the solid state. There are, however, interesting stmctural differences. The monolithiated tri-methylsilyl derivative, 3, forms a dimer like [LiPh(tmeda)]2, 7. The two cyclo-propenyl rings are nearly coplanar and are bridged quite symmetrically by the two Li ions. 

The additional chelating ligands in 4-6 lead to different types of self-assembly. 

6 Supramolecular Self-Assembly Caused by Ionic Interactions 

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Although trimers are quite rare, a more preferred type of supramolecular association in solvated alkali metal hydrocarbyls is the formation of tetramers in the solid state. Typical examples of cyclic tetramers are the sodium o-xylyl derivative [Na(o-xylyl)(tmeda)]4, 41, and the diphenylmethyl complex [NaCHPh2(tmeda)]4, 42 [41], The tetrameric THF adduct of -butylethynyllithium, 35, forms a heterocubane... 

The structures of base-free hydrocarbyls of the alkali metals have been investigated much less than those of the solvated species [4], The first example of an unsolvated dimer was [LiC(SiMe3)3]2,1 [5]. This pyrophoric compound was prepared by reacting the corresponding HgBr derivative with n-butylhthium. 

Yet another type of hydrocarbyl bridging has been established for some solvated triphenylmethyl derivatives of the heavier alkali metals. [NaCPh3(tmeda)] , 64, and [KCPh3(diglyme)] , 66 [57], are supramolecularly associated into zigzag chains by weak intermolecular sodium-carbon contacts. 

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