Group I. The Alkali and Coinage Metals

BY B. C. CROSSE

Structural and Bonding Studies. — Whereas lithium alkyls are highly associated in hydrocarbon solvents, the addition of tetrahydrofuran leads to a large conductivity increase, indicating ionization. The nature of the ions formed has been established by electrodialysis of labelled n-butyl-lithium solutions in tetrahydrofuran. An equilibrium is established between the solvated dimer, monomer, and free ions:

Bu- + Li+, THF [??] BuLi,THF [??] (BuLi)2, THF [??] Bu- + Li2Bu+, THF

The addition of lithium 2-methoxyethoxide or 2-dimethylaminoethoxide greatly enhances the ionic nature of lithium alkyls. The resulting electronic spectra are those expected for solvent-separated ion pairs. The presence of species of the type RLi,2MeOC2H4OLi is indicated, and the term 'co-ordination-agent-separated' ion pair is suggested, to distinguish them from solvent-separated species.

The structures of some aromatic ion pairs, including cyclopentadienyl-lithium and 1-phenylallyl-lithium, have been studied by n.m.r. spectroscopy. The 7Li chemical shift is related to the distance between the plane of the aromatic anion and the lithium cation located above its π-electron cloud. N.m.r. has also been used to study the nature of the bonding in benzyllithium and its derivative n-C5H11CPh2Li. The variation of the n.m.r. data with solvent points to an appreciable sp3 character for the α-carbons, but the variation is less marked in the diphenylhexyl compound, due to the larger π-system, which favours sp2 character. A similar study of n-C5H11CPh2Li and n-C3H7CPh2Li was interpreted in terms of a dimeric structure in benzene solution and solvent-separated ion pairs in tetrahydrofuran.

The sole X-ray structural determination was of the polynuclear compound LiBMe4. In this, each lithium and each boron atom is four-co-ordinate and linked to a terminal methyl group, two bridging methyls, and an almost linear bridging methyl group in which the lithium-carbon bond length is the shortest known. Measurements of 7Li spin–lattice relaxation times suggested that (Me3SiCH2Li)4 is distorted from tetrahedral symmetry, and so even is the methyl-lithium tetramer above 0 °C. A Raman study of the t-butyl-lithium tetramer indicated that the extent of metal–metal bonding in this compound is insignificant.

n-Pentyl- and n-octyl-lithium appear to exist in solution as hexamers; this is deduced from the extent of their lowering of the vapour pressure of benzene. In contrast, phenyl- and p-tolyl-lithium in ether solution appear not to be associated. A variable-temperature 7Li n.m.r. study of the exchange reaction between them showed that only two species exist, and these are in rapid equilibrium. If there were any association, one would expect also mixed species such as p-MexC6H4Li2Ph.

The exchange reaction between phenyl-lithium and bromobenzene has also been studied. The addition of lithium bromide had an inhibiting effect on the exchange of phenyl groups, and this was explained by the formation of a phenyl-lithium–lithium bromide complex, for which a 1:1 stoicheiometry was established. The previous irreproducibility of results from phenyl-lithium exchange reactions was therefore attributed to different methods of preparation, and the resulting presence or otherwise of this complex. In the halogen–metal exchange reaction of 1- and 2-fluoronaphthalenes with lithium, the presence of the fluoronaphthalene radical anions has been detected below -30 °C by e.s.r. spectroscopy.

Polylithio-compounds. — Some polynuclear arenes undergo facile polylithiation by n-butyl-lithium and NNN'N'-tetramethylethylenediamine (TMED). Deuteriation experiments and mass spectral analysis show that biphenyl and indene will substitute up to six hydrogen atoms by lithium, while anthracene and fluorene form traces of the perlithio-derivatives as well as less highly metallated species. In each case the predominant species contained about three lithium atoms per molecule.

A perlithio-compound C5Li4 was obtained from the butyl-lithium–TMED treatment of penta-l,3-diyne. Hydrolysis led to a mixture of penta-1,4-diyne, penta-1,2-dien-4-yne, and some of the initial diacetylene. Traces of the perlithio-compound C9Li8 may have been formed when 1-phenylpropyne was heated with a fifty-fold excess of butyl-lithium, but the main products were C9H2Li6 and C9H3Li5. Several other polylithio-species were also detected. The presence of TMED reduced the degree of lithiation. Butyllithium in ether at 0 °C gave only the monolithio-compound. Derivatization by quenching with water, deuterium oxide, formaldehyde, acetone, or trimethylchlorosilane gave products derived from 1- and 3-phenylpropynes and phenylallene. Methylation of the di- and tri-lithio-compounds followed by hydrolysis or silylation proved that attack occurs first at the benzylic carbon atom:

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Allenic structures have been suggested for the di-, tri- and tetra-lithio-species on the basis of their i.r. spectra. Attempts to polylithiate the aliphatic ring of 1,2,3-triphenylcyclopropane resulted in ring opening, giving a mixture of stilbenes after hydrolysis.

The reaction of dilithiomethane with various boron and silicon halides gave compounds of the type R2BCH2BR2 and (Me3Si)2CH2. The former are not stable and dismute into BR3 and (RBCH2)n.

Functionally substituted organic compounds, such as nitriles and sulphones, easily form gem-dilithio-derivatives on treatment with excess n-butyl-lithium. Subsequent reactions occur only at the α-carbon atom, despite resonance stabilization:

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Acetonitrile similarly gives the dilithio-compound Li2C2HN with t-butyl-lithium...