Phenyllithium-diethyl ether tetrameric

Fig. 7.2. Crystal structure of tetrameric phenyllithium diethyl etherate (a) tetrameric C4Li4 cluster (b) complete structure except for hydrogens. Reproduced from J. Am. Chem. Soc.,...

Phenyllithium dissolves in hexane by addition of TMEDA. Hie phenyllithium TMEDA adduct subsequently crystallizes out of solution as the dimer (113) corresponding to general structural type (16)." With diethyl ether solvation, phenyllithium exists as a solid tetramer (114). In ether solution PhLi is known to be either dimeric or tetrameric. Monomeric phenyllithium was successfully crystallized with PMDETA as the ligand. This monomer is depicted as (115). Note the difference in the coordination number of the carbanionic center in the monomer (115), the dimer (113), and the tetramer (114), i.e. one, two and three, respectively. 

An interesting mixed tetrameric complex containing three equivalents of phenyllithium and one equiv-alent of lithium bromide, i.e. [(PhLi)3-LiBr3Et20], depicted as (116), has been characterized." In this mixed aggregate the lithium atom diagonally opposite Ae bromide in the tetramer rem s unsolvated by an ether molecule. Recall that the cyclopropyllithium lithium bromide-diethyl ether complex (44) with... 

Fig. 6.1. Crystal structures of phenyllithium (a) dimeric structure incorporating tetramethylethylene-diamine (b) tetrameric structure incorporating diethyl ether. Reproduced from Chem. Ber., Ill, 3157 (1978) and J. Am. Chem. Soc., 105, 5320 (1983), by permission of Wiley-VCH and the American Chemical Society, respectively.

Phenyllithium is known to form, in equilibrium with the monomer, a dimer 25 (Scheme 1-21) held together by electron-deficient partial bonds. However, phenyllithium can also adopt the structure of a lithium lithiate ion pair 26 (Scheme 1-21). What makes the difference The solvent plays a capital role. As long as the solvation forces remain moderate as in diethyl ether, the doubly carbon-lithium-carbon bridged dimer 25 is energetically most favorable. In a 2 1 mixture of cylohexane and diethyl ether, phenyllithium even assembles a tetrameric cluster. " " in neat THF, the four-centered dimer 25 is found in equilibrium with the monomer. " However, in the presence of the powerful electron-donor hexamethylphosphoric triamide (HMPA), it is the ion pair 26 that coexists with the monomer. Analogous lithiate complexes have been identified with... 

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