Polymorphism of Organolithium Compounds

In recent years increasing effort has been devoted to the elucidation of solid state structures of organolithium species 135). This led at least in two cases to the discovery of polymorphism and thus, in a certain respect, to two isomers of a carbanion in the solid state. 

The first report to deal with polymorphs concerned the structure of the dianion 180 136). 

As shown in Fig. 1, Li(B) sits upon one of the two six-membered rings of the naphthalene part of 180, while Li(A) is located on the other side of the dianion below the Si-heterocyclus. 

The P form of 180, although the precision of its determination is lower than that of the a form, is shown in Fig. 2. In the p-phase Li(A) is also located below the six-membered heterocyclus containing Si(B). Li(B), however, sits outside the six-membered carbocyclus leading to a bonding situation which is similar to that in benzyllithium137). 

Different positions of the Li+ ions and the TMEDA molecules are also observed in the two phases of dilithium 1,2-diphenylbenzocyclobutadiene-diide TMEDA (181)138). The X-ray structure of phase I is shown in Fig. 3a. A,projection of 181-1 normal to the benzocyclobutadiene-diide rings is given in Fig. 3 b. One Li+ ion is below the four-membered ring, the other one sits on the other side of the dianion slightly outside the four-membered ring. The two TMEDA molecules attached to the Li+ ions are oriented more or less orthogonally to one another. 

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