Cyclopentadienyllithium complexes

Other delocalized anions have been investigated as well, such as complexes of indenyl and fluorenyllithium. These data are also included in Table 8. The sole investigated indenyllithium system was the TMEDA complex. It is known from X-ray crystallography that the lithium cation is located above the five-membered ring and that the TMEDA binds in a bidentate fashion . The x value is somewhat larger than for the corresponding cyclopentadienyllithium complexes (entry 9). 

In the DEE complex a chemical shift of —5.6 indicates that the ring current has a much more profound effect in this complex. However, the solid state structure was not known at that time. In the TMEDA complex, the lithium cation is postulated to be positioned above the central five-membered ring, and the Li chemical shift is —7.5 ppm, i.e. in the range of CIPs of cyclopentadienyllithium. In the THE complex, a shift of —2.6 ppm was observed. Again, no effect from the ring current is observed. However, based on the quadrupolar interaction this system was assigned as an SSIP, as discussed below. 

The effect of structure and solvation on the Li quadrupolar interaction was studied for a relatively large number of cyclopentadienyllithium derivatives of known sohd state structure . As mentioned above, these complexes have been shown to form several types of structures (Scheme 3) . Again, the factors determining the structure are the steric requirements of the substituents and ligands. For solubility reasons, most of the complexes studied by X-ray crystallography are substituted, often with large trimethylsilyl substituents. 

FIGURE 16. (a-c) Simulated and (d) experimental static Li NMR spectrum of the diglyme complex of cyclopentadienyllithium at 38.92 MHz. Reproduced with permission of John Wiley Sons Ltd. from Reference 158... 

The ability of the crown ether ligand, 12-crown-4, to separate the lithium cation from the organic moiety, thus stabilizing the SSIP structure, was also observed in the study dealing with cyclopentadienyllithium systems . As described in Section II.C.3, SSIP structures of these compounds are characterized by low x( Li) values. However, it is important to reahze that the variation found for x( Li) is basically caused by the local symmetry around the lithium cation and other highly symmetric situations will also lead to small x( Li) values. Examples are the sandwich compounds mentioned in Section n.C.3. It is thus necessary to consider also / Li and C chemical shift data in order to classify a certain complex as SSIP or CIP. 

The dynamic behavior of various solid organolithium complexes with TMEDA was investigated by variable-temperature and CP/MAS and Li MAS NMR spectroscopies. Detailed analysis of the spectra of the complexes led to proposals of various dynamic processes, such as inversion of the five-membered TMEDA-Li rings and complete rotation of the TMEDA ligands in their complex with the PhLi dimer (81), fast rotation of the ligands in the complex with cyclopentadienyllithium (82) and 180° ring flips in the complex with dilithium naphthalene (83) °. The significance of the structure of lithium naphthalene, dilithium naphthalene and their TMEDA solvation coiMlexes, in the function of naphthalene as catalyst for lithiation reactions, was discussed . ... 

Lithium [749,750,760-762] and sodium [750,760] organic compounds, lithium alcoholates [752,757,760-762], sodiomalonic diesters [755], complex bases from alkali imides and alcohols or alcoholates [756], phosphines [758,759], and others [751,753,754] have been used as initiators. It was found that with THF as solvent and fluorenyllithium or phenyllithium as initiator, molar mass is independent of initiator and monomer concentration. Relatively low masses of 2600 to 4200 were found. With DMF as solvent, the molecular mass increases with the monomer concentration at low (1.5mmol/L) initiator levels. With cyclopentadienyllithium or cyclopentadienyl sodium at high concentrations (68 mmol/L) and DMF as solvent, the molecular mass increases strongly with the monomer concentration. This is explained on the basis of a polyfunctionality of cyclopentadienyllithium and cyclopentadienyl sodium initiators. This view is supported by ozonolysis of the incorporated initiator, which leads to a decrease in the molar masses only of those polymers that were initiated by cyclopentadienyllithium or cyclopentadienyl sodium [750]. 

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