Cyclopentadienyllithium

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. 

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). 

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 . ... 

A freshly prepared solution of (t75-C5(CH3)5)Fe(acac) (7.9 mmol) prepared as described above, is cooled to — 78°C with a dry ice/acetone bath. A solution of cyclopentadienyllithium (0.566 g, 7.9 mmol Aldrich Chemical Co.) in 10 mL of anhydrous THF is added to the i75-C5(CH3)5Fe(acac) solution via syringe. After completion of the addition, the mixture has a yellowish-green color. 

Caution. Cyclopentadienyllithium is corrosive and moisture sensitive and must be handled with care under an inert atmosphere. 

Since the discovery of ferrocene and the development of metallocene chemistry, the compound cyclopentadienyllithium, C5H5Li, has been of considerable interest to both experimentalists and theoreticians. In its physical properties cyclopentadienyllithium qualitatively behaves like a typical salt. It is nearly insoluble in noncoordinating solvents and possesses a high melting (decomposition) point and a very low volatility. To our knowledge the crystal structure of C5H5Li is unknown. 

In tris(trimethylsilyl)cyclopentadienyllithium TMEDA (25) as well as in trimethylsilylcyclopentadienyllithium TMEDA (55) (compounds III), a symmetrical pentahapto coordination is observed in indenyllithium TMEDA (11) (IV) there is a tendency toward orbital-controlled trihapto coordination finally, in fluorenyllithium bis(quinuclidine) (12) (V) an allylic trihapto coordination is favored. 

The fulvene 145 offered the opportunity for a sequential inter-intramolecular carbolithiation the intermediate cyclopentadienyllithium 146 undergoes 3-exo cyclisation to give 147, a precursor to longifolene.71... 

Fig. 14. Molecular structure of the tetramethylethylenediamine adduct of trisftrimethyl-silyl)cyclopentadienyllithium, (Me3Si)3H2C5Li(tmeda) (147).

It was also reported that charge-delocalized organolithiums derived from carbon acids whose pK, are lower than ca. 20, such as cyclopentadienyllithium, indenyllithium and Tlithioacetophenone, do not form the corresponding lithium organoborates to any detectable extent... 

However, more recent analyses have emphasized the ionic character of lithium bonding to a greater extent (38-38b). Streitwieser (38), in particular, has insisted that lithium bonds have very little, if any, covalent character. Before presenting details of the crystal structures, the two interpretations will be compared with simple examples allyllithium and cyclopentadienyllithium. 

Cyclopentadienyllithium. The tt charge in the aromatic cyclopentadienyl anion is distributed equally to all five carbon atoms. A lithium counterion should thus electrostatically favor a central location (Csv, 26a) over the tt face. The same conclusion is reached on the basis of MO considerations (46). The six interstitial electron interactions involving the three cyclopentadienyl TT orbitals and those of corresponding symmetry on lithium (one of these is shown in 26b) also favor structure 26a. 

Two cyclopentadienyllithium compounds, 27 (45) and 28 (49), have been prepared and characterized, lie structural features of these compounds are in agreement with theoretical predictions (46). 

The reaction of starmylated dicyclopentadiene (111) with methyllithium gave dicyclopentadiene (113) and cyclopentadienyllithium (114) even at -78 C." The intermediate can be trapped by addition of iron(II) cMoride to give intermediate (115), which then cleaves off cyclopentadiene m one of the two dicyclopentadiene ligands in a rDA process to give (116) as the isolated product. It therefore appears that the rDA decomposition is only partially blocked by trapping with iron(II) chloride. 

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