A Remarkably Stable Dimer of Tetralithiocorannulene

The relatively simple NMR spectrum of S MLi does not provide much information about dynamic processes or possible states of aggregation. In an attempt to gain this kind of information, the tetralithio derivatives of 1-r-Bu- and I-j-Pr-co-rannulenes were also examined, and they appeared to show two different species each in similar but unequal abundance. Moreover, no significant changes were observed in the H and NMR from -80 °C to room temperature suggesting the lack of any dynamic processes. 

The explanation offered for this unusual spectroscopic behavior is the existence of stable, dimeric aggregates. Due to the symmetry of l-/ -corannulenes, face-to-face association results in two topologically different dimers (dl and meso). Moreover, the lack of any signal coalescence in either or NMR up to room temperature suggests that these dimers are unusually stable, and do not dissociate on the NMR time scale.  

The two Li NMR peaks coalesce into a single peak at room temperature (T. = -8 °C), indicating that the interior and exterior cations are able to exchange places without dissociation of the dimer The Li NMR spectra of both 1-i-Pr and 1-r-Bu corannulene /4Li exhibit quite analogous behavior. Moreover, tetraanion solutions generated from mixtures of corannulene and r-Bu-corannulene show the presence of mixed dimers in the H NMR spectrum.  

With this model, it is necessary to assume fast inversion of the two corannulene units within the dimer (or planarity) otherwise the top and bottom bowls would not be equivalent in the NMR spectra. MNDO calculations predict an activation enthalpy of approximately 13 kcal/mol for the concerted inversion process, and this value is inconsistent with the NMR behavior that shows no evidence of freezing out such a dynamic process. These calculations, however, do not take solvation of the exterior lithiums into account, and so the exterior lithiums were removed, and the system was calculated under conditions that would roughly model solvent separation of the lithium cations. Accordingly, the approximate barrier for inversion was lowered to approximately 1 kcal/mol, suggesting that solvation of the exterior lithium cations can indeed greatly reduce the inversion barrier.  

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