Chemical Reduction of Planar and Curved Polyarenes: From Group I to Group II Metals

摘要

To further advance energy storage materials research, the structural studies of carbonaceous compounds intercalated with alkali and alkaline earth metal ions are of great interest and importance. Structure-property relationships of electrode and superconductive materials can be better understood with the gained fundamental knowledge stemming from a synergistic use of solid state and solution studies. Due to the polymeric structures and insolubility in common solvent of intercalated carbonaceous materials, studies of structures and properties must be conducted on smaller subunits. Planar polycyclic aromatic hydrocarbons (PAHs) can serve as models for the infinite pi-carbon layers of graphite or graphene, while nonplanar PAHs have been utilized to explore the reactivity of carbon allotropes with curved pi-surfaces. Therefore, we have focused our work on the structural investigation of multi-charged fragments of both graphite and fullerenes, using planar and curved PAHs with different topologies.;In this work, we have accomplished the first structural characterization of monoreduced coronene (C24H12•-- ) isolated with Na+ countercation as a solvent-separated ion pair (SSIP). This has provided the first evaluation of the effect of the distribution of a negative charge over the coronene surface without the interference of metal binding. This has also allowed for the first structural evaluation of the Jahn-Teller (JT) effect in monoreduced coronene; the D 2h symmetrical distortion was found to fit the experimental data. The first structural characterization of monoreduced coronene isolated with Rb+ countercation as a contact ion pair (CIP) has revealed the role of secondary interactions, involving 18-crown-6 ether, in tuning the metal ion binding to the charged pi-surface. In addition, we have conducted the first X-ray structural characterization of bicorannulenyl dianion (C 40H182--) comprised of two fused corannulene bowls isolated with two alkali metal ions, Li+ and Cs +. The direct comparison has revealed two different binding modes, namely the "naked" form in the SSIP with small Li+ ions vs. the unique double concave metal coordination in the Cs product. Furthermore, the X-ray diffraction study of the bicorannulenyl dianion salts confirms that, upon acquisition of two electrons, the large biaryl is converted into a charged overcrowded ethylene, as predicted by prior solution studies. Next, the controlled preparation of rubrene (C42H28) mono-, di-, and tetraanions has also been achieved in our laboratory with a series of alkali metal ions ranging from Li to Cs. The X-ray diffraction studies of the resulting products revealed the distortion of the rubrene core upon stepwise acquisition of additional electrons. In-depth structural analysis of the tetrareduced rubrene crystallized with Li+ and Rb+ counterions illustrated the inherent flexibility of its tetracene core; although the effect of alkali metal ion size was not straightforward. Finally, we have started developing the synthetic approaches needed to utilize the Group 2 metals as reducing agents towards curved carbon pi-systems. The first reduction step has been achieved for corannulene (C20H10) using the lighter alkaline earth metals, such as magnesium and calcium. The resulting products have been isolated in the solid state and structurally characterized, revealing the tendency to produce SSIPs with solvated divalent countercations, similar to the previously observed trends for planar carbanions.;Overall, a diverse set of pi-systems of increasing structural complexity, ranging from planar coronene to bowl-shaped corannulene, bicorannulenyl and rubrene, have been investigated in stepwise reduction processes. These studies reveal notable carbon framework transformations, new metal binding trends and the important role of both primary and secondary interactions. The first exploration of Group 2 metals as reducing agents for non-planar PAHs has also been initiated, opening this field for further investigations.