From corannulene to larger carbon bowls: are they better for multiple metal encapsulation?

Stepwise Generation of Mono-, Di-, and Triply-Reduced Warped Nanographenes: Charge-Dependent Aromaticity, Surface Nonequivalence, Swing Distortion, and Metal Binding Sites

The stepwise chemical reduction of a molecular warped nanographene (WNG) having a negatively curved π-surface and defined C₈₀ H₃₀ composition with Cs metal used as the reducing and complexing agent allowed the isolation of three different reduced states with one, two, and three electrons added to its π-conjugated system. This provided a unique series of nanosized carbanions with increasing negative charge for in-depth structural analysis of consequences of controlled electron charging of non-planar nanographenes, using X-ray crystallographic and computational tools. The 3D molecular electrostatic potential (MEP) maps identified the negative charge localization at the central part of the WNG surface where selective coordination of Cs⁺ ions is confirmed crystallographically. In-depth theoretical investigation revealed a complex response of the WNG to the stepwise electron acquisition. The extended and contorted π-surface of the WNG undergoes subtle swinging distortions that are accompanied by notable changes in the electronic structure and site-dependent aromaticity of the resulting carbanions.

Graphitic supramolecular architectures based on corannulene, fullerene, and beyond

In this Feature Article, we survey the advances made in the field of fulleretic materials over the last five years. Merging the intriguing characteristics of fulleretic molecules with hierarchical materials can lead to enhanced properties of the latter for applications in optoelectronic, biomaterial, and heterogeneous catalysis sectors. As there has been significant growth in the development of fullerene- and corannulene-containing materials, this article will focus on studies performed during the last five years exclusively, and highlight the recent trends in designing fulleretic compounds and understanding their properties, that has enriched the repertoire of carbon-rich functional materials.

Chemical Reduction of a Nanosized [6]Cyclo-2,7-naphthylene Macrocycle

Chemical reduction of a naphthylene macrocycle, [6]cyclo-2,7-naphthylene ([6]CNAP, 1), with alkali metals, Li and K, revealed the accessibility of the doubly-reduced state of 1. The macrocyclic 1^2- anion was isolated in different coordination environments and crystallographically characterized. The single-crystal X-ray diffraction confirmed the formation of contact-ion complexes with one Li⁺ and two K⁺ ions in THF, and a "naked" dianion in the solvent-separated ion product with K⁺ ions in the presence of 18-crown-6 ether. The detailed structural analysis of 1^2- showed that the π-conjugation over the biaryl linkages between naphthylene panels were enhanced upon two-fold reduction, which was rationally explained by theoretical calculations.

Charging a Negatively Curved Nanographene and Its Covalent Network

This study explores a bottom-up approach toward negatively curved carbon allotropes from octabenzo[8]circulene, a negatively curved nanographene. Stepwise chemical reduction reactions of octabenzo[8]circulene with alkali metals lead to a unique highly reduced hydrocarbon pentaanion, which is revealed by X-ray crystallography suggesting a local view for the reduction and alkali metal intercalation processes of negatively curved carbon allotropes. Polymerization of the tetrabromo derivative of octabenzo[8]circulene by the nickel-mediated Yamamoto coupling reaction results in a new type of porous carbon-rich material, which consists of a covalent network of negatively curved nanographenes. It has a specific surface area of 732 m² g^-1 and functions as anode material for lithium ion batteries exhibiting a maximum capacity of 830 mAh·g^-1 at a current density of 100 mA·g^-1. These results indicate that this covalent network presents the key structural and functional features of negatively curved carbon allotropes.

Porphyrin-based systems containing polyaromatic fragments: decoupling the synergistic effects in aromatic-porphyrin-fullerene systems

In this work, we report a two-step synthesis that allows the introduction of four pyrene or corannulene fragments at the para position of meso-tetraarylporphyrins using a microwave-assisted quadruple Suzuki-Miyaura reaction. Placing the PAHs at this position, further from the porphyrin core, avoids the participation of the porphyrin core in binding with fullerenes. The fullerene hosting ability of the four new molecular receptors was investigated by NMR titrations and DFT studies. Despite having two potential binding sites, the pyrene derivatives did not associate with C60 or C70. In contrast, the tetracorannulene derivatives bound C60 and C70, although with modest binding constants. In these novel para-substituted systems, the porphyrin core acts as a simple linker that does not participate in the binding process, which allows the system to be considered as two independent molecular tweezers; i.e., the first binding event is not transmitted to the second binding site. This behavior can be considered a direct consequence of the decoupling of the porphyrin core from the binding event.

Coupling of two curved polyaromatic radical-anions: stabilization of dimers by counterions

In this study, a comprehensive theoretical investigation of both kinetic and thermodynamic stabilities was performed for dimeric dianionic systems (C₂₀H₁₀)₂^2- and (C₂₈H₁₄)₂^2-, neutralized by two alkali metal cations. The influence of the counterions was of primary interest. The impact of the additional/spectator ligand(s) was elucidated by considering adducts with four molecules of diglyme or two molecules of 18-crown-6 ether. Importantly, both types of systems - in the form of contact-ion pair (CIP) and solvent-separated ion pair (SSIP) - were considered. The SSIP set was augmented by the adduct, in which the dimeric dianionic species were neutralized with purely organic cations N(CH₃)₄⁺ and P(CH₃)₄⁺. Detailed analysis of the bonding revealed that the presence of the counterions made these systems thermodynamically stable. This finding is in sharp contrast with results obtained for isolated (PAH)₂^2- systems, which were previously found to be thermodynamically unstable, but kinetically persistent. The introduction of the alkali metal cations to the system significantly increases the ionic term (ΔE_elstat), whereas the repulsive ΔE_Pauli one was found to be substantially reduced. Considering that the orbital component (ΔE_orb) exhibited only a moderate decrease and the preparation energy (ΔE_prep) showed no changes, the above-mentioned changes in ΔE_elstat and ΔE_Pauli provided a clear explanation for the increase of the thermodynamic stability of the target species. Importantly, a clear correlation between the size of the alkali metal cation and stability of the target dimeric product was established. Thermodynamic stability of the system rises with a decrease in the size of M⁺ due to enlargement of the ΔE_orb. Evaluated energy barriers (as spin-crossing points between singlet and triplet energy surfaces) were found to be equal to +15.85 kcal mol^-1 and +18.5 kcal mol^-1 for [(Cs⁺)₂{(C₂₀H₁₀)₂^2-}] and [(Cs⁺)₂{(C₂₈H₁₄)₂^2-}], respectively, which is substantially higher than those calculated for isolated (PAH)₂^2- systems (+10.00 kcal mol^-1 for (C₂₀H₁₀)₂^2- and +12.35 kcal mol^-1 for (C₂₈H₁₄)₂^2-). Thus, this study identified the presence of counterions as the key factor, which have a dramatic influence on the thermodynamic and kinetic stabilities of the aimed dianionic dimeric systems, which are formed by two curved polyaromatic monoanion-radicals.

Self-Assembly of Bowl-Shaped Naphthalimide-Annulated Corannulene

The self-assembly of a bowl-shaped naphthalimide-annulated corannulene of high solubility has been studied in a variety of solvents by NMR and UV/Vis spectroscopy. Evaluation by the anti-cooperative K2-K model revealed the formation of supramolecular dimers of outstanding thermodynamic stability. Further structural proof for the almost exclusive formation of dimers over extended aggregates is demonstrated by atomic force microscopy (AFM) and diffusion ordered spectroscopy (DOSY) measurements as well as by theoretical calculations. Thus, herein we present the first report of a supramolecular dimer of an annulated corannulene derivative in solution and discuss its extraordinarily high thermodynamic stability with association constants up to >106 M-1 in methylcyclohexane, which is comparable to the association constants given for planar phthalocyanine and perylene bisimide dyes.

Mono-reduced Corannulene: To Couple and Not to Couple in One Crystal

One-electron reduction of corannulene, C₂₀ H₁₀ , with Li metal in diglyme resulted in crystallization of [{Li⁺ (diglyme)₂ }₄ (C₂₀ H₁₀ ^.- )₂ (C₂₀ H₁₀ -C₂₀ H₁₀ )^2- ] (1), as revealed by single-crystal X-ray diffraction. This hybrid product contains two corannulene monoanion-radicals along with a dianionic dimer, crystallized with four Li⁺ ions wrapped by diglyme molecules. The dimeric (C₂₀ H₁₀ -C₂₀ H₁₀ )^2- anion provides the first crystallographically confirmed example of spontaneous radical dimerization for C₂₀ H₁₀ ^.- . The C-C bond length between the two C₂₀ H₁₀ ^.- bowls of 1.588(5) Å is consistent with the single σ-bond character of the linker. The trans-disposition of two bowls in the centrosymmetric (C₂₀ H₁₀ -C₂₀ H₁₀ )^2- dimer is observed with the torsion angle around the central C-C bond of 180°. Comprehensive theoretical analysis of formation/decomposition processes of the dimeric dianion has been carried out in order to evaluate the nature of bonding and energetics of the C₂₀ H₁₀ ^.- coupling. It is found that such σ-bonded dimers are thermodynamically unstable due to large preparation energy and repulsive Pauli component of the bonding, but kinetically persistent due to a high energy barrier provided by the existing spin-crossing point.

Charging OBO-Fused Double [5]Helicene with Electrons

Chemical reduction of OBO-fused double[5]helicene with Group 1 metals (Na and K) has been investigated for the first time. Two doubly-reduced products have been isolated and structurally characterized by single-crystal X-ray diffraction, revealing a solvent-separated ion triplet (SSIT) with Na+ ions and a contact-ion pair (CIP) with K+ ion. As the key structural outcome, the X-ray crystallographic analysis discloses the consequences of adding two electrons to the double helicene core in the SSIT without metal binding and reveals the preferential binding site in the CIP with K+ counterions. In both products, an increase in the twisting of the double helicene core upon charging was observed. The negative charge localization at the central core has been identified by theoretical calculations, which are in full agreement with X-ray crystallographic and NMR spectroscopic results. Notably, it was confirmed that the two-electron reduction of OBO-fused double[5]helicene is reversible.