Site-Directed Dimerization of Bowl-Shaped Radical Anions to Form a σ-Bonded Dibenzocorannulene Dimer

Mixed-Valence BN-Doped Corannulene Trimer Radical Cations

Mixed-valence (MV) dimers have been extensively investigated, however, the structure and properties of purely organic MV trimers based on open-shell polycyclic aromatic hydrocarbons remain elusive. Herein, unprecedented MV BN-doped corannulene radical cations [BN-Cor1]3 ⋅⋅2+  ⋅ 2[BArylF 4 ]- and [BN-Cor2]3 ⋅⋅2+  ⋅ 2[BArylF 4 ]- were synthesized via chemical oxidation, and their structures were unambiguously confirmed by single-crystal X-ray diffraction. These uncommon radical cations consist of three corannulene cores and two [BArylF 4 ]- anions, and three corannulene motifs [BN-Cor1]3 ⋅⋅2+ and [BN-Cor2]3 ⋅⋅2+ in the unit cell exhibit a trimer structure with a slipped π-stacking configuration. Detailed structural analyses further revealed that the corannulene cores exhibit an infinite layered self-assembly configuration, allowing their potential applications as building blocks for molecular conductors. The detection of a forbidden transition (Δms =±2) by electron paramagnetic resonance (EPR) spectroscopy further confirmed the existence of two unpaired electrons in the π-trimers and the MV characteristic of these two species. Variable-temperature EPR and conductivity measurements suggested that the BN-doped π-trimers exhibited antiferromagnetic coupling and conductivity properties.

Reversible structural rearrangement of π-expanded cyclooctatetraene upon two-fold reduction with alkali metals

The chemical reduction of a π-expanded COT derivative, octaphenyltetrabenzocyclooctatetraene (1), with lithium or sodium metals in the presence of secondary ligands affords a new doubly-reduced product (1_TR^2-). The X-ray diffraction study revealed a reductive core rearrangement accompanied by the formation of a single C-C bond and severe twist of the central tetraphenylene core. The reversibility of two-electron reduction and core transformation is further confirmed by NMR spectroscopy and DFT calculations.

Antiaromaticity-Promoted Radical Anion stability in α-vinyl Heterocyclics

As an electron-rich species, radical anions have a wide range of applications in organic synthesis. In addition, aromaticity is an essential concept in chemistry that has attracted considerable attention from...

Tuning Magnetic Interactions Between Triphenylene Radicals by Variation of Crystal Packing in Structures with Alkali Metal Counterions

The monoanion of triphenylene (C₁₈H₁₂, 1) was generated in THF using several alkali metals (Na, K, Rb, and Cs) as reducing agents and crystallized with the corresponding cations in the presence of 18-crown-6 ether. The UV-vis spectroscopy points to the metal-dependent coordination environment of the triphenylene monoanion-radicals, 1^·-, in solution. The X-ray diffraction characterization confirmed the formation of a solvent-separated ion pair (SSIP) with sodium ions, [{Na⁺(18-crown-6)(THF)₂}(1^·-)] (2), and three contact-ion pair (CIP) complexes formed by larger alkali metal ions, [{K⁺(18-crown-6)}(1^·-)] (3), [{Rb⁺(18-crown-6)}(1^·-)] (4), and [{Cs⁺(18-crown-6)}(1^·-)] (5). Structural analysis of the series reveals a notable geometry perturbation of the triphenylene framework in 2 caused by one-electron acquisition, which is further enhanced by direct metal binding in 3-5. This has been correlated with the aromaticity changes and charge redistribution upon one-electron reduction of 1, as revealed by the computational studies. The EPR spectroscopy and magnetic susceptibility measurements confirm antiferromagnetic interactions corresponding to an S = 1/2 system in the solid state. The magnetic behavior of 3-5 correlates with the arrangement of triphenylene radicals in the crystal structures. All three compounds exhibit antiferromagnetic (AFM) interactions between S = 1/2 radicals in the solid state, but the exchange coupling in 4 and 5 is notably stronger than that in 3, which leads to AFM ordering at 3.8 K in 4 and at 2.0 K in 5. The magnetic phase transitions in 4 and 5 can be interpreted as originating from interactions between the chains of the AFM-coupled S = 1/2 radicals.

Synthesis and Structural Elucidation of Bisdibenzocorannulene in Multiple Redox States

We report an anti-folded bowl-shaped bisdibenzocorannulene (BDBC) featuring a new chair-cyclohexane-like hexagon as a bridge of two dibenzocorannulene moieties. The neutral compound showed multiple redox-active properties and could be converted to the corresponding redox states through chemical reduction or oxidation. Chemical reduction of BDBC by stoichiometric addition of metallic potassium in the presence of [18]crown-6 ether, provided a radical anion BDBC^.- and a dianion BDBC^2- , respectively; while chemical oxidation by silver hexafluoroantimonate(V), converted the neutral compound to an open-shell singlet diradical dication (BDBC^.. )²⁺ . The structural consequences of both electron-reduction and oxidation were closely related to the release of ring-strain of the bowl-shaped π-scaffold and imposed steric hindrance of the hexagonal bridge. In addition, the unusual open-shell nature of the dication could mainly be attributed to the changing of localized antiaromaticity in the closed-shell structure to delocalized character in the biradical, and thus the emergence of weakly bonded π-electrons.

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.

Dimerization of indenocorannulene radicals: imposing stability through increasing strain and curvature

One-electron reduction of bowl-shaped indenocorannulene affords a new stable dimeric dianion, as confirmed by single-crystal X-ray diffraction, NMR and UV-vis spectroscopy, and DFT calculations.

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.

Stable radical versus reversible σ-bond formation of (porphyrinyl)dicyanomethyl radicals

(Porphyrinyl)dicyanomethyl radicals were produced by oxidation of dicyanomethyl-substituted porphyrins with PbO2. These radicals constitute a rare example displaying stable radical versus dynamic covalent chemistry (DCC) depending upon the substitution position of the dicyanomethyl radical. meso-Dicyanomethyl-substituted radicals exist as stable monomeric species and do not undergo any dimerization processes either in the solid state or in solution. In contrast, β-dicyanomethyl-substituted radicals are isolated as σ-dimers that are stable in the solid-state but display reversible σ-dimerization behavior in solution; monomeric radical species exist predominantly at high temperatures, while σ-dimerization is favoured at low temperatures. This dynamic behaviour has been confirmed by variable-temperature 1H NMR, UV-vis and EPR measurements. The structures of the stable radical and σ-dimer have been revealed by single-crystal X-ray diffraction analysis. The observed different reactivities of the two (porphyrinyl)dicyanomethyl radicals have been rationalized in terms of their spin delocalization behaviours.

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

The effects of size, charge and symmetry of carbon π-bowls on their supramolecular assembly and metal ion intercalation trends are discussed.