Structure and Bonding in π-Stacked Perylenes: The Impact of Charge on Pancake Bonding.

Perylene (PER) is a prototype of polycyclic aromatic hydrocarbons (PAHs), which play a pivotal role in various functional and electronic materials due to favorable molecule-to-molecule overlaps, which enhance electronic transport. This study provides guidelines regarding the impact of molecular charge on pancake bonding, a form of strong π-stacking interaction. Pancake bonding significantly boosts interaction energies within the monopositive dimer ([(C20H12)2]•+ or PER2+), crucial for stabilizing aggregation and crystal formation. We discovered energetically feasible sliding and rotation pathways within the [(C20H12)2]•+ dimer, connecting different configurations found in the Cambridge Structural Database (CSD). The dimer's charge profoundly influences the pancake bond order (PBO) and the strength and structural preferences of pancake bonding. The most stable configuration is found in the monocationic state (PER2+), featuring a pancake bond order of 1/2 with one-electron multicenter bonding (1e/mc) with similar characteristics for charge -1. Increasing the total charge of the dimer to +2 or -2 leads to an unstable local minimum. Diverse distribution of pancake bonding types present in crystal structures is interpreted with modeling based on dimer computations with varying charges.

Unveiling the Multielectron Acceptor Properties of π-Expanded Pyracylene: Reversible Boat to Chair Conversion.

In this work, the chemical reduction of a hybrid pyracylene-hexa-peri-hexabenzocoronene (HPH) nanographene was investigated with different alkali metals (Na, K, Rb) to reveal its remarkable multielectron acceptor abilities. The UV-vis and 1H NMR spectroscopy monitoring of the stepwise reduction reactions supports the existence of all intermediate reduction states up to the hexaanion for HPH. Tuning the experimental conditions enabled the synthesis of the HPH anions with gradually increasing reduction states (up to -5) isolated with different alkali metal ions as crystalline materials. The single-crystal X-ray diffraction structure analysis demonstrates that the highly negatively charged HPH anions (-4 and -5) exhibit a drastic geometry change from boat-shaped (observed in the neutral parent, mono- and dianions) to a chair conformation, which was proved to be fully reversible by NMR spectroscopy. DFT calculations show that this geometry change is induced by an enhanced interaction between the coordinated metal ions and negatively charged HPH core in the chair conformation.

Bending a Cumulene with Electrons: Stepwise Chemical Reduction and Structural Study of a Tetraaryl[4]Cumulene.

Chemical reduction of a [4]cumulene with cesium metal was explored, and the structural changes stemming from electron acquisition are detailed using X-ray crystallography. It is found that the [4]cumulene undergoes dramatic geometric changes upon stepwise reduction, including bending of the cumulenic core and twisting of the endgroups from orthogonal to planar. The structural deformation is consistent with early theoretical reports that suggest that the twisting should occur upon reduction of both even and odd [n]cumulenes. The current results, on the other hand, are inconsistent with a previous experimental study of a [3]cumulene in which the predicted twisting is not observed upon reduction. DFT calculations reveal that the barrier to deformation is an order of magnitude lower in a [3]cumulene than a [4]cumulene, allowing the barrier to be overcome in the solid-state.

Homoleptic Rare-Earth-Metal Sandwiches with Dibenzo[a,e]cyclooctatetraene Dianions.

A family of rare-earth complexes [RE(III) = Y, La, Gd, Tb, Dy, and Er] with doubly reduced dibenzo[a,e]cyclooctatetraene (DBCOT) has been synthesized and structurally characterized. X-ray diffraction analysis confirms that all products of the [RE(DBCOT)(THF)4][RE(DBCOT)2] composition consist of the anionic sandwich [RE(DBCOT)2]- and the cationic counterpart [RE(DBCOT)(THF)4]+. Within the sandwich, two elongated π decks are slightly bent toward the metal center (avg. 7.3°) with a rotation angle of 35.9-37.6°. The RE(III) ion is entrapped between the central eight-membered rings of DBCOT2- in a η8 fashion. The trends in the RE-COT bond lengths are consistent with the variations of the ionic radii of RE(III) centers. The 1H NMR spectra of the diamagnetic Y(III) and La(III) analogues illustrate the distinct solution behavior for the cationic and anionic parts in this series. Magnetic measurements for the Dy analogue reveal single-molecule magnetism, which was rationalized by considering the effect of crystal-field splitting for both building units analyzed by electronic structure calculations.

Orbital Nature of Carboionic Monoradicals Made from Diradicals.

The electronic, optical, and solid state properties of a series of monoradicals, anions and cations obtained from starting neutral diradicals have been studied. Diradicals based on s-indacene and indenoacenes, with benzothiophenes fused and in different orientations, feature a varying degree of diradical character in the neutral state, which is here related with the properties of the radical redox forms. The analysis of their optical features in the polymethine monoradicals has been carried out in the framework of the molecular orbital and valence bond theories. Electronic UV-Vis-NIR absorption, X-ray solid-state diffraction and quantum chemical calculations have been carried out. Studies of the different positive-/negative-charged species, both residing in the same skeletal π-conjugated backbone, are rare for organic molecules. The key factor for the dual stabilization is the presence of the starting diradical character that enables to indistinctively accommodate a pseudo-hole and a pseudo-electron defect with certainly small reorganization energies for ambipolar charge transport.

Twisted Diindeno-Fused Dibenzo[a,h]anthracene Derivatives and their Dianions.

We report a facile synthesis of diindeno-fused dibenzo[a,h]anthracene derivatives (DIDBA-2Cl, DIDBA-2Ph, and DIDBA-2H) with different degrees of non-planarity using three substituents (chloro, phenyl, and hydrogen) of various sizes. The planarization of their cores, as evidenced by the decreased end-to-end torsional angles, was confirmed by X-ray crystallography. Their enhanced energy gaps with twisting were investigated by a combination of spectroscopic and electrochemical methods with density functional theory, which showed a transition from singlet open-shell to closed-shell configuration. Moreover, their doubly reduced states, DIDBA-2Ph2- and DIDBA-2H2- , were achieved by chemical reduction. The structures of dianions were identified by X-ray crystallographic analysis, which elucidated that the electron charging further distorted the backbones. The electronic structure of the dianions was demonstrated by experimental and theoretical approaches, suggesting decreased energy gaps with larger non-planarity, different from the neutral species.

Localized Antiaromaticity Hotspot Drives Reductive Dehydrogenative Cyclizations in Bis- and Mono-Helicenes.

We describe reductive dehydrogenative cyclizations that form hepta-, nona-, and decacyclic anionic graphene subunits from mono- and bis-helicenes with an embedded five-membered ring. The reaction of bis-helicenes can either proceed to the full double annulation or be interrupted by addition of molecular oxygen at an intermediate stage. The regioselectivity of the interrupted cyclization cascade for bis-helicenes confirms that relief of antiaromaticity is a dominant force for these facile ring closures. Computational analysis reveals the unique role of the preexisting negatively charged cyclopentadienyl moiety in directing the second negative charge at a specific remote location and, thus, creating a localized antiaromatic region. This region is the hotspot that promotes the initial cyclization. Computational studies, including MO analysis, molecular electrostatic potential maps, and NICS(1.7)ZZ calculations, evaluate the interplay of the various effects including charge delocalization, helicene strain release, and antiaromaticity. The role of antiaromaticity relief is further supported by efficient reductive closure of the less strained monohelicenes where the relief of antiaromaticity promotes the cyclization even when the strain is substantially reduced. The latter finding significantly expands the scope of this reductive alternative to the Scholl ring closure.

Doubly-Reduced Pentacene in Different Coordination Environments: X-ray Crystallographic and Theoretical Insights into Structural and Electronic Changes.

Chemical reduction of pentacene (C22 H14 , 1) with Group 1 metals ranging from Li to Cs revealed that 1 readily undergoes a two-fold reduction to afford a doubly-reduced 12- anion in THF. With the help of 18-crown-6 ether used as a secondary coordinating agent, five π-complexes of 12- with different alkali metal counterions have been isolated and fully characterized. This series of complexes enables the first evaluation of alkali-metal ion binding patterns and structural changes of the 12- dianion based on the crystallographically confirmed examples. The difference in coordination of the smallest Li+ ion vs. heavier Group 1 congeners has been demonstrated. In addition, the use of benzo-15-crown-5 in the reaction of 1 with Na metal allowed the isolation of the unique solvent-separated ion product with a "naked" dianion, 12- . The detailed structural analyses of the series revealed the C-C bond alteration and core deformation of pentacene upon two-fold reduction and complexation. The negative charge localization at the central six-membered ring of 12- identified by theoretical calculations corroborates with the X-ray crystallographic results. Subsequent in-depth theoretical analysis provided a detailed description of changes in the electronic structure and aromaticity of pentacene upon reduction.

Dianion and Dication of Tetracyclopentatetraphenylene as Decoupled Annulene-within-an-Annulene Models.

The dianion and dication of tetramesityl-substituted tetracyclopentatetraphenylene, a circulene consisting of alternating five- and six-membered rings, have been generated by reduction with alkali metals and oxidation with antimony(V) halides, respectively. They are theoretically predicted to adopt double annulenoid structures called annulene-within-an-annulene models in which the outer and inner conjugation circuits are significantly decoupled. The theoretical structures were experimentally proven by X-ray crystallographic analyses and the electronic configurations were supported by MCD spectra. Based on the 13 C NMR chemical shifts, negative and positive charges are shown to be located mainly at the outer periphery, indicating that the dianion and dication have delocalized 22-π and 18-π electron outer perimeters, respectively, and 8-π electron structure at the inner ring. Notably, the dianion has an open-shell character, whereas the dication has a closed-shell ground state.

Site-Specific Reduction-Induced Hydrogenation of a Helical Bilayer Nanographene with K and Rb Metals: Electron Multiaddition and Selective Rb+ Complexation.

The chemical reduction of π-conjugated bilayer nanographene 1 (C138 H120 ) with K and Rb in the presence of 18-crown-6 affords [K+ (18-crown-6)(THF)2 ][{K+ (18-crown-6)}2 (THF)0.5 ][C138 H122 3- ] (2) and [Rb+ (18-crown-6)2 ][{Rb+ (18-crown-6)}2 (C138 H122 3- )] (3). Whereas K+ cations are fully solvent-separated from the trianionic core thus affording a "naked" 1.3 - anion, Rb+ cations are coordinated to the negatively charged layers of 1.3 - . According to DFT calculations, the localization of the first two electrons in the helicene moiety leads to an unprecedented site-specific hydrogenation process at the carbon atoms located on the edge of the helicene backbone. This uncommon reduction-induced site-specific hydrogenation provokes dramatic changes in the (electronic) structure of 1 as the helicene backbone becomes more compressed and twisted upon chemical reduction, which results in a clear slippage of the bilayers.

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.

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.

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 12- 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 12- 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.

Reduction of π-Expanded Cyclooctatetraene with Lithium: Stabilization of the Tetra-Anion through Internal Li+ Coordination.

The chemical reduction of a π-expanded polycyclic framework comprising a cyclooctatetraene moiety, octaphenyltetrabenzocyclooctatetraene, with lithium metal readily affords the corresponding tetra-anion instead of the expected aromatic dianion. As revealed by X-ray crystallography, the highly contorted tetra-anion is stabilized by coordination of two internally bound Li+ , while two external cations remain solvent separated. The variable-temperature 7 Li NMR spectra in THF confirm the presence of three types of Li+ ions and clearly differentiate internal binding, consistent with the crystal structure. Density-functional theory calculations suggest that the formation of the highly charged tetra-reduced carbanion is stabilized through Li+ coordination under the applied experimental conditions.

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.

Compressing Double [7]Helicene by Successive Charging with Electrons.

Chemical reduction of a benzo-fused double [7]helicene (1) with two alkali metals, K and Rb, provided access to three different reduced states of 1. The doubly-reduced helicene 12- has been characterized by single-crystal X-ray diffraction as a solvent-separated ion triplet with two potassium counterions. The triply- and tetra-reduced helicenes, 13- and 14- , have been crystallized together in an equimolar ratio and both form the contact-ion complexes with two Rb+ ions each, leaving three remaining Rb+ ions wrapped by crown ether and THF molecules. As structural consequence of the stepwise reduction of 1, the central axis of helicene becomes more compressed upon electron addition (1.42 Šin 14- vs. 2.09 Šin 1). This is accompanied by an extra core twist, as the peripheral dihedral angle increases from 16.5° in 1 to 20.7° in 14- . Theoretical calculations provided the pattern of negative charge build-up and distribution over the contorted helicene framework upon each electron addition, and the results are consistent with the X-ray crystallographic and NMR spectroscopic data.

Negative Charge as a Lens for Concentrating Antiaromaticity: Using a Pentagonal "Defect" and Helicene Strain for Cyclizations.

Incorporation of a five-membered ring into a helicene framework disrupts aromatic conjugation and provides a site for selective deprotonation. The deprotonation creates an anionic cyclopentadienyl unit, switches on conjugation, leads to a >200 nm red-shift in the absorbance spectrum and injects a charge into a helical conjugated π-system without injecting a spin. Structural consequences of deprotonation were revealed via analysis of a monoanionic helicene co-crystallized with {K+ (18-crown-6)(THF)} and {Cs+ 2 (18-crown-6)3 }. UV/Vis-monitoring of these systems shows a time-dependent formation of mono- and dianionic species, and the latter was isolated and crystallographically characterized. The ability of the twisted helicene frame to delocalize the negative charge was probed as a perturbation of aromaticity using NICS scans. Relief of strain, avoidance of antiaromaticity, and increase in charge delocalization assist in the additional dehydrogenative ring closures that yield a new planarized decacyclic dianion.

Record Alkali Metal Intercalation by Highly Charged Corannulene.

The need for advanced energy storage technologies demands the development of new functional materials. Novel carbon-rich and carbon-based materials of different structural topologies attract significant attention in this regard. Attractive systems include a unique class of bowl-shaped polycyclic aromatic hydrocarbons that map onto fullerene surfaces and are thus often referred to as fullerene fragments, buckybowls, or π-bowls. Importantly, carbon bowls are able to acquire multiple electrons in stepwise reduction reactions producing sets of successively reduced carbanions. The resulting negatively charged π-bowls exhibit unique supramolecular assembly and metal intercalation patterns that only recently have begun to be uncovered. First, we have resolved the long-standing mystery behind the supramolecular structure formed by a highly reduced fullerene fragment called corannulene (C20H104-) with multiple lithium ions, using X-ray crystallography coupled with NMR spectroscopy and theoretical calculations. This work provided a new paradigm for lithium ion intercalation between the curved carbon π-surfaces and facilitated understanding of the lithium ion storage mechanism in carbonaceous matrices. Next, we have initiated a new research direction, an investigation of the mixed alkali metal reduction reactions using bowl-shaped corannulene as a remarkable multielectron reservoir and unique ligand with open convex and concave π-surfaces. As a result, we have revealed the cooperative effect of lithium with heavier Group 1 metals in reduction and self-assembly processes of corannulene. Moreover, we have discovered a new class of organometallic supramolecules having heterometallic cores with high nuclearity and charge such as Li3M36+ and LiM56+ (M = K, Rb, and Cs) sandwiched between two tetrareduced corannulene decks. The resulting triple-decker supramolecular assemblies, fully characterized by X-ray diffraction and spectroscopic methods, were found to exhibit a record ability of the highly charged corannulene π-surfaces to be fully engaged in intercalation of multiple metal ions. Based on this unique ability, curved π-ligands with extended carbon frameworks are expected to show remarkable potential for alkali metal storage compared to flat polycyclic arenes. Notably, a previously unseen mode of internal lithium binding revealed in the heterobimetallic sandwiches is accompanied by unprecedented negative shifts (up to -25 ppm) in 7Li NMR spectra. Based on in-depth analysis of NMR data, augmented by DFT calculations, we have rationalized the observed experimental trends and proposed the mechanism of stepwise alkali metal substitution reactions. Furthermore, we have correlated the origin of the record 7Li NMR shifts with unique electronic structures of these novel supramolecular aggregates. Herein we present comprehensive analysis of unusual structural and electronic features of remarkable heterometallic self-assemblies formed by tetrareduced corannulene, using a wealth of our recent experimental and computational results. This work uncovers unique potential of highly negatively charged bowl-shaped π-ligands for new supramolecular chemistry and materials chemistry applications.

Multiple C-H Bond Activations in Corannulene by a Dirhenium Complex.

The reaction of Re2 (CO)8 (µ-C6 H5 )(µ-H), 1 with corannulene (C20 H10 ) yielded the product Re2 (CO)8 (µ-H)(µ-η2 -1,2-C20 H9 ), 2 (65 % yield) containing a Re2 metalated corannulene ligand formed by loss of benzene from 1 and the activation of one of the CH bonds of the nonplanar corannulene molecule by an oxidative-addition to 1. The corannulenyl ligand has adopted a bridging η2 -σ+π coordination to the Re2 (CO)8 grouping. Compound 2 reacts with a second equivalent of 1 to yield three isomeric doubly metalated corannulene products: Re2 (CO)8 (µ-H)(µ-η2 -1,2-µ-η2 -10,11-C20 H8 )Re2 (CO)8 (µ-H), 3 (35 % yield), Re2 (CO)8 (µ-H)(µ-η2 -2,1-µ-η2 -10,11-C20 H8 )Re2 (CO)8 (µ-H), 4 (12 % yield), and Re2 (CO)8 (µ-H)(µ-η2 -1,2-µ-η2 -11,10-C20 H8 )Re2 (CO)8 (µ-H), 5 (12 % yield), by a second CH activation on a second rim double bond on the corannulene molecule. The isomers differ by the relative orientations of the coordinated Re2 (CO)8 (µ-H) groupings. All new products were characterized structurally by single crystal X-ray diffraction analysis.

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) Šis 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.