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The two-fold reduction of tetrabenzo[a,c,e,g]cyclooctatetraene (TBCOT, or tetraphenylene, 1) with K, Rb, and Cs metals reveals a distinctive core transformation pathway: a newly formed C-C bond converts the central eight-membered ring into a twisted core with two fused five-membered rings. This C-C bond of 1.589(3)-1.606(6) Å falls into a single σ-bond range and generates two perpendicular π-surfaces with dihedral angles of 110.3(9)°-117.4(1)° in the 1 TR 2- dianions. As a result, the highly contorted 1 TR 2- ligand exhibits a "butterfly" shape and could provide different coordination sites for metal-ion binding. The K-induced reduction of 1 in THF affords a polymeric product with low solubility, namely [{K + (THF)} 2 (1 TR 2- )] (K 2 -1 TR 2- ). The use of a secondary ligand facilitates the isolation of discrete complexes with heavy alkali metals, [Rb + (18-crown-6)] 2 [1 TR 2- ] (Rb 2 -1 TR 2- ) and [Cs + (18-crown-6)] 2 [1 TR 2- ] (Cs 2 -1 TR 2- ). Both internal and external coordination are observed in K 2 -1 TR 2- , while the bulky 18-crown-6 ligand only allows external metal binding in Rb 2 -1 TR 2- and Cs 2 -1 TR 2- . The reversibility of the two-fold reduction and bond rearrangement is demonstrated by NMR spectroscopy. Computational analysis shows that the heavier alkali metals enable effective charge transfer from the 1 TR 2- TBCOT dianion, however, the aromaticity of the polycyclic ligand remains largely unaffected., (© 2025 Wiley-VCH GmbH.)

The chemical reduction of a pyracylene-hexa- peri -hexabenzocoronene-(HBC)-fused nanographene TPP was investigated with K and Rb metals to reveal its multi-electron acceptor abilities. The in situ reaction of TPP with the above alkali metals, monitored by UV-vis-NIR and 1 H NMR spectroscopy, evidenced the stepwise reduction process. The use of different solvents and secondary ligands enabled isolation of single crystals of three different reduced states of TPP with 1, 2, and 3 electrons added to its π-system. This provided a unique set of carbanions with gradually increasing negative charge for in-depth structural analysis of the outcomes of controlled electron addition to a non-planar and asymmetric nanographene, using X-ray crystallographic, spectroscopic, and theoretical tools. EPR spectroscopy measurements of the mono- and triply-reduced TPP products revealed distinct EPR splitting patterns. DFT calculations demonstrated a notable difference in the spin density distribution of these two open-shell products and provided insights into experimental EPR data. Moreover, the influence of the counterions on the "naked" TPP anions was illustrated computationally., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Helicenes of increasing dimensions and complexity have recently burst into the scene due to their unique structures coupled with interesting chiral, optical, and conducting properties. The helicene-related research has quickly progressed from fundamental curiosity to a diverse range of applications in organic catalysis, optoelectronic devices, chiroptical switches, sensors, and energy storage. The in-depth understanding of electron accepting properties of helicenes should further advance their materials chemistry applications, however, previous reports only relied on spectrocopic and electrochemical studies, while their structural changes weren't extensively discussed. Therefore, we initiated a broad investigation of chemical reduction behaviour of helicenes ranging in size and properties coupled with X-ray diffraction characterization of the reduced products. The responses of helicenes with different structures to the stepwise electron addition were investigated using a combination of X-ray crystallography, spectroscopic methods, and calculations. This study revealed topology- and charge-dependent consequences of chemical reduction ranging from reversible geometry perturbation to irreversible core transformation and site-specific reactivity of helicenes in addition to original alkali metal coordination patterns. This overview is focused on the crystallographically confirmed examples stemming from chemical reduction reactions of different helicenes with alkali metals. The opened discussion should stimulate further exploration of reactivity and complexation of novel π-expanded and heteroatom-doped helicenes based on the revealed structure-property correlations, thus advancing their applications as intriguing new materials., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Chemical reduction of a vertically expanded pentacene, TIPS- peri -pentacenopentacene (TIPS-PPP), with sodium metal in THF readily afforded a doubly-reduced product isolated as [{Na + (THF) 3 } 2 (TIPS-PPP2-)]. Single-crystal X-ray diffraction revealed the formation of a π-complex of TIPS-PPP2- with two {Na + (THF) 3 } moieties. The sodium ion is coordinated to three C-atoms at the most negatively charged edge sites and at the lateral ethynyl group. The delocalisation of the charge on the ethynyl function is accompanied by its notable elongation ( Δ = 0.015 Å) coupled with the contraction of the adjacent single C-C bond ( Δ = 0.033 Å). Bond length analysis supported by Harmonic Aromaticity Oscillator Model (HOMA) shows that the dianion can be written with four aromatic sextets in agreement with the Clar representation. Although TIPS-PPP2- has 36 π-electrons (4 n ), it does not show a global magnetic paratropic response. The rings with the most negative charge density have a very low paratropic response, while the other rings have a low diatropic response. Overall, the dianion is a non-aromatic molecule.

A new and unique π-stacking triphenylene trimer cation radical unit appears in the crystal structure of a newly synthesized salt with an oligomeric gallium(iii) chloride, [(C 18 H 12 ) 3 ]˙ + (Ga 3 Cl 10 ) - , which is the first triphenylene aggregate observed. The structure is attributed to a shared electron distributed over the trimer displaying π-stacking pancake bonding. Computational modeling rationalizes the appearance of a "chain-shaped" rather than a "star-shaped" gallium chloride anion as well as the reasons why the trimer, rather than a radical cation aggregate of different size, is preferred in this system. Moreover, the calculations allowed evaluation of larger cationic triphenylene radical π-stacked aggregates. Additional stabilization due to the shared single unpaired electron is calculated to remain significant at 5-7 kcal mol -1 for aggregates as large as 5-6 units., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

π-Conjugated macrocycles behave differently from analogous linear chains because their electronic wavefunctions resemble a quantum particle on a ring, leading to aromaticity or anti-aromaticity. [18]Annulene, (CH) 18 , is the archetypal non-benzenoid aromatic hydrocarbon. Molecules with circuits of 4n + 2 π electrons, such as [18]annulene (n = 4), are aromatic, with enhanced stability and diatropic ring currents (magnetic shielding inside the ring), whereas those with 4n π electrons, such as the dianion of [18]annulene, are expected to be anti-aromatic and exhibit the opposite behaviour. Here we use 1 H NMR spectroscopy to re-evaluate the structure of the [18]annulene dianion. We also show that it can be reduced further to an aromatic tetraanion, which has the same shape as the dianion. The crystal structure of the tetraanion lithium salt confirms its geometry and reveals a metallocene-like sandwich, with five Li + cations intercalated between two [18]annulene tetraanions. We also report a heteroleptic sandwich, with [18]annulene and corannulene tetraanion decks., (© 2024. The Author(s).)

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 1 H 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.

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 DBCOT 2- in a η 8 fashion. The trends in the RE-COT bond lengths are consistent with the variations of the ionic radii of RE(III) centers. The 1 H 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.

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., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

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 ([(C 20 H 12 ) 2 ] •+ or PER 2 + ), crucial for stabilizing aggregation and crystal formation. We discovered energetically feasible sliding and rotation pathways within the [(C 20 H 12 ) 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 (PER 2 + ), 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.

As a planar subunit of C 60 -fullerene, truxene (C 27 H 18 ) represents a highly symmetrical rigid hydrocarbon with strong blue emission. Herein, we used truxene as a model to investigate the chemical reactivity of a fullerene fragment with alkali metals. Monoanion, dianion, and trianion products with different alkali metal counterions were crystallized and fully characterized, revealing the core curvature dependence on charge and alkali metal coordination. Moreover, a 1 proton nuclear magnetic resonance study coupled with computational analysis demonstrated that deprotonation of the aliphatic CH 2 segments introduces aromaticity in the five-membered rings. Importantly, the UV-vis absorption and photoluminescence of truxenyl anions with different charges reveal intriguing charge-dependent optical properties, implying variation of the electronic structure based on the deprotonation process. An increase in aromaticity and π-conjugation yielded a red shift in the absorption and photoluminescent spectra; in particular, large Stokes shifts were observed in the truxenyl monoanion and dianion with high emission quantum yield and time of decay. Overall, stepwise deprotonation of truxene provides the first crystallographically characterized examples of truxenyl anions with three different charges and charge-dependent optical properties, pointing to their potential applications in carbon-based functional materials., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

[2 4 ]Paracyclophanetetraene is a classic example of a macrocyclic hydrocarbon that becomes globally aromatic on reduction to the di-anion, and switches to globally anti-aromatic in the tetra-anion. This redox activity makes it promising as an electrode material for batteries. Here, we report the solid-state structures of the di- and tetra-anions of this cyclophane, in several coordination environments. The changes in bond length on reduction yield insights into the global aromaticity of the di-anion (26π electrons), and anti-aromaticity of the tetra-anion (28π electrons), that were previously deduced from NMR spectra of species generated in situ . The experimental geometries of the aromatic di-anion and anti-aromatic tetra-anion from X-ray crystallographic data match well with gas-phase calculated structures, and reproduce the low symmetry expected in the anti-aromatic ring. Comparison of coordinated and naked anions confirms that metal coordination has little effect on the bond lengths. The UV-vis-NIR absorption spectra show a sharp intense peak at 878 nm for the di-anion, whereas the tetra-anion gives a broad spectrum typical of an anti-aromatic system., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

A series of dibenzannulated phenyl-annulated [4,2] peri -acenoacenes have been synthesized in three straightforward steps from 4,10-dibromoanthanthrone (vat orange 3). The phenyl bisannulation of [4,2] peri -acenoacene provides extra stability by increasing the overall aromatic character of the molecules, and allows for a 45-80% increase of the molar extinction coefficient ( ε ) compared to their [5,2] peri -acenoacene isomers. Depending on the substituents attached to the π-conjugated core, some derivatives exhibit strong aggregation in the solid state with association constant ( K a ) up to 255 M -1 , resulting in a significant broadening of the absorption spectrum and a substantial decrease of the bandgap value (more than 0.3 V) from solution to the solid state. One [4,2] peri -acenoacene derivative was doubly reduced using cesium and the crystal structure of the resulting salt has been obtained. Field-effect transistors showing a temperature-dependent hole mobility have been tested., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

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-2Ph 2- and DIDBA-2H 2- , 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., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Aromaticity is one of the most deeply rooted concepts in chemistry. But why, if two-thirds of existing compounds can be classified as aromatic, is there no consensus on what aromaticity is? σ-, π-, δ-, spherical, Möbius, or all-metal aromaticity… why are so many attributes needed to specify a property? Is aromaticity a dubious concept? This perspective aims to reflect where the aromaticity community is and where it is going., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

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.

The chemical reduction of a corannulene-based molecular nanographene, C 76 H 64 (1), with Na metal in the presence of 18-crown-6 afforded the doubly-reduced state of 1. This reduction provokes a distortion of the helicene core and has a significant impact on the aromaticity of the system.

The synthesis of a novel family of homoleptic COT-based heterotrimetallic self-assemblies bearing the formula [LnKCa(COT) 3 (THF) 3 ] (Ln(iii) = Gd, Tb, Dy, Ho, Er, Tm, and Yb) is reported followed by their X-ray crystallographic and magnetic characterization. All crystals conform to the monoclinic P 2 1 / c space group with a slight compression of the unit cell from 3396.4(2) Å 3 to 3373.2(4) Å 3 along the series. All complexes exhibit a triple-decker structure having the Ln(iii) and K(i) ions sandwiched by three COT 2- ligands with an end-bound {Ca 2+ (THF) 3 } moiety to form a non-linear (153.5°) arrangement of three different metals. The COT 2- ligands act in a η 8 -mode with respect to all metal centers. A detailed structural comparison of this unique set of heterotrimetallic complexes has revealed consistent trends along the series. From Gd to Yb, the Ln to ring-centroid distance decreases from 1.961(3) Å to 1.827(2) Å. In contrast, the separation of K(i) and Ca(ii) ions from the COT-centroid (2.443(3) and 1.914(3) Å, respectively) is not affected by the change of Ln(iii) ions. The magnetic property investigation of the [LnKCa(COT) 3 (THF) 3 ] series (Ln(iii) = Gd, Tb, Dy, Ho, Er, and Tm) reveals that the Dy, Er, and Tm complexes display slow relaxation of their magnetization, in other words, single-molecule magnet (SMM) properties. This behaviour is dominated by thermally activated (Orbach-like) and quantum tunneling processes for [DyKCa(COT) 3 (THF) 3 ] in contrast to [ErKCa(COT) 3 (THF) 3 ], in which the thermally activated and Raman processes appear to be relevant. Details of the electronic structures and magnetic properties of these complexes are further clarified with the help of DFT and ab initio theoretical calculations., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

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.

The chemical reduction of π-conjugated bilayer nanographene 1 (C 138 H 120 ) 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 ][C 138 H 122 3- ] (2) and [Rb + (18-crown-6) 2 ][{Rb + (18-crown-6)} 2 (C 138 H 122 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., (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Chemical reduction of pentacene (C 22 H 14 , 1) with Group 1 metals ranging from Li to Cs revealed that 1 readily undergoes a two-fold reduction to afford a doubly-reduced 1 2- anion in THF. With the help of 18-crown-6 ether used as a secondary coordinating agent, five π-complexes of 1 2- 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 1 2- 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, 1 2- . 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 1 2- 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., (© 2021 Wiley-VCH GmbH.)

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., (© 2021 Wiley-VCH GmbH.)

The stepwise chemical reduction of a molecular warped nanographene (WNG) having a negatively curved π-surface and defined C 80 H 30 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., (© 2021 Wiley-VCH GmbH.)

The monoanion of triphenylene (C 18 H 12 , 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 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., (© 2021 Wiley-VCH GmbH.)

The consequences of four-electron addition to [8]cycloparaphenylene ([8]CPP, 1 ) have been evaluated crystallographically, revealing a significant core deformation. The structural analysis exposes an elliptical distortion observed upon electron transfer, with the deformation parameter (D.P.) increased by 28% in comparison with neutral [8]CPP. The C-C bond length alteration pattern also indicates a quinoidal structural rearrangement upon four-fold reduction. The large internal cavity of [8]CPP 4- allows the encapsulation of two {K + (THF) 2 } cationic moieties with two additional cations bound externally in the solid-state structure of [{K + (THF) 2 } 4 ([8]CPP 4- )]. The experimental structural data have been used as a benchmark for the comprehensive theoretical description of the geometric changes and electronic properties of the highly-charged [8]CPP 4- nanohoop in comparison with its neutral parent. While neutral [8]CPP and the [8]CPP 2- anion clearly show aromatic behavior of all six-membered rings, subsequent addition of two more electrons completely reverses their aromatic character to afford the highly-antiaromatic [8]CPP 4- anion, as evidenced by structural, topological, and magnetic descriptors. The disentanglement of electron transfer from metal binding effects allowed their contributions to the overall core perturbation of the negatively-charged [8]CPP to be revealed. Consequently, the internal coordination of potassium cations is identified as the main driving force for drastic elliptic distortion of the macrocyclic framework upon reduction., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

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 m 2 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.

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., (© 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

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 1 2- 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, 1 3- and 1 4- , 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 Å in 1 4- vs. 2.09 Å in 1). This is accompanied by an extra core twist, as the peripheral dihedral angle increases from 16.5° in 1 to 20.7° in 1 4- . 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., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Chemical reduction of several cycloparaphenylenes (CPPs) ranging in size from [8]CPP to [12]CPP has been investigated with potassium metal in THF. The X-ray diffraction characterization of the resulting doubly-reduced [ n ]CPPs provided a unique series of carbon nanohoops with increasing dimensions and core flexibility for the first comprehensive structural analysis. The consequences of electron acquisition by a [ n ]CPP core have been analyzed in comparison with the neutral parents. The addition of two electrons to the cyclic carbon framework of [ n ]CPPs leads to the characteristic elliptic core distortion and facilitates the internal encapsulation of sizable cationic guests. Molecular and solid-state structure changes, alkali metal binding and unique size-dependent host abilities of the [ n ]CPP 2- series with n = 6-12 are discussed. This in-depth analysis opens new perspectives in supramolecular chemistry of [ n ]CPPs and promotes their applications in size-selective guest encapsulation and chemical separation., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

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., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

One-electron reduction of corannulene, C 20 H 10 , with Li metal in diglyme resulted in crystallization of [{Li + (diglyme) 2 } 4 (C 20 H 10 .- ) 2 (C 20 H 10 -C 20 H 10 ) 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 (C 20 H 10 -C 20 H 10 ) 2- anion provides the first crystallographically confirmed example of spontaneous radical dimerization for C 20 H 10 .- . The C-C bond length between the two C 20 H 10 .- 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 (C 20 H 10 -C 20 H 10 ) 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 C 20 H 10 .- 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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Mono- and dianions of 2-tert-butyl-3a 2 -azapentabenzo[bc,ef,hi,kl,no]corannulene (1 a) were synthesized by chemical reduction with sodium and cesium metals, and crystallized as the corresponding salts in the presence of 18-crown-6 ether. X-ray diffraction analysis of the sodium salt, [{Na + (18-crown-6)(THF) 2 } 3 {Na + (18-crown-6)(THF)}(1 a 2- ) 2 ], revealed the presence of a naked dianion. In contrast, controlled reaction of 1 a with Cs allowed the isolation of singly and doubly reduced forms of 1 a, both forming π-complexes with cesium ions in the solid state. In [{Cs + (18-crown-6)}(1 a - )]⋅THF, asymmetric binding of the Cs + ion to the concave surface of 1 a - is observed, whereas in [{Cs + (18-crown-6)} 2 (1 a 2- )], two Cs + ions bind to both the concave and convex surfaces of the dianion. The present study provides the first successful isolation and characterization of the reduced products of heteroatom-containing buckybowl molecules., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A highly volatile molecular precursor, dibismuth(ii) (tetra)trifluoroacetate, was successfully utilized for the gas phase growth of twin-free single crystals of bismuth. This allowed high-quality single-crystal X-ray diffraction data to be collected, providing accurate structural parameters for elemental bismuth under ambient conditions.

Two new organometallic cyclooctatetraenyl complexes of the type [M2(COT)3(THF)2] (M = Y and La) have been prepared, using optimized synthetic procedures, and fully characterized by X-ray diffraction analysis, IR and 1H NMR spectroscopies. The structures can be represented as formed by the double-decker [M(COT)2]- anion with an asymmetrically bound cationic [M(COT)(THF)2]+ unit. The COT rings in the anionic sandwich are not equidistant from the metal with the M-COTcentroid distances measuring at 1.991(5) Å and 2.074(5) Å for [Y(COT)2]-vs. 2.045(4) Å and 2.154(5) Å for [La(COT)2]-. The sandwich fragments are η2-coordinated to the second metal center with the average M-C distances of 2.837(4) Å and 2.879(5) Å for yttrium and lanthanum complexes, respectively. The M-COTcentroid distances in the cationic unit are 1.962(4) Å for the former and 2.009(2) Å for the latter.

Carbon bowls having two distinctly different faces, convex and concave, represent a unique class of non-planar π-ligands with defined molecular compositions and structures. The smallest π-bowl called corannulene is known to exhibit the record alkali metal ion binding abilities upon multi-electron acquisition. In contrast, the potential of larger carbon bowls with extended π-surfaces in self-assembly with multiple metal ions is yet to be revealed. This Frontier article opens a discussion of the effect of size, charge and symmetry of the bowl-shaped π-ligands on their supramolecular assembly and metal ion intercalation trends.

The reaction of Re 2 (CO) 8 (μ-C 6 H 5 )(μ-H), 1 with corannulene (C 20 H 10 ) yielded the product Re 2 (CO) 8 (μ-H)(μ-η 2 -1,2-C 20 H 9 ), 2 (65 % yield) containing a Re 2 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 η 2 -σ+π coordination to the Re 2 (CO) 8 grouping. Compound 2 reacts with a second equivalent of 1 to yield three isomeric doubly metalated corannulene products: Re 2 (CO) 8 (μ-H)(μ-η 2 -1,2-μ-η 2 -10,11-C 20 H 8 )Re 2 (CO) 8 (μ-H), 3 (35 % yield), Re 2 (CO) 8 (μ-H)(μ-η 2 -2,1-μ-η 2 -10,11-C 20 H 8 )Re 2 (CO) 8 (μ-H), 4 (12 % yield), and Re 2 (CO) 8 (μ-H)(μ-η 2 -1,2-μ-η 2 -11,10-C 20 H 8 )Re 2 (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 Re 2 (CO) 8 (μ-H) groupings. All new products were characterized structurally by single crystal X-ray diffraction analysis., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The chemical reduction of a [3]cumulene ([3]TrTol) has been explored using alkali metals. Mono- and doubly reduced forms of [3]TrTol were isolated as solvent-separated ion pairs with {Na(18-crown-6)THF 2 } + and {K(18-crown-6)THF 2 } + counterions and crystallographically characterized. This allowed analysis of structural parameters of the "naked" anions of [3]TrTol without interference from metal binding. The dianion of [3]TrTol was also isolated as a contact-ion complex with {Cs(18-crown-6)} + cations, thereby adding the effect of metal coordination to the core. Structural comparisons of anions to the neutral molecule, [3]TrTol, outline monotonic increases in bond-length alternation (BLA) upon stepwise reduction. The greatest BLA value is found for the contact-ion complex, which shows an alternating sequence of short and long carbon-carbon bonds, consistent with the structure of an alkyne. In contrast to studies on tetraphenyl[3]cumulene, the cumulenic framework of [3]TrTol remains planar in all the derivatives., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

An X-ray diffraction study of [6]cycloparaphenylene (1), crystallized under solvent-free conditions, revealed a unique solid state structure with tight packing of individual molecules that minimizes empty internal space. The controlled chemical reduction of this highly strained nanohoop with Group 1 metals resulted in the first isolation and structural characterization of its mono- and dianions, allowing for the evaluation of core transformations for the series ranging from 10 to 11- and 12-.

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 (C 20 H 10 4- ) 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 Li 3 M 3 6+ and LiM 5 6+ (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 7 Li 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 7 Li 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.

Designed site-directed dimerization of the monoanion radicals of a π-bowl in the solid state is reported. Dibenzo[a,g]corannulene (C 28 H 14 ) was selected based on the asymmetry of the charge/spin localization in the C 28 H 14 .- anion. Controlled one-electron reduction of C 28 H 14 with Cs metal in diglyme resulted in crystallization of a new dimer, [{Cs + (diglyme)} 2 (C 28 H 14 -C 28 H 14 ) 2- ] (1), as revealed by single crystal X-ray diffraction study performed in a broad range of temperatures. The C-C bond length between two C 28 H 14 .- bowls (1.560(8) Å) measured at -143 °C does not significantly change upon heating of the crystal to +67 °C. The single σ-bond character of the C-C linker is confirmed by calculations. The trans-disposition of two bowls in 1 is observed with the torsion angles around the central C-C bond of 172.3(5)° and 173.5(5)°. A systematic theoretical evaluation of dimerization pathways of C 28 H 14 .- radicals confirmed that the trans-isomer found in 1 is energetically favored., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Molecular and electronic structure, reduction electron transfer and coordination abilities of a polycyclic aromatic hydrocarbon (PAH) having a planar naphtho-group fused to the corannulene bowl have been investigated for the first time using a combination of theoretical and experimental tools. A direct comparison of naphtho[2,3-a]corannulene (C 28 H 14 , 1) with parent corannulene (C 20 H 10 , 2) revealed the effect of framework topology change on electronic properties and aromaticity of 1. The presence of two reduction steps for 1 was predicted theoretically and confirmed experimentally. Two reversible one-electron reduction processes with the formal reduction potentials at -2.30 and -2.77 V versus Fc +/0 were detected by cyclic voltammetry (CV) measurements, demonstrating accessibility of the corresponding mono- and dianionic states of 1. The products of the singly and doubly reduced napththocorannulene were prepared using chemical reduction with Group 1 metals and isolated as sodium and rubidium salts. Their X-ray diffraction study revealed the formation of "naked" mono- and dianions crystallized as solvent-separated ion products with one or two sodium cations as [Na + (18-crown-6)(THF) 2 ][C 28 H 14 - ] and [Na + (18-crown-6)(THF) 2 ] 2 [C 28 H 14 2- ] (3⋅THF and 4⋅THF, respectively). The dianion of 1 was also isolated as a contact-ion complex with two rubidium countercations, [{Rb + (18-crown-6)} 2 (C 28 H 14 2- )] (5⋅THF). The structural consequences of adding one and two electrons to the carbon framework of 1 are compared for 3, 4 and 5. Changes in aromaticity and charge distribution stemming from the stepwise electron acquisition are discussed based on DFT computational study., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Two series of aromatic compounds with perfluoroalkyl (R F ) groups of increasing length, 1,3,5,7-naphthalene(R F ) 4 and 1,3,5,7,9-corannulene(R F ) 5 , have been prepared and their electronic properties studied by low-temperature photoelectron spectroscopy (PES) (for gas-phase electron affinity measurements). These and many related compounds were also studied by DFT calculations. The data demonstrate unambiguously that the electron-withdrawing ability of R F substituents increases significantly and uniformly from CF 3 to C 2 F 5 to n-C 3 F 7 to n-C 4 F 9 ., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Of the five possible indenofluorene regioisomers, examples of a fully conjugated indeno[1,2-a]fluorene scaffold have so far remained elusive. This work reports the preparation and characterization of 7,12-dimesitylindeno[1,2-a]fluorene as a highly reactive species. Experimental and computational data support the notion of a molecule with pronounced diradical character that exists in a triplet ground state. As such, both NICS and ACID calculations suggest that the indeno[1,2-a]fluorene scaffold is weakly Baird aromatic. Reduction of the unstable red solid with Cs metal produces the dianion of the title compound, from which single crystals could be obtained and X-ray data acquired, thus fully corroborating the proposed indeno[1,2-a]fluorene hydrocarbon core., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The first comprehensive theoretical investigation of structural, energetic, and electronic changes in a sumanene skeleton, C 21 H 12 , upon a step-wise deprotonation process is performed. This study is complemented by a detailed consideration of aromaticity in target bowl-shaped systems, including neutral sumanene and its three deprotonated anions, namely C 21 H 11 1- , C 21 H 10 2- , and C 21 H 9 3- . In order to obtain the most reliable and method-independent characteristics, a set of aromatic descriptors of different nature has been applied. It included structure-based HOMA, topological descriptors PDI and FLU, as well as magnetic NICS and ACID. The calculation results reveal that the neutral sumanene can be best described as mechanically bent triphenylene, in which π-conjugation is mostly localized over three peripheral 6-membered rings. Sequential deprotonation changed the system from the localized mono-anionic to semi-localized di-anionic, and eventually to the fully delocalized tri-anionic sumanenyl species. Structural changes, namely, bond equalization upon the deprotonation process, are in excellent agreement with alterations observed in electronic structures and aromaticity. Deprotonation results in a significant reduction of the barrier for a bowl-to-bowl transition only in the tri-anionic sumanenyl system, whereas the first and the second deprotonation steps show no notable effect. This clearly indicates that only complete aromatization of the sumanene core in C 21 H 9 3- leads to a substantial increase of bowl flexibility.

The use of two alkali metals having the greatest ion size mismatch, Li and Cs, in the reduction of bowl-shaped corannulene (C 20 H 10 , 1), results in the preparation of a heterobimetallic organometallic self-assembly, [Cs(diglyme)] 2 //[Li 3 Cs 3 (C 20 H 10 ) 2 (diglyme) 2 ] (2). The X-ray crystallographic investigation of 2 revealed the formation of a triple-decker sandwich having a highly-charged hexanuclear core, (Li 3 Cs 3 ) 6+ , held between two tetrareduced corannulene decks. The sandwich is capped by external cesium ions filling the concave cavities of corannulene bowls. The average depth of two C 20 H 10 4- anions of 0.754(14) Å is significantly greater in 2 compared to the values of 0.241(2)-0.355(2) Å observed in the homometallic lithium sandwiches formed by tetrareduced corannulene, thus illustrating the unique flexibility of its carbon framework to adapt to different internal and external coordination environments.

Only a few cyclooctatetraene dianion (COT) π-complexes of lanthanides have been crystallographically characterized. This first single-crystal X-ray diffraction characterization of a scandium(III) COT chloride complex, namely di-μ-chlorido-bis[(η 8 -cyclooctatetraene)(tetrahydrofuran-κO)scandium(III)], [Sc 2 (C 8 H 8 ) 2 Cl 2 (C 4 H 8 O) 2 ] or [Sc(COT)Cl(THF)] 2 (THF is tetrahydrofuran), (1), reveals a dimeric molecular structure with symmetric chloride bridges [average Sc-Cl = 2.5972 (7) Å] and a η 8 -bound COT ligand. The COT ring is planar, with an average C-C bond length of 1.399 (3) Å. The Sc-C bond lengths range from 2.417 (2) to 2.438 (2) Å [average 2.427 (2) Å]. Direct comparison of (1) with the known lanthanide (Ln) analogues (La, Ce, Pr, Nd, and Sm) illustrates the effect of metal-ion (M) size on molecular structure. Overall, the M-Cl, M-O, and M-C bond lengths in (1) are the shortest in the series. In addition, only one THF molecule completes the coordination environment of the small Sc III ion, in contrast to the previously reported dinuclear Ln-COT-Cl complexes, which all have two bound THF molecules per metal atom.