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.

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.

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.

Structural and Electronic Effects of Stepwise Reduction of a Tetraaryl[3]Cumulene.

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)THF2 }+ and {K(18-crown-6)THF2 }+ 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.

Highly strained [6]cycloparaphenylene: crystallization of an unsolvated polymorph and the first mono- and dianions.

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-.

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.

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

Designed site-directed dimerization of the monoanion radicals of a π-bowl in the solid state is reported. Dibenzo[a,g]corannulene (C28 H14 ) was selected based on the asymmetry of the charge/spin localization in the C28 H14.- anion. Controlled one-electron reduction of C28 H14 with Cs metal in diglyme resulted in crystallization of a new dimer, [{Cs+ (diglyme)}2 (C28 H14 -C28 H14 )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 C28 H14.- 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 C28 H14.- radicals confirmed that the trans-isomer found in 1 is energetically favored.

Fusing a Planar Group to a π-Bowl: Electronic and Molecular Structure, Aromaticity and Solid-State Packing of Naphthocorannulene and its Anions.

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 (C28 H14 , 1) with parent corannulene (C20 H10 , 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 ][C28 H14- ] and [Na+ (18-crown-6)(THF)2 ]2 [C28 H142- ] (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 (C28 H142- )] (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.

Experimental and DFT Studies of the Electron-Withdrawing Ability of Perfluoroalkyl (RF ) Groups: Electron Affinities of PAH(RF )n Increase Significantly with Increasing RF Chain Length.

Two series of aromatic compounds with perfluoroalkyl (RF ) groups of increasing length, 1,3,5,7-naphthalene(RF )4 and 1,3,5,7,9-corannulene(RF )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 RF substituents increases significantly and uniformly from CF3 to C2 F5 to n-C3 F7 to n-C4 F9 .

Synthesis of the Unknown Indeno[1,2-a]fluorene Regioisomer: Crystallographic Characterization of Its Dianion.

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.

Tuning the separation and coupling of corannulene trianion-radicals through sizable alkali metal belts.

The first heterobimetallic sandwich-type aggregate formed by bowl-shaped corannulene trianion-radicals, C20H10˙3-, has been synthesized using mixed-metal reduction of C20H10. The product was crystallographically characterized to reveal the self-assembly of [Cs+//(C20H103-)/4K+/(C20H103-)//Cs+], in which two triply-charged corannulene decks encapsulate a rectangle of four potassium ions (the K···K separations are 4.212(4) and 5.185(4) Å), with the exterior concave bowl cavities being selectively filled by one cesium ion each. In order to provide insights into the geometrical features and electronic structure of this novel mixed-metal organometallic self-assembly, an in-depth theoretical investigation has been carried out. Specifically, the influence of internal metal binding on the geometry and magnetic coupling of C20H10˙3- radicals is investigated for Group 1 metals. This study reveals that replacement of the sandwiched potassium ions with larger (Cs) and smaller (Li) ions allows variation of the size of the encapsulated metal belts, and thus enables tuning of the coupling of C20H10˙3- radicals.

Double Concave Cesium Encapsulation by Two Charged Sumanenyl Bowls.

The controlled reaction of Na and Cs, two alkali metals of different ionic sizes and binding abilities, with sumanene (C21 H12 ) affords a novel type of organometallic sandwich [Cs(C21 H11- )2 ]- , which crystallized as a solvent-separated ion pair with a [Na(18-crown-6)(THF)2 ]+ cation (where THF=tetrahydrofuran). The unprecedented double concave encapsulation of a metal ion by two bowl-shaped sumanenyl anions in [Cs(C21 H11- )2 ]- was revealed crystallographically. Evaluation of bonding and energetics of the remarkable product was accomplished computationally (B2PLYP-D/TZVP/ZORA), providing insights into its formation.

Mixing Li and Cs in the reduction of corannulene for the assembly of a cesium-capped sandwich with a hexanuclear heterometallic core.

The use of two alkali metals having the greatest ion size mismatch, Li and Cs, in the reduction of bowl-shaped corannulene (C20H10, 1), results in the preparation of a heterobimetallic organometallic self-assembly, [Cs(diglyme)]2//[Li3Cs3(C20H10)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, (Li3Cs3)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 C20H104- 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.

A Biradical Balancing Act: Redox Amphoterism in a Diindenoanthracene Derivative Results from Quinoidal Acceptor and Aromatic Donor Motifs.

The reduced and oxidized states of an open-shell diindeno[b,i]anthracene (DIAn) derivative have been investigated by experimental and theoretical techniques. As a result of moderate biradical character and the ability of cyclopenta-fused scaffolds to stabilize both positive and negative charges, DIAn exhibits rich redox chemistry with four observable and isolable charged states. Structural and electronic properties of the DIAn system are brought to light by UV-vis-NIR and Raman spectroelectrochemical measurements. Aromatization of the diindeno-fused anthracene core upon successive single-electron injections is revealed through single-crystal X-ray diffraction of radical anion and dianion salts. We present a rare case where the pseudoaromatic/quinoidal ground state of a neutral biradical polycyclic hydrocarbon leads to a stable cascade of five redox states. Our detailed investigation of the transformation of molecular structure along all four redox events provides a clearer understanding of the nature of charge carriers in ambipolar organic field-effect transistors.

Supramolecular trap for a transient corannulene trianion.

The first structural characterization of the transient triply-reduced state of corannulene (C20H10) is accomplished. The X-ray crystallographic study reveals that the C20H10˙3- trianions, generated by corannulene reduction with metallic cesium, form a novel type of supramolecular sandwich-type assembly, [Cs+//(C20H103-)/4Cs+/(C20H103-)//Cs+]. In the product, two triply-charged corannulene decks encapsulate a rectangle of four cesium ions with the external concave bowl cavities being filled by one cesium ion each. The structural investigation is augmented by in-depth theoretical calculations to provide insights into the geometrical features and electronic structure of this unique organometallic self-assembly.

Monoreduced 1,2-dihydrocorannulene versus the parent corannulene.

The monoanion of dihydrogenated corannulene isolated in the form of its potassium salt, namely tris(diglyme-κ(3)O,O',O'')potassium hexacyclo[11.5.2.0(4,17).0(7,16).0(10,15).0(14,18)]icosa-1,3,5,7(16),8,10(15),11,13,17-nonaenide, [K(C6H14O3)3](C20H12), has been structurally characterized for the first time. The X-ray study confirms the previous NMR spectroscopic prediction that the two H atoms are attached to the same six-membered ring to form 1,2-dihydrocorannulene, thus destroying the aromaticity of only one arene ring of the corannulene core. The direct comparison of (C20H12)(-) with the parent corannulene anion, (C20H10)(-), is provided to illustrate the geometry perturbations caused by rim hydrogenation.

Convex and Concave Encapsulation of Multiple Potassium Ions by Sumanenyl Anions.

Herein we report the novel complex consisting of di- and tripotassiumsumanenide, K7(C21H10(2-))2(C21H9(3-))·8THF (2), which was prepared by the treatment of sumanene (C21H12, 1) with excess K metal in THF. The X-ray structural determination revealed unique self-assembly of six potassium ions sandwiched by convex faces of two sumanenyl trianions in addition to novel interaction involving all 15 carbon atoms of three Cp-like moieties on the concave surface of the sumanenyl bowls outside the sandwich. The unique structural features of 2 are rationalized with the help of DFT calculations.

Self-assembly of tetrareduced corannulene with mixed Li-Rb clusters: dynamic transformations, unique structures and record 7Li NMR shifts.

Self-assembly processes of the highly reduced bowl-shaped corannulene generated by the chemical reduction with a binary combination of alkali metals, namely Li-Rb, have been investigated by variable-temperature 1H and 7Li NMR spectroscopy. The formation of several unique mixed metal sandwich products based on tetrareduced corannulene, C20H104- (14-), has been revealed followed by investigation of their dynamic transformations in solutions. Analysis of NMR data allowed to propose the mechanism of stepwise alkali metal substitution as well as to identify experimental conditions for the isolation of intermediate and final supramolecular products. As a result, two new triple-decker aggregates with a mixed Li-Rb core, [{Rb(THF)2}2]//[Li3Rb2(C20H10)2{Li+(THF)}] (2) and [{Rb(diglyme)}2]//[Li3Rb3(C20H10)2(diglyme)2]·0.5THF (3·0.5THF), have been crystallized and structurally characterized. The Li3Rb2-product has an open coordination site at the sandwich periphery and thus is considered transient on the way to the Li3Rb3-sandwich having the maximized intercalated alkali metal content. Next, the formation of the LiRb5 self-assembly with 14- has been identified by 7Li NMR as the final step in a series of dynamic transformations in this system. This product was also isolated and crystallographically characterized to confirm the LiRb5 core. Notably, all sandwiches have their central cavities, located in between the hub-sites of two C20H104- decks, occupied by an internal Li+ ion which exhibits the record high negative shift (ranging from -21 to -25 ppm) in 7Li NMR spectra. The isolation of three novel aggregates having different Li-Rb core compositions allowed us to look into the origin of the unusual 7Li NMR shifts at the molecular level. The discussion of formation mechanisms, dynamic transformations as well as unique electronic structures of these remarkable mixed alkali metal organometallic self-assemblies is provided and supported by DFT calculations.

An unsolvated buckycatcher and its first dianion.

The X-ray crystallographic study of C60H28 consisting of two tethered corannulene bowls revealed a unique solid-state packing based on tight convex-concave π-π interactions. The controlled reduction of C60H28 resulted in the isolation and structural characterization of its dianion in the form of the rubidium salt that shows an entrapment of counterions by an anionic pincer.

Clamshell opening in the mixed-metal supramolecular aggregates formed by fourfold reduced corannulene for maximizing intercalated metal content.

The first members of a new class of supramolecular organometallic compounds with mixed-alkali-metal cluster cores, LiK5 and Li3 K3 , sandwiched between two four-fold reduced corannulene decks are prepared and fully characterized. The triple-decker supramolecular anions, [(C20 H10 (4-) )(LiK5 )(6+) (C20 H10 (4-) )](2-) and [(C20 H10 (4-) )(Li3 K3 )(6+) (C20 H10 (4-) )](2-) , illustrate a record ability of bowl-shaped and highly charged corannulene to provide all its sites, five benzene rings fused to a central five-membered ring, for binding of six alkali-metal ions. The previously unseen engagement of the hub-site of C20 H10 (4-) in lithium binding is accompanied by unprecedented shifts up to -24 ppm in (7) Li NMR spectra. The discussion of product formation mechanism, augmented by calculations, is provided.