Of the five possible indenofluorene regioisomers, examples of a fully conjugated indeno1,2‐afluorene scaffold have so far remained elusive. This work reports the preparation and characterization of ...7,12‐dimesitylindeno1,2‐afluorene 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 indeno1,2‐afluorene 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 indeno1,2‐afluorene hydrocarbon core.
Not a matter of IF but when: Described is the preparation of the last elusive indenofluorene (IF) regioisomer. While the body of circumstantial spectroscopic data support formation of the highly reactive triplet diradicaloid, definitive structural proof is provided by X‐ray diffraction analysis of crystals of the dianion, obtained by reduction of the indeno1,2‐afluorene with Cs metal.
Conspectus 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 (C20H10 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 Li3M3 6+ and LiM5 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 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.
An X-ray diffraction study of 6cycloparaphenylene (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-.
Lithium-coordinated polyaromatic anions such as tetrareduced corannulene, C 20 H 10 4- (1 4- ), are useful substrates to model and ultimately improve the graphitic electrodes in lithium-ion (Li + ) ...batteries. Previous studies suggested that 1 4- forms dimers encasing four Li + ions in solution. Here, we report a single-crystal x-ray diffraction analysis confirming the formation of a sandwich-type supramolecular aggregate with a high degree of alkali metal intercalation. In contrast to the prior model, our data reveal that five Li + ions are sandwiched between the two tetrareduced corannulene decks, and 7 Li nuclear magnetic resonance spectroscopy delineates a conserved structure in tetrahydrofuran solution. Remarkably, the sandwich is robust in both solution and solid states even in the presence of crown ethers that compete for Li + coordination. These results should help elucidate Li + intercalation motifs between curved carbon surfaces more broadly.
The controlled reaction of Na and Cs, two alkali metals of different ionic sizes and binding abilities, with sumanene (C21H12) affords a novel type of organometallic sandwich Cs(C21H11−)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(C21H11−)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.
It takes two to tango: The unprecedented double concave encapsulation of a metal ion by two bowl‐shaped carbanions is achieved by a synthetic strategy that capitalizes on the differing binding abilities of sodium and cesium alkali metals with respect to curved π‐surfaces.
The stepwise chemical reduction of a molecular warped nanographene (WNG) having a negatively curved π‐surface and defined C80H30 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.
Stepwise chemical reduction of a molecular warped nanographene (WNG, 1) with Cs metal affords the mono‐, di‐, and triply‐reduced nanosized carbanions. In‐depth X‐ray crystallographic analysis and comprehensive theoretical investigations of this series of negatively charged nanographenes reveal a very complex response and core flexibility upon multi‐electron charging.
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 13C 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.
Dianion and dication of a tetracyclopentatetraphenylene derivative were generated and investigated as the annulene‐within‐an‐annulene models. Experiments based on X‐ray crystallography and 1H NMR, 13C NMR, and MCD spectroscopies indicate that the outer and inner conjugation circuits are significantly decoupled, strongly supporting the theoretically predicted structures.
The chemical reduction of a 3cumulene (3TrTol) has been explored using alkali metals. Mono‐ and doubly reduced forms of 3TrTol 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 3TrTol without interference from metal binding. The dianion of 3TrTol 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, 3TrTol, 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 tetraphenyl3cumulene, the cumulenic framework of 3TrTol remains planar in all the derivatives.
Cumulene or alkyne? The reduction of a 3cumulene using Na and K in the presence of 18‐crown‐6 gives the mono‐ and doubly reduced cumulenes as solvent‐separated ion pairs. The analogous reduction with Cs affords a contact‐ion complex. Crystallographic analysis of all the species shows dramatically increased bond‐length alternation upon reduction, while the cumulenic framework remains essentially planar in all cases.
The reduced and oxidized states of an open-shell diindenob,ianthracene (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.
The first members of a new class of supramolecular organometallic compounds with mixed‐alkali‐metal cluster cores, LiK5 and Li3K3, sandwiched between two four‐fold reduced corannulene decks are ...prepared and fully characterized. The triple‐decker supramolecular anions, (C20H104−)(LiK5)6+(C20H104−)2− and (C20H104−)(Li3K3)6+(C20H104−)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 C20H104− in lithium binding is accompanied by unprecedented shifts up to −24 ppm in 7Li NMR spectra. The discussion of product formation mechanism, augmented by calculations, is provided.
Pearls in a clamshell: A new class of supramolecular organometallic compounds with mixed‐alkali‐metal clusters, LiK5 and Li3K3, sandwiched between two highly reduced corannulene decks have been prepared and characterized. They show a coordination record for corannulene tetraanions able to bind six alkali‐metal ions. Previously unseen engagement of the hub‐site of C20H104− in lithium binding is accompanied by unprecedented shifts up to −24 ppm in 7Li NMR spectra.
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![Highly strained [6]cyclopar...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/1359-7345/small "Highly strained [6]cycloparaphenylene: crystallization of an unsolvated polymorph and the first mono- and dianions")
