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Chemical reduction of pentacene (C₂₂H₁₄,1) with Group 1 metals ranging from Li to Cs revealed that1readily undergoes a two‐fold reduction to afford a doubly‐reduced1²⁻anion in THF. With the help of 18‐crown‐6 ether used as a secondary coordinating agent, five π‐complexes of1²⁻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 the1²⁻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 of1with Na metal allowed the isolation of the unique solvent‐separated ion product with a “naked” dianion,1²⁻. The detailed structural analyses of the series revealed the C−C bond alteration and core deformation of pentacene upon two‐fold reduction and complexation. The negative charge localization at the central six‐membered ring of1²⁻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.

 

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‐temperature⁷Li 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.

 

Chemical reduction of a benzo‐fused double [7]helicene (1) with two alkali metals, K and Rb, provided access to three different reduced states of1. The doubly‐reduced helicene1²⁻has been characterized by single‐crystal X‐ray diffraction as a solvent‐separated ion triplet with two potassium counterions. The triply‐ and tetra‐reduced helicenes,1³⁻and1⁴⁻, 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 of1, the central axis of helicene becomes more compressed upon electron addition (1.42 Å in1⁴⁻vs. 2.09 Å in1). This is accompanied by an extra core twist, as the peripheral dihedral angle increases from 16.5° in1to 20.7° in1⁴⁻. 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.

 

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.

 

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.

 

Mono‐ and dianions of 2‐tert‐butyl‐3a²‐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 a²⁻)₂], revealed the presence of a naked dianion. In contrast, controlled reaction of1 awith Cs allowed the isolation of singly and doubly reduced forms of1 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 of1 a⁻is observed, whereas in [{Cs⁺(18‐crown‐6)}₂(1 a²⁻)], 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.

 

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. 

One-electron reduction of bowl-shaped indenocorannulene, C 26 H 12 , with Rb metal in THF affords [{Rb + (18-crown-6)} 2 (C 26 H 12 –C 26 H 12 ) 2− ]·4THF, as confirmed by single-crystal X-ray diffraction. The product consists of a dimeric σ-bonded dianion (C–C, 1.568(7) Å) having two endo -η 6 coordinated {Rb + (18-crown-6)} moieties (Rb–C, 3.272(4)–3.561(4) Å). The (C 26 H 12 –C 26 H 12 ) 2− dimer represents the first crystallographically confirmed example of spontaneous coupling for indenocorannulene monoanion radicals, C 26 H 12 ˙ − . Comprehensive theoretical investigation of the new dimer confirms the single σ-bond character of the linker and reveals a significant increase of both thermodynamic and kinetic stability of [σ-(C 26 H 12 ) 2 ] 2− in comparison with analogues formed by such π-bowls as corannulene and its dibenzo-derivative. The in-depth computational analysis and direct comparison of the series demonstrates the effect of curvature on radical coupling processes, allowing control over stability and reactivity of bowl-shaped π-radicals. 

A bio-orthogonal chemistry-based approach for fluorescent labelling of ribosomal RNA is described. It involves an adenosine analogue modified with trans -cyclooctene and masked 5′-phosphate group using aryl phosphoramidate. The incorporation into rRNA has been confirmed using agarose gel electrophoresis, as well as a highly sensitive UHPLC-MS/MS method. Fluorescent labelling of rRNA has been achieved in live HeLa cells via an inverse electron demand Diels–Alder reaction with a tetrazine conjugated to an Oregon Green fluorophore. This communication describes the stepwise approach that led to the development and characterization of the probe. The results demonstrate a new strategy towards development of future fluorescent probes to investigate the biochemistry of nucleic acids.