Synthesis and Characterization of a Cyclic Trimer of a Chiral Spirosilabifluorene.

A chiral shape-persistent macrocyclic compound (Si-[3]), designed by the C/Si substitution in the spiro-atom of spirobifluorene in the cyclic trimer (C-[3]), has been successfully synthesized in this study. The C/Si substitution made the spiro-conjugation and energy levels of HOMO and LUMO decrease. Due to the silicon substitution, the macrocyclic compound Si-[3] was able to be degraded by fluoride ions, but its reaction rate was slower than that of the unsubstituted spirosilabifluorene, showing the chemical stability of Si-[3]. Furthermore, the chiroptical properties of Si-[3] with D3-symmetric macrocyclic structure were investigated, and (P,P,P)-Si-[3] showed a high emission quantum yield (Φf=80 %) and moderate dissymmetry factor of circularly polarized luminescence (CPL) (glum,exp=-1.2×10-3). According to the time-dependent density-functional theory (TD-DFT) calculations using polarizable continuum model (PCM), the bright CPL from Si-[3] was explained by a planarization of one bisilafluorenyl moiety at the excited state, which is responsible for the almost fully-allowed radiative transition with a short emission lifetime of τf=1.89 ns.

Intramolecular Electron Transfer in Multi-Redox Systems Based on Cyclic [3]Spirobifluorenylene Compound.

Cyclic [3]spirobifluorenylene with bulky alkyl groups at the ends (1) was designed and synthesized to investigate the electron transfer phenomena in a π-conjugated system including orthogonal π-conjugated chains. The three bifluorenyl units in 1 are conjugated to each other via spiro-conjugation, resulting in the splitting of the HOMO levels to a small extent. Therefore, the SOMO-HOMO gap of the radical cation species is small, which is considered to allow the facile intramolecular electron transfer. The electronic properties of 1 and its partial structures were characterized by absorption and fluorescence measurements and electrochemical analysis. From the electrochemical oxidation, the interchain Coulombic repulsion was observed. In the TD-DFT calculations for the radical cation species of 1, the geometry-featured interchain electronic transitions were visualized by NTO calculations. The radical cation species of 1 generated by chemical oxidation with SbCl5 exhibited a broadened and lower-energy NIR absorption band exceeding 2000 nm. Considering the results of the TD-DFT calculations, the NIR band of the radical cation of 1 was attributed to the intramolecular electron transfer processes among the bifluorenyl units in the macrocycle. ESR experiments also indicated the delocalization of a spin of 1⋅+ in the whole molecule via hole hopping in the ESR time scale at room temperature. This work demonstrates the usefulness of spiro-conjugation as a bridging unit in molecular wires to facilitate smooth electron transfer.

Multiply exo-Methylated Corannulenes.

The curved π-conjugated surface of bowl-shaped corannulene has been multiply methylated to form exo-di-, -tetra-, and -hexamethylated corannulenes. The multimethylations became possible through in-situ iterative reduction/methylation sequences that involve the reduction of corannulenes using sodium to form the anionic corannulene species, and the subsequent SN 2 reaction of the anionic species with reduction-resistant dimethyl sulfate. X-ray diffraction analyses, NMR, MS, UV-Vis measurements, and DFT calculations have revealed the molecular structures of the multimethylated corannulenes and the sequence of the multimethylation. This work has the potential to contribute to the controlled synthesis and characterizations of multifunctionalized fullerenes.

Multiply exo-Methylated Corannulenes.

Invited for the cover of this issue are Shinobu Aoyagi (Nagoya City University), Takahiro Sasamori (University of Tsukuba), and Hideki Yorimitsu (Kyoto University). The image depicts multiply exo-methylated corannulenes (flowers) synthesized from corannulene (central large flower) by the reduction with sodium (soil) to form anionic corannulenes and the subsequent methylation with reduction-resistant dimethyl sulfate (butterflies). Read the full text of the article at 10.1002/chem.202301557.

Overlooked Factors Required for Electrolyte Solvents in Li-O2 Batteries: Capabilities of Quenching 1 O2 and Forming Highly-Decomposable Li2 O2.

Although sufficient tolerance against attack by superoxide radicals (O2 - ) has been mainly recognized as an important property for Li-O2 battery (LOB) electrolytes, recent evidence has revealed that other critical factors also govern the cyclability, prompting a reconsideration of the basic design guidelines of LOB electrolytes. Here, we found that LOBs equipped with a N,N-dimethylacetamide (DMA)-based electrolyte exhibited better cyclability compared with other standard LOB electrolytes. This superior cyclability is attributable to the capabilities of quenching 1 O2 and forming highly decomposable Li2 O2 . The 1 O2 quenching capability is equivalent to that of a tetraglyme-based electrolyte containing a several millimolar concentration of a typical chemical quencher. Based on these overlooked factors, the DMA-based electrolyte led to superior cyclability despite its lower O2 - tolerance. Thus, the present work provides a novel design guideline for the development of LOB electrolytes.

Electrochemical and spectroscopic properties of twisted dibenzo[g,p]chrysene derivatives.

Dibenzo[g,p]chrysene (DBC), which consists of a twisted naphthalene core with four fused benzene rings, is a promising framework for organic electronic materials. Therefore, the research for structure-property relationships is important for the design of DBC-based materials. Here, the electrochemical and spectroscopic properties of DBC derivatives were investigated, and the effects of substituents and torsion of the naphthalene moiety were examined based on density functional theory (DFT) calculations. All the substituted DBC derivatives showed higher oxidation potentials than that for DBC-H, even for compounds that contained an electron-donating group such as DBC-Me and DBC-SMe. DFT calculations clearly indicate that these higher oxidation potentials are due to the ineffective conjugation of the MeO group, which is oriented perpendicular to the benzene ring because of the steric repulsion of substituents on both sides. More specifically, the inductive effect of the MeO group is dominant rather than the mesomeric effect when the substituent is located at both sides of the MeO group. Concerning the torsion of the naphthalene moiety, the twisting results in a slight increase in the HOMO and a slight lowering of the LUMO. The twisting effect is much smaller than the conjugation effect of the MeO group. Absorption spectra of all the substituted DBC derivatives also showed a red-shift as compared to that for DBC-H. Concerning the luminescence, a strong photoluminescence was observed for DBC-H and DBC-Si.

Linear [3]Spirobifluorenylene: An S-Shaped Molecular Geometry of p-Oligophenyls.

An S-shaped p-oligophenyl derivative was synthesized, as exemplified by linear [3]spirobifluorenylene (R,P,R/S,M,S)-1, which consisted of two p-sexiphenyls tied at the spiro-carbons of three spirobifluorenes. The direct linking of spirobifluorenes results in the formation of strained p-oligophenyl chains, which support each other to maintain the strained structure. This curved structure was successfully constructed by Suzuki-Miyaura cross-coupling of the corresponding spirobifluorene building blocks. Chiral (R,P,R)-1 and (S,M,S)-1 were also synthesized using chiral monomers, and their chiroptical properties were investigated by CD and CPL spectroscopy. The benzene rings in (R,P,R/S,M,S)-1 are distorted with the largest bend angle of 11.6°. The strain energy of (S,M,S)-1 was estimated to be 88.4 kcal/mol, which lies between those for [6] and [7]CPP. Through-space orbital interactions (spiroconjugation) were evaluated by TD-DFT calculations, and the results suggest the contribution of a 15 nm red-shift in (S,M,S)-1 in comparison with the single strand molecule 10. In addition, spiroconjugation is discussed in terms of molecular orbitals, and the splitting of both HOMO and LUMO was revealed by the calculations. The LUMO splitting is particularly interesting, since it is in contrast to the degenerate LUMOs in a simple spirobifluorene. Desymmetrization based on the distortion of the spirobifluorene moieties induces the LUMO splitting.

Polyaniline-Induced Arylation with Arenediazonium Salts Derived from Anilines.

A catalytic amount of a reduced form of polyaniline (a redox-active π-conjugated polymer) was found to induce C-H direct arylation of (hetero)arenes with arenediazonium salts prepared from anilines with methanesulfonic acid (MeSO3 H) and tert-butyl nitrite (tBuONO). The difficult part of this method is the coexistence of an oxidant and a reductant in this sequential diazotization and arylation system; diazotization requires weak oxidants such as alkyl nitrites, whereas the arylation is induced by a reductant. This was achieved by the careful control of the amount of tBuONO (1.0 equivalent) for the diazotization step, and sequential arylation using 5 mol % of the polyaniline. The reaction took place under mild conditions without any metals or strong bases at room temperature, and the amino group is a formal leaving group. The scope of the substrates demonstrates the versatility in the combination of anilines with a variety of functional groups and several (hetero)arenes. Two-directional arylation for the synthesis of an unsymmetrical 1,4-diarylated (furyl and pyrrolyl groups) benzene was achieved, using mono-Boc-protected 1,4-phenylenediamine as a substrate. This shows potential for the synthesis of more complicated oligoarene compounds.

Syntheses, Properties, and Catalytic Activities of Metal(II) Complexes and Free Bases of Redox-Switchable 20π, 19π, and 18π 5,10,15,20-Tetraaryl-5,15-diazaporphyrinoids.

In spite of significant advances in redox-active porphyrin-based materials and catalysts, little attention has been paid to 20π and 19π porphyrins because of their instability in air. Here we report the meso-modification of 5,10,15,20-tetraarylporphyrin with two nitrogen atoms, which led to redox-switchable 20π, 19π, and 18π 5,10,15,20-tetraaryl-5,15-diazaporphyrinoids (TADAPs). Three kinds of metal(II) complexes and free bases of TADAP were prepared by the metal-templated annulation of the corresponding metal-bis(dipyrrin) complexes. The inductive and resonance effects of the meso-nitrogen atoms on the aromatic, optical, electrochemical, and magnetic properties of the entire TADAP π-systems were assessed by using various spectroscopic measurements and density functional theory calculations. The aromaticity and π-π* electronic transition energies of the TADAPs varied considerably, and were shown to be dependent on the oxidation states of the π-systems. In contrast to the isoelectronic 5,10,15,20-tetraarylporphyrin derivatives, the 20π and 19π TADAPs were chemically stable under air. In particular, the 19π TADAP radical cations were extremely stable towards dioxygen, moisture, and silica gel. This reflected the low-lying singly occupied molecular orbitals of their π-systems and the efficient delocalization of their unshared electron spin. The capability of MgTADAP to catalyze aerobic biaryl formation from aryl Grignard reagents was demonstrated, which presumably involved a 19π/20π redox cycle.

Radical Ions of a π-Bowl Sumanene: Effects of Strained Structure on the Electronic Transitions.

Strained polyaromatic carbon molecules exhibit interesting properties owing to enhanced interactions between p orbitals of sp2 carbons; sumanene (SUM) is one of these bowl-shaped π-conjugated molecules (π-bowls). In the present study, the radical cation and radical anion of SUM were characterized by radiation chemical methods, that is, pulse radiolysis and γ-ray radiolysis. Absorption spectra of the SUM radical cation and radical anion in a wide spectral region ranging from the visible to near-IR were successfully obtained. Absorption spectra of the SUM radical cation and radical anion are similar in shape to those of the corresponding species of triphenylene (TP), which possesses a planar structure and the same π-electron system as SUM. However, the SUM radical anion showed lower peak shifts than the TP radical anion. Theoretical calculations revealed that the MOs responsible for the electronic transitions of the SUM radical anion are different from those of the TP radical anion, in contrast to the radical cations. These results demonstrate that the strains in the molecular geometries affect the electronic transitions of radical ions.

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.

Palladium(II)-Catalyzed Dehydroboration via Generation of Boron Enolates.

The PdII -catalyzed dehydroboration of boron enolates generated from ketones and 9-iodo-9-borabicyclo[3.3.1]nonane was achieved, providing a synthetically versatile protocol from ketones to α,ß-unsaturated ketones. The PdII compound employed in this reaction worked catalytically in the presence of Cu(OAc)2 . The high trans-selectivity of the olefinic moiety was observed. Aryl halide moieties (-Br and -Cl) remained intact for this reaction in spite of the presence of a Pd species. An ester substrate could also be applied when a stoichiometric amount of PdII was used. The crossover reactions using boron and silyl enolates revealed that the oxidation reaction is much faster than the Saegusa-Ito reaction.

Selective Intermolecular Oxidative Cross-Coupling of Enolates.

Selective intermolecular oxidative cross-coupling of enolates, which is a bond-forming reaction between carbanion equivalents, remains as an unsolved issue despite its potential utility for the direct synthesis of unsymmetrical 1,4-diones. The main difficulty derives from the unavoidable homo-coupling. Our strategy depends on the selective one-electron oxidation of one enolate to afford an electrophilic carbonyl α-radical species, followed by trapping with another enolate. The present study demonstrates the selective oxovanadium(V)-induced cross-coupling between boron and silyl enolates.

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.

Polyaniline-Induced C-H Arylation of Arenes with Arenediazonium Salts.

A reduced form of polyaniline has been shown to induce direct arylation of an arenediazonium salt with an arene (Gomberg-Bachmann reaction) to give the cross-coupling product in moderate to good yields under mild conditions. Various arenediazonium salts and arenes, including heteroarenes such as furans, thiophenes, and pyrroles, are employed for the reaction. The most favorable combination of substrates is an electron-poor arenediazonium salt with an electron-rich heteroarene. Investigation of the mechanism by reactions with radical scavengers and experiments on kinetic isotope effects indicated the occurrence of a radical chain reaction initiated by one-electron reduction of an arenediazonium salt by the polyaniline. Only 1 mol % (based on aniline tetramer) of the polyaniline is required for the cross-coupling reaction to occur. This reaction proceeds under metal-free conditions and with no need for photonic activation.

Chemistry of sumanene.

Sumanene (C21 H12 ) is a bowl-shaped π-conjugated molecule with C3v symmetry, consisting of alternating benzene rings and cyclopentadiene rings around the central benzene ring. The structure corresponds to the smallest C3v-symmetric fragment for the structural motifs of fullerenes or end-caps of carbon nanotubes. The presence of three sp(3)-hybridized benzylic sites is one of the most characteristic structural features in sumanene, which allows the different chemistry from that of corannulene. Since our first synthesis in 2003, we have engaged in the study of sumanene. This article summarizes our continuous study, including the synthesis, structure, dynamics, derivatization, complexation with transition metals, charge carrier mobility, and conversion to nitrogen-doped graphitic carbon, of sumanene.

Construction of a hemifullerene skeleton: a regioselective intramolecular oxidative cyclization.

A two-step synthesis of a strained π bowl, with hemifullerene skeleton from sumanene, was achieved in a high yield. The first step is a base-promoted condensation reaction with a benzophenone compound, bis(3,5-dimethylphenyl)methanone. The second step is the regioselective intramolecular oxidative cyclization, which is a key reaction for the hemifullerene skeleton synthesis. This regioselective cyclization is likely to be under thermodynamic control. This strategy will allow facile synthesis of various highly strained π bowls.

Sumanenyl metallocenes: synthesis and structure of mono- and trinuclear zirconocene complexes.

Mono- and trinuclear sumanenyl zirconocene complexes Cp(sumanenyl)ZrCl2, Cp*(sumanenyl)ZrCl2, and (C21H9)[(Cp*)ZrCl2]3 were successfully synthesized. In the mononuclear complexes, convex binding with disturbed η(5)-bonding was confirmed in a solid state. As demonstrated in the trinuclear complex, the multibenzylic anion of sumanene offers a novel ligand for the multinuclear metallocenes.

Bowl inversion of surface-adsorbed sumanene.

Bowl-shaped π-conjugated compounds offer the possibility to study curvature-dependent host-guest interactions and chemical reactivity in ideal model systems. For surface-adsorbed π bowls, however, only conformations with the bowl opening pointing away from the surface have been observed so far. Here we show for sumanene on Ag(111) that both bowl-up and bowl-down conformations can be stabilized. Analysis of the molecular layer as a function of coverage reveals an unprecedented structural phase transition involving a bowl inversion of one-third of the molecules. On the basis of scanning tunneling microscopy (STM) and complementary atomistic simulations, we develop a model that describes the observed phase transition in terms of a subtle interplay between inversion-dependent adsorption energies and intermolecular interactions. In addition, we explore the coexisting bowl-up and -down conformations with respect to host-guest binding of methane. STM reveals a clear energetic preference for methane binding to the concave face of sumanene.

Synthesis of heterodinuclear hemisalen complexes on a hexaarylbenzene scaffold and their application for the cross-pinacol coupling reaction.

Intermolecular cross-pinacol coupling reaction between aliphatic and aromatic aldehydes by using heterodinuclear hemisalen complexes 1cis with vanadium(V) and titanium(IV) on a hexaarylbenzene scaffold is reported. Our ligand design is based on the individual activation of two aldehydes by vanadium and titanium, which are positioned with a suitable space on the rigid scaffold. Ligands such as 1cis were synthesized by Diels-Alder addition and decarbonylation reaction, followed by condensation of dialdehyde 3cis with various aminophenols. The influence of the substituents on the ligands on the pinacol coupling reaction was investigated. As a result, the reductive coupling reaction between aliphatic and aromatic aldehydes by using a catalytic amount of 1cis in the presence of Me3 SiCl and Zn provided the corresponding cross-coupled 1,2-diol in good yields with high cross-selectivity.