Peculiar All‐Metal σ‐Aromaticity of the [Au2Sb16]4− Anion in the Solid State

Gas‐phase clusters are deemed to be σ‐aromatic when they satisfy the 4n+2 rule of aromaticity for delocalized σ electrons and fulfill other requirements known for aromatic systems. While the range of n values was shown to be quite broad when applied to short‐lived clusters found in molecular‐beam experiments, stability of all‐metal cluster‐like fragments isolated in condensed phase was previously shown to be mainly ascribed to two electrons (n=0). In this work, the applicability of this concept is extended towards solid‐state compounds by demonstrating a unique example of a storable compound, which was isolated as a stable [K([2.2.2]crypt)]⁺ salt, featuring a [Au₂Sb₁₆]^4? cluster core possessing two all‐metal aromatic AuSb₄ fragments with six delocalized σ electrons each (n=1). This discovery pushes the boundaries of the original idea of Kekulé and firmly establishes the usefulness of the σ‐aromaticity concept as a general idea for both small clusters and solid‐state compounds.     

A Perylene Bisimide Cyclophane as a “Turn‐On” and “Turn‐Off” Fluorescence Probe

A rigid, covalently linked perylene‐3,4:9,10‐tetracarboxylic acid bisimide (PBI) cyclophane was synthesized by imidization of a bay‐substituted perylene bisanhydride with p‐xylylenediamine. The interchromophoric distance of approximately 6.5 Å establishes an ideal rigid cavity for the encapsulation of large aromatic compounds such as perylene and anthracene with binding constants up to 4.6×10⁴ M ^?1 (in CHCl₃). For electron‐poor guest molecules, the complexation process is accompanied by a significantly increased fluorescence, whereas the emission intensity is dramatically quenched by more electron‐rich guests because of the formation of charge‐transfer complexes. Furthermore, the influence of the PBI core twist on the binding constant results in a remarkable selectivity towards more flexible aromatic guest molecules.     

Synthesis of a Tetrabenzotetraaza[8]circulene by a “Fold‐In” Oxidative Fusion Reaction

Tetrabenzotetraaza[8]circulene ( 1 ) has been synthesized in good yield by a “fold‐in” oxidative fusion reaction of a 1,2‐phenylene‐bridged cyclic tetrapyrrole. X‐ray diffraction analysis of 1 has revealed a planar square structure with a central cyclooctatetraene (COT) core that shows little alternation of the bond lengths. Despite these structural features, 1 shows aromatic‐like character, such as sharp absorption bands, high fluorescence quantum yields (Φ_F=0.55 in THF), and a single exponential fluorescence decay with τ_F=3.8 ns. These observations indicate a dominant contribution of an [8]radialene‐like π conjugation and hence aromatic character of the local aromatic segments in 1 .     

Synthesis and photophysical properties of 2,5,8,11‐tetrakis(5‐hexylthiophen‐2‐yl)tetrathieno[2,3‐a:3′,2′‐c:2′′,3′′‐f:3′′′,2′′′‐h]naphthalene

The title compound, C₅₈H₆₄S₈, has been prepared by Pd‐catalysed direct C—H arylation of tetrathienonaphthalene (TTN) with 5‐hexyl‐2‐iodothiophene and recrystallized by slow evaporation from dichloromethane. The crystal structure shows a completely planar geometry of the TTN core, crystallizing in the monoclinic space group P2₁/c. The structure consists of slipped π‐stacks and the interfacial distance between the mean planes of the TTN cores is 3.456 (5) Å, which is slightly larger than that of the comparable derivative of tetrathienoanthracene (TTA) with 2‐hexylthiophene groups. The packing in the two structures is greatly influenced by both the aromatic core of the structure and the alkyl side chains.     

Facile synthesis of hyperbranched poly(p‐phenyleneethynylene‐alt‐m‐phenyleneethynylene) with narrow dispersity and high branching degree by sonogashira polymerization of an AB2 monomer

An AB₂ monomer PhBr₂  C  C  Ph  C  CH containing one acetylene group and two bromide groups was efficiently synthesized by a strategy based on the different reactivity between aromatic iodide and bromide in Sonogashira reaction. The Sonogashira polymerization of PhBr₂  C  C  Ph  C  CH was investigated to get hyperbranched poly(p‐phenyleneethynylene‐alt‐m‐phenyleneethynylene) (hb‐PMPE) in terms of the effects of monomer addition method, core molecule with different functionality, and ratio of [monomer]/[core molecule]. The results showed that narrow dispersities (D) (D: 1.23∼1.50) were obtained by slow monomer addition and with core molecule. Bifunctional core molecule induced narrower dispersity than monofunctional core molecule. The molecular weight of hb‐PMPE increased with increasing ratio of [monomer]/[core molecule], however, a negative deviation from calculated value was observed. The dispersity slightly increased with increasing [monomer]/[core molecule]. When the ratio of [monomer]/[core molecule] was below 50/1, monomodal distribution was observed; whereas when the ratio increased to 70/1, bimodal distribution was obtained. All the polymers showed degrees of branching (DBs) around 0.6. The hb‐PMPEs showed one major absorption band with λ_max around 330 nm, and emission band with λ_max around 390 nm. All the polymers showed relative quantum yields (Φ_r) above 0.5 in dilute THF solution. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 96–104     

Synthesis of aromatic polyethersulfone‐based graft copolyacrylates via ATRP catalyzed by FeCl2/isophthalic acid

The atom transfer radical polymerization (ATRP) catalyzed by the FeCl₂/isophthalic acid system was used for the preparation of novel aromatic polyethersulfone (PSF)‐based graft copolymers in N,N‐dimethylformamide (DMF), such as aromatic PSF‐graft‐poly(methyl methacrylate), aromatic PSF‐graft‐polymethylacrylate, and aromatic PSF‐graft‐poly(butyl acrylate). The route consisted of two steps. The first step included the chloromethylation of aromatic PSF, and the second step involved the ATRP of acrylate monomers using chloromethylated aromatic PSF as the macroinitiator and FeCl₂/isophthalic acid as the catalyst in DMF. Characterization data by gel permeation chromatography, DSC, IR, ¹H NMR, and thermogravimetric analysis confirmed that the graft copolymerization was successful. Only one glass‐transition temperature (T_g) was observed for aromatic PSF‐graft‐poly(methyl methacrylate), and two T_g's were detected for aromatic PSF‐graft‐methyl acrylate and aromatic PSF‐graft‐poly(butyl acrylate). © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2943–2950, 2001     

Three‐arm star block copolymers of aromatic polyether and polystyrene from chain‐growth condensation polymerization,atom transfer radical polymerization,and click reaction

Well‐defined (AB)₃ type star block copolymer consisting of aromatic polyether arms as the A segment and polystyrene (PSt) arms as the B segment was prepared using atom transfer radical polymerization (ATRP), chain‐growth condensation polymerization (CGCP), and click reaction. ATRP of styrene was carried out in the presence of 2,4,6‐tris(bromomethyl)mesitylene as a trifunctional initiator, and then the terminal bromines of the polymer were transformed to azide groups with NaN₃. The azide groups were converted to 4‐fluorobenzophenone moieties as CGCP initiator units by click reaction. However, when CGCP was attempted, a small amount of unreacted initiator units remained. Therefore, the azide‐terminated PSt was then used for click reaction with alkyne‐terminated aromatic polyether, obtained by CGCP with an initiator bearing an acetylene unit. Excess alkyne‐terminated aromatic polyether was removed from the crude product by means of preparative high performance liquid chromatography (HPLC) to yield the (AB)₃ type star block copolymer (M_n = 9910, M_w/M_n = 1.10). This star block copolymer, which contains aromatic polyether segments with low solubility in the shell unit, exhibited lower solubility than A₂B or AB₂ type miktoarm star copolymers. In addition, the obtained star block copolymer self‐assembled to form spherical aggregates in solution and plate‐like structures in film. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

1,2,3‐Trimethoxy‐4‐[(E)‐2‐phenylvinyl]benzene and (E,E)‐1,4‐bis(2,3,4‐trimethoxyphenyl)buta‐1,3‐diene

The stilbene derivative 1,2,3‐trimethoxy‐4‐[(E)‐2‐phenylvinyl]benzene, C₁₇H₁₈O₃, (I), and its homocoupling co‐product (E,E)‐1,4‐bis(2,3,4‐trimethoxyphenyl)buta‐1,3‐diene, C₂₂H₂₆O₆, (II), both have double bonds in trans conformations in their conjugated linkages. In the structure of stilbene (I), the aromatic rings deviate significantly from coplanarity, in contrast with coproduct (II), the core of which is rigorously planar. The deviation in stilbene (I) seems to be driven by intermolecular electrostatic interactions. Diene (II) sits on a crystallographic inversion centre, which bisects the conjugated linkage.     

From Hexaoxy‐[6]Pericyclynes to Carbo‐Cyclohexadienes,Carbo‐Benzenes,and Dihydro‐Carbo‐Benzenes: Synthesis,Structure, and Chromophoric and Redox Properties

When targeting the quadrupolar p‐dianisyltetraphenyl‐carbo‐benzene by reductive treatment of a hexaoxy‐[6]pericyclyne precursor 3 with SnCl₂/HCl, a strict control of the conditions allowed for the isolation of three C₁₈‐macrocyclic products: the targeted aromatic carbo‐benzene 1 , a sub‐reduced non‐aromatic carbo‐cyclohexadiene 4 A , and an over‐reduced aromatic dihydro‐carbo‐benzene 5 A . Each of them was fully characterized by its absorption and NMR spectra, which were interpreted by comparison with calculated spectra from static structures optimized at the DFT level. According to the nucleus‐independent chemical shift (NICS) value (NICS≈?13 ppm), the macrocyclic aromaticity of 5 A is indicated to be equivalent to that of 1 . This is confirmed by the strong NMR spectroscopic deshielding of the ortho‐CH protons of the aryl substituents, but also by the strong shielding of the internal proton of the endocyclic trans‐CH?CH double bond that results from the hydrogenation of one of the C?C bonds of 3 . Both the aromatics 1 and 5 A exhibit a high crystallinity, revealed by SEM and TEM images, which allowed for a structural determination by using an X‐ray microsource. A good agreement with calculated molecular structures was found, and columnar assemblies of the C₁₈ macrocycles were evidenced in the crystal packing. The non‐aromatic carbo‐cyclohexadiene 4 A is shown to be an intermediate in the formation of 1 from 3 . It exhibits a remarkable dichromism in solution, which is related to the occurrence of two intense bands in the visible region of its UV/Vis spectrum. These properties could be attributed to the dibutatrienylacetylene (DBA) unit that occurs in the three chromophores, but which is not involved in a macrocyclic π‐delocalization in 4 A only. A versatile redox behavior of the carbo‐chromophores is evidenced by cyclic voltammetry and was analyzed by calculation of the ionization potential, electron affinity, and frontier molecular orbitals.     

Exfoliation of Graphite into Graphene in Polar Solvents Mediated by Amphiphilic Hexa‐peri‐hexabenzocoronene

A water‐soluble surfactant consisting of hexa‐peri‐hexabenzocoronene (HBC) as hydrophobic aromatic core and hydrophilic carboxy substituents was synthesized. It exhibited a self‐assembled nanofiber structure in the solid state. Profiting from the π interactions between the large aromatic core of HBC and graphene, the surfactant mediated the exfoliation of graphite into graphene in polar solvents, which was further stabilized by the bulky hydrophilic carboxylic groups. A graphene dispersion with a concentration as high as 1.1 mg L^?1 containing 2–6 multilayer nanosheets was obtained. The lateral size of the graphene sheets was in the range of 100–500 nm based on atomic force microscope (AFM) and transmission electron microscope (TEM) measurements.     

Rhodium‐catalyzed ortho‐C‐H olefination of aromatic aldehydes employing transient directing strategy

A Rhodium(III)‐catalyzed ortho‐C‐H olefination of aromatic aldehydes in the presence of catalytic amount of TsNH₂ has been developed. The in situ generated imine intermediate from aldehyde and TsNH₂ worked as a transient directing group. Both electron‐rich and electron‐deficient aromatic aldehydes were tolerated, affording the corresponding products in moderate to good yields. Importantly, the present protocol provides a straightforward access to olefinated aromatic aldehydes with aldehydes as the simple starting materials.     

N‐Fusion Approach in Construction of Contracted Carbaporphyrinoids: Formation of N‐Fused Telluraporphyrin

Insertion of PCl₃ into 5,10,15,20‐tetraaryl‐21‐telluraporphyrin leads to a phosphorus complex of N‐fused dihydrotelluraporphyrin with an inverted tellurophene ring. Its CNN coordination core places the macrocycle in the family of contracted carbaporphyrinoids. A cycle of direct transformations affords an elegant triangle of three mutually convertible N‐fused porphyrinoids, with distinct spectroscopic features: antiaromatic, nonaromatic and aromatic. The nonaromatic species has a dome shaped skeleton which forms in the solid state a ball and socket structure with C₆₀.     

Exploiting the Aromatic Chameleon Character of Fulvenes for Computational Design of Baird‐Aromatic Triplet Ground State Compounds

Due to the reversal in electron counts for aromaticity and antiaromaticity in the closed‐shell singlet state (normally ground state, S₀) and lowest ππ* triplet state (T₁ or T₀), as given by Hückel's and Baird's rules, respectively, fulvenes are influenced by their substituents in the opposite manner in the T₁ and S₀ states. This effect is caused by a reversal in the dipole moment when going from S₀ to T₁ as fulvenes adapt to the difference in electron counts for aromaticity in various states; they are aromatic chameleons. Thus, a substituent pattern that enhances (reduces) fulvene aromaticity in S₀ reduces (enhances) aromaticity in T₁, allowing for rationalizations of the triplet state energies (E_T) of substituted fulvenes. Through quantum chemical calculations, we now assess which substituents and which positions on the pentafulvene core are the most powerful for designing compounds with low or inverted E_T. As a means to increase the π‐electron withdrawing capacity of cyano groups, we found that protonation at the cyano N atoms of 6,6‐dicyanopentafulvenes can be a route to on‐demand formation of a fulvenium dication with a triplet ground state (T₀). The five‐membered ring of this species is markedly Baird‐aromatic, although less than the cyclopentadienyl cation known to have a Baird‐aromatic T₀ state.     

A Hybrid of Corannulene and Azacorannulene: Synthesis of a Highly Curved Nitrogen‐Containing Buckybowl

A highly curved nitrogen‐containing buckybowl, which can be considered as a corannulene/azacorannulene hybrid, was synthesized and characterized. This molecule has a polycyclic aromatic C₄₀N core, corresponding to a partial azafullerene structure C_80−xN_x (x=1,2,3…), and exhibits interesting properties that arise from its large and highly curved π surface and the embedded nitrogen atom, which include association with C₆₀, a lower LUMO level relative to azapentabenzocorannulene, and the formation of a radical cationic species upon oxidation.     

Synthesis and characterization of novel fluorinated polyimides derived from 4,4′‐[2,2,2‐trifluoro‐1‐(3‐trifluoromethylphenyl)ethylidene]diphthalic anhydride and aromatic diamines

A novel fluorinated aromatic dianhydride, 4,4′‐[2,2,2‐trifluoro‐1‐(3‐trifluoromethyl‐phenyl)ethylidene]diphthalic anhydride (TFDA) was synthesized by coupling of 3′‐trifluoromethyl‐2,2,2‐trifluoroacetophenone with o‐xylene under the catalysis of trifluoromethanesulfonic acid, followed by oxidation of KMnO₄ and dehydration. A series of fluorinated aromatic polyimides derived from the novel fluorinated aromatic dianhydride TFDA with various aromatic diamines, such as p‐phenylenediamine (p‐PDA), 4,4′‐oxydianiline (ODA), 1,4‐bis(4‐aminophenoxy)benzene (p‐APB), 1,3‐bis(4‐amino‐phenoxy)benzene (m‐APB), 4‐(4‐aminophenoxy)‐3‐trifluoromethylphenylamine (3FODA) and 1,4‐bis(4‐amino‐2‐trifluoromethylphenoxy)benzene (6FAPB), were prepared by polycondensation procedure. All the fluorinated polyimides were soluble in many polar organic solvents such as NMP, DMAc, DMF, and m‐cresol, as well as some of low boiling point organic solvents such as CHCl₃, THF, and acetone. Homogeneous and stable polyimide solutions with solid content as high as 35–40 wt % could be achieved, which were prepared by strong and flexible polyimide films or coatings. The polymer films have good thermal stability with the glass transition temperature of 232–322 °C, the temperature at 5% weight loss of 500–530 °C in nitrogen, and have outstanding mechanical properties with the tensile strengths of 80.5–133.2 MPa as well as elongations at breakage of 7.1–12.6%. It was also found that the polyimide films derived from TFDA and fluorinated aromatic diamines possess low dielectric constants of 2.75–3.02, a low dissipation factor in the range of 1.27–4.50 × 10^?3, and low moisture absorptions <1.3%. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4143–4152, 2004     

A DFT study on 1,4‐dihydro‐1,4‐azaborinine annulated linear polyacenes: Absorption spectra,singlet‐triplet energy gap,aromaticity, and HOMO–LUMO energy modulation

Linear polyacene (LPA) mimics containing multiple heterocycles have been computationally designed by annulating 1,4‐dihydro‐1,4‐azaborinine moieties to benzene (aB₁–aB₅), naphthalene (aN₁–aN₅), anthracene (aA₁–aA₅), and tetracene (aT₁–aT₅) cores. DFT studies conducted on them using M06L/6‐311++G(d,p) method reveal a perfect planar structure for all and suggest the utilization of nitrogen lone pairs for aromatic π‐electron delocalization. The computed values of aromaticity indices such as HOMA, NICS, and dehydrogenation energy (E _dh) of heterocycles support strong aromatic character for each six‐membered ring in the LPA mimics. On the basis of the minimum value of the molecular electrostatic potential (V _min) observed on each LPA unit in the LPA mimics, the extended delocalization of π‐electrons is verified. The energetic parameter E _dh showed strong linear correlation with HOMA, NICS and V _min parameters, which strongly supports the multidimensional character of aromaticity in LPA mimics. The electronic property modification is shown by the theoretical absorption spectra data and singlet‐triplet energy gap (ΔE _ST). The bandgap and ΔE _ST tunings are achieved for LPA mimics by selecting appropriate number of azaborinine type units and the size of LPA core used for annulation. © 2017 Wiley Periodicals, Inc.     

Me‐BIPAM for the Synthesis of Optically Active 3‐Aryl‐3‐hydroxy‐2‐oxindoles by Ruthenium‐catalyzed Addition of Arylboronic Acids to Isatins

A chiral O‐linked C₂‐symmetric bidentate phosphoramidite (Me‐BIPAM) was found to be efficient for the ruthenium‐catalyzed addition of arylboronic acids to isatins. Asymmetric synthesis of 3‐aryl‐3‐hydroxy‐2‐oxindoles by 1,2‐addition of arylboronic acids to isatins was carried out in the presence of [RuCl₂(PPh₃)₃]/(R,R)‐Me‐BIPAM and KF, resulting in an enantioselectivity as high as 90 % ee. It was found that the reaction with N‐protected isatins proceeds with high yields and good enantioselectivities. The best protective groups on the nitrogen atom were different depending on the substituents on the aromatic ring. The use of a N‐benzyl group resulted in excellent enantioselectivities in many substrates compared with other groups.     

Methyl 2‐benzamido‐4‐(3,4‐dimethoxyphenyl)‐5‐methylbenzoate and N‐{5‐benzoyl‐2‐[(Z)‐2‐methoxyethenyl]‐4‐methylphenyl}benzamide

Methyl 2‐benzamido‐4‐(3,4‐dimethoxyphenyl)‐5‐methylbenzoate, C₂₄H₂₃NO₅, (Ia), and N‐{5‐benzoyl‐2‐[(Z)‐2‐methoxyethenyl]‐4‐methylphenyl}benzamide, C₂₄H₂₁NO₃, (IIa), were formed via a Diels–Alder reaction of appropriately substituted 2H‐pyran‐2‐ones and methyl propiolate or (Z)‐1‐methoxybut‐1‐en‐3‐yne, respectively. Each of these cycloadditions might yield two different regioisomers, but just one was obtained in each case. In (Ia), an intramolecular N—H...O hydrogen bond closes a six‐membered ring. A chain is formed due to aromatic π–π interactions, and a three‐dimensional framework structure is formed by a combination of C—H...O and C—H...π(arene) hydrogen bonds. Compound (IIa) was formed not only regioselectively but also chemoselectively, with just the triple bond reacting and the double bond remaining unchanged. Compound (IIa) crystallizes as N—H...O hydrogen‐bonded dimers stabilized by aromatic π–π interactions. Dimers of (IIa) are connected into a chain by weak C—H...π(arene) interactions.     

Structural insights into methyl‐ or methoxy‐substituted 1‐(α‐aminobenzyl)‐2‐naphthol structures: the role of C—H…π interactions

Aminobenzylnaphthols are a class of compounds containing a large aromatic molecular surface which makes them suitable candidates to study the role of C—H…π interactions. We have investigated the effect of methyl or methoxy substituents on the assembling of aromatic units by preparing and determining the crystal structures of (S,S)‐1‐{(4‐methylphenyl)[(1‐phenylethyl)amino]methyl}naphthalen‐2‐ol, C₂₆H₂₅NO, and (S,S)‐1‐{(4‐methoxyphenyl)[(1‐phenylethyl)amino]methyl}naphthalen‐2‐ol, C₂₆H₂₅NO₂. The methyl group influenced the overall crystal packing even if the H atoms of the methyl group did not participate directly either in hydrogen bonding or C—H…π interactions. The introduction of the methoxy moiety caused the formation of new hydrogen bonds, in which the O atom of the methoxy group was directly involved. Moreover, the methoxy group promoted the formation of an interesting C—H…π interaction which altered the orientation of an aromatic unit.     

6‐Acetonyldi­hydro­avicine

The title compound, 1‐(5‐methyl‐5,6‐di­hydro­[1,3]­dioxolo­[4′,5′:4,5]­benzo­[c][1,5]­dioxolo­[4,5‐j]­phenanthridin‐6‐yl)­acetone, C₂₃H₁₉NO₅, isolated from the stem bark of Zanthoxy­lum rhoifolium, crystallizes as a racemate in space group P. The structure shows two aromatic ring systems, each terminated by a five‐membered dioxole ring, coupled by an N‐containing ring. The core of the mol­ecule is almost planar; the planes of the two ring systems form an angle of 18.42 (6)°. The packing shows the mol­ecules parallel to each other and about 3.5 Å apart with graphite‐type interactions. The N‐methyl and acetone groups, which are anti with respect to one another, lie out of the plane and pack in spaces between neighbouring mol­ecules.