Reversible Adsorption of Ammonia in the Crystalline Solid of a CO2H-Functionalized Cyclic Oligophenylene.

Ammonia (NH3) is a viable candidate for the storage and distribution of hydrogen (H2) due to its exceptional volumetric and gravimetric hydrogen energy density. Therefore, it is desirable to develop NH3 storage materials that exhibit robust stability across numerous adsorption-desorption cycles. While porous materials with polymeric frameworks are often used for NH3 capture, achieving reversible NH3 uptake remains a formidable challenge, primarily due to the high reactivity of NH3. Here, we advocate the use of CO2H-functionalized cyclic oligophenylene 1a with high chemical stability as a novel single-molecule-based adsorbent for NH3. Simple reprecipitation of 1a selectively yielded microporous crystalline solid 1a (N). Crystalline 1a (N) adsorbs up to 8.27 mmol/g of NH3 at 100 kPa and 293 K. Adsorbed NH3 in the pore of 1a (N) has a packing density of 0.533 g/cm3 at 293 K, which is close to the density of liquid NH3 (0.681 g/cm3 at 240 K). Crystalline 1a (N) also exhibits reversible NH3 adsorption over at least nine cycles, sustaining its storage capacity (1st cycle: 8.27 mmol/g; 9th cycle: 8.25 mmol/g at 100 kPa and 293 K) and crystallinity. During each desorption cycle, NH3 was removed from 1a (N) under reduced pressure (∼65 Pa), leaving <3% of the total uptake, and 1a (N) was fully purged under dynamic vacuum conditions (∼5 × 10-4 Pa at 293 K for 1 h) before the subsequent adsorption cycles. Thus, microporous crystalline 1a (N) can reliably adsorb and desorb NH3 repeatedly, which avoids the need for heat-based activation between cycles.

M/P Helicity Switching and Chiral Amplification in Double-Helical Monometallofoldamers.

Short-stranded double-helical monometallofoldamers capable of M/P-switching were constructed by the complexation of two strands, each with two L-shaped units linked by a 2,2'-bipyridine, with a Zn(II) cation. The helix terminals of the "double-helical form" folded by π-π interactions can unfold in solution to equilibrate with the "open forms" that are favored at higher temperatures. Interestingly, the helical chirality of the monometallofoldamers with chiral side chains induced a single-handed helix sense and controlled M/P-switching depending on achiral solvent stimuli. For instance, the (M)-helicity was favored in nonpolarized solvents (toluene, hexane, Et2O), whereas the (P)-helicity was favored in Lewis basic solvents (acetone, DMSO). Circular dichroism (CD) and rotating-frame overhauser enhancement spectroscopy (ROESY) measurements revealed that the conformational change of the chiral side chains due to interaction of Lewis basic solvents with the double helices induced helicity bias. These novel double-helical monometallofoldamers possessed a stable helical structure and exhibited switchable chiroptical properties (gabs ∼ 10-3-10-2). In addition, the chiral strand exhibited chiral transfer and amplification abilities through the formation of chiral heteroleptic double-helical monometallofoldamers when mixed with an achiral strand.

Recognition of Amino Acid Salts by Temperature-Dependent Allosteric Binding with Stereodynamic Urea Receptors.

Positive homotropic artificial allosteric systems are important for the regulation of cooperativity, selectivity and nonlinear amplification. Stereodynamic homotropic allosteric receptors can transmit and amplify induced chirality by the first ligand binding to axial chirality between two chromophores. We herein report stereodynamic allosteric urea receptors consisting of a rotational shaft as the axial chirality unit, terphenyl units as structural transmission sites and four urea units as binding sites. NMR titration experiments revealed that the receptor can bind two carboxylate guests in a positive homotropic allosteric manner attributed to the inactivation by intramolecular hydrogen-bonding between urea units within the receptor. In addition, the VT-CD spectra observed upon binding of the urea receptor with l- or D-amino acid salts in MeCN showed interesting temperature-dependent Cotton effects, based on the differences of the receptor shaft unit and the guest structure. The successful discrimination of hydrocarbon-based side chains of amino acid salts indicated that the input of chiral and steric information for the guest was amplified as outputs of the Cotton effect and the temperature-dependence of VT-CD spectra through cooperativity of positive allosteric binding.

A dodecamethoxy[6]cycloparaphenylene consisting entirely of hydroquinone ethers: unveiling in-plane aromaticity through a rotaxane structure.

[n]Cycloparaphenylenes ([n]CPPs, where n is the number of phenylene groups), consisting of 1,4-linked phenylene unit, have attracted much attention due to their cyclic π-conjugated structures and physical properties. However, functionalizing of the benzene rings of smaller [n]CPPs (n < 7) has been a challenge due to ring strain and steric hindrance of the substituents that hampers their synthesis. Here we show successful synthesis of a new [6]CPP derivative with twelve methoxy groups at the 2,5-positions of all benzene rings by utilizing our developed CPP synthesis method via a macrocyclic gold complex. This molecule exhibited a significantly higher oxidation potential caused by the electron-donating ability of the methoxy groups and the tubular molecular conformation, allowing facile oxidation to give dicationic species with in-plane aromaticity. Furthermore, this molecule successfully included with the guest molecules with a flexible alkyl chain in the cavity, enabling the creation of a CPP-based rotaxane, which exploited its mechanically interlocked molecular structure to the first experimental observation that the in-plane aromaticity in the center of the macrocycle.

Structural Interconversion Based on Intramolecular Boroxine Formation.

A novel structural interconversion unit based on intramolecular boroxine formation has been developed. A macrocyclic triboronic acid consisting of three phenylboronic acid units linked by covalent linkers preferentially underwent intramolecular rather than intermolecular boroxine formation, resulting in a quantitative formation of tricyclic boroxine. This structural transformation was accompanied by changes in the polarity, flexibility, and size of the molecule. Dynamic interconversion between the macrocyclic triboronic acid and the tricyclic boroxine was achieved by simple heating/cooling, whereas no boroxine formation occurred upon heating when three boronic acid units were not connected by linkers. Thermodynamic analysis revealed that the entropic advantage of the intramolecular boroxine formation process resulted in these unique features. The entropically stabilized tricyclic boroxine also shows high stability with respect to hydrolysis.

Endo-Functionalized Cyclic Oligophenylenes: Synthesis and Complexation with a Chiral Phosphoric Acid.

The synthesis of endo-functionalized cyclic oligophenylenes in which adjacent benzene rings are perpendicular to one another is described. Annulation precursors, OH- or NH2-functionalized quinquephenyl diboronic acids, and septiphenyl dibromo compounds were systematically prepared by using a diprotected biphenyl-3,4'-diyl diboronic acid as a key compound. Four endo-functionalized cyclic oligophenylenes were synthesized by annulation of the precursors in dilute conditions through Suzuki-Miyaura cross-coupling. X-ray analysis of the macrocycle revealed the unique 1D channel packing structure formed by connecting the nanometer-sized cavity of the macrocycle. Furthermore, NH2-functionalized macrocycles could bind a chiral phosphoric acid in the cavity in CDCl3 solution.

Boroxine template for macrocyclization and postfunctionalization.

A novel synthetic strategy for large macrocyclic molecules using boroxine formation was developed. For this, the threefold intramolecular olefin metathesis of 3,5-bis(alkenyloxy)phenylboroxines with various lengths of alkenyl chains, formed by the dehydration of the corresponding boronic acid substrates, together with treatment with pinacol, was used to produce 39-, 45-, and 51-membered macrocyclic compounds with three boronate units. The boroxine moiety functions as a covalent template but can also be used to postmodify the macrocycle. Boroxine-templated macrocyclization implemented in this way does not require the addition of template molecules and simplifies the synthetic procedure.

Dynamic Au-C σ-Bonds Leading to an Efficient Synthesis of [n]Cycloparaphenylenes (n = 9-15) by Self-Assembly.

The transmetalation of the digold(I) complex [Au2Cl2(dcpm)] (1) (dcpm = bis(dicyclohexylphosphino)methane) with oligophenylene diboronic acids gave the triangular macrocyclic complexes [Au2(C6H4) x (dcpm)]3 (x = 3, 4, 5) with yields of over 70%. On the other hand, when the other digold(I) complex [Au2Cl2(dppm)] (1') (dppm = bis(diphenylphosphino)methane) was used, only a negligible amount of the triangular complex was obtained. The control experiments revealed that the dcpm ligand accelerated an intermolecular Au(I)-C σ-bond-exchange reaction and that this high reversibility is the origin of the selective formation of the triangular complexes. Structural analyses and theoretical calculations indicate that the dcpm ligand increases the electrophilicity of the Au atom in the complex, thus facilitating the exchange reaction, although the cyclohexyl group is an electron-donating group. Furthermore, the oxidative chlorination of the macrocyclic gold complexes afforded a series of [n]cycloparaphenylenes (n = 9, 12, 15) in 78-88% isolated yields. The reorganization of two different macrocyclic Au complexes gave a mixture of macrocyclic complexes incorporating different oligophenylene linkers, from which a mixture of [n]cycloparaphenylenes with various numbers of phenylene units was obtained in good yields.

Ring-Over-Ring Deslipping From Imine-Bridged Heterorotaxanes.

Ring-over-ring slippage and ring-through-ring penetration are important processes in the construction of ring-in-ring multiple interlocked architectures. We have successfully observed "ring-over-ring deslipping" on the rotaxane axle by exploiting the dynamic covalent nature of imine bonds in imine-bridged heterorotaxanes R1 and R2 with two macrocycles of different ring sizes on the axle. When the imine bridges of R1 were cleaved, a hydrolyzed hetero[4]rotaxane [4]R1' was formed as an intermediate under dynamic equilibrium, and the larger 38-membered macrocycle M was deslipped over the 24-membered ring (24C8 or DB24C8) to dissociate into a [3]rotaxane [3]R3 and a macrocycle M. The time dependent NMR measurement and the determined thermodynamic parameters revealed that the rate-limiting step of the deslipping process was attributed to steric hindrance between two rings and reduced mobility of M due to proximity to the crown ether, which was bound to the anilinium on the axle molecule.

Spacer-Dependent Cooperativity of Helicity in Fluorescent Bishelical Foldamers Based on L-Shaped Dibenzopyrrolo[1,2-a][1,8]naphthyridine.

For the construction of helical foldamers composed of π-frameworks, the choice of appropriate π-π stacking units and π-spacers connecting them is important. The transfer of helicity between the minimal helix structural units is also an essential factor in the construction of homochiral helical foldamers. Tetramers 4 a-4 d, which have four L-shaped dibenzopyrrolo[1,2-a]naphthyridine units, were synthesized to investigate the interplay and cooperativity of the helical structures. Tetramer 4 a bridged with a biphenyl unit formed a homochiral bishelical structure with π-π stacking between the L-shaped units (3.3 Å), consisting only of (P,P)- and (M,M)-enantiomers without the (P,M)-diastereomer, owing to interplay through the axial chirality of biphenyl unit in the solid state. Similarly, in solution, thermodynamic stabilization of the two helix formations worked cooperatively to favor the bishelical form of 4 a. Furthermore, bishelical foldamer 4 a emitted intense fluorescence (Φ=0.86).

Short-step synthesis and chiroptical properties of polyaza[5]-[9]helicenes with blue to green-colour emission.

We have developed a two- or three-step synthesis of polyaza[5]-[9]helicenes by consecutive N-H/C-H coupling with a hypervalent iodine reagent. We further evaluated their photophysical properties, and circularly polarized luminescence properties and found that glum values increase with the number of fused rings up to 2.7 × 10-2, which is the highest value among [9]helicenes.

Fluorescent Short-Stranded Helical Foldamers Based on L-shaped Dibenzopyrrolo[1,2-a][1,8]naphthyridine.

Helical structures were constructed by using π-spacer-bridged dimers of dibenzopyrrolo[1,2-a][1,8]naphthyridine, which has a highly fluorescent L-shaped π-extended skeleton. Three dimers with biphenylene (dimer 1), phenanthrene (dimer 2), and m-phenylene (dimer 3) spacers, as well as a fixed-helical dimer 4 where two quinolone rings were covalently cross-linked, were designed and prepared. 1 H NMR and ROESY spectra revealed that dimers 1 and 2 adopted helical forms in solution, whereas dimer 3 did not. The helical conformation of 1 was strengthened by addition of either polar or nonpolar solvents to the chloroform solution, which suggested that π-π stacking was the main contributor to the stabilization of the helical structure. All of the dimers, including fixed-helical dimer 4, emitted fluorescence with high quantum yields (ϕ=0.79-0.86).

Dibenzopyrrolo[1,2-a][1,8]naphthyridines: Synthesis and Structural Modification of Fluorescent L-Shaped Heteroarenes.

The L-shaped, π-extended pentacycle dibenzopyrrolo[1,2-a][1,8]naphthyridine and its derivatives were synthesized using two methods: fully intramolecular [2 + 2 + 2] cycloaddition and oxidative aromatization using substituted carbodiimide and modification of an electron-rich indole ring of an L-shaped skeleton via electrophilic reaction and cross-coupling. These L-shaped compounds emitted fluorescence in high quantum yield. The position of substituents affected the fluorescence color through two different mechanisms, π-conjugation and skeletal distortion, which caused the substituted L-shaped compounds to emit fluorescence in a variety of colors and to exhibit solvato-fluorochromism.

Facile Two-Step Synthesis of 1,10-Phenanthroline-Derived Polyaza[7]helicenes with High Fluorescence and CPL Efficiency.

A facile two-step synthesis of aza[7]helicenes possessing a 6-5-6-6-6-5-6 skeleton from commercially available 2,9-dichloro-1,10-phenanthroline via double amination with aniline derivatives followed by hypervalent iodine reagent-mediated intramolecular double-NH/CH couplings was developed. Single-crystal X-ray analyses of the helicenes revealed unique structures, including both a significantly twisted center and planar terminals of the skeleton. The azahelicenes show high fluorescent quantum yields (Φs) under both neutral (Φ: 0.25-0.55) and acidic conditions (Φ: up to 0.80). An enantiomerically pure aza[7]helicene showed high circularly polarized luminescence (CPL) activity under both neutral and acidic conditions (glum : up to 0.009).

Bis(10-methylacridinium)s as a versatile platform for redox-active functionalized dyes and novel structures.

10-Methylacridinium is a stable aromatic cation exhibiting yellow-orange color and strong green fluorescence (FL), and can be used as a versatile building block in constructing a wide variety of novel π-electron systems. The dications with the two chromophores connected by a proper π spacer undergo reversible "dyrex" (dynamic redox) behavior, and C-C bond formation/cleavage is accompanied by their redox reactions. The prototype is biphenyl-2,2'-diylbis(10-methylacridinium), which exhibits electrochromic response with ON/OFF switching of FL during the reversible interconversion with the di(spiroacridan)-type electron donor. Slight structural alteration under the concept of "MFMS" (maximum function on the minimum skeleton) endowed the dyrex system with metal-binding or chiroptical properties, which could be modified by the redox reactions. Some of the di(spiroacridan) derivatives generated from bis(10-methylacridinium)s have extreme structural parameters, such as the greatest C-C bond length ever reported.

Crystal structures and redox responses coupled with ion recognition of p-benzoquinone- and hydroquinone-fused [18]crown-6.

The crystal structures and redox properties of p-benzoquinone (BQ)-fused [18]crown-6 1 and bis-BQ-fused [18]crown-6 2 were examined. The anion radicals of these BQ molecules were stabilized by addition of metal ions, through effective electrostatic interactions between the negatively charged BQ moiety and positively charged ion-capturing [18]crown-6 unit. The electrostatic interactions and solvation energy played important roles in determining the magnitudes of anodic redox shifts in the reduction potentials. Regular π-stacking of BQ units and regular arrays of [18]crown-6 units were observed in crystal 2, in which one-dimensional π-electron columns were separated by ionic channels. The hydroquinone-fused [18]crown-6 molecule 3 and a new BQ- and phenol-fused [18]crown-6 derivative 4 were obtained as single crystals. The molecular conformations of [18]crown-6 in crystal 3 and hydrated crystal 3⋅H2 O were different from each other.

Preparation and structure of acenaphthylene-1,2-diyldi(9-acridine) derivatives with a long C=C bond.

Due to purely steric effects of acridine units, acenaphthylene-1,2-diyldi(9-acridine) has a long C=C bond [1.3789(19) Å] while maintaining its sp(2) hybridized nature and bond order.

Total synthesis of pectenotoxin-2.

Pectenotoxin-2 (PTX2) is a shellfish toxin and has a non-anomeric spiroacetal, which is not stabilized by an anomeric effect. The selective construction of the non-anomeric spiroacetal has been a major problem in the synthesis of PTX2. Described herein is the stereoselective total synthesis of PTX2 via the isomerization of anomeric spiroacetal pectenotoxin-2b (PTX2b). The synthesis of PTX2b was achieved by a simple process including sulfone-mediated assembly of spirocyclic and bicyclic acetals and subsequent macrocyclization by ring-closing olefin metathesis. Finally, the selective construction of PTX2 was accomplished by the early termination of a dynamic transition process to equilibrium in the acid-catalyzed isomerization of anomeric PTX2b. [6,6]-Spiroacetal pectenotoxin-2c (PTX2c) was also synthesized from PTX2b. The cytotoxicity assay of the synthetic compounds against HepG2 and Caco2 cancer cells showed a potency of the order: PTX2≫PTX2b>PTX2c.

A new variant of fused cyclic ether synthesis based on Ireland-Claisen rearrangement and RCM.

A new variant of fused cyclic ether synthesis based on Ireland-Claisen rearrangement and ring-closing olefin metathesis (RCM) was developed. The Ireland-Claisen rearrangement of a (Z)-3-alkoxyprop-2-en-1-yl glycolate ester having a cyclic ether on the oxygen at C3 of the (Z)-prop-2-en-1-yl group stereoselectively produced an anti-alpha,beta-dialkoxyester, which was successfully transformed to a fused bicyclic ether via a reaction sequence including RCM.

A C(3)-symmetric chiroptical molecular propeller based on hexakis(phenylethynyl)benzene with a threefold terephthalamide: stereospecific propeller generation through the cooperative transmission of point chiralities on the host and guest upon complexation.

Propeller-shaped dynamic helicity was generated in a hexakis(phenylethynyl)benzene framework that preferred a particular sense to afford a strong CD signal, which was realized by the cooperative transmission of point chiralities upon complexation with a chiral guest through a threefold binding site presented by a syn-formed terephthalamide.