Arene Analogues Research Articles

Umbrella boronyl cluster B10O: A new candidate for the transition-metal-like bonding model of boron

We report a new candidate B10O with the transition-metal-like boron atom in the boron monoxide clusters family. It adopts an umbrella architecture with a structural configuration of B8-B-BO. Significantly, the central boron atom is coordinated with both arene analog (B8) and carbonyl analog (BO), demonstrating its transition-metal property. Chemical bonding analyses suggest that the upper B9 (B8-B) possesses the double (6σ + 6π) aromaticity, which gives rise to the extraordinary stability of B10O molecular umbrella. As a dynamically stabilized global minimum, the neutral B10O molecular umbrella is promising to be synthesized in the condensed phase.

Mixed Thioether/Amine‐Functionalized Expanded Calixarenes, Related Macropolycycles, and Metal Complexes thereof

AbstractA series of tetraaza‐expanded calix[4]arene analogues (2–4), in which two of the four aryl groups are ethylene‐protected thiophenol components were prepared from tetraaldehyde 1. Two of these strapped macrocycles were characterized crystallographically, as well as the dipalladium complex of 4. The same tetraaldehyde had been previously used for the synthesis of a macrotricycle (5) and a macropentacycle (6), the coordination properties of which have been investigated. In particular, ligand 6 was shown to be able to encapsulate a Cu+ or a Ag+ ion in the tetrahedral coordination pocket offered by a bridgehead nitrogen atom and the three proximal thioether sulfur atoms.

Thiapillar[6]arene: Synthesis, Functionalization, and Properties

AbstractThe design and synthesis of new macrocycles with well-defined cavities represent a promising avenue for the development of new supramolecular hosts. Moreover, the ability to diversify a macrocycle through chemical manipulations enables the fine-tuning and tailoring of properties. In this report, the synthesis and functionalization of thiapillar[6]arene, a pillar[6]arene analogue in which the bridging methylene groups are replaced by sulfurs, are described. First, we demonstrate the scalable synthesis of the parent thiapillar[6]arene. Next, the diversification of thiapillar[6]arene is demonstrated via functionalization of the phenols and oxidation of the sulfur atoms. The solid-state structures of two thiapillar[6]arene derivatives are reported, and the effect of sulfur oxidation state on the macrocyclic conformation is discussed. All sulfone derivatives described were found to demonstrate high luminescence quantum yields (ΦF = 0.43–0.66) in CH2Cl2 with emission maxima between λ = 404 and 462 nm. Lastly, assessment of the electrochemical properties of the sulfone derivatives by square-wave voltammetry revealed electron-accepting ability owing to the oxidation of the sulfur atoms, with four reduction events observed for the analogues surveyed. Overall, this work implicates thiapillar[6]arene as a modular scaffold amenable for further applications in host–guest chemistry and sensing.

Bottom-Up Solid-State Molecular Assembly via Guest-Induced Intermolecular Interactions.

The manipulation of molecular motions to construct highly ordered supramolecular architectures from chaos in the solid state is considered to be far more complex and challenging in comparison to that in solution. In this work, a bottom-up molecular assembly approach based on a newly designed skeleton-trimmed pillar[5]arene analogue, namely the permethylated leggero pillar[5]arene MeP[5]L, is developed in the solid state. An amorphous powder of MeP[5]L can take up certain guest vapors to form various ordered linker-containing solid-state molecular assemblies, which can be further used to construct a thermodynamically favored linker-free superstructure upon heating. These approaches are driven by vapor-induced solid-state molecular motions followed by a thermally triggered phase-to-phase transformation. The intermolecular interactions play a crucial role in controlling the molecular arrangements in the resulting assemblies. This research will open new insights into exploring controllable molecular motions and assemblies in the solid state, providing new perspectives in supramolecular chemistry and materials.

Thioderivatives of Resorcin[4]arene and Pyrogallol[4]arene: Are Thiols Tolerated in the Self-Assembly Process?

Three novel thiol bearing resorcin[4]arene and pyrogallol[4]arene derivatives were synthesized. Their properties were studied with regards to self-assembly, disulfide chemistry, and Brønsted acid catalysis. This work demonstrates that (1) one aromatic thiol on the resorcin[4]arene framework is tolerated in the self-assembly process to a hexameric hydrogen bond-based capsule, (2) thio-derivatized resorcin[4]arene analogs can be covalently linked through disulfides, and (3) the increased acidity of aromatic thio-substituent is not sufficient to replace HCl as cocatalyst for capsule catalyzed terpene cyclizations.

Calixarene-Based lead receptors: an NMR, DFT and X-Ray synergetic approach

ABSTRACT The conformational changes of the homooxa adamantyl ketones 1 and 2, as well as of the calix[4]arene analogue derivative 3, all in the cone conformation, upon Pb2+ complexation were investigated by 1H, 13C, 207Pb NMR and proton spin-lattice relaxation times (T 1) experiments. The X-ray crystal structures of ketones 1 and 3 were determined. The inclusion of an acetonitrile molecule in the hydrophobic cavity of the ligands was observed in the solid state. In solution, inclusion was observed only in the case of Pb2+ ⊂ ketone 3 complex. DFT calculations were also performed to complement the NMR conformational analysis and to bring further insights to the cation complexation. The data confirmed the formation of 1:1 complexes between Pb2+ and the ligands, and that the cation is located inside the cavity defined by the phenoxy and carbonyl oxygen atoms. In general, the ligand conformations became closer to a regular cone upon complexation, with the binding models found for the three ketones through the NMR studies corroborated by the DFT calculations.

Theoretical design and exploration of low-valent uranium metallocenes via manipulating cyclopentadienyl substituent

Over a century of investigations on uranium complexes resulted in great progress in establishing their fundamental properties and developing applications. However, the in-depth understanding of uranium chemistry, particularly in record low + II valence, is still challenging due to rare crystallographically-identified U(II) complexes, extremely low stability and rigid synthetic conditions. Herein, to further expand experimental findings, various substituted-cyclopentadienyl uranium complexes, [U(CpX)3]n (X = Me, H, Cl, SiMe3 and 2SiMe3; n = -1 and 0), have been explored using relativistic density functional theory. It is shown that the reduction from [U(CpX)3] to [U(CpX)3]– slightly strengthens the U-Cp bonding, while the great enhancement is found in previously reported uranium arene analogues. The + III oxidation state is assigned to the metal of [U(CpX)3]; in contrast, it is approximately + II valence for [U(CpX)3]– because of non-negligible electron density residing on the CpX ligands, whose amount positively correlates with the electron-withdrawing ability of X substituent. Calculated reduction potentials range from −3.02 to −2.74 V while considering solvation and spin–orbit coupling effects. The largest value of the chloride complex couples is attributed to the better balance of electronic and steric effects.

Oxygen‐ and Nitrogen‐Embedded Zigzag Hydrocarbon Belts

AbstractDespite the aesthetically appealing structures and tantalizing physical and chemical properties, zigzag hydrocarbon belts and their heteroatom‐embedded analogues remain challenging synthetic targets. We report herein the synthesis of diverse O/N‐doped zigzag hydrocarbon belts based on selective bridging of the fjords of resorcin[4]arene derivatives through intramolecular SNAr and palladium‐catalyzed intermolecular C−N bond formation reactions. Preorganized conformations of mono‐macrocyclic, half‐belt and quasi‐belt compounds were revealed to facilitate cyclization reactions to construct heteroatom‐linked octahydrobelt[8]arenes. The acquired products had strained square‐prism‐shaped belt structures in which all six‐membered heterocyclic rings adopted an unusual boat conformation with equatorially configured alkyl groups. The unprecedented heteroatom‐bearing belts also exhibited different photophysical and redox properties to those of octahydrobelt[8]arene analogues.

A p-tert-Tutyldihomooxacalix[4]arene Based Soft Gel for Sustained Drug Release in Water.

P-tert-butyldihomooxacalix[4]arene is a well-known calix[4]arene analog in which one CH2 bridge is replaced by one -CH2OCH2- group. Thus, dihomooxacalix[4]arene has a slightly larger cavity than that of calix[4]arene and usually possesses a more flexible cone conformation, and the bridged oxygen atom might provide additional binding sites. Here, we synthesized a new functional p-tert-butyldihomooxacalix[4]arene 1 through Ugi reaction with good yield (70%), starting from condensed p-tert-butyldihomooxacalix[4]arene O-alkoxy–substituted benzaldehydes, benzoic acid, benzylamine, and cyclohexyl isocyanide. Proton nuclear magnetic resonance spectroscopy (1H NMR), 13C NMR, IR, and diffusion-ordered 1H NMR spectroscopy (DOSY) methods were used to characterize the structure of 1. Then soft gel was prepared by adding 1 into cyclohexane directly. It shows remarkable thermoreversibility and can be demonstrated for several cycles. As is revealed by scanning electron microscopy (SEM) images, xerogel showed highly interconnected and homogeneous porous network structures, and hence, the gel is suitable for storage and controlled release.

The enhancement of luminescent properties of Tb3+ complexes with tetra-1,3-diketone ligands promoted by the tetrathiacalix[4]arene scaffold

This is the first report on the synthesis and characterization of tetra-1,3-diketone derivatives of tetrthia- and calix[4]arenes substituted by acetylacetonyl or dipivaloylmethanyl moieties at the upper rim and bearing hydroxyl groups at the lower rim as ligands for Tb3+ complexes. The spatial structure and tautomeric content for the synthesized ligands have been determined by means of X-ray, IR and NMR spectroscopy. Comparison of the sensitizing properties of tetrathiacalix[4]arenes and their calix[4]arene analogues on the Tb3+-centered luminescence in DMF solutions has been performed. A substantial enhancement of the luminescent properties of Tb3+- complexes with tetra-1,3-diketone ligands promoted by the tetrathiacalix[4]arene scaffold was established.

Rational synthesis of benzimidazole[3]arenes by CuII-catalyzed post-macrocyclization transformation.

A new series of calix[n]arene analogues, benzimidazole[3]arenes, was rationally synthesized by CuII-catalyzed post-macrocyclization transformation of a tris(o-phenylenediamine) macrocycle, and fully characterized by NMR, MS, and single-crystal X-ray diffraction (XRD) analyses. The resulting syn- and anti-benzimidazole[3]arenes have a bowl-shaped and a warped structure, respectively, in their crystalline states, and both display a dynamic inversion behavior in solution. This modification resulted in strong fluorescence due to the generated benzimidazole moieties. The mechanistic study of the post-macrocyclization transformation demonstrated that the formation of both benzimidazole[3]arenes was catalyzed, via triimine intermediates, by CuII ions in air through oxidation and cyclization of the tris(o-phenylenediamine) macrocycle.

Transfer and Dynamic Inversion of Coassembled Supramolecular Chirality through 2D-Sheet to Rolled-Up Tubular Structure.

Transfer and inversion of supramolecular chirality from chiral calix[4]arene analogs (3D and 3L) with an alanine moiety to an achiral bipyridine derivative (1) with glycine moieties in a coassembled hydrogel are demonstrated. Molecular chirality of 3D and 3L could transfer supramolecular chirality to an achiral bipyridine derivative 1. Moreover, addition of 0.6 equiv of 3D or 3L to 1 induced supramolecular chirality inversion of 1. More interestingly, the 2D-sheet structure of the coassembled hydrogels formed with 0.2 equiv of 3D or 3L changed to a rolled-up tubular structure in the presence of 0.6 equiv of 3D or 3L. The chirality inversion and morphology change are mainly mediated by intermolecular hydrogen-bonding interactions between the achiral and chiral molecules, which might be induced by reorientations of the assembled molecules, confirmed by density functional theory calculations.

Potassium Binding Adjacent to Cationic Transition-Metal Fragments: Unusual Heterobimetallic Adducts of a Calix[4]arene-Based Thione Ligand.

The synthesis of cationic rhodium and iridium complexes of a bis(imidazole-2-thione)-functionalized calix[4]arene ligand and their surprising capacity for potassium binding are described. In both cases, uptake of the alkali metal into the calix[4]arene cavity occurs despite adverse electrostatic interactions associated with close proximity to the transition-metal fragment [Rh+···K+ = 3.715(1) Å; Ir+···K+ = 3.690(1) Å]. The formation and constituent bonding of these unusual heterobimetallic adducts have been interrogated through extensive solution and solid-state characterization, examination of the host-guest chemistry of the ligand and its upper-rim unfunctionalized calix[4]arene analogue, and use of density functional theory based energy decomposition analysis.

Easy Access to Difluoromethylene‐Containing Arene Analogues through Palladium‐Catalysed C–H Olefination

An efficient palladium‐catalysed ortho‐C–H olefination of α,α‐difluorophenylacetic acid derivatives using 8‐aminoquinoline as a bidentate directing group has been developed. A range of olefinated arenes can thus be synthesized in a concise way. This reaction provides an easy and straightforward route to a panel of difluoromethylated arene analogues in moderate to good yields, with a satisfactory tolerance of common functional groups. Transformation of the products into a variety of other difluoromethylene‐containing compounds demonstrates the utility of this method.

Differential Recognition of Anions with Selectivity towards F(-) by a Calix[6]arene-Thiourea Conjugate Investigated by Spectroscopy, Microscopy, and Computational Modeling by DFT.

Anion recognition studies were performed with triazole-appended thiourea conjugates of calix[6]arene (i.e., compound (6) L) by absorption and (1) H NMR spectroscopy by using nineteen different anions. The composition of the species of recognition was derived from ESI mass spectrometry. The absorption spectra of compound (6) L showed a new band at λ=455 nm in the presence of F(-) due to a charge transfer from the anion to the thiourea moiety and the absorbance increases almost linearly in the concentration range 5 to 200 μm. This is associated with a strong visual color change of the solution. Other anions, such as H2 PO4 (-) and HSO4 (-) , exhibit a redshift of the λ=345 nm band and the spectral changes are associated with the formation of an isosbestic point at λ=343 nm. (1) H NMR studies further confirm the binding of F(-) efficiently to the thiourea group among the halides by shifting the thiourea proton signals downfield followed by their disappearance after the addition of more than one equivalent of F(-) . The other anions also showed interactions with compound (6) L, however, their binding strength follows the order F(-) >CO3 (2-) >H2 PO4 (-) ≈CH3 COO(-) >HSO4 (-) . The NMR spectral changes clearly revealed the anion-binding region of the arms in case of all these anions. The anion binding to compound (6) L indeed stabilizes a flattened-cone conformation as deduced based on the calix-aromatic proton signals and was further confirmed by VT (1) H NMR experiments. The stabilization of the flattened-cone conformation was further augmented by the interaction of the butyl moiety of the nBu4 N(+) counterion. The structural features of the anion-bound species were demonstrated by DFT computations and the resultant structures carried the features that were predicted based on the (1) H NMR spectroscopic measurements. In addition, SEM images showed a marigold flower-type morphology for compound (6) L and this has been transformed into a chain-like structure of connected spherical particles in the presence of F(-) . The anion-induced microstructural features are reflective of the binding strength, size, and shape of the anions. The binding strengths of the anions by compound (6) L were further compared with that of compound (4) L, a calix[4]arene analogue of compound (6) L, in order to address the role of the number of arms built on the calixarene platform based on absorption spectroscopy, (1) H NMR spectroscopy, and DFT computations and it was found that compound (6) L is a better receptor for F(-) , which extends its interactions from all the three arms.

Recognition of nitrate by the ammonium derivative of a 20-membered dioxadiaza-calix[4]arene analog—solution and solid-state study of the anion binding properties

A 20-membered dioxadiaza-calix[4]arene analog (1) was combined with a series of inorganic acids to explore the anion-inclusion properties of the corresponding ammonium dication [1-H2]2+. The neutral calix[4]arene analog 1 and six salts of the receptor in the mono- or diprotonated form, [1-H2]Cl2 (2), [1-H2]Br2 (3), [1-H2][MeOSO3]2 (4), [1-H2][NO3]2 (5), [1-H2][ClO4]2 (6), and [1-H][ClO4] (6b), were structurally characterized by single-crystal X-ray diffraction analysis, which enabled a detailed comparative analysis of the non-covalent interactions. The structural comparison showed that the different anions induce conformational changes of the macrocyclic structure, but only NO3− gave a true inclusion complex. Additionally, the tetramethylammonium derivative of 1, [1-Me4]I2 (7), was prepared and structurally characterized.1H NMR titration studies of [1-H2]2+ in an aqueous–polar solvent mixture (H2O/MeOH, 9:1 v/v, pH 5.5) with a series of organic and inorganic anions gave association constants varying from 1.4 to 84.6 M−1.

Synthetic supercontainers exhibit distinct solution versus solid state guest-binding behavior.

The phase-dependent host-guest binding behavior of a new family of synthetic supercontainers has been probed in homogeneous solution and at liquid-liquid, solid-liquid, and solid-gas interfaces. The synthetic hosts, namely, type II metal-organic supercontainers (MOSCs), are constructed from the assembly of divalent metal ions, 1,4-benzenedicarboxylate (BDC) linker, and sulfonylcalix[4]arene-based container precursors. One member of the MOSCs, MOSC-II-tBu-Ni, which is derived from Ni(II), BDC, and p-tert-butylsulfonylcalix[4]arene (TBSC), crystallizes in the space group R3 and adopts pseudo face-centered cubic (fcc) packing, whereas other MOSCs, including TBSC analogue MOSC-II-tBu-Co, p-tert-pentylsulfonylcalix[4]arene (TPSC) analogues MOSC-II-tPen-Ni/Co, and p-tert-octylsulfonylcalix[4]arene (TOSC) analogues MOSC-II-tOc-Ni/Mg/Co, all crystallize in the space group I4/m and assume a pseudo body-centered cubic (bcc) packing mode. This solid-state structural diversity is nevertheless not reflected in their solution host-guest chemistry, as evidenced by the similar binding properties of MOSC-II-tBu-Ni and MOSC-II-tBu-Co in solution. Both MOSCs show comparable binding constants and adsorb ca. 7 equiv of methylene blue (MB) and ca. 30 equiv of aspirin in chloroform. In contrast, the guest-binding behavior of the MOSCs in solid state reveals much more variations. At the solid-liquid interface, MOSC-II-tBu-Co adsorb ca. 5 equiv of MB from an aqueous solution at a substantially faster rate than MOSC-II-tBu-Ni does. However, at the solid-gas interface, MOSC-II-tBu-Ni has higher gas uptake than MOSC-II-tBu-Co, contradicting their overall porosity inferred from the crystal structures. This discrepancy is attributed to the partial collapse of the solid-state packing of the MOSCs upon solvent evacuation. It is postulated that the degree of porosity collapse correlates with the molecular size of the MOSCs, i.e., the larger the MOSCs, the more severe they suffer from the loss of porosity. The same principle can rationalize the negligible N2 and O2 adsorption seen in the larger MOSC-II-tPen-Co and MOSC-II-tOC-Ni/Mg/Co molecules. MOSC-II-tPen-Ni features an intermediate molecular size and endures a partial structural collapse in such a way that the resulting pore dimension permits the inclusion of kinetically smaller O2 (3.46 Å) but excludes larger N2 (3.64 Å), explaining the observed remarkable O2/N2 adsorption selectivity.

Triptycene-derived calix[6]arene analogues: synthesis, structure and complexation with paraquat derivatives

A new kind of triptycene-derived calix[6]arene analogue was synthesized by two different routes. The triptycene-derived calix[6]arene analogues show highly symmetric structures and fixed conformations in solution, and can self-assemble into organic tubular structures with aromatic rings as the wall in the solid state. Moreover, the calix[6]arene analogue with two 1,8-dihydroxyltriptycene moieties in the cis position was also found to form stable complexes with paraquat derivatives with different terminal functional groups. Consequently, a [2]rotaxane assembly based on the calix[6]arene analogue was further constructed.

Water-soluble arene ruthenium complexes: From serendipity to catalysis and drug design

Shortly after the discovery of benzene ruthenium dichloride and some controversy about its polymeric or dimeric nature in the 1960s, the hydrolysis of this material in water to give a mixture of benzene ruthenium aqua complexes was discovered. However, it took a long time until this reaction and the hydrolysis of other arene analogs were used as an entry to the synthesis of water-soluble arene ruthenium complexes. These complexes are able to activate molecular hydrogen in aqueous solution and allow the design of arene ruthenium bioconjugates. They can serve as catalysts or catalyst precursors for hydrogenation and transfer hydrogenation reactions in water and they are at present one of the most promising classes of metal complexes to replace cisplatin in future cancer therapy, due to their inherent cytotoxicity and their good cellular uptake, conditioned by well balanced lipophilic and hydrophilic properties.

Solution and solid-state conformations of 1,5-pyridine and 1,5-phenanthroline-bridged p-tert-butylcalix[8]arene derivatives

Graphical abstract An extension of the method for the regioselective introduction of nitrogen-containing heterocycles at the 1,5-phenolic positions of p - tert -butylcalix[8]arene previously described for 2,6-bis(chloromethyl)pyridine allowed the use of 2,9-bis(bromomethyl)-1,10-phenanthroline as a selective 1,5-dialkylating agent. The 1,5-dimethylpyridine derivative was previously characterized as a relatively rigid dicesium complex [ 1 (4H) 2− ·2Cs + ], its metal-free counterpart 1 (6H) is herein described as a fluxional species in solution, as evidenced by VT–NMR and theoretical studies. The 1,5-[2,9-bis(methylene)-1,10-phenanthroline] -p - tert -butylcalix[8]arene analogue 2 (6H) was characterized spectroscopically in solution as a relatively rigid cone conformer at 390 K. The identity of both metal-free 1 (6H) and the monocesium complex [ 2 (5H) − ·Cs + ] was established in the solid state by X-ray crystallography, confirming the 1,5-substitution of the latter. The conformational behavior of 1 (6H) and 2 (6H) in solution is described in terms of the enthalpic and entropic contributions by semiempirical methods.