Ammonium halide selective ion pair recognition and extraction with a chalcogen bonding heteroditopic receptor

The first example of a heteroditopic receptor capable of cooperative recognition and extraction of ammonium salt (NH4X) ion-pairs is described. Consisting of a bidentate 3,5-bis-tellurotriazole chalcogen bond donor binding cleft, the appendage of benzo-15-crown-5 (B15C5) substituents to the tellurium centres facilitates binding of the ammonium cation via a co-facial bis-B15C5 sandwich complex, which serves to switch on chalcogen bonding-mediated anion binding potency. Extensive quantitative ion-pair recognition 1H NMR titration studies in CD3CN/CDCl3 (1 : 1, v/v) solvent media reveal impressive ion-pair binding affinities towards a variety of ammonium halide, nitrate and thiocyanate salts, with the heteroditopic receptor displaying notable ammonium halide salt selectivity. The prodigious solution phase NH4X recognition also translates to efficient solid-liquid and liquid-liquid extraction capabilities.

Manipulating Molecular Self-Assembly Process at the Solid-Liquid Interface Probed by Scanning Tunneling Microscopy

The phenomenon of ordered self-assembly on solid substrates is a topic of interest in both fundamental surface science research and its applications in nanotechnology. The regulation and control of two-dimensional (2D) self-assembled supra-molecular structures on surfaces have been realized through applying external stimuli. By utilizing scanning tunneling microscopy (STM), researchers can investigate the detailed phase transition process of self-assembled monolayers (SAMs), providing insight into the interplay between intermolecular weak interactions and substrate-molecule interactions, which govern the formation of molecular self-assembly. This review will discuss the structural transition of self-assembly probed by STM in response to external stimuli and provide state-of-the-art methods such as tip-induced confinement for the alignment of SAM domains and selective chirality. Finally, we discuss the challenges and opportunities in the field of self-assembly and STM.

Allosteric and Electrostatic Cooperativity in a Heteroditopic Halogen Bonding Receptor System

A halogen bonding (XB) heteroditopic receptor, consisting of a 1,3-bis-iodo-triazole benzene XB anion binding site covalently appended via a flexible methylene group with two benzo-15-crown-5 (B15C5) cation binding moieties, and its hydrogen bonding receptor analogue, are used to delineate the mechanisms of cooperativity for alkali metal halide ion-pair recognition. Extensive cation, anion and ion-pair ¹ H NMR titration investigations demonstrate the combination of allosteric pre-organisation, via 1 : 1 stoichiometric intramolecular potassium and rubidium metal cation bis-B15C5 sandwich complexation, in concert with favourable electrostatics and XB potency, results in a remarkable enhancement of halide anion binding affinity by a factor of at least 700. By contrast, a notably diminished halide anion affinity enhancement factor of just 15 is observed with the corresponding 1 : 1 stoichiometric sodium cation complexed receptor system, where the smaller sized cation singly occupies one B15C5 unit and only an electrostatic contribution to cooperativity is possible. These observations serve to illustrate that allosteric pre-organisation capability, electrostatic attraction and XB mediated anion recognition are important strategic design features to incorporate in future high-fidelity heteroditopic ion-pair receptor development.

Stereospecific Formation of the rctt Isomer of Bis-crown-Containing Cyclobutane upon [2 + 2] Photocycloaddition of an (18-Crown-6)stilbene Induced by Self-Assembly via Hydrogen Bonding

The formation and the spectroscopic and structural properties of 1:1 and 2:1 (ligand-to-dication) complexes of an (18-crown-6)stilbene with ethane-1,2-diammonium diperchlorate in MeCN were studied by UV-vis and NMR spectroscopy and by density functional theory calculations. Prolonged UV irradiation of 2:1 mixtures of the crown stilbene and the diammonium salt led to the formation of two main photoproducts, namely, the single syn-"head-to-head" photodimer of the crown stilbene (rctt cyclobutane) due to supramolecular-assisted [2 + 2] photocycloaddition and a crown ether derivative of phenanthrene due to a photoinduced electrocyclization reaction. The rctt cyclobutane was isolated by preparative photolysis, followed by chromatography. The selectivity of the [2 + 2] photocycloaddition is explained by supramolecular pre-organization of crown stilbene molecules into the 2:1 complexes that have a pseudo-sandwich structure with stacking interactions between the stilbene moieties.

Cyclometallated Platinum(II) Complexes with Small Crown Ether Rings: Appropriate Choice of the Bridging Diphosphane to Coordinate Potassium Cations

This account reports the synthesis and structural characterization of the first cyclometallated platinum(II) complex that coordinates a potassium cation in a sandwich arrangement via two 15-crown-5 ether rings within the same molecule. The cooperation of the two small crown ether moieties allows the entrapment of the non-ideal potassium ion. The reaction of the parent thiosemicarbazone ligand 3,4-(C₈H₁₆O₅)C₆H₃C(Me)=NN-(H)C(=S)NHMe, 1, containing the crown ether ring, with K₂[PtCl₄], or alternatively with PtCl₂(DMSO)₂, and subsequent treatment with the diphosphanes Ph₂PCH₂PPh₂ (dppm) and Ph₂PC(=CH₂)PPh₂ (vdpp) produced the double nuclear platinacycles 3a, 3b, and 4, probably via formation of the 2a and 2b intermediates. Complex 3a with the K⁺ cation in a sandwich coordination was slightly mixed with 3b lacking any K⁺. Alternatively, reaction of 1 with K₂[PtCl₄] or with PtCl₂(DMSO)₂ followed by the diphosphane Ph₂PC(=CH₂)PPh₂ (vdpp) only gave the dinuclear phosphane-bridged compound 4; this highlights the importance of choosing the right diphosphane ligand. Density functional theory calculations (B3LYP-D3/LANL2DZ-ECP-6.311++G**) revealed similar affinities for both dppm and vdpp derivatives to coordinate potassium cations. Crystal structure analysis was performed for compounds 3a and 4.

Selective Potassium Chloride Recognition, Sensing, Extraction, and Transport Using a Chalcogen-Bonding Heteroditopic Receptor

Chalcogen bonding (ChB) is rapidly rising to prominence in supramolecular chemistry as a powerful sigma (σ)-hole-based noncovalent interaction, especially for applications in the field of molecular recognition. Recent studies have demonstrated ChB donor strength and potency to be remarkably sensitive to local electronic environments, including redox-switchable on/off anion binding and sensing capability. Influencing the unique electronic and geometric environment sensitivity of ChB interactions through simultaneous cobound metal cation recognition, herein, we present the first potassium chloride-selective heteroditopic ion-pair receptor. The direct conjugation of benzo-15-crown-5 ether (B15C5) appendages to Te centers in a bis-tellurotriazole framework facilitates alkali metal halide (MX) ion-pair binding through the formation of a cofacial intramolecular bis-B15C5 M⁺ (M⁺ = K⁺, Rb⁺, Cs⁺) sandwich complex and bidentate ChB···X^– formation. Extensive quantitative ¹H NMR ion-pair affinity titration experiments, solid–liquid and liquid–liquid extraction, and U-tube transport studies all demonstrate unprecedented KCl selectivity over all other group 1 metal chlorides. It is demonstrated that the origin of the receptor’s ion-pair binding cooperativity and KCl selectivity arises from an electronic polarization of the ChB donors induced by the cobound alkali metal cation. Importantly, the magnitude of this switch on Te-centered electrophilicity, and therefore anion-binding affinity, is shown to correlate with the inherent Lewis acidity of the alkali metal cation. Extensive computational DFT investigations corroborated the experimental alkali metal cation–anion ion-pair binding observations for halides and oxoanions.

Improved alkali metal ion capturing utilizing crown ether-based diblock copolymers in a sandwich-type complexation

The compexation behavior of metals with free crown ethers (CE) and diblock copolymer-based CE is investigated. The latter shows at least 10 000 times stronger complexation than free CEs. On this basis, a highly stable CE complex within the polymer for efficient extraction of metal ions from low concentrations, e.g. lithium in seawater, is presented.

Alkali metal complexes of bis-o-xylyl-(17-crown-5): from a dinuclear monomer and a dinuclear polymer to sandwich polymer

The self-assembly reactions of bis-o-xylyl-(17-crown-5) with alkali metal salts afforded binding mode (good fit, perching and sandwich)-dependent supramolecular complexes, including a sandwich-type cesium(i) coordination polymer.

Femtosecond excited state dynamics of a stilbene–viologen charge transfer complex assembled via host–guest interaction

The ultrafast photoinduced electron transfer in a supramolecular charge transfer complex was studied.

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

The [2 + 2] photocycloaddition is undisputedly the most important and most frequently used photochemical reaction. In this review, it is attempted to cover all recent aspects of [2 + 2] photocycloaddition chemistry with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated separately in sections 2 and 4 , whereas the vast majority of photocycloaddition reactions which occur by direct excitation or sensitization are divided within section 3 into individual subsections according to the photochemically excited olefin.

Direct metal-free synthesis of diarylamines from 2-nitropropane via the twofold C–H functionalization of arenes

Synthesis of symmetric diarylamines via a twofold intermolecular electrophilic C–H functionalization of electron-rich arenes by umpolung-activated nitroalkane in polyphosphoric acid is demonstrated.

Influence of the anion nature on styryl dye crystal packing and feasibility of the direct and back [2 + 2] photocycloaddition reactions without single crystal degradation

By X-ray diffractometry, [2 + 2]-photocycloaddition reaction and its back reaction were investigated.