Anion‐Responsive Diguanidinium‐Based Chiral Organogelators

The chiral bicyclic diguanidinium chloride 1 forms gels in aromatic apolar solvents. The gels were characterized at different levels of organization, from the macroscopic to the molecular level by using microscopy, spectroscopy, and powder X‐ray diffraction. The dependency on chirality has been highlighted by circular dichroism and electron microscopy. Furthermore, the gel has been shown to be effectively responsive to anionic stimuli, thus allowing the reversible control of the organic‐phase gelation in contact with different salted aqueous solutions.     

Anion‐Binding Catalysis by Electron‐Deficient Pyridinium Cations

A new activation principle in organocatalysis is presented: halide binding through Coulombic interactions. This mode of catalysis was realized by using 3,5‐di(carbomethoxy)pyridinium ions that carry an additional electron‐withdrawing substituent on the nitrogen atom, for example, pentafluorobenzyl or cyanomethyl. For the N‐pentafluorobenzyl derivative, Coulombic interaction with the pyridinium moiety is complemented in the solid state by anion–π interactions with the perfluorophenyl ring. Bromide and chloride are bound by these cations in a 1:1 stoichiometry. Catalysis of the C? C coupling between 1‐chloroisochroman (and related electrophiles) with silyl ketene acetals occurs at ?78 °C and at low catalyst loading (2 mol %).     

An Oxidizable Anion‐Excluding Polymer‐Overlayer for Oxygen Electrodes

Here we describe an anion excluding ion‐permeable membrane, which we evaluate on an O₂‐electroreducing cathode poised at a strongly oxidizing potential, near the reversible potential of the O₂/H₂O half cell. The bioelectrocatalyst of the O₂ cathode consists of the cross‐linked electrostatic adduct of a polycationic redox hydrogel and bilirubin oxidase (BOD), a polyanion at neutral at pH 7.3. If an uncured Nafion dispersion is applied on this bioelectrocatalyst, the polyanionic Nafion displaces the BOD in the electrostatic adduct, de‐wiring the BOD. We show here that insertion of a polycationic poly(acrylamide‐co‐vinylimidazole) (PAA‐PVI) between the bioelectrocatalyst and the Nafion prevents the dewiring of BOD. The resulting bi‐layer membrane effectively excludes the urate, thiocyanate and NADH anions.     

Chaotropic‐Anion‐Induced Supramolecular Self‐Assembly of Ionic Polymeric Micelles

Traditional micelle self‐assembly is driven by the association of hydrophobic segments of amphiphilic molecules forming distinctive core–shell nanostructures in water. Here we report a surprising chaotropic‐anion‐induced micellization of cationic ammonium‐containing block copolymers. The resulting micelle nanoparticle consists of a large number of ion pairs (≈60 000) in each hydrophobic core. Unlike chaotropic anions (e.g. ClO₄^?), kosmotropic anions (e.g. SO₄^2?) were not able to induce micelle formation. A positive cooperativity was observed during micellization, for which only a three‐fold increase in ClO₄^? concentration was necessary for micelle formation, similar to our previously reported ultra‐pH‐responsive behavior. This unique ion‐pair‐containing micelle provides a useful model system to study the complex interplay of noncovalent interactions (e.g. electrostatic, van der Waals, and hydrophobic forces) during micelle self‐assembly.     

Anion‐aided Dynamic One‐pot Self‐assembly of Rectangular Metallacycles

The tetracopper(I) complex [{Cu₂(μ‐dppm)₂}₂(μ‐1,4‐O₂CC₆H₄ (CO₂ )₂)](BF₄ )₂ ( 1 (BF₄ )₂) and 1,2‐bis(4‐pyridyl)ethane (bpa) can establish a dynamic equilibrium in CH₂Cl₂ . From the equilibrium mixture containing 1 (BF₄ )₂ and bpa with the molar ratio 1 (BF₄ )₂/bpa of 1:1, a supramolecular compound [{Cu₂(μ‐dppm)₂}₂(μ‐1,4‐C₆H₄ (CO₂ )₂)(μ‐bpa)]₂(BF₄ )₄ ( 2 (BF₄ )₄) was obtained as single crystals. The crystal structure was determined by X‐ray crystallography to reveal presence of one anion inside a cationic rectangular metallacycle { 2 ⊂ BF₄ }³⁺. Both structural evidence and DFT ‐calculated results indicate that the F atoms of the anion exert weak electrostatic attraction with hydrogen atoms of the bound bpa as the framework of the cationic metallacycle. The attractive interactions apparently play an important role in stabilizing some dynamically self‐assembled precursors so as to form the final anion‐included metallacycle. Without the electrostatic help from the anion, the self‐assembly of the empty metallacycle may be hindered by a rather large endothermic free energy. The favorable electrostatic stabilization is present not only for a anion but also for other anions such as , , and even when the flexible bpa is replaced by rigid 4,4′‐bipyridine (bpy). Based on the DFT results, the metallacycle 2 (BF₄ )₄ can be easily prepared in a one‐pot reaction of [Cu(MeCN )₄](BF₄ ) with three ligands.     

Cationic Metallo‐Polyelectrolytes for Robust Alkaline Anion‐Exchange Membranes

Chemically inert, mechanically tough, cationic metallo‐polyelectrolytes were conceptualized and designed as durable anion‐exchange membranes (AEMs). Ring‐opening metathesis polymerization (ROMP) of cobaltocenium‐containing cyclooctene with triazole as the only linker group, followed by backbone hydrogenation, led to a new class of AEMs with a polyethylene‐like framework and alkaline‐stable cobaltocenium cation for ion transport. These AEMs exhibited excellent thermal, chemical and mechanical stability, as well as high ion conductivity.     

Magnetic Multistability in an Anion‐Radical Pimer

Radical pimers are the simplest and most important models for studying charge‐transfer processes and provide deep insight into π‐stacked organic materials. Notably, radical pimer systems with magnetic bi‐ or multistability may have important applications in switchable materials, thermal sensors, and information‐storage media. However, no such systems have been reported. Herein, we describe a new pimer consisting of neutral N‐(n‐propyl) benzene triimide ([BTI‐3C]) and its anionic radical ([BTI‐3C]^?.) that exhibits rare magnetic multistability. The crystalline pimer was readily synthesized by reduction of BTI‐3C with cobaltocene (CoCp₂). The transition occurred with a thermal hysteresis loop that was 27 K wide in the range of 170–220 K, accompanied by a smaller loop with a width of 25 K at 220–242 K. The magnetic multistability was attributed to slippage of the π‐stacked BTI structures and entropy‐driven conformational isomerization of the side propyl chains in the crystalline state during temperature variation.     

Anion‐Induced Shuttling of a Naphthalimide Triazolium Rotaxane

The anion‐templated synthesis of a rotaxane structure, incorporating the new naphthalimide triazolium motif, is described and the interlocked host shown to exhibit selective, uni‐directional, anion‐induced shuttling. Initial pseudorotaxane investigations demonstrate the ability of a naphthalimide triazolium threading component to form interpenetrated assemblies with counter‐anion‐dependent co‐conformations. ¹H NMR studies reveal that the shuttling behaviour of the analogous rotaxane host system is controlled by selective anion binding and by the nature of the solvent conditions. Complete macrocycle translocation only occurs upon the recognition of the smaller halide anions (chloride and bromide). The rotaxane solid‐state crystal structure in the presence of chloride is in agreement with the solution‐phase co‐conformation. The sensitivity of the axle naphthalimide absorbance band to the position of the macrocycle component within the interlocked structure enabled the molecular motion to be observed by UV/Vis spectroscopy, and the chloride‐induced shuttling of the rotaxane was reversed upon silver hexafluorophosphate addition.     

An Ionophore‐Based Anion‐Selective Optode Printed on Cellulose Paper

A general anion‐sensing platform is reported based on a portable and cost‐effective ion‐selective optode and a smartphone detector equipped with a color analysis app. In contrast to traditional anion‐selective optodes using a hydrophobic polymer and/or plasticizer to dissolve hydrophobic sensing elements, the new optode relies on hydrophilic cellulose paper. The anion ionophore and a lipophilic pH indicator are inkjet‐printed and adsorbed on paper and form a “dry” hydrophobic sensing layer. Porous cellulose sheets also allow the sensing site to be modified with dried buffer that prevents any sample pH dependence of the observed color change. A highly selective fluoride optode using an Al^III‐porphyrin ionophore is examined as an initial example of this new anion sensing platform for measurements of fluoride levels in drinking water samples. Apart from Lewis acid–base recognition, hydrogen bonding recognition is also compatible with this sensing platform.     

Chloride‐Anion‐Templated Synthesis of a Strapped‐Porphyrin‐Containing Catenane Host System

The synthesis, structure and anion‐recognition properties of a new strapped‐porphyrin‐containing [2]catenane anion host system are described. The assembly of the catenane is directed by discrete chloride anion templation acting in synergy with secondary aromatic donor–acceptor and coordinative pyridine–zinc interactions. The [2]catenane incorporates a three‐dimensional, hydrogen‐bond‐donating anion‐binding pocket; solid‐state structural analysis of the catenane?chloride complex reveals that the chloride anion is encapsulated within the catenane’s interlocked binding cavity through six convergent CH????Cl and NH???Cl hydrogen‐bonding interactions and solution‐phase ¹H NMR titration experiments demonstrate that this complementary hydrogen‐bonding arrangement facilitates the selective recognition of chloride over larger halide anions in DMSO solution.     

Peptide Stapling with Anion‐π Catalysts

We report design, synthesis and evaluation of a series of naphthalenediimides (NDIs) that are bridged with short peptides. Reminiscent of peptide stapling technologies, the macrocycles are conveniently accessible by a chromogenic nucleophilic aromatic substitution of two bromides in the NDI core with two thiols from cysteine sidechains. The dimension of core‐bridged NDIs matches that of one turn of an α helix. NDI‐stapled peptides exist as two, often separable atropisomers. Introduction of tertiary amine bases in amino‐acid sidechains above the π‐acidic NDI surface affords operational anion‐π catalysts. According to an enolate chemistry benchmark reaction, anion‐π catalysis next to peptides occurs with record chemoselectivity but weak enantioselectivity. Catalytic activity drops with increasing distance of the amine base to the NDI surface, looser homocysteine bridges, mismatched, shortened and elongated α‐helix turns, and acyclic peptide controls. Elongation of isolated turns into short α helices significantly increases activity. This increase is consistent with remote control of anion‐π catalysis from the α‐helix macrodipole.     

Dual Role of the 1,2,3‐Triazolium Ring as a Hydrogen‐Bond Donor and Anion–π Receptor in Anion‐Recognition Processes

Several bis(triazolium)‐based receptors have been synthesized as chemosensors for anion recognition. The central naphthalene core features two aryltriazolium side‐arms. NMR experiments revealed differences between the binding modes of the two triazolium rings: one triazolium ring acts as a hydrogen‐bond donor, the other as an anion–π receptor. Receptors 9²⁺?2BF₄ ^? (C₆H₅), 11²⁺?2BF₄ ^? (4‐NO₂?C₆H₄), and 13²⁺?2BF^4? (ferrocenyl) bind HP₂O₇^3? anions in a mixed‐binding mode that features a combination of hydrogen‐bonding and anion–π interactions and results in strong binding. On the other hand, receptor 10²⁺?2 BF₄ ^? (4‐CH₃O?C₆H₄) only displays combined Csp₂?H/anion–π interactions between the two arms of the receptors and the bound anion rather than triazolium (CH)⁺???anion hydrogen bonding. All receptors undergo a downfield shift of the triazolium protons, as well as the inner naphthalene protons, in the presence of H₂PO₄^? anions. That suggests that only hydrogen‐bonding interactions exist between the binding site and the bound anion, and involve a combination of cationic (triazolium) and neutral (naphthalene) C?H donor interactions. Theoretical calculations relate the electronic structure of the substituent on the aromatic group with the interaction energies and provide a minimum‐energy conformation for all the complexes that explains their measured properties.     

Surface‐Functionalized,Ring‐Opening Metathesis Polymerization‐Derived Monoliths for Anion‐Exchange Chromatography

Ring‐opening metathesis polymerization (ROMP) was used for the synthesis of monolithic capillary columns with inner diameters of 200 µm. The resulting polymeric monoliths were characterized by inverse size‐exclusion chromatography (ISEC). Surface functionalization was carried out in situ using 2‐(N,N‐dimethylaminoethyl)norborn‐5‐ene‐2‐ylcarboxylic amide ( 1 ). The resulting functionalized monoliths were successfully used in anion‐exchange chromatography of oligodeoxynucleotides.