Self‐Assembly and Disassembly of Vesicles as Controlled by Anion–π Interactions

Anion–π interactions have been widely studied as new noncovalent driving forces in supramolecular chemistry. However, self‐assembly induced by anion–π interactions is still largely unexplored. Herein we report the formation of supramolecular amphiphiles through anion–π interactions, and the subsequent formation of self‐assembled vesicles in water. With the π receptor 1 as the host and anionic amphiphiles, such as sodium dodecylsulfate (SDS), sodium laurate (SLA), and sodium methyl dodecylphosphonate (SDP), as guests, the sequential formation of host–guest supramolecular amphiphiles and self‐assembled vesicles was demonstrated by SEM, TEM, DLS, and XRD techniques. The intrinsic anion–π interactions between 1 and the anionic amphiphiles were confirmed by crystal diffraction, HRMS analysis, and DFT calculations. Furthermore, the controlled disassembly of the vesicles was promoted by competing anions, such as NO₃^?, Cl^?, and Br^?, or by changing the pH value of the medium.     

The Role of the Ethynyl Substituent on the π–π Stacking Affinity of Benzene: A Theoretical Study

Herein, we report a high‐level theoretical study (SCS‐RI‐MP2(full)/aug‐cc‐pVTZ) examining the stacking affinity of 1,3,5‐triethynylbenzene. The stacking properties of this compound are compared to those of benzene and 1,3,5‐trifluorobenzene. The results indicate that the ethynyl substituent improves the stacking affinity of the arene, since the binding energies for the stacked ethynyl‐substituted arene dimers are higher than those of both benzene and the fluoro‐substituted arene. This interesting behaviour has been studied by examining the energetics, geometries and electron charge density features of the complexes. A query in the Cambridge Structural Database returned several X‐ray crystal structures containing π–π stacking interactions of 1,3,5‐triethynylaryls that strongly agree with the theoretical results.     

Solution and Solid‐State Studies on the Halide Binding Affinity of Perfluorophenyl‐Armed Uranyl–Salophen Receptors Enhanced by Anion–π Interactions

The enhancement of the binding between halide anions and a Lewis acidic uranyl–salophen receptor has been achieved by the introduction of pendant electron‐deficient arene units into the receptor skeleton. The association and the occurrence of the elusive anion–π interaction with halide anions (as tetrabutylammonium salts) have been demonstrated in solution and in the solid state, providing unambiguous evidence on the interplay of the concerted interactions responsible for the anion binding.     

Anion–π and Cation–π Interactions on the Same Surface

Herein, we address the question whether anion–π and cation–π interactions can take place simultaneously on the same aromatic surface. Covalently positioned carboxylate–guanidinium pairs on the surface of 4‐amino‐1,8‐naphthalimides are used as an example to explore push–pull chromophores as privileged platforms for such “ion pair–π” interactions. In antiparallel orientation with respect to the push–pull dipole, a bathochromic effect is observed. A red shift of 41 nm found in the least polar solvent is in good agreement with the 70 nm expected from theoretical calculations of ground and excited states. Decreasing shifts with solvent polarity, protonation, aggregation, and parallel carboxylate–guanidinium pairs imply that the intramolecular Stark effect from antiparallel ion pair–π interactions exceeds solvatochromic effects by far. Theoretical studies indicate that carboxylate–guanidinium pairs can also interact with the surfaces of π‐acidic naphthalenediimides and π‐basic pyrenes.     

Dramatic Enhancement of Binding Affinities Between Foldamer‐Based Receptors and Anions by Intra‐Receptor π‐Stacking

As a synthetic model for intra‐protein interactions that reinforce binding affinities between proteins and ligands, the energetic interplay of binding and folding was investigated using foldamer‐based receptors capable of adopting helical structures. The receptors were designed to have identical hydrogen‐bonding sites for anion binding but different aryl appendages that simply provide additional π‐stacking within the helical backbones without direct interactions with the bound anions. In particular, the presence of electron‐deficient aryl appendages led to dramatic enhancements in the association constant between the receptor and chloride or nitrate ions, by up to three orders of magnitude. Extended stacking within the receptor contributes to the stabilization of the entire folding structure of complexes, thereby enhancing binding affinities.     

Self‐Assembled Tetragonal Prismatic Molecular Cage Highly Selective for Anionic π Guests

The metal‐directed supramolecular synthetic approach has paved the way for the development of functional nanosized molecules. In this work, we report the preparation of the new nanocapsule 3? (CF₃SO₃)₈ with a A₄B₂ tetragonal prismatic geometry, where A corresponds to the dipalladium hexaazamacrocyclic complex Pd‐1 , and B corresponds to the tetraanionic form of palladium 5,10,15,20‐tetrakis(4‐carboxyphenyl)porphyrin ( 2 ). The large void space of the inner cavity and the supramolecular affinity for guest molecules towards porphyrin‐based hosts converts this nanoscale molecular 3D structure into a good candidate for host–guest chemistry. The interaction between this nanocage and different guest molecules has been studied by means of NMR, UV/Vis, ESI‐MS, and DOSY experiments, from which highly selective molecular recognition has been found for anionic, planar‐shaped π guests with association constants (K_a) higher than 10⁹ M ^?1, in front of non‐interacting aromatic neutral or cationic substrates. DFT theoretical calculations provided insights to further understand this strong interaction. Nanocage 3? (CF₃SO₃)₈ can not only strongly host one single molecule of M(dithiolene)₂ complexes (M=Au, Pt, Pd, and Ni), but also can finely tune their optical and redox properties. The very simple synthesis of both the supramolecular cage and the building blocks represents a step forward for the development of polyfunctional supramolecular nanovessels, which offer multiple applications as sensors or nanoreactors.     

Selective Co‐Encapsulation Inside an M6L4 Cage

There is broad interest in molecular encapsulation as such systems can be utilized to stabilize guests, facilitate reactions inside a cavity, or give rise to energy‐transfer processes in a confined space. Detailed understanding of encapsulation events is required to facilitate functional molecular encapsulation. In this contribution, it is demonstrated that Ir and Rh‐Cp‐type metal complexes can be encapsulated inside a self‐assembled M₆L₄ metallocage only in the presence of an aromatic compound as a second guest. The individual guests are not encapsulated, suggesting that only the pair of guests can fill the void of the cage. Hence, selective co‐encapsulation is observed. This principle is demonstrated by co‐encapsulation of a variety of combinations of metal complexes and aromatic guests, leading to several ternary complexes. These experiments demonstrate that the efficiency of formation of the ternary complexes depends on the individual components. Moreover, selective exchange of the components is possible, leading to formation of the most favorable complex. Besides the obvious size effect, a charge‐transfer interaction may also contribute to this effect. Charge‐transfer bands are clearly observed by UV/Vis spectrophotometry. A change in the oxidation potential of the encapsulated electron donor also leads to a shift in the charge‐transfer energy bands. As expected, metal complexes with a higher oxidation potential give rise to a higher charge‐transfer energy and a larger hypsochromic shift in the UV/Vis spectrum. These subtle energy differences may potentially be used to control the binding and reactivity of the complexes bound in a confined space.     

Interpenetrated Cage Structures

This Review covers design strategies, synthetic challenges, host–guest chemistry, and functional properties of interlocked supramolecular cages. Some dynamic covalent organic structures are discussed, as are selected examples of interpenetration in metal–organic frameworks, but the main focus is on discrete coordination architectures, that is, metal‐mediated dimers. Factors leading to interpenetration, such as geometry, flexibility and chemical makeup of the ligands, coordination environment, solvent effects, and selection of suitable counter anions and guest molecules, are discussed. In particular, banana‐shaped bis‐pyridyl ligands together with square‐planar metal cations have proven to be suitable building blocks for the construction of interpenetrated double‐cages obeying the formula [M₄L₈]. The peculiar topology of these double‐cages results in a linear arrangement of three mechanically coupled pockets. This allows for the implementation of interesting guest encapsulation effects such as allosteric binding and template‐controlled selectivity. In stimuli‐responsive systems, anionic triggers can toggle the binding of neutral guests or even induce complete structural conversions. The increasing structural and functional complexity in this class of self‐assembled hosts promises the construction of intelligent receptors, novel catalytic systems, and functional materials.     

The Origin of the Room‐Temperature Stability of [TTF].+⋅⋅⋅[TTF].+ Long,Multicenter Bonds Found in Functionalized π‐[R‐TTF]22+ Dimers Included in the Cucurbit[8]uril Cavity

A computational study is performed to identify the origin of the room‐temperature stability, in aqueous solution, of functionalized π‐[R‐TTF]₂²⁺ dimers (TTF=tetrathiafulvalene; R=(CH₂OCH₂)₅CH₂OH) included in the cavity of a cucurbit[8]uril (CB[8]) molecule. π‐[R‐TTF]₂²⁺ dimers in pure water are weakly stable, and are mostly dissociated at room temperature. Upon addition of CB[8] to an aqueous π‐[R‐TTF]₂²⁺ solution, a (π‐[R‐TTF]₂?CB[8])²⁺ inclusion complex is formed. The same complex is obtained after the sequential inclusion of two [R‐TTF]^.+ monomers in the CB[8] molecule. Both processes are thermodynamically and kinetically allowed. π‐[R‐TTF]₂²⁺ dimers dissolved in pure water present a [TTF]^.+???[TTF]^.+ long, multicenter bond, similar to that already identified in π‐[TTF]₂²⁺ dimers dissolved in organic solvents. Upon their inclusion in CB[8], the strength and other features of the [TTF]^.+???[TTF]^.+ long, multicenter bond are preserved. The room temperature stability of the π‐[R‐TTF]₂²⁺ dimers included in CB[8] is shown to originate in the π‐[R‐TTF]₂²⁺???CB[8] interaction, the strength of which comes from a strongly attractive electrostatic component and a dispersion component. Such a dominant electrostatic term is caused by the strongly polarized charge distribution in CB[8], the geometrical complementarity of the π‐[R‐TTF]₂²⁺ and CB[8] geometries, and the amplifying effect of the 2+ charge in π‐[R‐TTF]₂²⁺.     

Water Structure Recovery in Chaotropic Anion Recognition: High‐Affinity Binding of Dodecaborate Clusters to γ‐Cyclodextrin

Dodecaborate anions of the type B₁₂X₁₂^2? and B₁₂X₁₁Y^2? (X=H, Cl, Br, I and Y=OH, SH, NH₃⁺, NR₃⁺) form strong (K_a up to 10⁶ L mol^?1, for B₁₂Br₁₂^2?) inclusion complexes with γ‐cyclodextrin (γ‐CD). The micromolar affinities reached are the highest known for this native CD. The complexation exhibits highly negative enthalpies (up to ?25 kcal mol^?1) and entropies (TΔS up to ?18.4 kcal mol^?1, both for B₁₂I₁₂^2?), which position these guests at the bottom end of the well‐known enthalpy‐entropy correlation for CDs. The high driving force can be traced back to a chaotropic effect, according to which chaotropic anions have an intrinsic affinity to hydrophobic cavities in aqueous solution. In line with this argument, salting‐in effects revealed dodecaborates as superchaotropic dianions.     

Compressed Corannulene in a Molecular Cage

Self‐assembled coordination cages can be employed as a molecular press, where the bowl‐shaped guest corannulene (C₂₀H₁₀) is significantly flattened upon inclusion within the hydrophobic cavity. This is demonstrated by the pairwise inclusion of corannulene with naphthalene diimide as well as by the dimer inclusion of bromocorannulene inside the box‐like host. The compressed corannulene structures are unambiguously revealed by single‐crystal X‐ray analysis.     

Orbital Effect‐Induced Anomalous Anion–π Interactions between Electron‐Rich Aromatic Hydrocarbons and Fluoride

Anion–π interactions generally exist between an anion and an electron‐deficient π‐ring because of the electron‐accepting character of the ring. In this paper, we report orbital effect‐induced anomalous binding between electron‐rich π systems and F^? through anion–π interactions calculated at the MP2/6‐31+G(d,p) and ωB97X‐D/6‐31+G(d,p) levels of theory. We find that anion–π interactions between F^? and electron‐rich π rings increase markedly with increasing number of π electrons and size of the π rings. This is contrary to intuition because anion–π interactions would be expected to gradually decrease because of gradually increasing Coulombic repulsion between the negative charge of the anions and gradually increasing number of π electrons of the aromatic surfaces. Energy decomposition analysis showed that the key to this anomalous effect is the more effective delocalization of negative charge to the unoccupied π* orbitals of larger π rings, which is termed an “orbital effect”.