Putting anion-π interactions into perspective

Supramolecular chemistry is a field of scientific exploration that probes the relationship between molecular structure and function. It is the chemistry of the noncovalent bond, which forms the basis of highly specific recognition, transport, and regulation events that actuate biological processes. The classic design principles of supramolecular chemistry include strong, directional interactions like hydrogen bonding, halogen bonding, and cation-π complexation, as well as less directional forces like ion pairing, π-π, solvophobic, and van der Waals potentials. In recent years, the anion-π interaction (an attractive force between an electron-deficient aromatic π system and an anion) has been recognized as a hitherto unexplored noncovalent bond, the nature of which has been interpreted through both experimental and theoretical investigations. The design of selective anion receptors and channels based on this interaction represent important advances in the field of supramolecular chemistry. The objectives of this Review are 1) to discuss current thinking on the nature of this interaction, 2) to survey key experimental work in which anion-π bonding is demonstrated, and 3) to provide insights into the directional nature of anion-π contact in X-ray crystal structures.     

Combined cation-π and anion-π interactions for zwitterion recognition

Brothers and enemies: Anion-π and cation-π interactions act in a synergistic way when gathered in the molecular cavity of a hemicryptophane host, affording an efficient contribution (-170?kJ?mol(-1)) in zwitterion recognition. NMR titration experiments and calculations reveal the positioning of the guest in the cavity of the heteroditopic receptor. This study emphasizes the importance of anion-π bonds in host-guest chemistry.     

CH-anion versus anion-π interactions in the crystal and in solution of pentafluorobenzyl phosphonium salts

A series of phosphonium salts with pentafluorobenzyl substituents have been synthesized and were investigated in the crystal as well as in solution. The solid state structures of 1a, 1b and 2d reveal the presence of anion-π as well as CH-anion interactions. The two attractive, yet competitive forces seem to act in concert and a directing effect of the CH interaction on the relative position between anion and π-system is observed. The search for anion-π interactions in solution failed. Only CH-anion interactions proved to be important in solution.     

Al4(2-); the anion-π interactions and aromaticity in the presence of counter ions

The influence of presence of counter ions and π-complexation with benzene on the bonding and magnetic properties of Al(4)(2-), the most studied all-metal cluster, is studied here. It is shown that complexation by either counter ions or benzene decreases the delocalization index between Al atoms and the magnitude of bond magnetizability, that is a Quantum Theory of Atoms in Molecules, QTAIM, -based magnetic index of aromaticity. Benzene forms two types of π-complexes with the Al(4) framework; CH-π (T-shaped) complexes and parallel π-π stacking (PPS) complexes. It is shown that variation in the π-charge of the Al(4) framework affects the relative stability of the T-shaped/PPS complexes. Free Al(4)(2-) forms a stable T-shaped anion-π complex with benzene but in the presence of cations, formation of PPS complexes is more favourable, energetically. It is suggested that this property could be used for designing molecular switches and tuneable anion sensors.     

The strength and selectivity of perfluorinated nano-hoops and buckybowls for anion binding and the nature of anion-π interactions

Perfluorinated cycloparaphenylenes (F-[n]CPP, n = 5–8), boron nitride nanohoop (F-[5]BNNH), and buckybowls (F-BBs) were proposed as anion receptors via anion-π interactions with halide anions (Cl⁻, Br⁻ and I⁻), and remarkable binding strengths up to −294.8 kJ/mol were computationally verified. The energy decomposition approach based on the block-localized wavefunction method, which combines the computational efficiency of molecular orbital theory and the chemical intuition of ab initio valence bond theory, was applied to the above anion-π complexes, in order to elucidate the nature and selectivity of these interactions. The overall attraction is mainly governed by the frozen energy component, in which the electrostatic interaction is included. Remarkable binding strengths with F-[n]CPPs can be attributed to the accumulated anion-π interactions between the anion and each conjugated ring on the hoop, while for F-BBs, additional stability results from the curved frameworks, which distribute electron densities unequally on π-faces. Interestingly, the strongest host was proved to be the F-[5]BNNH, which exhibits the most significant anisotropy of the electrostatic potential surface due to the difference in the electronegativities of nitrogen and boron. The selectivity of each host for anions was explored and the importance of the often-overlooked Pauli exchange repulsion was illustrated. Chloride anion turns out to be the most favorable anion for all receptors, due to the smallest ionic radius and the weakest destabilizing Pauli exchange repulsion.     

Energetic effects between halogen bonds and anion-π or lone pair-π interactions: a theoretical study

Energetic effects between halogen bonds and anion-π or lone pair-π interactions have been investigated by means of ab initio MP2 calculations. 1,4-diiodo-perfluorobenzene, a very effective building block for crystal engineering based on halogen bonding, is selected in this work both as electron-deficient π aromatic ring and as halogen bond donor. Additive and diminutive effects are observed when halogen bonds and anion-π/lone pair-π interactions coexist in the same complex, which can be ascribed to the same direction of charge transfer for the two interactions. These effects have been analyzed in detail by the structural, energetic, and AIM properties of the complexes. Finally, experimental evidence of the combination of the interactions has been obtained from the Cambridge Structural Database.     

From attraction to repulsion: anion-π interactions between bromide and fluorinated phenyl groups

Anion-π interactions in crystals of fluorobenzyl ammonium salts depend on the degree of fluorination at the aromatics.     

Multiple fluorescence color transitions mediated by anion-π interactions and C-F covalent bond formation

The emission changes of fluorescent dyes under the influence of environmental changes or interaction with analytes are the basis for designing ratiometric fluorescent probes and logic gates. However, it is rare that only one external stimulus induces continuous fluorescent color changes in a fluorescent dye. In this paper, we report a cage-like molecule formed by two benzene rings and three imidazolium salts which produces continuous fluorescence wavelength changes when interacted with fluoride ions. Fluoride ions are first bound to the center of the cage under the action of anion-π interaction, and the (C?H)⁺···F^– type ionic hydrogen bonds induce the blue-shift of fluorescence. The subsequent formation of C-F covalent bonds with fluoride ions makes the fluorophore wavelength continue to blue-shift, and finally obtains continuous multiple fluorescence changes caused by a single external stimulus. According to the fluorescence wavelength and intensity, six different fluorescence signal channels can be obtained, which can be encoded as six numbers from 0 to 5. We expect that this reaction process can find applications in quantitative anion recognition and molecular counters.     

Unexpected nonadditivity effects in anion-π complexes

Several complexes of fluorine-substituted ethyne, ethene, butadiene, benzene, and [n]radialenes (n = 3-5) with two anions have been optimized at the RI-MP2/aug-cc-pVTZ level of theory. The additivity of the anion-π interaction was studied depending on the number of double bonds and fluorine atoms. Interesting nonadditivity effects were observed in the aromatic and antiaromatic complexes, which were analyzed by partitioning the total interaction energy into individual components, using Bader's theory of "atoms in molecules" and changes in the aromatic character of the ring upon complexation.     

Cooperativity of H-bonding and anion-π interaction in the binding of anions with neutral π-acceptors

A rare anion-π complex between bromide and a neutral receptor is reported and related receptor systems are studied with a series of anions. The interaction is observed in the solid state and in solution, and further evidence for it is obtained by a computational study.     

Fluoride ion sensing by an anion-π interaction

We report the discovery of a supramolecular interaction (anion-π and charge/electron transfer, CT/ET) involving fluoride ion and π-electron deficient colorless naphthalene diimide (NDI) receptors. Strong electronic interactions between lone-pair electrons of F(-) ion and π*-orbitals of the NDI unit lead to an unprecedented F(-)→NDI ET event, which produces an orange colored NDI(?-) radical anion. Further reduction of NDI(?-) by another F(-) ion produces a pink colored NDI(2-) dianion, rendering NDI a colorimetric F(-) sensor. Preorganization of two NDI units in overlapping positions using folded linkers improves their selectivity and sensitivity for the F(-) ion significantly, allowing F(-) detection at nM concentration in 85:15 DMSO/H(2)O solutions.     

Ion pair receptors based on anion-π interaction

A novel type of ditopic ion pair receptors based on anion-π interaction is reported. Oxacalix[2]arene[2]triazine azacrowns were synthesized efficiently from a one-pot reaction between 2,4-dichloro-4-methoxytriazine and 3,5-dihydroxybenzaldehyde followed by condensation with a diamine and reduction of bisimine; they acted as selective ditopic receptors to recognize ion pairs.     

Contribution of cation-π interactions in iminium catalysis

Ab initio calculations were carried out for a benzyl-substituted iminium cation derived from (E)-crotonaldehyde and a chiral imidazolidinone that was developed as an organocatalyst by MacMillan et al. At the MP2 level of theory it is predicted that the phenyl group is close to the iminium moiety in the most stable conformer, suggesting that the cation-π interaction contributes to the stabilization of this conformer. Energy decomposition analyses on model systems indicate that the electrostatic and polarization terms make significant contribution to the attractive interactions between the benzene ring and the iminium cation.     

Relative substituent position on the strength of π-π stacking interactions

It was observed that the relative position of the arene substituents has a profound influence on the strength of π-π stacking in the 9-benzyl-substituted triptycene system. A new series of model compounds (3a-i) capable of revealing quantitatively π-π stacking interactions was studied. This series of compounds (3a-i) has an ortho-substituted methyl group in one of the two interacting arenes and the syn/anti ratios were determined and compared to a series previously studied compounds (4a-i) that have a para methyl group on the corresponding arene. A greater than 50% increase in the strength of π-π stacking interactions was observed with the methyl group in the ortho position comparing to that in the para position. No difference in π-π stacking interactions was observed when the other aromatic ring was a pentafluorobenzoate group.     

Induction-driven stabilization of the anion-π interaction in electron-rich aromatics as the key to fluoride inclusion in imidazolium-cage receptors

Intermolecular interactions that involve aromatic rings are key processes in both chemical and biological recognition. It is common knowledge that the existence of anion-π interactions between anions and electron-deficient (π-acidic) aromatics indicates that electron-rich (π-basic) aromatics are expected to be repulsive to anions due to their electron-donating character. Here we report the first concrete theoretical and experimental evidence of the anion-π interaction between electron-rich alkylbenzene rings and a fluoride ion in CH(3)CN. The cyclophane cavity bridged with three naphthoimidazolium groups selectively complexes a fluoride ion by means of a combination of anion-π interactions and (C-H)(+)···F(-)-type ionic hydrogen bonds. (1)H NMR, (19)F NMR, and fluorescence spectra of 1 and 2 with fluoride ions are examined to show that only 2 can host a fluoride ion in the cavity between two alkylbenzene rings to form a sandwich complex. In addition, the cage compounds can serve as highly selective and ratiometric fluorescent sensors for a fluoride ion. With the addition of 1 equiv of F(-), a strongly increased fluorescence emission centered at 385 nm appears at the expense of the fluorescence emission of 2 centered at 474 nm. Finally, isothermal titration calorimetry (ITC) experiments were performed to obtain the binding constants of the compounds 1 and 2 with F(-) as well as Gibbs free energy. The 2-F(-) complex is more stable than the 1-F(-) complex by 1.87 kcal mol(-1), which is attributable to the stronger anion-π interaction between F(-) and triethylbenzene.     

Associative combination of lone pair-π, π-π and anion-π interactions observed in a ternary system comprising Mg(II)-malonate-2-aminopyridine-hexafluoridophosphate

One Mg(II) malonate complex with protonated 2-aminopyridine and hexafluoridophosphate as counterions, (C₅H₇N₂)₄[Mg(C₃H₂O₄)₂(H₂O)₂](PF₆)₂ (1) [C₅H₇N₂ = protonated 2-aminopyridine, C₃H₄O₄ = malonic acid] has been synthesized from purely aqueous media just by mixing the reactants in their stoichiometric proportion and its crystal structure has been determined by single-crystal X-ray diffraction. The role of weak forces like lone pair?π and anion?π interactions in influencing the self-assembly process appears to be of importance. A rare combination of lone pair?π and anion?π interactions in 1, of the type lone pair?π/π?π/π?anion, is observed, and this unusual supramolecular network is fully described here.     

π-π interactions in the self-assembly of melamine and barbituric acid derivatives

Self-assembly of a pair of complementary molecular components, 5-(4-dodecyloxyben-zylidene)-(1H,3H)-2,4,6-pyrimidinetrione (PB12) and 4-amino-2,6-didodecylamino-1, 3, 5-triazine (M12) was studied by cyclic voltammogram, surface photovoltage spectroscopy, fluorescence spectroscopy, FTIR and X-ray diffraction. It is found that after mixing equimolar amount of PB12 and M12 at room temperature, not only triply complementary hydrogen bonds are formed between PB12 and M12 but also further self-assembly of the supermolecules based on network of hydrogen bonds occurs via π-π interactions. During the self-assembly of the supermolecules, π-π interactions are induced by delocalized interactions between the HOMO of M12 and the LUMO of PB12, resulting in the formation of a supramolecular nanotube with a layered structure bearing a d value of 0.41 nm and PB12 and M12are arranged alternatively between adjacent supermolecules.     

π-π interactions in the self-assembly of melamine and barbituric acid derivatives

Self-assembly of a pair of complementary molecular components, 5-(4-dodecyloxyben-zylidene)-(1H,3H)-2, 4,6-pyrimidinetrione (PB₁₂)and 4-amino-2,6-didodecylamino-1, 3, 5-triazine (M₁₂) was studied by cyclic voltammogram, surface photovoltage spectroscopy, fluorescence spectroscopy, FTIR and X-ray diffraction. It is found that after mixing equimolar amount of PB₁₂ and M₁₂ at room temperature, not only triply complementary hydrogen bonds are formed between PB₁₂and M₁₂ but also further self-assembly of the supermolecules based on network of hydrogen bonds occurs via π-π interactions. During the self-assembly of the supermolecules, π -π interactions are induced by delocalized interactions between the HOMO of M₁₂ and the LUMO of PB₁₂, resulting in the formation of a supramolecular nanotube with a layered structure bearing a d value of 0.41 nm and PB₁₂ and M₁₂are arranged alternatively between adjacent supermolecules.