π -Type and σ -type cation- π complexes of atomic cations

MP2/6-31+G* calculations were performed on the cation- complexes of ethylene, cyclobutadiene and benzene with a number of atomic cations. It was found that except B⁺ all the atomic cations form -type cation- complexes with ethylene. On the other hand, with cyclobutadiene Li⁺, N⁺, Na⁺, P⁺ and K⁺ form -type complexes, whereas H⁺, F⁺, and Cl⁺ form covalent -type complexes. With benzene Li⁺, B⁺, Na⁺, Al⁺, and K⁺ form -type complexes whereas H⁺, F⁺, and Cl⁺ form -type complexes. It was concluded that the driving force to form the -type complex is chemical bonding, and that for metal cations to form -type complexes is non-covalent interaction.     

Insight into the cation-π interaction at the metal binding site of the copper metallochaperone CusF

The periplasmic Cu(+)/Ag(+) chaperone CusF features a novel cation-π interaction between a Cu(+)/Ag(+) ion and Trp44 at the metal binding site. The nature and strength of the Cu(+)/Ag(+)-Trp44 interactions were investigated using computational methodologies. Quantum-mechanical (QM) calculations showed that the Cu(+) and Ag(+) interactions with Trp44 are of similar strength (~14 kcal/mol) and bond order. Quantum-mechanical/molecular-mechanical (QM/MM) calculations showed that Cu(+) binds in a distorted tetrahedral coordination environment in the Trp44Met mutant, which lacks the cation-π interaction. Molecular dynamics (MD) simulations of CusF in the apo and Cu(+)-bound states emphasized the importance of the Cu(+)-Trp44 interaction in protecting Cu(+) from water oxidation. The protein structure does not change over the time scale of hundreds of nanoseconds in the metal-bound state. The metal recognition site exhibits small motions in the apo state but remains largely preorganized toward metal binding. Trp44 remains oriented to form the cation-π interaction in the apo state and faces an energetic penalty to move away from the metal ion. Cu(+) binding quenches the protein's internal motions in regions linked to binding CusB, suggesting that protein motions play an essential role in Cu(+) transfer to CusB.     

Two significantly different conformations in crystal: formation of a molecular dimer governed by cation-π interactions

Two significantly different conformations were observed in crystal of 1, which form an unsymmetrical molecular dimer governed by cation-π interactions between a pyridinium cation and a phenyl ring, whereas compound 2 forms a head-to-tail type of dimer.     

Thiacalix[4]arene-alkali metal assemblies: crystal structures and guest-binding capabilities of supramolecular architectures supported by metal coordination and cation-π interactions

To investigate alkali metal complexation with sulfur-linked calixarene analogues and their guest-binding properties for gaseous organic guest molecules, we elucidated a crystal structure of a cesium complex with p-H-thiacalix[4]arene (1·4H) ligands and guest-binding properties of the cesium complex (2) and the previously reported rubidium complex (3). In crystals of the complex 2, a ‘sandwich-like’ binuclear complex was formed by inter-molecular coordination of cesium cations to the thiacalixarene molecules and methanol molecules, mutually interacting by aromatic-H?S hydrogen bonding and alkali metal cation-π interactions between the alkali metal cation and thiacalixarene aromatic rings outside of the cavities. On the guest-binding behaviors both complexes 2 and 3 toward organic guest molecules, methanol, ethanol, and 1-propanol as polar molecules, the complex 2 has no methanol adsorption ability, but the complex 3 showed vapor adsorption properties for all guest molecules. In particular, both complexes exhibited a high adsorption capability toward ethanol molecule. As results of gaseous guest adsorption measurements for alcohol molecules, the guest-binding of these complexes are significantly different because the properties depend heavily on structural natures between complexes 2 and 3.     

The electrochemically controlled sorption of d - metal cations by ion exchangers based on titanium phosphate

A study of the sorption of d-metal ions by titanium phosphate on a platinum electrode shows that the potential of the platinum electrode significantly influences the uptake of cations by the ion-exchanger. Cathodic polarization leads to a significant increase of the sorption whereas anodic polarization is accompanied by desorption. Therefore it is possible to perform cycles of sorption and desorption simply by switching the potential. The results obtained can be explained by the fact that the layer of titanium phosphate acts as a phase which extracts protons or hydroxide ions. The electrochemical generation of hydroxide ions at negative potentials can, especially at less acidic pH, lead to the formation of metal hydroxides in the ion-exchanger phase. This interference is less important at a lower solution pH. Received: 5 March 1997 / Accepted: 3 April 1998     

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.     

Cation···π interactions: QTAIM and NBO studies on the interaction of alkali metal cations with heteroaromatic rings

The cation···π interactions of alkali metal cations (Li⁺, Na⁺, and K⁺) with five-membered heteroaromatic rings [furan(C₄H₄O), thiophene(C₄H₄S), pyrrole(C₄H₅N)] were examined by high level ab initio calculations, to investigate the different roles of C₄H₄O, C₄H₄S, and C₄H₅N as the electron donor, the influential factors that affect these interactions, the nature of this kind of cation···π interaction, and to determine topological and energetical properties to characterize these interactions. The sulfur atom in C₄H₄S plays a certain role in the cation···π interactions except the C–C π bond, which is different from C₄H₄O and C₄H₅N. The size of cation and the character of heteroaromatic ring are two influential factors that affect the cation···π interactions. The studied cation···π interactions can be classified as “closed-shell” and noncovalent interactions. The electron density and its Laplacian at the bond critical points and ring critical points generated upon complexation are useful measurements for the strength of cation···π interactions.     

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-chromatographic separation of metal cations in the presence of α-hydroxyisobutyric acid

Separations of metal cations on a column packed with the strongly acidic cation exchanger Separon SGX CX were investigated in the presence of alpha-hydroxyisobutyric acid (HIBA) in the mobile phase. A retention model based on the general theory of side equilibria was elaborated and relations describing dependences of capacity factors of analytes on the compositon of the mobile phase were derived. Effects of HIBA concentration and pH of the mobile phase on the analyte retention were studied in detail. Stability constants of divalent metal cations (Cd(2+), Co(2+), Mn(2+), Ni(2+) and Zn(2+)) with HIBA were calculated from the experimental dependences of the reciprocal values of capacity factors on the ligand concentration.     

Accurate prediction of cation-π interaction energy using substituent effects

Substituent effects on cation-π interactions have been quantified using a variety of Φ-X···M(+) complexes where Φ, X, and M(+) are the π-system, substituent, and cation, respectively. The cation-π interaction energy, E(M(+)), showed a strong linear correlation with the molecular electrostatic potential (MESP) based measure of the substituent effect, ΔV(min) (the difference between the MESP minimum (V(min)) on the π-region of a substituted system and the corresponding unsubstituted system). This linear relationship is E(M(+)) = C(M(+))(ΔV(min)) + E(M(+))' where C(M(+)) is the reaction constant and E(M(+))' is the cation-π interaction energy of the unsubstituted complex. This relationship is similar to the Hammett equation and its first term yields the substituent contribution of the cation-π interaction energy. Further, a linear correlation between C(M(+))() and E(M(+))()' has been established, which facilitates the prediction of C(M(+)) for unknown cations. Thus, a prediction of E(M(+)) for any Φ-X···M(+) complex is achieved by knowing the values of E(M(+))' and ΔV(min). The generality of the equation is tested for a variety of cations (Li(+), Na(+), K(+), Mg(+), BeCl(+), MgCl(+), CaCl(+), TiCl(3)(+), CrCl(2)(+), NiCl(+), Cu(+), ZnCl(+), NH(4)(+), CH(3)NH(3)(+), N(CH(3))(4)(+), C(NH(2))(3)(+)), substituents (N(CH(3))(2), NH(2), OCH(3), CH(3), OH, H, SCH(3), SH, CCH, F, Cl, COOH, CHO, CF(3), CN, NO(2)), and a large number of π-systems. The tested systems also include multiple substituted π-systems, viz. ethylene, acetylene, hexa-1,3,5-triene, benzene, naphthalene, indole, pyrrole, phenylalanine, tryptophan, tyrosine, azulene, pyrene, [6]-cyclacene, and corannulene and found that E(M)(+) follows the additivity of substituent effects. Further, the substituent effects on cationic sandwich complexes of the type C(6)H(6)···M(+)···C(6)H(5)X have been assessed and found that E(M(+)) can be predicted with 97.7% accuracy using the values of E(M(+))' and ΔV(min). All the Φ-X···M(+) systems showed good agreement between the calculated and predicted E(M(+))() values, suggesting that the ΔV(min) approach to substituent effect is accurate and useful for predicting the interactive behavior of substituted π-systems with cations.     

Geometric effects in olefinic cation-π interactions with alkali metals: a computational study

Although cation-π interactions commonly involve aromatic or heteroaromatic rings as the source of π-electrons, isolated and nonconjugated olefins are equally effective donors of π-electron density. Previous comparisons of these π-electron sources have indicated that the net energy of the binding interactions is not a simple additive function of the number of π-bonds involved. For instance, the enthalpy of binding (ΔH°) of Li(+), Na(+), or K(+) cations to two ethylene molecules or to one benzene molecule is approximately the same, despite the 4:6 ratio of π-electrons involved. This present density functional theory study indicates that geometric factors can partially account for the proportionally greater interaction energies of olefins, but whether they are symmetrically placed around the cation or grouped on one hemisphere has little effect on the binding energy. Instead, flexible ligands that permit olefinic π-electrons to be oriented more favorably toward the metal than those in rigid aromatic rings can be correlated with greater bonding. For Li(+) complexes, this appears to be an appreciable factor, although it is less significant with Na(+) and K(+) complexes. For all three cations, stronger polarization interactions with olefins compared to arenes contribute to the strength of cation-π interactions involving olefinic π-bonds.     

Enhanced catalytic activity on titanosilicate molecular sieves controlled by cation-π interactions

A new class of heterogeneous catalytic systems utilizing cation-guest interactions was designed based on microporous titanosilicate molecular sieves. Introducing heavier alkali metal cations on ion-exchange sites of the framework resulted in a significant enhancement of the catalytic activity for oxidation of cyclohexene and styrene, whereas such an enhancement was not observed in oxidation of cyclohexane without π systems. Distinct relationships between the catalytic activities and intermolecular interaction energies which were determined by IR spectroscopic and computational approaches clearly evidenced the predominance of the cation-π interaction in this catalytic system.     

Plasmonics for the study of metal ion–protein interactions

The study of metal–protein interactions is an expanding field of research investigated by bioinorganic chemists as it has wide applications in biological systems. Very recently, it has been reported that it is possible to study metal–protein interactions by immobilizing biomolecules on metal surfaces and applying experimental approaches based on plasmonics which have usually been used to investigate protein–protein interactions. This is possible because the electronic structure of metals generates plasmons whose properties can be exploited to obtain information from biomolecules that interact not only with other molecules but also with ions in solution. One major challenge of such approaches is to immobilize the protein to be studied on a metal surface with preserved native structure. This review reports and discusses all the works that deal with such an expanding new field of application of plasmonics with specific attention to surface plasmon resonance, highlighting the advantages and drawbacks of such approaches in comparison with other experimental techniques traditionally used to study metal–protein interactions.

Effects of metal–ligand coordination on the self-assembly behaviour of a crown ether functionalised perylenetetracarboxylic diimide

A novel perylenetetracarboxylic diimide (PDI) derivative, N,N′-di(4′-benzo-15-crown-5-ether)-1,7-di(4-tert-butyl-phenoxy)perylene-3,4;9,10-tetracarboxylic diimide (CRPDI), has been synthesised and characterised. Dimerisation of CRPDI is induced by the presence of K⁺ in CHCl₃ or spontaneously occurs in methanol, as revealed by absorption and emission spectroscopy. In particular, the formation of co-facial dimer in the presence of K⁺ proceeds in a three-stage process, as indicated by absorption spectroscopy. The belt- and rope-like nanostructures of CRPDI fabricated from methanol and CHCl₃ solution in the presence of K⁺ are obtained by scanning electron microscopy. Furthermore, the conductivity of the rope-like nanostructures from the cation-induced dimeric species is more than ca. 1 order of magnitude higher than the belt-like nanostructures from the solvent-induced dimeric species. The present result represents the further effort towards realisation of controlling and tuning the morphology of self-assembled nanostructures of PDI derivatives through molecular design and synthesis. It will be valuable for the design and preparation of PDI-based nano-(opto)electronic devices with good performance due to the close relationship between the molecular ordering and dimensions of nanostructures and the performance of nanodevices.     

Empirical formulation and parameterization of cation-π interactions for protein modeling

Cation-π interaction is comparable and as important as other main molecular interaction types, such as hydrogen bond, electrostatic interaction, van der Waals interaction, and hydrophobic interaction. Cation-π interactions frequently occur in protein structures, because six (Phe, Tyr, Trp, Arg, Lys, and His) of 20 natural amino acids and all metallic cations could be involved in cation-π interaction. Cation-π interactions arise from complex physicochemical nature and possess unique interaction behaviors, which cannot be modeled and evaluated by existing empirical equations and force field parameters that are widely used in the molecular dynamics. In this study, the authors present an empirical approach for cation-π interaction energy calculations in protein interactions. The accurate cation-π interaction energies of aromatic amino acids (Phe, Tyr, and Try) with protonated amino acids (Arg and Lys) and metallic cations (Li(+), Na(+), K(+), and Ca(2+)) are calculated using B3LYP/6-311+G(d,p) method as the benchmark for the empirical formulization and parameterization. Then, the empirical equations are built and the parameters are optimized based on the benchmark calculations. The cation-π interactions are distance and orientation dependent. Correspondingly, the empirical equations of cation-π interactions are functions of two variables, the distance r and the orientation angle θ. Two types of empirical equations of cation-π interactions are proposed. One is a modified distance and orientation dependent Lennard-Jones equation. The second is a polynomial function of two variables r and θ. The amino acid-based empirical equations and parameters provide simple and useful tools for evaluations of cation-π interaction energies in protein interactions.     

A computational study of cation-π interactions in polycyclic systems: exploring the dependence on the curvature and electronic factors

Density functional theory (B3LYP) calculations with double and triple-ζ quality basis sets were performed on the Li⁺ and Na⁺ π-complexes of corannulene 2, sumanene 3CH₂, heterosumanenes 3X, triphenylene 4 and heterotrindenes 5X. The metal ions bind to both convex and concave faces of buckybowls, with a consistent preference to bind to the convex surface by about 1-4 kcal/mol. The metal ion complexation with the π-framework of the central six-membered ring span wider range compared to benzene, indicating the control of size, curvature and electronic perturbations over the strength of cation-π interactions. Computations show that the bowl-to-bowl inversion barriers are only slightly altered upon metal complexation, indicating the continuity of bowl-to-bowl inversion despite metal complexation. We have calculated the binding energies of model systems, triphenylene (4) and heterotrindenes (5X), which indicate that the interaction energies are controlled by electronic factors. While the inversion barrier is dependent mainly on the size of the heteroatom, the extent of binding is independent of the size of the atom or the bowl depth.     

Use of Butler–Volmer treatment to assess the capability of the voltammetric ion sensors: Application to a PPy/DBS film for cations detection

In this work, the Butler–Volmer formalism has been used to obtain a new equation to assess the calibration of voltammetric ion sensors. This new method postulates a direct relationship between the mid point reversible potential, E_R, and the logarithm of the electrolyte concentration. In the same way as the other relationships based on the Nernst equation, a positive slope is expected for a cationic exchanging system, and a negative one for an anionic sensor. However, the theoretical slope proposed in this work includes the electron-transfer coefficient, which allows us to explain the slopes frequently reported in the literature non coincident with the ideal value estimated from the Nernst equation: 2.303RT/nF. Also, comments are made on results from other literature with respect to the new equation, and an application of this method to PPy/DBS films as a cation sensor is carried out.     

4 + 4] photodimerization of azaanthracenes in both solution and solid phase controlled by cation-π interactions

Regio- and stereoselective [4 + 4] photodimerization reactions of 1- and 2-azaanthracenes were performed in both methanol solution and solid phases to give anti-HT dimers in high yields. In these reactions, intermolecular cation-π interactions between the pyridinium cation and the benzene ring play a key role in preorientation prior to the photodimerization reactions.