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.     

Hg2＋-Selective Fluorescent Chemosensor Based on Cation-π Interaction

Two sulphur-containing 4-aminonaphthalimide derivatives were investigated as Hg2＋ fluorescent chemosensors. In CH3CN, both sensors present a remarkable fluorescence enhancement to Cu2＋ and Fe3＋, but a selective fluorescence quenching to Hg2＋ among the other metal ions. A cation-π interaction between Hg〉 and the naphthalimide moiety was proposed and confirmed By the density tunetional theory（DFT）.     

Self-assembly of Bis[2-(2-hydroxyphenyl)-pyridine]Copper(Ⅱ) Induced by C-H…π and π…π Stacking Interaction

Introduction The control of molecular assembly in the solid state is an important theme of modern chemistry.It is in this regard that there is an activity in the area of supramolecular structures at present.The self-assembly of molecules can form well-defined supramolecular structures under the influence of drive forces such as hydrogen bonds[1-3],metal-ligand coordination bonds[4-6] and π…π stacking interactions[7-10].Word et al.have described the co-ordination chemistry of polydentate chelating ligands which contain mixed pyridine-phenol donor sets[11].Some unusual structures of transition metal pyridine-phenol complexes have been established in which non-covalent interactions such as hydrogen bonding and π…π stacking appear to play a dominant part.These observations suggest that it might be possible to construct supramolecular structures with a metal pyridine-phenol system.To explore this idea we have begun to investigate the self-assembly properties of metal pyridine-phenol complexes.Herein we present the self-assembly properties of Cu(pp)2[pp=2-(2-hydroxyphenol)-pyridine] under different conditions.     

Crystalline 2π Aromatic Azadiboriridinylium: A BN Analogue of Cyclopropenylium Cation

N-Substitution of a thermally unstable diboratriazole 1 with a trimethylsilyl group affords a remarkably stable diboratriazole derivative 2 . Ring contraction of 2 with an N-heterocyclic carbene accompanied by the release of N₂ as well as 1,4-hydrogen shift affords a carbene-stabilized azadiboriridine 3 . Abstraction of the H−B_3mem hydride in 3 with methyl trifluoromethanesulfonate leads to the isolation of a hitherto unknown azadiboriridinylium 4 , the first BN analogue of cyclopropenylium cation. X-ray diffraction analysis and computational studies confirmed the delocalization of π electrons over the B₂N three-membered ring, indicating the 2π aromatic feature. Compound 4 undergoes ring expansion reactions with azobenzene and pyridazine to furnish triazadiborolidinylium species 5 and 6 , the latter of which possesses a cationic B₂N₃ ring with a pronounced 6π aromatic property. Moreover, the reaction of 4 with a diazo compound produces a cationic B₂N₃C pentafulvene derivative 7 .     

Ring currents that survive bond alternation in constrained 8π and 6π monocycles

Current density maps are computed at the ipsocentric CTOCD-DZ/6-31G^**//RHF/6-31G^** level for angle-constrained planar 1,3,5,7-cyclooctatetraenes (COT) and benzene. Constraint of α(CCH) angles to 90° in D_4h COT (3b) leads to endo-4 and exo-4 valence isomers. The exo structure, with CH bonds perpendicular to long sides of the octagon, is lower in energy by 202 kJ/mol and has stronger bond alternation (ΔR 0.265 Å). However, endo-4, exo-4 and COT 3b at its best planar geometry all sustain intense paratropic ring currents, attributed to the rotational character of the HOMO–LUMO transition, and consistent, mutatis mutandis, with the diatropic current in D_3h benzene 5.     

Intramolecular MLOH/π and MLNH/π interactions in crystal structures of metal complexes

Intramolecular metal-ligand OH/π (MLOH/π) and metal-ligand NH/π (MLNH/π) interactions in transition metal complexes between aqua or ammine ligand and ligand containing a C6-aromatic ring were investigated in crystal structures deposited in the Cambridge Structural Database (CSD). These intramolecular interactions appear in 38 structures with aqua ligand as the hydrogen atom donor and in 10 structures with ammine ligand as the hydrogen atom donor. Among all these complexes only one is negatively charged, 14 are positively charged and 33 are neutral indicating that the overall charge of the molecule has an influence on the XH/π (X = O or N) interactions. Energy estimated by DFT calculations is approximately 19 kJ mol⁻¹ for the MLOH/π interactions and approximately 15 kJ mol⁻¹ for the MLNH/π interactions. Dedicated to Professor Milan Melník on the occasion of his 70th birthday     

π plasmon modes inC 60 clusters

We study collective excitations of electrons in the fullerene molecule, by using the sum rule approach and linear response theory. The results for the excitation spectrum are discussed in relation to experimental data and to other theoretical approaches.Unité de Recherche des Universités Paris XI et Paris VII associée au CNRS     

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.     

Kinetics and thermochemistry of [2π + 2σ + 2σ]-cycloaddition of quadricyclane to tetracyanoethylene

Kinetic data for the unusual [2π + 2σ + 2σ]-cycloaddition of quadricyclane to tetracyanoethylene in toluene have been obtained for the first time. The same reaction in 1,4-dioxane appears to be the most exothermic among known cycloaddition reactions. The entropy of activation and reaction volume differ only slightly from the corresponding parameters of conventional Diels–Alder reactions.     

Research progress in cation-π interactions

Cation-π interaction is a potent intermolecular interaction between a cation and an aromatic system,which has been viewed as a new kind of binding force,as being compared with the classical interactions(e.g. hydrogen bonding,electrostatic and hydrophobic interactions). Cation-π interactions have been observed in a wide range of biological contexts. In this paper,we present an overview of the typical cation-π interactions in biological systems,the experimental and theoretical investigations on cation-π interactions,as well as the research results on cation-π interactions in our group.     

Functionally Important Aromatic-Aromatic and Sulfur-π Interactions in the D2 Dopamine Receptor

The recently published crystal structure of the D3 dopamine receptor shows a tightly packed region of aromatic residues on helices 5 and 6 in the space bridging the binding site and what is thought to be the origin of intracellular helical motion. This highly conserved region also makes contacts with residues on helix 3, and here we use double mutant cycle analysis and unnatural amino acid mutagenesis to probe the functional role of several residues in this region of the closely related D2 dopamine receptor. Of the eight mutant pairs examined, all show significant functional coupling (Ω > 2), with the largest coupling coefficients observed between residues on different helices, C3.36/W6.48, T3.37/S5.46, and F5.47/F6.52. Additionally, three aromatic residues examined, F5.47, Y5.48, and F5.51, show consistent trends upon progressive fluorination of the aromatic side chain. These trends are indicative of a functionally important electrostatic interaction with the face of the aromatic residue examined, which is likely attributed to aromatic-aromatic interactions between residues in this microdomain. We also propose that the previously determined fluorination trend at W6.48 is likely due to a sulfur-π interaction with the side chain of C3.36. We conclude that these residues form a tightly packed structural microdomain that connects helices 3, 5, and 6, thus forming a barrier that prevents dopamine from binding further toward the intracellular surface. Upon activation, these residues likely do not change their relative conformation, but rather act to translate agonist binding at the extracellular surface into the large intracellular movements that characterize receptor activation.     

A Neutral Three-Membered 2π Aromatic Disilaborirane and the Unique Conversion into a Four-Membered BSi2N-Ring

We report the design, synthesis, structure, bonding, and reaction of a neutral 2π aromatic three-membered disilaborirane. The disilaborirane is synthesized by a facile one-pot reductive dehalogenation of amidinato-silylene chloride and dibromoarylborane with potassium graphite. Despite the tetravalent arrangement of atoms around silicon, the three-membered silicon-boron-silicon ring is aromatic, as evidenced by NMR spectroscopy, nucleus independent chemical shift calculations, first-principles electronic structure studies using density functional theory (DFT) and natural bond orbital (NBO) based bonding analysis. Trimethylsilylnitrene, generated in situ, inserts in the Si−Si bond of disilaborirane to obtain a four-membered heterocycle 1-aza-2,3-disila-4-boretidine derivative. Both the heterocycles are fully characterized by X-ray crystallography.     

Planar Tetracoordinate Carbon in 6 σ+2π Double Aromatic CBe42-Derivatives

As a typical electron deficient element,beryllium is potentially suitable for designing the species with novel non-classical planar hypercoordinate carbon due t...     

2D and 3D supramolecular assemblies of double cyclopalladated azobenzenes realized by C-H?Cl-Pd, π?π and C-H?π interactions

The double cyclopalladated complex with azobenzene, μ-[(E)-1,2-diphenyldiazene-C^2,8, N^1,2]-di-[chloro(dimethylsulfoxide)palladium(II)]; (DMSO)PdCl(μ-C₆H₄NNC₆H₄)(DMSO)PdCl (1) and its analogous complex with DMF as ancillary ligand, (DMF)PdCl(μ-C₆H₄NNC₆H₄)(DMF)PdCl; μ-[(E)-1,2-diphenyldiazene-C^2,8,N^1,2]-di-[chloro(dimethylformamide)palladium(II)] (2a) were synthesized and the function of cyclopalladated moiety in molecular assembling in the solid state is illustrated by their crystal packings. The polymorphism of 2a and 2b is discussed. The crystal structures reveal assemblies with molecular components self-organized by C-H?Cl-Pd hydrogen bonds, π?π, and C-H?π interactions. The double cyclopalladated complexes of azobenzene, with two Pd-Cl moieties participating in the hydrogen bond formation and π-conjugated system involved in the π?π or C-H?π interactions, represent a new class of building blocks for construction of solid state supramolecular assemblies.     

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.     

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.     

On the directionality of anion-π interactions

The directionality of two important noncovalent interactions involving aromatic rings (namely anion-π and cation-π) is investigated. It has been recently published that the anion-π interactions observed in X-ray structures where the anion is located exactly over the center of the ring are scarce compared to cation-π interactions. To explain this behavior, we have analyzed how the interaction energy (RI-MP2/aug-cc-pVDZ level of theory) is affected by moving the anion from the center of the ring to several directions in anion-π complexes of chloride with either hexafluorobenzene or trifluoro-s-triazine. We have compared the results with the directionality of the cation-π interaction in the sodium-benzene complex. The results are useful to explain the experimental differences between both ion-π interactions. We have also computed the van der Waals radii of several halide anions and we have compared them to the neutral halogen atoms.