Interplay between anion-pi and hydrogen bonding interactions

The interplay between two important noncovalent interactions involving aromatic rings is studied by means of high level ab initio calculations. They demonstrate that synergistic effects are present in complexes where anion-pi and hydrogen bonding interactions coexist. These synergistic effects have been studied using the "atoms-in-molecules" theory and the Molecular Interaction Potential with polarization partition scheme. The present study examines how these two interactions mutually influence each other.     

A theoretical study of anion-pi interactions in seven-membered rings.

Several complexes of tropylium (1) with anions are optimized at the RI-MP2(full)/6-31++G** level of theory. This binding unit can interact very favorably with anions, and it combines the strength of the electrostatic interaction with the directionality of the anion-pi interaction. The complexes of 1 with anions are characterized by means of the Bader theory of "atoms-in-molecules," and the physical nature of the interaction has been analyzed by means of the molecular interaction potential with polarization tool. Experimental evidence of anion-pi interactions involving seven-membered rings has been found in the solid state.     

Rigid oligonaphthalenediimide rods as transmembrane anion-pi slides

We report the design, synthesis, and evaluation of rigid oligonaphthalenediimide (O-NDI) rods that are expected to act as transmembrane anion-pi slides. Studies in fluorogenic large unilamellar egg yolk phosphatidylcholine vesicles reveal that rigid O-NDI rods mediate anion-selective transport with a rare halide VI selectivity sequence (Cl- > F- > Br- > I-). This and decreasing activity, selectivity, and halide sequence with increasing positive charge of the rod termini support the occurrence of anion-pi interactions. A strong anomalous mole fraction effect in Cl-/I- mixtures is in agreement with the existence of multiple active sites along the anion-pi slide and multi-anion hopping as a mechanism of transport. The strong inverted NDI quadruple moment found by DFT calculations is in excellent agreement with these results.     

Structural versatility of anion-pi interactions in halide salts with pentafluorophenyl substituted cations

A series of pentafluorophenyl substituted ammonium, iminium, amidinium, and phosphonium halides are presented which show extensive anion-pi interactions. Hereby, the well-known anion-donor-pi-acceptor as well as "eta6" anion-pi-complex type interactions are observed. The latter is supported by fixation of the anion on top of the aromatic system through hydrogen bonding. This arrangement was investigated by theoretical methods showing a highly attractive anion-pi interaction. In addition an eta2-type coordination of the anions to only two C-atoms of the electron-deficient ring system is described.     

Probing anion-pi interactions in 1-D Co(II), Ni(II), and Cd(II) coordination polymers containing flexible pyrazine ligands

Two flexible thioether-containing heterocyclic ligands bis(2-pyrazylmethyl)sulfide (L1) and 2-benzylsulfanylmethylpyrazine (L2) have arene rings with differing pi-acidities which were used to probe anion-pi binding in five 1-D coordination polymers formed from the metal salts Co(ClO4)2, Ni(NO3)2, and Cd(NO3)2. In {[Co(L1)(MeCN)2](ClO4)2}infinity (1), {[Ni(L1)(NO3)2]}infinity (2), and {[Cd2(L1)(MeCN)(H2O)(NO3)4].H2O}infinity (3.H2O), the symmetrical ligand L1 was bound facially to the metal center and was bridged through a pyrazine donor to an adjacent metal forming a polymer chain. The folding of L1 formed U-shaped pi-pockets in 1 and 3.H2O which encapsulated free and bound anions, respectively. The anions interacted with the pi-acidic centers in a variety of different binding modes including anion-pi-anion and pi-anion-pi sandwiching. A wider pi-pocket was formed in 2 which also contained anion-pi interactions. The polymer chains in 2 were interdigitated through a rare type of complementary T-shaped N(pyrazine)...pi interaction. In {[Co(L2)(H2O)3](ClO4)2.H2O}infinity (4.H2O) and {[Cd(L2)(H2O)(NO3)2]}infinity (5), the unsymmetrical ligand L2 chelated the metal center and bridged through a pyrazine donor to an adjacent metal forming a polymer chain. The ligand arrangement resulted in the anions in both structures being involved in only anion-pi-anion sandwich interactions. In 4.H2O, the noncoordinated ClO4- anions interacted with only one chain while in 5 the coordinated NO3- anions acted as anion-pi supramolecular synthons between chains. Comparison between the polymers formed with ligands L1 and L2 showed that only the more pi-acidic ring was involved in the anion-pi interactions.     

Anion-arene adducts: C-H hydrogen bonding, anion-pi interaction, and carbon bonding motifs

This article summarizes experimental and theoretical evidence for the existence of four distinct binding modes for complexes of anions with charge-neutral arenes. These include C-H hydrogen bonding and three motifs involving the arene-pi system-the noncovalent anion-pi interaction, weakly covalent sigma interaction, and strongly covalent sigma interaction.     

Metal-organic frameworks exhibiting strong anion-pi interactions

Coordination polymers offer a significant potential for applications in adsorption, guest and anion recognition and sensing. Their structure commonly provides binding sites for such specific interactions as pi-pi stacking and XH...pi hydrogen bonding. The latter reflects the ability of the pi-cloud to interact with positively polarized atoms. An electrostatic interaction between anionic species and electron deficient heterocycles, which parallels the above binding scheme, is also possible and very recently the existence of anion-pi interactions was proved in the solid state and in solution. This effect may be significant also for biomolecule/solution interfaces, as it occurs in protein structures. In fact, such interactions could be especially relevant for host-guest chemistry of coordination polymers, particularly for functionalization of hydrophobic crystal cavities and for the design of geometrically rigid anion receptors. However, typical electron deficient heterocycles such as 1,3,5-triazines and 1,2,4,5-tetrazines are very weak donors and they are hardly suitable for bridging metal ions and the generation of coordination frameworks. As a system that combines efficient donor properties towards transition metal ions and a pronounced ability for anion-pi interactions we have developed unsubstituted pyridazino[4,5-d]pyridazine, which was readily accessible by a novel one-pot synthesis involving inverse electron demand Diels-Alder cycloaddition (Scheme 1). Unusual anion binding properties of the ligand may be clearly related to its electron-deficiency (LUMO energy -1.591 vs. -0.288 eV for the parent pyridazine), influenced also by N-coordination to such Lewis acids as metal ions.     

Rigid-rod anion-pi slides for multiion hopping across lipid bilayers

Shape-persistent oligo-p-phenylene-N,N-naphthalenediimide (O-NDI) rods are introduced as anion-pi slides for chloride-selective multiion hopping across lipid bilayers. Results from end-group engineering and covalent capture as O-NDI hairpins suggested that self-assembly into transmembrane O-NDI bundles is essential for activity. A halide topology VI (Cl > F > Br approximately I, Cl/Br approximately Cl/I > 7) implied strong anion binding along the anion-pi slides with relatively weak contributions from size exclusion (F >or= OAc). Anomalous mole fraction effects (AMFE) supported the occurrence of multiion hopping along the pi-acidic O-NDI rods. The existence of anion-pi interactions was corroborated by high-level ab initio and DFT calculations. The latter revealed positive NDI quadrupole moments far beyond the hexafluorobenzene standard. Computational studies further suggested that anion binding occurs at the confined, pi-acidic edges of the sticky NDI surface and is influenced by the nature of the phenyl spacer between two NDIs. With regard to methods development, a detailed analysis of the detection of ion selectivity with the HPTS assay including AMFE in vesicles is provided.     

A coordination polymer strategy for anion encapsulation: anion-pi interactions in (4,4) nets formed from Ag(I) salts and a flexible pyrimidine ligand

Anions encapsulated by a uniform mode of anion-pi binding in isomorphous (4,4) nets formed from Ag(I) salts and bis(4-pyrimidylmethyl)sulfide appear to be structurally directing.     

Anion-pi interactions in a carousel copper(II)-triazine complex

Unprecedented anion-pi interactions are revealed for the electron-poor triazine rings in [L2(CuCl)3][CuCl4]Cl (L = hexakis(pyridin-2-yl)-[1,3,5]-triazine-2,4,6-triamin), where both the chloride ion and a Cl atom of [CuCl4]2- are located approximately 3.15 A above the ring centroids, in excellent agreement with theoretical predictions for a Cl-...triazine complex. This confirms the importance of attractive anion-pi interactions for the supramolecular assembly of complexes with pi-electron-deficient heteroaromatics.     

Anion-pi interactions in cyanuric acids: a combined crystallographic and computational study

Several structures of pi complexes of isocyanuric acid and of several thio derivatives with anions have been computed by using high level ab initio calculations. The nature of the complexes has been studied by means of the method of molecular interaction potential with polarization (MIPp) and Bader's theory of atoms-in-molecules. These molecules form favorable complexes with anions and can be used as binding units for building receptors for the molecular recognition of anions. In several cases, the anion-pi interaction has been demonstrated experimentally by means of X-ray crystallography.     

Interplay between cation-pi, anion-pi and pi-pi interactions.

The interplay between three important noncovalent interactions involving aromatic rings is studied by means of high level ab initio calculations. They demonstrate that very strong synergic effects are present in complexes where either cation–π or anion–π and π‐‐π interactions coexist. These strong synergic effects have been studied using the “atoms in molecules” theory and the physical nature of the interactions investigated by means of the molecular interaction potential with polarization (MIPp).     

Anion-pi interaction augments halide binding in solution

1H NMR spectroscopic data and complementary theoretical predictions suggest that a designed receptor exhibits the anion-pi interaction in solution.     

The role of hydrogen bonding in the selectivity of L-cysteine methyl ester (CYSM) and L-cysteine ethyl ester (CYSE) for chloride ion

The interaction of cysteamine (CY), L-cysteine methyl ester (CYSM), and L-cysteine ethyl ester (CYSE) with nitrate, sulfate, perchlorate, dihydrogen phosphate, and chloride ions was investigated using surface enhanced Raman spectroscopy (SERS). CYSM and CYSE are chemical derivatives of CY. These thiols have a quaternary ammonium group to attract the anions to the SERS surface. Dihydrogen phosphate did not interact with these cationic thiols. The CY interaction with perchlorate, nitrate, and sulfate is stronger than the interaction with chloride. However, replacing a hydrogen on the carbon adjacent to the quaternary ammonium group with either a methyl or ethyl ester group results in stronger complexation with chloride ion than with either sulfate or nitrate ion. In the case of CYSM, the chloride interaction is five times stronger than the interaction with perchlorate. Molecular modeling indicates that the high selectivity of CYSM/CYSE for chloride is due to hydrogen bonding between the chloride ion and the hydrogen of the CH3 moeities of adjacent ester groups.     

Degradation of polymer mixtures. IV. Blends of poly(vinyl acetate) with poly(vinyl chloride) and other chlorine-containing polymers

The degradation of the binary polymer blends, poly(vinyl acetate)/poly(vinyl chloride), poly(vinyl acetate)/poly(vinylidene chloride) and poly(vinyl acetate)/polychloroprene has been studied by using thermal volatilization analysis, thermogravimetry, evolved gas analysis for hydrogen chloride and acetic acid, and spectroscopic methods. For the first two systems named, strong interaction occurs in the degrading blend, but the polychloroprene blends showed no indication of interaction. In the PVA/PVC and PVA/PVDC blends, hydrogen chloride from the chlorinated polymer causes substantial acceleration in the deacetylation of PVA. Acetic acid from PVA destabilizes PVC but has little effect in the case of PVDC because of the widely differing degradation temperatures of PVA and PVDC. The presence of hydrogen chloride during the degradation of PVA results in the formation of longer conjugated sequences, and the regression in sequence length at high extents of deacetylation found for PVA degraded alone is not observed.     

Spectroscopic evidence for pi-pi interaction between poly(diallyl dimethylammonium) chloride and multiwalled carbon nanotubes

The interaction between multiwalled carbon nanotubes (MWCNTs) and aqueous poly(diallyl dimethylammonium) chloride (PDDA) was studied by X-ray photoelectron (XPS) and photoacoustic Fourier transform infrared (PA-FTIR) spectroscopies. We have found that the mild sonication of MWCNTs in aqueous PDDA results in a significant improvement of CNT dispersibility and greatly enhances their adhesion to Au and Si substrates. The MWCNT-PDDA interaction is due to the presence of an unsaturated contaminant in the PDDA chain, as confirmed by both XPS and PA-FTIR, which enters into a pi-pi interaction with the CNTs. Electrostatic group repulsions of the coated CNTs then provide the dispersibility and adhesion.     

Dielectric,ultrasonic and x-ray diffraction studies on poly(vinyl chloride)-chlororubber-20 blends

Poly(vinyl chloride)-chlororubber-20 blends have been studied for compatibility by dielectric, ultrasonic and X-ray diffraction techniques. It has been found by both ultrasonic and dielectric techniques that poly(vinyl chloride) forms compatible blends with chlororubber-20 over a wide range of composition. X-ray diffraction studies indicate polymer-polymer interaction and reduction in the crystallinity of poly(vinyl chloride) by the incorporation of chlororubber-20. These results agree with earlier observations and have been explained in terms of the molecular and morphological behaviour of the blends.     

Mechanical properties of blends of PAMAM dendrimers with poly(vinyl chloride) and poly(vinyl acetate)

Hybrid blends of poly(amidoamine) PAMAM dendrimers with two linear high polymers, poly(vinyl chloride), PVC, and poly(vinyl acetate), PVAc, are reported. The interaction between the blend components was studied using dynamic mechanical analysis, xenon nuclear magnetic resonance (NMR) spectroscopy, and tensile property measurements. The data suggest a much higher degree of interaction between components of PVAc-containing blends compared to those containing PVC. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 2111–2117, 1998     

Self-assembly of a chiral carbonate- and cytidine-containing dodecanuclear copper(II) complex: a multiarm-supplied globular capsule

A dodecanuclear copper(II) globular-shaped structure has been obtained with the cytidine nucleoside and the templating carbonate anion. It shows receptor properties through anion-cation and multiple anion-pi interactions toward ClO 4 (-) as well as an overall antiferromagnetic coupling.