Spectroscopic, structural, and theoretical studies of halide complexes with a urea-based tripodal receptor

A urea-based tripodal receptor L substituted with p-cyanophenyl groups has been studied for halide anions using (1)H NMR spectroscopy, density functional theory (DFT) calculations, and X-ray crystallography. The (1)H NMR titration studies suggest that the receptor forms a 1:1 complex with an anion, showing a binding trend in the order of fluoride > chloride > bromide > iodide. The interaction of a fluoride anion with the receptor was further confirmed by 2D NOESY and (19)F NMR spectroscopy in DMSO-d(6). DFT calculations indicate that the internal halide anion is held by six NH···X interactions with L, showing the highest binding energy for the fluoride complex. Structural characterization of the chloride, bromide, and silicon hexafluoride complexes of [LH(+)] reveals that the anion is externally located via hydrogen bonding interactions. For the bromide or chloride complex, two anions are bridged with two receptors to form a centrosymmetric dimer, while for the silicon hexafluoride complex, the anion is located within a cage formed by six ligands and two water molecules.     

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.     

Binding of molecular hydrogen to halide anions: A computational exploration of eco-friendly materials for hydrogen storage

Using density functional theory calculations, we investigate the structures of the complexes derived from the interaction of molecular hydrogen to halide anions. The bromide anion can bind up to seven hydrogen molecules while both fluoride and chloride anions form stable complexes with up to six hydrogen molecules. According to the results of QTAIM analyses, closed shell interactions are operative in these complexes.     

Boundaries of Anion/Naphthalenediimide Interactions: From Anion-π Interactions to Anion-Induced Charge-Transfer and Electron-Transfer Phenomena

The recent emergence of anion-π interactions has added a new dimension to supramolecular chemistry of anions. Yet, after a decade since its inception, actual mechanisms of anion-π interactions remain highly debated. To elicit a complete and accurate understanding of how different anions interact with π-electron-deficient 1,4,5,8-naphthalenediimides (NDIs) under different conditions, we have extensively studied these interactions using powerful experimental techniques. Herein, we demonstrate that, depending on the electron-donating abilities (Lewis basicity) of anions and electron-accepting abilities (π-acidity) of NDIs, modes of anion-NDI interactions vary from extremely weak non-chromogenic anion-π interactions to chromogenic anion-induced charge-transfer (CT) and electron-transfer (ET) phenomena. In aprotic solvents, electron-donating abilities of anions generally follow their Lewis basicity order, whereas π-acidity of NDIs can be fine-tuned by installing different electron-rich and electron-deficient substituents. While strongly Lewis basic anions (OH(-) and F(-)) undergo thermal ET with most NDIs, generating NDI(?-) radical anions and NDI(2-) dianions in aprotic solvents, weaker Lewis bases (AcO(-), H(2)PO(4)(-), Cl(-), etc.) often require the photoexcitation of moderately π-acidic NDIs to generate the corresponding NDI(?-) radical anions via photoinduced ET (PET). Poorly Lewis basic I(-) does not participate in thermal ET or PET with most NDIs (except with strongly π-acidic core-substituted dicyano-NDI) but forms anion/NDI CT or anion-π complexes. We have looked for experimental evidence that could indicate alternative mechanisms, such as a Meisenheimer complex or CH···anion hydrogen-bond formation, but none was found to support these possibilities.     

Effect of microhydration on the guanidinium···benzene interaction

The effect of microhydration on the interaction of guanidinium cation with benzene has been studied by employing ab initio calculations. Four different structural arrangements were considered for the guanidinium···benzene interaction to which up to six water molecules were added. T-shaped structures are usually the most stable, but as water molecules are included the energy differences with the parallel structures decrease, reaching a point where parallel complexes are even more stable than T-shaped ones. Therefore, the inclusion of water molecules promotes a change in the structure of the cation···π contact. The analysis reveals that these stability changes are more related with the structure of the hydrating water molecules than to a modulation of the cation···π interaction. Already with three water molecules, one water molecule in the T-shaped complex has to be located in the second solvation shell, whereas in parallel structures this occurs with four water molecules. As a consequence energy differences among structures decrease. The calculations show that the nature of the interaction is almost unaffected in T-shaped structures, whereas an important dispersion increment is observed in parallel ones, though its overall effect is small.     

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.     

Quantification of binding affinities of essential sugars with a tryptophan analogue and the ubiquitous role of C-H···π interactions

The role of noncovalent interactions in carbohydrate recognition by aromatic amino acids has long been reported. To develop a molecular understanding of noncovalent interactions in the recognition process, we have examined a series of binary complexes between 3-methylindole (3-MeIn) and sugars. In particular, the geometries and binding affinities of 3-MeIn with α/β-D-glucose, β-D-galactose, α-D-mannose and α/β-L-fucose are obtained using the MP2(full)/6-31G(d,p) and the M06/TZV2D//MP2/6-31G(d,p) level of theories. The conventional hydrogen bonding such as N-H···O and C-H···O as well as nonconventional O-H···π and C-H···π type of interactions is, in general, identified as responsible for the moderately strong interaction energies. Large variations in the position-orientations of 3-MeIn with respect to saccharide are noticed, within the same sugar family, as well as across different sugar series. Furthermore, complexes with large differences in their geometries are recognized as capable of exhibiting very similar interaction energies, underscoring the significance of exhaustive conformation sampling, as carried out in the present study. These observations are readily attributed to the differences in the efficiency of the type of interactions enlisted above. The highest and lowest interaction energies, upon inclusion of 50% BSSE correction, are found to be -16.02 and -6.22 kcal mol(-1), respectively, for α-D-glucose (1a) and α-L-fucose (5j). While more number of prominent conventional hydrogen bonding contacts remains as a characteristic feature of the strongly bound complexes, the lower end of the interaction energy spectrum is dominated by multiple C-H···π interactions. The complexes exhibiting as many as four C-H···π contacts are identified in the case of α/β-D-glucose, β-D-galactose, and α/β-L-fucose with an interaction energy hovering around -8 kcal mol(-1). The presence of effective C-H···π interactions is found to be dependent on the saccharide configuration as well as the area of the apolar patch constituted by the C-H groups. The study offers a comprehensive set of binary complexes, across different saccharides, which serves as an illustration of the significance and ubiquitous nature of C-H···π interactions in carbohydrate binding in saccharide-protein complexes.     

Oligoether-strapped calix[4]pyrrole: an ion-pair receptor displaying cation-dependent chloride anion transport

A ditopic ion-pair receptor (1), which has tunable cation- and anion-binding sites, has been synthesized and characterized. Spectroscopic analyses provide support for the conclusion that receptor 1 binds fluoride and chloride anions strongly and forms stable 1:1 complexes ([1·F](-) and [1·Cl](-)) with appropriately chosen salts of these anions in acetonitrile. When the anion complexes of 1 were treated with alkali metal ions (Li(+), Na(+), K(+), Cs(+), as their perchlorate salts), ion-dependent interactions were observed that were found to depend on both the choice of added cation and the initially complexed anion. In the case of [1·F](-), no appreciable interaction with the K(+) ion was seen. On the other hand, when this complex was treated with Li(+) or Na(+) ions, decomplexation of the bound fluoride anion was observed. In contrast to what was seen with Li(+), Na(+), K(+), treating [1·F](-) with Cs(+) ions gave rise to a stable, host-separated ion-pair complex, [F·1·Cs], which contains the Cs(+) ion bound in the cup-like portion of the calix[4]pyrrole. Different complexation behavior was seen in the case of the chloride complex, [1·Cl](-). Here, no appreciable interaction was observed with Na(+) or K(+). In contrast, treating with Li(+) produces a tight ion-pair complex, [1·Li·Cl], in which the cation is bound to the crown moiety. In analogy to what was seen for [1·F](-), treatment of [1·Cl](-) with Cs(+) ions gives rise to a host-separated ion-pair complex, [Cl·1·Cs], in which the cation is bound to the cup of the calix[4]pyrrole. As inferred from liposomal model membrane transport studies, system 1 can act as an effective carrier for several chloride anion salts of Group 1 cations, operating through both symport (chloride+cation co-transport) and antiport (nitrate-for-chloride exchange) mechanisms. This transport behavior stands in contrast to what is seen for simple octamethylcalix[4]pyrrole, which acts as an effective carrier for cesium chloride but does not operates through a nitrate-for-chloride anion exchange mechanism.     

Anion binding in the C3v-symmetric cavity of a protonated tripodal amine receptor: potentiometric and single crystal X-ray studies

Tris(2-aminoethyl)amine (tren) based pentafluorophenyl-substituted tripodal L, tris[[(2,3,4,5,6-pentafluorobenzyl)amino]ethyl]amine receptor is synthesized in good yield and characterized by single crystal X-ray diffraction analysis. Detailed structural aspects of binding of different anionic guests toward L in its triprotonated form are examined thoroughly. Crystallographic results show binding of fluoride in the C(3v)-symmetric cavity of [H(3)L](3+) where spherical anion fluoride is in tricoordinated geometry via (N-H)(+)···F interaction in the complex [H(3)L(F)]·[F](2)·2H(2)O, (3). In the case of complexes [H(3)L(OTs)]·[OTs](2), (4) and [H(3)L(OTs)]·[NO(3)]·[OTs], (5), tetrahedral p-toluenesulphonate ion is engulfed in the cavity of [H(3)L](3+) via (N-H)(+)···O interactions. Interestingly, complex [(H(3)L)(2)(SiF(6))]·[BF(4)](4)·CH(3)OH·H(2)O, (6) shows encapsulation of octahedral hexafluorosilicate in the dimeric capsular assembly of two [H(3)L](3+) units, via a number of (N-H)(+)···F interactions. The kinetic parameters of L upon binding with different anions are evaluated using a potentiometric study in solution state. The potentiometric titration experiments in a polar protic methanol/water (1:1 v/v) binary solvent system show high affinity of the receptor toward more basic fluoride and acetate anions, with a lesser affinity for other inorganic anions (e.g., chloride, bromide, nitrate, sulfate, dihydrogenphosphate, and p-toluenesulphonate).     

Influence of Different Organic Carboxylate Anions on the Crystal Structure of Silver(Ⅰ)Coordination Polymers

The reaction of AgNO3 , 4,4′-bipyridine (bpy) and 2,2′-bipyridine-3,3′-dicarboxylic acid (H2bpdc)/2,2′-biquinoline-4,4′-dicarboxylic acid (H2bqdc)/1,3-benzenedicarboxylic acid (H2bdc) gave rise to block-like crystals of [Ag4(bpy)2(bpdc)2]·13H2O(1), [Ag2(bpy)(bqdc)(H2O)]·4.5H2O(2) and [Ag2(bpy)2(H2O)2](bdc)·3H2O(3) by slow evaporation. All the three complexes contain sandwich-like crystal structures, in which anionic sheets built up from different anions (bpdc2- , bqdc2- and bdc2- ) and lattice water molecules via rich hydrogen-bonding interactions are inserted between the cationic silver complex layers, and the abundant Ag···Ag, Ag···N and π-π stacking interactions further strengthen the 3D frameworks. The lattice water molecules are situated among the framework of crystal structure and stabilized by rich hydrogen-bonding interactions, and lattice water molecules may play a role in the orientation of organic anions in the crystal packing. Additionally, the thermal properties of 1, 2 and 3 were also discussed in detail.     

New Type of Aromatic π-Systems for Anion Recognition: Strong Anion-π and C−H⋅⋅⋅Anion Interactions Between Halides and Aromatic Ligands in Half-Sandwich Compounds

Half-sandwich compounds of benzene, cyclopentadienyl, pentamethylcyclopentadienyl, and indenyl were studied as a new type of aromatic π-systems for interactions with halide anions. Although uncoordinated benzene forms only C−H⋅⋅⋅anion interactions, and hexafluorobenzene forms only anion-π interactions, aromatic ligands in half-sandwich compounds can form both types of interactions, because their entire electrostatic potential surface is positive. These aromatic ligands can form stronger anion-π interactions than organic aromatic molecules, as a consequence of more pronounced dispersion and induction energy components. Moreover, C−H⋅⋅⋅anion interactions of aromatic ligands are stronger than anion-π interactions, and significantly stronger than C−H⋅⋅⋅anion interactions of benzene. Our study shows that transition-metal coordination can make aromatic moieties suitable for strong interactions with anions, and gives insight into the design of new anion receptors.     

Effects of the biological backbone on stacking interactions at DNA-protein interfaces: the interplay between the backbone···π and π···π components

The (gas-phase) MP2/6-31G*(0.25) π···π stacking interactions between the five natural bases and the aromatic amino acids calculated using (truncated) monomers composed of conjugated rings and/or (extended) monomers containing the biological backbone (either the protein backbone or deoxyribose sugar) were previously compared. Although preliminary energetic results indicated that the protein backbone strengthens, while the deoxyribose sugar either strengthens or weakens, the interaction calculated using truncated models, the reasons for these effects were unknown. The present work explains these observations by dissecting the interaction energy of the extended complexes into individual backbone···π and π···π components. Our calculations reveal that the total interaction energy of the extended complex can be predicted as a sum of the backbone···π and π···π components, which indicates that the biological backbone does not significantly affect the ring system through π-polarization. Instead, we find that the backbone can indirectly affect the magnitude of the π···π contribution by changing the relative ring orientations in extended dimers compared with truncated dimers. Furthermore, the strengths of the individual backbone···π contributions are determined to be significant (up to 18 kJ mol(-1)). Therefore, the origin of the energetic change upon model extension is found to result from a balance between an additional (attractive) backbone···π component and differences in the strength of the π···π interaction. In addition, to understand the effects of the biological backbone on the stacking interactions at DNA-protein interfaces in nature, we analyzed the stacking interactions found in select DNA-protein crystal structures, and verified that an additive approach can be used to examine the strength of these interactions in biological complexes. Interestingly, although the presence of attractive backbone···π contacts is qualitatively confirmed using the quantum theory of atoms in molecules (QTAIM), QTAIM electron density analysis is unable to quantitatively predict the additive relationship of these interactions. Most importantly, this work reveals that both the backbone···π and π···π components must be carefully considered to accurately determine the overall stability of DNA-protein assemblies.     

Neutral acyclic anion receptor with thiadiazole spacer: halide binding study and halide-directed self-assembly in the solid state

A halide binding study of a newly synthesized neutral acyclic receptor LH(2) with a thiadiazole spacer has been methodically performed both in solution and in the solid state. Crystal structure analysis of the halide complexes elucidate the fact that fluoride forms an unusual 1:1 hyrogen-bonded complex with monodeprotonated receptor, whereas in the case of other congeners, such as chloride and bromide, the receptor binds two halide anions along with formation of a halide-bridged 1D polymeric chain network by participation of N-H···X(-) and aromatic C-H···X(-) hydrogen-bonding (where X = Cl and Br) interactions. The presence of a rigid thiadiazole spacer presumably opens up enough space for capturing two halide anions by a single receptor molecule, where the coordinated -NH protons are pointed in the same direction with respect to the spacer and eventually favor formation of halide (Cl(-) and Br(-)) induced polymeric architecture, although no obvious chloride- or bromide-directed polymeric assembly is found in solution. A significant red shift of 243 nm in the absorption spectra of LH(2) was solely observed in the presence of excess fluoride anion, which enables LH(2) as an efficient colorimetric sensor for optical detection of fluoride anion (yellow to blue). Furthermore, spectroscopic titration experiments with increasing equivalents of fluoride anion suggest formation of a H-bonded complex with subsequent stepwise deprotonation of two N-H groups, which can be visually monitored by a change in color from yellow to blue via pink.     

The structure of water in p-sulfonatocalix[4]arene

Tetrasodium p-sulfonatocalix[4]arene exists as a hydrate with approximately 14 water molecules and has three polymorphic modifications, all of which contain a water molecule in the molecular cavity that is engaged in OH···π interactions. Single-crystal neutron structures are reported for two of these three forms and reveal a "compressed" water molecule with short OH bonds. Partial atomic charges and hardness analysis (PACHA) calculations based on the neutron coordinates give an OH···π interaction energy of 6.9-7.5 kJ mol(-1). The PACHA analysis also reveals the dominance of the charge-assisted hydrogen bonds from the Na(+)-coordinated water molecules. The instability of the crystal towards dehydration can be traced to an uncoordinated lattice water site. The remarkable calixarene-Na(+)-hydrate motif is conserved almost unchanged across all three polymorphs. A single-crystal neutron structure is also reported for pentasodium p-sulfonatocalix[4]arene·12H(2)O, which exhibits an intracavity water molecule that is engaged in both OH···π and OH···O hydrogen bonding. The shorter covalent bond to the hydrogen atom that forms the interaction with the aromatic ring is again apparent.     

苯甲酰氨基脲的合成及其阴离子识别

设计合成了N-(取代苯甲酰氨基)脲衍生物(取代基＝p-OC2H5, H, p-Cl) 1～3, 应用吸收光谱法考察了受体分子与阴离子如 , F－, 等的相互作用, 考察了取代基对受体分子与阴离子亲合力和结合选择性的调控或改善能力. 结果表明, 该类受体分子与阴离子通过氢键形成阴离子配合物, 乙腈中受体分子1对F－表现出极高的响应选择性. Job作图法表明1与F－的结合计量比为1∶1, 1H NMR滴定结果为受体分子与阴离子间的氢键作用本质提供了直接证据, 初步探讨了F－响应选择性的原因.     

Host-guest supramolecular interactions in the coordination compounds of 4,4'-azobis(pyridine) with MnX2 (X = NCS–, NCNCN–, and PF6(–)): structural analyses and theoretical study

Three new Mn(II) coordination compounds {[Mn(NCNCN)(2)(azpy)]·0.5azpy}(n) (1), {[Mn(NCS)(2)(azpy)(CH(3)OH)(2)]·azpy}(n) (2), and [Mn(azpy)(2)(H(2)O)(4)][Mn(azpy)(H(2)O)(5)]·4PF(6)·H(2)O·5.5azpy (3) (where azpy = 4,4'-azobis(pyridine)) have been synthesized by self-assembly of the primary ligands, dicyanamide, thiocyanate, and hexafluorophosphate, respectively, together with azpy as the secondary spacer. All three complexes were characterized by elemental analyses, IR spectroscopy, thermal analyses, and single crystal X-ray crystallography. The structural analyses reveal that complex 1 forms a two-dimensional (2D) grid sheet motif. These sheets assemble to form a microporous framework that incorporates coordination-free azpy by host-guest π···π and C-H···N hydrogen bonding interactions. Complex 2 features azpy bridged one-dimensional (1D) chains of centrosymmetric [Mn(NCS)(2)(CH (3)OH)(2)] units which form a 2D porous sheet via a CH(3)···π supramolecular interaction. A guest azpy molecule is incorporated within the pores by strong H-bonding interactions. Complex 3 affords a 0-D motif with two monomeric Mn(II) units in the asymmetric unit. There exist π···π, anion···π, and strong hydrogen bonding interactions between the azpy, water, and the anions. Density functional theory (DFT) calculations, at the M06/6-31+G* level of theory, are used to characterize a great variety of interactions that explicitly show the importance of host-guest supramolecular interactions for the stabilization of coordination compounds and creation of the fascinating three-dimensional (3D) architecture of the title compounds.     

Moderate and advanced intramolecular proton transfer in urea-anion hydrogen-bonded complexes

The study of the interactions of the three urea-based receptors AH, BH(+) and CH(2+) with a variety of anions, in MeCN, has made it possible to verify the current view that hydrogen bonding is frozen proton transfer from the donor (the urea N-H fragment in this case) to the acceptor (the anion X(-)). The poorly acidic, neutral receptor AH establishes two equivalent hydrogen bonds N-H···X(-), with all anions, including CH(3)COO(-) and F(-), in which moderate proton transfer from N-H to the anion takes place. The strongly acidic, dicationic receptor CH(2+) forms, with most anions, complexes in which two inequivalent hydrogen bonds are present: one involving moderate proton transfer (N-H···X(-)) and one in which advanced proton transfer has taken place, described as N(-)···H-X. The degree of proton advancement is directly related to the basic tendencies of the anion. The cationic receptor BH(+) of intermediate acidic properties only forms complexes with two inequivalent hydrogen bonds (moderate+advanced proton transfer) with CH(3)COO(-) and F(-), and complexes with two equivalent hydrogen bonds (moderate proton transfer) with all the other anions. Moreover, [B···HF] and [C···HF](+), on addition of a second F(-) ion, lose the bound HF molecule to give HF(2)(-). Release of CH(3)COOH, with the formation of [CH(3)COOH···CH(3)COO](-), also takes place with the [B···CH(3)COOH] complex in the presence of a large excess of anion.     

A theoretical exploration of unexpected amine···π interactions

Counterintuitive amine lone pair···π interactions are computationally revealed by MP2 and CCSD(T) methods, attractive lone pair···π interactions are observed when the lone pair of nitrogen points toward the π system. Symmetry adapted perturbation theory (SAPT) calculations and atoms in molecules (AIM) analyses were performed and the origin of the calculated attractive interaction between nitrogen lone pairs and π rings is discussed. Dispersion effects were revealed to play a crucial role in the attractive lone pair···π interaction.     

Long-chain ammonium fluoride salts as universal analytical reagents for total anion determination

Lipophilic quaternary ammonium cations are insoluble in water when paired with any anion except fluoride or hydroxide. This phenomenon is the basis for a novel method for total anion determination. Tetra-n-octylammonium fluoride (TOAF) is used for the direct titration of any anion or mixture of anions with the exception of hydroxide. Anions for which no direct analytical method exists, e.g., nitrate or hydrogenfluoride (HF₂^?), are simply assayed by conductimetric titration with TOAF reagent.