Structural Versatility of Anion−π Interactions in Halide Salts with Pentafluorophenyl Substituted Cations

In Situ Conversion of Ligand to a Coordination Polymer via a Core@Shell Crystal: A Multi-Step Phase-Dependent Structural Transformation

Two pseudopolymorphic 1D coordination polymers of the formulas [Cd(3,3'-pytz)(CH3OH)2(ClO4)2]n (1) and [Cd(3,3'-pytz)(CH3CN)2(ClO4)2]n (2) have been prepared using the electron-deficient 3,6-bis(pyridin-3-yl)-1,2,4,5-tetrazine (3,3'-pytz) ligand and cadmium perchlorate in the chloroform/methanol and chloroform/acetonitrile solvent system, respectively. It was observed that compounds 1 and 2 experienced one-step (CPreagent → CPproduct) single-crystal-to-powder structural transformation to the pure water-coordinated compound [Cd(3,3'-pytz)(H2O)2(ClO4)2]n (3) by absorbing water vapor from air (solid-gas phase transformation). Interestingly, compounds 1, 2, and 3 undergo a different transformation path and show an in situ unique three-step (CPreagent → CPproduct → Ligandintermediate → CPproduct) single-crystal-to-single-crystal (SCSC) structural transformation process through soaking in deionized water (solid-liquid phase transformation). In this fascinating transformation, we report for the first time the direct conversion of a ligand into a coordination polymer by a rare core-shell pathway in a solid-liquid phase transformation. In this process, we obtained compound {[Cd(3,3'-pytz)(H2O)4](3,3'-pytz)2(ClO4)2(H2O)6}n (4) (single-crystal = S, crystal = C, or microcrystal = P) as mixed compounds of core-shell L@4C and 4S or core-shell L@4P and 4P for compounds (1 and 2) and 3, respectively.

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.

Unfurling Anion-π Interactions Involving Graphynes

Ever since the inception of anion-π interactions, their nature and functional relevance have intrigued researchers. We address the twin challenge of elucidation of the role of extended conjugation and design of all-carbon neutral anion receptors by computations on the anion-π complexes of the halide ions with graphynes. Leveraging on the extended π-conjugation effects, we unfurl the functional relevance of graphynes as anion receptors using descriptors such as electrostatic potential, quadrupole moments, molecular polarizabilities and binding energies. Further, employing natural energy decomposition analysis, we assert that anion-π interactions are not merely dominated by electrostatic interactions. The polarization components do indeed play a crucial role in governing the binding of the anions to the graphynes.

Three in One: The Versatility of Hydrogen Bonding Interaction in Halide Salts with Hydroxy-Functionalized Pyridinium Cations

The paradigm of supramolecular chemistry relies on the delicate balance of noncovalent forces. Here we present a systematic approach for controlling the structural versatility of halide salts by the nature of hydrogen bonding interactions. We synthesized halide salts with hydroxy-functionalized pyridinium cations [HOCn Py]+ (n=2, 3, 4) and chloride, bromide and iodide anions, which are typically used as precursor material for synthesizing ionic liquids by anion metathesis reaction. The X-ray structures of these omnium halides show two types of hydrogen bonding: 'intra-ionic' H-bonds, wherein the anion interacts with the hydroxy group and the positively charged ring at the same cation, and 'inter-ionic' H-bonds, wherein the anion also interacts with the hydroxy group and the ring system but of different cations. We show that hydrogen bonding is controllable by the length of the hydroxyalkyl chain and the interaction strength of the anion. Some molten halide salts exhibit a third type of hydrogen bonding. IR spectra reveal elusive H-bonds between the OH groups of cations, showing interaction between ions of like charge. They are formed despite the repulsive interaction between the like-charged ions and compete with the favored cation-anion H-bonds. All types of H-bonding are analyzed by quantum chemical methods and the natural bond orbital approach, emphasizing the importance of charge transfer in these interactions. For simple omnium salts, we evidenced three distinct types of hydrogen bonds: Three in one!

About the relevance of anion-π interactions in water

Anion-π interactions are emerging as exotic features with potential applications in chemistry. In the last years, their relevance in living systems has been outlined, and so far there is no concluding significant evidence recognized about the participation of anion-π interactions in water because anion-π sensors contain large aromatic hydrophobic surfaces with limited solubility. By transforming a neutral heterocycle (for example quinoline) into its corresponding salt (quinolinium), we have been able to overcome these solubility issues, and new cationic water-soluble fluorophores have been prepared. Herein, we used N-alkylated heterocycles as π-acidic surfaces to shed light on the nature of anion-π in water by the direct measurement of the fluorescence and UV/Vis spectra in combination with DFT and X-ray analyses.

Influence of Aromatic Cations on the Structural Arrangement of Hg(II) Halides

Understanding the structure and arrangement of hybrid metal halides and their contribution to the optoelectronic properties is, thus far, a challenging topic. In particular, new materials composed of d¹⁰ metal halides and pyridinium cations are still largely unexplored. Therefore, we report the synthesis and characterization of six Hg(II) salts built up from (Hg₂Cl₆)^2- or (HgX₄)^2- anions (X = Cl, Br, I) and 2,2'-bipyridium (2,2'-Hbipy)⁺, 2,2'-bipyridine-1,1'-diium (2,2'-H₂bipy)²⁺, or 1,10-phenantrolinium (1,10-Hphen)⁺ cations, using the same experimental conditions. All of them have been characterized by PXRD, EA, FTIR-ATR, and ¹H NMR spectroscopies; single-crystal X-ray diffraction; and TG/DTA determinations. The study of their packing via Hirshfeld surface analysis and 3D deformation density mapping revealed the contributions of the intermolecular interactions to the structural arrangement, notably, the effect of the cation planarity on them. Successively, periodic DFT calculations showed that (i) the valence and conducting bands are mainly composed of the p orbitals of the halide and the organic cation, respectively, and (ii) the corresponding band gap depends mainly on the halide.

Discriminative Behavior of a Donor-Acceptor-Donor Triad toward Cyanide and Fluoride: Insights into the Mechanism of Naphthalene Diimide Reduction by Cyanide and Fluoride

A new molecular donor-acceptor-donor (D-A-D) triad, comprised of an electron deficient 1,4,5,8-naphthalene tetracarboxylic diimide (NDI) unit covalently connected to two flanking photosensitizers, i.e., a bis-heteroleptic Ru(II) complex of 1,10-phenanthroline and pyridine triazole hybrid ligand, is described. The single crystal X-ray structure of the perchlorate salt of the triad demonstrates that the electron deficient NDI unit can act as a host for anions via anion-π interaction. Detailed solution-state studies indicate that fluoride selectively interacts with the D-A-D triad to form a dianionic NDI, NDI^2-, via a radical anion, NDI^•-. On the contrary, cyanide reduces the NDI moiety to NDI^•-, as confirmed by UV-vis, NMR, and EPR spectroscopy. Further, femtosecond transient absorption spectroscopic studies reveal a low luminescence quantum yield of the D-A-D triad attributable to the photoinduced electron transfer (PET) process from the photoactive Ru(II) center to the NDI unit. Interestingly, the triad displays "OFF-ON" luminescence behavior in the presence of fluoride by restoring the Ru(II) to phenanthroline/pyridine-triazole-based MLCT emission, whereas cyanide fails to show a similar property due to a different redox process operational in the latter. The reduction of NDI in the presence of fluoride and cyanide in different polar solvents indicates that involvement of such deprotonated solvents in the electron transfer mechanism may not be operative in our present system. Low-temperature kinetic studies support the formation of a charge transfer associative transient species, which likely allows overcoming the thermodynamically uphill barrier for the direct electron transfer mechanism.

Soft-donor dipicolinamide derivatives for selective actinide(iii)/lanthanide(iii) separation: the role of S- vs. O-donor sites

A dithiopicolinamide analog selectively extracts Am(iii) over Eu(iii) under acidic conditions.

Vesicles Constructed with Chiral Amphiphilic Oxacalix[2]arene[2]triazine Derivatives for Enantioselective Recognition of Organic Anions

Chiral amphiphilic oxacalix[2]arene[2]triazine derivatives 1-3 bearing l-prolinol moieties were synthesized. The self-assembly behavior of the chiral macrocyclic amphiphiles was investigated. SEM, TEM, and DLS measurements demonstrated that 1 formed stable vesicles (size of ∼90 nm), whereas 2 and 3 formed micelles. As monitored by DLS, vesicles composed of 1 showed selective response to the chiral anions (2S, 3S)-2,3-dihydroxysuccinate (d-tartrate), S-mandelate and S-(+)-camphorsulfonate over their enantiomers. DFT calculations revealed that the enantioselectivity arises from cooperative anion-π interactions and hydrogen bonding between the chiral electron-deficient cavity and the organic anions.

Probing and evaluating anion–π interaction in meso-dinitrophenyl functionalized calix[4]pyrrole isomers

We report the comprehensive structural and binding isotherm analyses of meso-3,5-dinitrophenyl-substituted calix[4]pyrroles with a variety of anions, displaying anion–π interactions.

Selective separation behavior of graphene flakes in interaction with halide anions in the presence of an external electric field

The adsorption of halide anions in the absence, and presence, of a perpendicularly external electric field on the C₅₄H₁₈ graphene surface has been investigated using M06-2X/6-31G(d,p) density functional theory (DFT).

Experimental investigation of anion–π interactions – applications and biochemical relevance

Anion–π interactions, intuitively repulsive forces, turned from controversial to a well-established non-covalent interaction over the past quarter of a century.

Interplay between Beryllium Bonds and Anion-π Interactions in BeR2:C6X6:Y- Complexes (R = H, F and Cl, X = H and F, and Y = Cl and Br)

A theoretical study of the beryllium bonds in BeR₂:C₆X₆ (R = H, F, Cl and X = H and F) has been carried out by means of MP2/aug′-cc-pVDZ computational methods. In addition, the ternary complexes BeR₂:C₆X₆:Y⁻ (Y = Cl and Br) have been analyzed. Geometric, energetic and electronic aspects of the complexes have been taken into account. All the parameters analyzed provide a clear indication of favorable cooperativity in both interactions observed, beryllium bond and aromatic ring:anion interaction.

The pentafluorophenyl group as π-acceptor for anions: a case study

The present study gives a comprehensive insight into anion-π interactions in the solid state, focusing on purely organic and charge-neutral fluorophenyl groups bearing a positive charge located at a side chain. The detailed statistical analysis of a series of structural data sets shows the geometrical variability of anion-π bonding in the solid state. It reveals the directing substituents at the arene as key elements for the positional preferences of anions above π-systems. The structural variety of the interaction between anions and electron-deficient arenes is considered by use of the hapticity concept. Together with new evaluation criteria, two helpful tools to understand and describe anion-π interactions in the solid are used.

Photoelectron spectroscopy and theoretical studies of anion–π interactions: binding strength and anion specificity

Direct experimental measurements of non-covalent interactions between various anions and a π-electron-deficient cavity show significant binding strength and anion specificity.

Perfluoro-1,1′-biphenyl and perfluoronaphthalene and their derivatives as π-acceptors for anions

Perfluoro-1,1′-biphenyl and perfluoronaphthalene are identified experimentally as π-acceptors for anions by studying the parent systems as well as cationic derivatives.

Orbital effect-induced anomalous anion-π interactions between electron-rich aromatic hydrocarbons and fluoride

Anion-π interactions generally exist between an anion and an electron-deficient π-ring because of the electron-accepting character of the ring. In this paper, we report orbital effect-induced anomalous binding between electron-rich π systems and F(-) through anion-π interactions calculated at the MP2/6-31+G(d,p) and ωB97X-D/6-31+G(d,p) levels of theory. We find that anion-π interactions between F(-) and electron-rich π rings increase markedly with increasing number of π electrons and size of the π rings. This is contrary to intuition because anion-π interactions would be expected to gradually decrease because of gradually increasing Coulombic repulsion between the negative charge of the anions and gradually increasing number of π electrons of the aromatic surfaces. Energy decomposition analysis showed that the key to this anomalous effect is the more effective delocalization of negative charge to the unoccupied π* orbitals of larger π rings, which is termed an "orbital effect".

Assessing the impact of anion–π effects on phenylalanine ion structures using IRMPD spectroscopy

The gas-phase structures of two halide-bound phenylalanine anions (PheX⁻, X = Cl⁻ or Br⁻) and five fluorinated derivatives have been identified using infrared multiple photon dissociation (IRMPD) spectroscopy.

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.

Aromatic rings in chemical and biological recognition: energetics and structures

This review describes a multidimensional treatment of molecular recognition phenomena involving aromatic rings in chemical and biological systems. It summarizes new results reported since the appearance of an earlier review in 2003 in host-guest chemistry, biological affinity assays and biostructural analysis, data base mining in the Cambridge Structural Database (CSD) and the Protein Data Bank (PDB), and advanced computational studies. Topics addressed are arene-arene, perfluoroarene-arene, S⋅⋅⋅aromatic, cation-π, and anion-π interactions, as well as hydrogen bonding to π systems. The generated knowledge benefits, in particular, structure-based hit-to-lead development and lead optimization both in the pharmaceutical and in the crop protection industry. It equally facilitates the development of new advanced materials and supramolecular systems, and should inspire further utilization of interactions with aromatic rings to control the stereochemical outcome of synthetic transformations.