N. Jiten Singh

Imidazolium receptors for the recognition of anions

This tutorial review covers imidazolium receptors for anion recognition according to their topological and structural classification, and includes benzene tripodal, cyclophane and calix-imidazolium, fluorescent imidazolium, ferrocenyl imidazolium, cavitand and calixarene, and polymeric imidazolium systems.

Unique sandwich stacking of pyrene-adenine-pyrene for selective and ratiometric fluorescent sensing of ATP at physiological pH.

A pincer-like benzene-bridged sensor 1 with a pyrene excimer as a signal source and imidazolium as a phosphate anion receptor was synthesized and investigated for ATP sensing. A unique switch of excimer vs monomer pyrene fluorescence of 1 is observed in the presence of ATP due to the charcteristic sandwich pi-pi stacking of pyrene-adenine-pyrene. On the other hand, four other bases of nucleoside triphosphates such as GTP, CTP, UTP, and TTP can interact only from the outside with the already stabilized stacked pyrene-pyrene dimer of 1, resulting in excimer fluorescence quenching. The fluorescent intensity ratio of monomer-to-excimer for 1 upon binding with ATP (I(375)/I(487)) is much larger than that upon binding with ADP and AMP. This difference is large enough to discriminate ATP from ADP and AMP. As one of the biological applications, sensor 1 is successfully applied to the ATP staining experiments. Sensor 1 is also applied to monitor the hydrolysis of ATP and ADP by apyrase. The results indicate that 1 is a useful fluorescent sensor for investigations of ATP-relevant biological processes.

Comprehensive Energy Analysis for Various Types of π-Interaction

We have investigated various types of π-interactions, where one of the interacting π-systems is represented by an aromatic benzene molecule. The system includes Rg-π, CH-π, π-π(D), π-π(T), H-π(T), π(+)-π(D), π(+)-π(T), H(+)-π(T), π(+2)-π(D), M(+)-π, and M(+2)-π complexes, where Rg denotes a rare gas or noble atom, M denotes a metal, and D/T indicates displaced-stacked/T-shaped structure. The microsolvation effect is also considered. We note that the interaction between a cationic π system and a neutral π system (πcation-π interaction) is so far ambiguously considered as either π-π or cation-π interaction. In terms of total binding energy, the πcation-π interaction is weaker than the cation-π interaction, but much stronger than the π-π interaction. When the hydrophilic (N-H)(+) or (C-H)(+) group in a singly charged π(+) system (as in protonated histidine, arginine, pyridine, or dimethyl imidazolium) interacts with a π-system, the complex favors a T-shaped form [π(+)-π(T) complex]. However, in the presence of polar solvating molecules or counteranions, these species interact with the (N-H)(+)/(C-H)(+) group, while the π(+) system interacts with the neutral aromatic ring. Then, the displaced-stacked form [π(+)-π(D) complex] is favored or otherwise nearly isoenergetic to the π(+)-π(T) form. The π(+)-π systems are stabilized mainly by both dispersion and electrostatic energies. Ternary diagrams using either attractive energy components or both attractive and repulsive energy components show that the π(+)-π(D) complexes have more contribution from dispersion energy but less contribution from induction energy than the π(+)-π(T) complexes, while both complexes have similar percentage contributions from electrostatic and exchange energy components. In particular, the π(+)-π(D) complexes are found to be distinctly different from the π-π complexes and the non-π organic or metallic cation-π complexes.

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.

Hydrogen-Release Mechanisms in Lithium Amidoboranes

Alkali-metal amidoboranes have been recently highlighted as materials that satisfy many of the criteria required to make hydrogen-storage media. It is, therefore, crucial for us to understand the dehydrogenation mechanism of these materials for further development towards making successful hydrogen-storage media. In the present study, we attempt to shed light on the mechanisms involved in the loss of one molar equivalent of H(2) from solid lithium amidoboranes by using high-level ab initio calculations of monomeric and dimeric compounds in the gas phase. In the lithium amidoborane dimer, H(2) is released by the formation of LiH, which is followed by a redox reaction of the dihydrogen bond formed between the strongly basic H(-) in LiH and H(delta+) bonded to N. In the dehydrogenation process, the Li cation catalyzes the intermolecular N-B bond formation; this could lead to new pathways for N-B polymerization. After the release of the first molecule of H(2), a Li cation binds to a nitrogen atom, resulting in a lowering of the energy barrier for the second dehydrogenation process per dimer. These results will be useful for the design of future hydrogen-storage media.

Designing Ionophores and Molecular Nanotubes Based on Molecular Recognition

In this mini-review we briefly describe intermolecular interactions ranging from hydrogen bonding to ionic interactions to aromatic interactions. Manifestation of these interaction forces is in the design and realization of various ionophores with chemo-sensing capability for biologically important cations and anions. We also explain how the understanding of hydrogen bonding and π-interactions has led to the design of self-assembled organic nanotubes. We further discuss the conformational changes between stacked and edge-to-face conformers in benzoquinone-benzene complexes, which are controlled by alternating electrochemical potential. The resulting flapping motion illustrates a promising pathway toward the design of nanomechanical devices.

Calix[ n ]imidazolium as a new class of positively charged homo-calix compounds

Macrocycles based on neutral calixarenes and calixpyrroles have been extensively explored for ion binding, molecular assembly and related applications. Given that only these two types of calix compounds and their analogs are available, the introduction of new forms of widely usable calix macrocycles is an outstanding challenge. Here we report the quadruply/quintuply charged imidazole-based homo-calix compounds, calix[4/5]imidazolium. The noncovalent (C-H)+/π+-anion interactions of the imidazolium rings with anions inside and outside the cone are the stabilizing factors for crystal packing, resulting in self-assembled arrays of cone-shaped calix-imidazolium molecules. Calix[4]imidazolium senses fluoride selectively even in aqueous solutions. Calix[5]imidazolium recognizes neutral fullerenes through π+–π interactions and makes them soluble in water, which could be useful in fullerene chemistry. Not only derivatization and ring expansion of calix[n]imidazolium, but also their utilization in ionic liquids, carbene chemistry and nanographite/graphene exfoliation could be exploited.

A successive layer-by-layer assembly of supramolecular frameworks driven by a novel type of face-to-face π+–π+ interactions

The solid-state complex [PTPH3](NO3)3·2(HNO3) (1) has been synthesized and characterized by X-ray studies, where PTPH3 is the triply protonated form of 4′-(4-pyridyl)-2,2′:6′,2′′-terpyridine (PTP). The solid-state structure of the complex reveals that the π+–π+ interactions are the major driving force in the crystal packing while π+–π, π–π and π–anion interactions assist the overall stabilization of self-assembly. Complex 1 exhibits two different π-stack layers, where layer 1 is generated through π+–π+ interactions and the mutual forces of π+–π+ and π+–π form layer 2. The interaction energies of the main driving forces (π+–π+, π+–π and π–anion interactions) observed in the crystal structure have been calculated using dispersion-corrected density functional theory (DFT-D). An analysis of the Hirshfeld surface of complex 1 shows the intermolecular interactions involved within the crystal structure and corresponding quantitative information are presented by fingerprint plots.

Molecular architecture using novel types of non-covalent π-interactions involving aromatic neutrals, aromatic cations and π-anions

A solid-state complex utilizing non-covalent interactions between two aromatic cations is synthesized and characterized. The X-ray study of the structure shows that the anion templated π+–π+ interactions are the major driving force in the crystal packing, while π+–π, π–π, π–anion and π+–anion interactions assist the overall stabilization of self-assembly. In addition, we also identify the cation-mediated non-covalent interaction between two π anions (π−–π− interaction). The interaction energies of the important driving forces (π+–π+, π+–π, π–anion, π+–anion, and π−–π− interactions) observed in the crystal structure are calculated using dispersion-corrected density functional theory (DFT-D).

How Different Are Aromatic π Interactions from Aliphatic π Interactions and Non-π Stacking Interactions?

We compare aromatic π interactions with aliphatic π interactions of double- and triple-bonded π systems and non-π stacking interactions of single-bonded σ systems. The model dimer systems of acetylene (C2H2)2, ethylene (C2H4)2, ethane (C2H6)2, benzene (C6H6)2, and cyclohexane (C6H12)2 are investigated. The ethylene dimer has large dispersion energy, while the acetylene dimer has strong electrostatic energy. The aromatic π interactions are strong with particularly large dispersion and electrostatic energies, which would explain why aromatic compounds are frequently found in crystal packing and molecular self-engineering. It should be noted that the difference in binding energy between the benzene dimer (aromatic-aromatic interactions) and the cyclohexane dimer (aliphatic-aliphatic interactions) is not properly described in most density functionals.