Yohei Haketa

Oriented Salts: Dimension‐Controlled Charge‐by‐Charge Assemblies from Planar Receptor–Anion Complexes

Salts, ionic compounds comprising cations (positive ions) and anions (negative ions), are essential materials for biotic activities. They are also utilized as inorganic minerals for industry. The appropriate arrangement of charged species through electrostatic interactions is a significant issue for constructing ordered nanoscale architectures in various states. For example, most inorganic, organic, and inorganic–organic hybrid salts use electrostatic interactions between ions to form organized three-dimensional (3D) crystal structures. The 3D structures defined herein include not only crystals of isomeric space groups in a cubic system but also non-isomeric crystals. Appropriate pairs of cations and anions yield ionic liquids, which are partially ordered but essentially nondimensional (0D) states. In ionic liquids, bulky geometries of both the cationic and the anionic species effectively prevent crystallization owing to significantly weaker ionic interactions. In contrast to crystals and liquids from ions, soft materials formed by electrostatic interactions between charged components have been reported as liquid crystals on the basis of ionic mesogens. 4] For example, Kato et al. reported various ionic liquid crystals comprising alkyl-substituted imidazolium salts, which afford columnar structures and have ionic conduction. Compared to such ionic mesophases, in which the locations of either cations or anions cannot be confirmed, more rigidly organized structures with a certain level of mobility in their building subunits are also useful for various applications such as ferroelectric materials. In contrast to bulky components, planar cationic and anionic molecules effectively interact with each other and form charge-by-charge assemblies composed of alternately stacking charged components. Aided by supplementary van der Waals interactions along with electrostatic and p–p interactions, dimension-controlled charge-by-charge assemblies will form not only crystals but also soft materials such as supramolecular gels, liquid crystals, and other organized structures. In comparison to p-conjugated cations, which are often based on sp-hybridized planar geometries, p-conjugated planar anionic species are required to delocalize their excess electrons, for example by depositing them in aromatic systems, to prevent them from suffering an electrophilic attack. Focusing on these perspectives, one of the strategies for forming planar anions is the complexation of electronically neutral p-conjugated anion receptors and spherical halide anions. As p-conjugated planes for associating with halide anions, BF2 complexes of 1,3-dipyrrolyl-1,3-propanediones efficiently bind spherical anions with inversion of pyrrole rings (1 and 2 ; Scheme 1 a). 8] Receptor 1 affords single crystals, which are composed of 1D columnar structures of alternately stacking chloride and bromide complexes and tetrapropylammonium (TPA) countercations, and are prepared from a hydrocarbon solvent. In contrast to these crystal states, an alkyl-substituted receptor 2 exhibits the formation of anion-responsive supramolecular octane gel, which is temporally transformed into a solution state by the addition of tetrabutylammonium (TBA) salts owing to the formation of soluble ion pairs comprising fairly aliphatic TBA cations and receptor–anion complexes. 9] Therefore, the introduction of planar cations in place of bulky TBA cations may form fine-tuned supramolecular organized structures as soft materials using p–p stacking and electrostatic interactions along with van der Waals forces. Herein, we present the [*] Y. Haketa, Prof. Dr. H. Maeda College of Pharmaceutical Sciences, Institute of Science and Engineering, Ritsumeikan University Kusatsu 525–8577 (Japan) Fax: (+ 81)77-561-2659 E-mail: maedahir@ph.ritsumei.ac.jp

From helix to macrocycle: anion-driven conformation control of π-conjugated acyclic oligopyrroles.

Anion-responsive pyrrole-based linear receptor oligomers were newly synthesized and their anion-driven dynamic conformation changes were investigated. Phenylene-bridged dimers and a tetramer of dipyrrolyldiketone boron complexes as π-conjugated acyclic anion receptors formed anion-driven helical structures in the solid and solution states. In fact, single-crystal X-ray analyses of the receptor-anion complexes exhibited various helical structures, such as [1+1]- and [1+2]-type single helices and a [2+2]-type double helix according to the lengths of oligomers and the existence of terminal aryl substituents. Anion-binding modes and behaviors of the oligomers in solution state were also examined by (1)H NMR and UV/Vis spectra along with ESI-TOF MS. Differences in the binding modes were observed in the solid and solution states. The oligomers showed augmented anion-binding constants and anion-tunable electronic and optical properties in comparison with the monomer receptor. A negative cooperative effect in the tetramer was observed in the second anion binding of the [1+2]-type single helix due to electrostatic repulsion between two anions captured in the helix. Further, an anion-template coupling reaction from the linear dimer provided a receptor macrocycle, which was obtained as a Cl(-) complex with distinct electronic and optical properties. The macrocycle exhibited extremely high anion-binding constants (>10(10) m(-1) in CH(2)Cl(2)) through multiple hydrogen bonding.

Asymmetric Induction in the Preparation of Helical Receptor–Anion Complexes: Ion‐Pair Formation with Chiral Cations

A number of helical structures have been reported. Foldamers form helical structures in response to chemical stimuli such as neutral molecules, metal cations, and anions. The ability to prepare enantiomerically enriched helical foldamers is crucial for applying helical structures to functional materials with chiroptical properties. One strategy for preparing enantiomerically enriched helices is the direct attachment of chiral moieties to the foldamers. In addition, the introduction of a chiral guest species can also induce the preferential formation of one diastereomer of the resulting complex through specific noncovalent interactions between the guest and the host system. Electrostatic interactions between oppositely charged species can occur in the absence of specific interactions. Therefore, a challenging way to make a compound fold into an enantiomerically pure chiral structure is to use electrostatic interactions between an achiral ion and an enantiomerically pure chiral counterion. In fact, chiral anions have been used for the preparation of enantiomerically pure metal helicates. 8] Conversely, the association of chiral cations with helixforming compounds that contain receptor and anionic moieties has led to the formation of enantiomerically pure helical structures. The chiroptical properties of receptor– anion helical complexes that form through hydrogen bonding can be difficult to examine because they can undergo more facile interconversion between enantiomeric helical forms compared to metal-based helices that form through coordination bonds. This fast interconversion is not a problem when one diastereomer of an ion pair consisting of a helical receptor–anion complexes and chiral counter cations is more stable than the other because then only one enantiomeric helix structure predominates in solution. p-Conjugated molecules that form helical structures in the presence of anions include boron complexes of 1,3-dipyrrolyl1,3-propanediones. These complexes, an example being 1a (Scheme 1a), bind anions through dynamic conformational changes involving rotation of the bond between the carbonyl group and the pyrrole moiety, thus resulting in helical oligomers (e.g., 2a and 2b, Scheme 1b). 11b, c,e] These helical oligomers were observed in the solid state and were comprised of alternately stacking negatively and positively charged species, that is, oligomer–anion complexes and counter cations, respectively. Anion complexes of the receptor-containing oligomers could be formed in enantiomerically enriched state in solution through ion pairing with optically active cations. In this paper, we report the preparation of enantiomerically enriched anionic helices that form electrostatic interactions with chiral counter cations; we also describe the chiroptical properties of these helices such as their circularly polarized luminescence (CPL). 13] Chiral p-conjugated cations are suitable candidates for inducing asymmetry in helix formation owing to their ability to form interactions with p-conjugated receptor–anion complexes. Therefore, we focused on the chiral binaphthylammonium Cl and Br salts, RR·X and SS·X (X = Cl and Br) (Scheme 1c), which Ooi, Kameda, and Maruoka reported as being efficient phase-transfer catalysts in enantioselective reactions. The formation of 1:1 receptor–anion complexes in solution can be followed by analyzing electronic spectra. Upon the addition of RR·Cl (1.5 equivalents) to 2b in CH2Cl2 (1 mm) at 20 and 70 8C, the UV/Vis absorption bands associated with 2 b at 514 and 523 nm decreased and those at [*] Dr. Y. Haketa, Y. Bando, Prof. Dr. H. Maeda College of Pharmaceutical Sciences, Ritsumeikan University Kusatsu 525–8577 (Japan) E-mail: maedahir@ph.ritsumei.ac.jp

Heteroaryl-substituted C3-bridged oligopyrroles: potential building subunits of anion-responsive pi-conjugated oligomers.

Acyclic anion receptors (BF 2 complexes of dipyrrolyldiketones) with heteroaryl moieties synthesized by using cross-coupling reactions have unique electronic states, as observed in UV/vis absorption spectra. Among heteroaryl-substituted derivatives, a pyrrolyl-substituted receptor exhibits an extremely high affinity for anions in solution.

Discotic columnar mesophases derived from 'rod-like' π-conjugated anion-responsive acyclic oligopyrroles

Rod-like pi-conjugated anion-responsive acyclic oligopyrroles have been reported to form stacked disk-like components that result in the formation of discotic columnar mesophases as thermotropic liquid crystals.

Synthesis, Crystal Structures, and Supramolecular Assemblies of Pyrrole-Based Anion Receptors Bearing Modified Pyrrole β-Substituents

Dipyrrolyldiketone BF(2) complexes acting as acyclic anion receptors form supramolecular assemblies with structures and properties that are dependent on the pyrrole β-substituents. In particular, although β-alkyl substituents interfered with the formation of stable gel states, the introduction of fluorine moieties induced a stable supramolecular gel when compared to that of β-unsubstituted receptor.

Electronic and Optical Properties in the Solid‐State Molecular Assemblies of Anion‐Responsive Pyrrole‐Based π‐Conjugated Systems

On the basis of the chemistry in solution, the solid-state structures and the corresponding electronic and optical properties of dipyrrolyldiketone boron complexes as π-conjugated acyclic anion receptors have been investigated. Solid-state assemblies of the receptors exhibit anion-dependent properties that are in sharp contrast to those in the solution state. Anion complexation, along with structural differences in the pyrrole subunits such as a benzo-fused pyrrole, plays an essential role not only in the formation of the assembled modes but also in determining electronic and optical properties, as well as the charge-carrier mobilities. In addition, by anion complexation, inclusion of the counter cations into the crystals has also been found to be one of the essential factors to determine the properties.

Charge‐Based Assemblies Comprising Planar Receptor–Anion Complexes with Bulky Alkylammonium Cations

Charge-by-charge assembly: On the basis of a planar receptor-anion complex, bulky tetraalkylammonium cations were found to construct charge-by-charge assemblies comprising alternately stacked positively and negatively charged species. The number and length of long alkyl chains in the cations determine the states of assembled structures, resulting in the formation of a library of charge-based materials (see scheme).

Chirality Induction by Formation of Assembled Structures Based on Anion-Responsive π-Conjugated Molecules

Anion-responsive π-conjugated compounds having chiral alkyl chains were synthesized. Circular dichroism (CD) and circularly polarized luminescence (CPL) were observed in the solution-state assemblies of the chiral anion receptors and those of their anion complexes as salts of a planar triazatriangulenium cation. The CD and CPL spectral patterns of the ion-pair-based assemblies were completely opposite to those of the anion-free assemblies, and this suggests that anion binding and subsequent ion pairing change the chirality of the assembly modes.

Charge-by-charge assemblies based on planar anion receptors

This critical overview proposes a new concept, a charge-by-charge assembly, which consists of alternately stacked positively and negatively charged species, by introducing examples of planar anionic structures in the solid state. The preparation of planar organic anions is not easy; however, complexation of planar anion receptor molecules with inorganic anions is found to be an efficient strategy for constructing them. As suitable motifs, dipyrrolyldiketone boron complexes exhibit various receptor–anion complexes and charge-by-charge assemblies in the solid state. The concept discussed here can be applied to useful soft materials as well as crystals by modifying the anion receptors and cations.