Hiroki Takahagi

Guest‐Induced Dynamic Self‐Assembly of Two Diastereomeric Cage‐Like Boronic Esters

Guest-induced dynamic self-assembly of several different macrocyclic host molecules starting from the same set of components is an active area of research in the field of supramolecular chemistry. In case that chiral, racemic components are employed, formation of diastereomeric host molecules becomes possible, which makes the dynamic selfassembly more intriguing. However, the diastereoselective self-assembly using racemic substrates is still rare, and moreover, selective formation of each diastereomer by the addition of an appropriate guest molecule has remained as a challenging subject due to the difficulty in discriminating diastereomeric structures by non-covalent host–guest interactions. We recently reported the dynamic self-assembly of macrocyclic boronic esters based on the reversible formation of boronic ester in the presence of appropriate guest molecules. 4] In this system, two types of macrocyclic boronic ester of different size could be produced diastereoselectively by the addition of appropriate guest molecules, for which precipitation of the complex played an important role for selective assembly. To realize more precise discrimination of host–guest interaction, we thought of using 1,3,5-benzenetri(boronic acid) 2 instead of 1,4-benzenedi(boronic acid), because the expected cage-like structure has an internal space surrounded by chiral tetrols from three directions. In this paper, we report disubstituted benzene-induced selective formation of two diastereomeric cage-like boronic esters from racemic tetrol 1 and 1,3,5-benzenetri(boronic acid) 2. When racemic tetrol 1 and 0.70 molar amount of tri(boronic acid) 2 were mixed in methanol at room temperature (Scheme 1), the mixture became gradually heterogeneous, but the filtrated solid was soluble in several organic solvents such as chloroform, toluene, etc. FAB-MS and H, C NMR spectral data suggested the formation of a single, highly symmetric product, 3:2 complex of tetrol 1 and tri(boronic acid) 2 (abbreviated as homo-[3+ 2]). It was also confirmed by H NMR that methanol was not included in this isolated homo-[3+2] . It should be noted that in the previous ACHTUNGTRENNUNG[2+2] formation from 1,4-benzenedi(boronic acid) and tetrol 1, addition of appropriate guest molecules was necessary and polymeric boronates insoluble in all organic solvents examined were obtained in methanol. On the other hand, this homo-[3+2] structure is thought to be thermodynamically more stable than polymeric boronates and was constructed without adding guest molecules. Single crystal of this homo-[3 +2] suitable for X-ray diffraction analysis was successfully obtained by the slow vapor diffusion of n-pentane into a toluene solution of homo-[3+ 2] . As shown in Figure 1, the complex has the expected cage-like structure and is composed of three molecules of tetrol 1 and two molecules of tri(boronic acid) 2, and one molecule of toluene was observed in its internal space, suggesting the presence of p–p interactions with the phenyl

Crystallization‐Controlled Dynamic Self‐Assembly and an On/Off Switch for Equilibration Using Boronic Ester Formation

Macrocyclic boronic esters of different sizes can be prepared selectively from the same starting diboronic acid and 1,2-diol by means of an interesting dynamic self-assembly phenomena. More specifically, two kinds of macrocyclic boronic esters could be formed diastereoselectively and nearly quantitatively under neutral conditions by the addition of an appropriate guest molecule that acts as a template. Although a mixture of tetrol 1 and di(boronic acid) 2 in methanol gave only insoluble polymeric boronic esters, a soluble macrocyclic boronic ester, homo-[2+2], was obtained selectively in the presence of toluene as a guest molecule. Furthermore, when benzene was employed as a guest molecule, the selective formation of another macrocyclic boronic ester, hetero-[3+3], occurred. Interestingly, each of these macrocycles could be converted into the other in the presence of methanol and the appropriate guest molecule; however, under aprotic conditions, guest molecules encaged by the macrocyclic boronic ester could be exchanged without affecting its structure. Thus the presence or absence of a protic solvent could be used as a regulator to switch on or off the dynamic equilibrium of the system. In addition, investigation of the effect of reaction time, direct observation of the reaction mixture by NMR spectroscopy, and carrying out the reaction using optically active tetrol suggested that precipitation plays an essentially important role in the selective formation of the macrocyclic boronic esters. Thus, although both of [2+2] and [3+3] were present as solutes in the reaction mixture, the type of added guest molecule induced the selective precipitation of only one form of macrocyclic boronic ester, hence displacing the equilibrium of the system.

Enhancement of host-guest interactions using rationally designed macrocyclic boronic esters with a naphthalene core.

Efficient inclusion of electron-deficient aromatic guest molecules in an organic solvent utilizing π-stacking interactions was achieved by using two kinds of macrocyclic boronic esters, 1,4-naph-[2+2] and 1,5-naph-[2+2], which were easily prepared by self-assembly of 1,4-naphthalenediboronic acid (3) or 1,5-naphthalenediboronic acid (4) and racemic tetrol 1 with an indacene framework in a protic solvent. The X-ray crystallographic analyses revealed that the tilt angles of the two naphthalene rings are different: that of 1,4-naph-[2+2] is about 15° and that of 1,5-naph-[2+2] is about 0°. Owing to the parallel alignment of two aromatic rings, 1,5-naph-[2+2] has a much higher binding ability than 1,4-naph-[2+2]. This knowledge could be useful for the design of the new host molecules in organic solvents.

Open channels in porous molecular crystals: host–guest structures and interactions

Porous organic materials have attracted much attention in the material science field due to their functions such as gas sorption, catalytic abilities, and others. One of the interesting families of the materials is Porous Molecular Crystal (PMC). It is constructed by discrete organic molecules forming a framework structure in the crystal. When intrinsic porous molecule such as cage or ring molecules stack columnar in crystal, a porous channel should be formed as open pores in the PMC.

Photochromism change by conformation control in macrocyclic boronic ester cavity

N-salicylideneaniline derivatives are known to show photochromism by UV light, and it depends on the molecular conformation in the crystal. The twisted molecule is photochromic but the planar one is not[1,2]. N-salicylidene-2-aminopyridine (2SAP) always has a planar conformation due to the chemical structure without steric hindrance, therefore 2SAP is known as non-photochromic. However, by confining the molecule in a cavity of the macrocyclic boronic ester 1[3], the conformation and photochromism can be controlled. The inclusion crystal of 1 (homo-parallel form) has a special feature to have a channel type crystal structure in which the macrocyclic ring aligned one-dimensionally to include guest molecules. Interestingly, the crystals can reversibly absorb/desorb guests maintaining the crystal structure, because the channel structure is robust. To exchange the included guests, the crystal of 1(homo-parallel form) was immersed in the melt of 2SAP at 90 ̊C. The obtained inclusion crystal showed photochromism. In the crystal structure, the shape of residual electron density for 2SAP indicated the conformation of 2SAP was twisted due to the confining in the cavity, which is the reason for photochromism. On the other hand, the recrystallization of 1 and 2SAP from THF / hexane solution unexpectedly gave hetero-antiparallel form of 1 because of the dynamic covalent bond formation in the boronic esters (Fig.). As this hetero-antiparallel form has planar cavity in the molecule, the conformation of the included guest molecule, 2SAP, is also planar as always seen. Therefore the inclusion crystal is non-photochromic. Thus, photochromism change of 2SAP was realized by conformational control through confining in the cavity of macrocyclic boronic ester.

Guest Inclusion Crystal structures of Macrocyclic Boronic Esters

Macrocyclic compound has been attracting increasing attention because of their application for guest absorption and storage, guest selectivity, and reaction environment, which would utilize void space in the compound. Recently, such macrocyclic compound, boronic esters, has found to be formed as dynamic self-assembly of organic molecules through solvent dependent dynamic covalent bond formation between racemic polyol and planar 1,4-benzen(boronic acid)[1]. Thus, it is important to determine the crystal structure of the macrocyclic boronic esters with incorporated guest molecule to reveal the features of the compounds. In this study, structures of boronic ester of 1,4naphthalene(boronic acid) (1) are presented and compared. The boronic ester with toluene guest molecule crystalized in monoclinic system, C2/c, Z=4, V=5099.7(6) Å3. As expected, toluene molecule was accommodated within a ring sandwiched by two naphthalene rings suggesting π-π interaction (ca. 3.6 Å separations). It is interesting that other structures of 1 with 1,4-dicyanobenzene, chloroform, and THF also have isomorphic structures to 1 with toluene. It clearly shows the guest inclusion ability of this boronic ester by weak intermolecular interactions. In the crystal structures, the boronic ester aligned along b-axis forming one-dimensional stacking with channel structure filled with guest molecules. Also, 1 with chloroform has a pseudo-polymorph phase (P21/c, V=5780.8(13) Å3) that has two additional chloroform molecules inside and outside of the ring; however, it also shows similar one-dimensional stacking structure with channel, implying this boronic ester has an easily stacking molecular shape. Although, the molecules have similar [2+2] ring structure, dihedral angle between two facing naphthalene rings is different in 1 with toluene, which is smaller as 14.1° than 22 to 24° in other structures. It may indicate a flexibility of the macrocyclic ring.

Guest-induced fluorescence property of macrocyclic boronic ester

Macrocyclic boronic esters (1) are obtained as a self-assembled molecule by condensation reaction between rac-tetrol (2) and 1,4naphthalenediboronic acid (3) in the presence of toluene molecule [1]. In the crystal, this macrocyclic molecules form a charasteristic one dimensional channel structure that accommodates various small molecules. Interestingly, reversible desorption / absorption phenomena of guest molecules is observed without significant crystal packing change, meaning this crystal may have guest storage, separation, and catalytic abilities. In the course of exploring further functional aspects of the molecule, we give fluorescence property to this crystal by inclusion of acene molecules into this robust one dimensional channel structure. Naphthalene inclusion crystal was obtained by the diffusion method. The crystal structure is isostructural to known crystals, that is, a naphthalene molecule is included in a channel and sandwiched by two naphthalene moieties of the macrocyclic molecule (inter planar distance is about 3.6 angstrom). Under UV light, a blue color fluorescence observed in this crystal, suggesting the guest naphthalene molecule contributes the fluorescence property. After heating by 200 degrees C, the naphthalene was released to leave isostructural apohost crystal without fluorescence property. However, by naphthalene vapor exposure to the apohost crystal, the fluorescence property was recovered, which means naphthalene desorption and absorption are possible in crystalline state. Moreover Anthracene and Tetracene inclusion crystal were obtained, and they also showed light blue and yellow color fluorescence under UV light, respectively. Thus, the fluorescence function was successfully realized by inclusion of acene molecule in the one dimensional channel of the crystals, and furthermore the fluorescent color can be controlled by changing acene molecules.

Macrocyclic Boronic Ester: Porous and Host-Guest Structures Revealed by SDPD

Diboronic acid and racemic tetrol are found to form a self-assembled macrocyclic boronic ester in the presence of appropriate guest molecules[1]. In the crystal, stacking of macrocyclic ring is observed to form infinite channel structure accommodating guest molecules by supramolecular interactions. In such structure, it is important to investigate the guest uptake and release mechanism via the crystal structure determination of the guest-free apohost. However, the apohost crystal can only be obtained by guest release process by heating, which results to form powder crystals. In this study, the structure of apohost was determined by “Structure Determination from Powder X-ray Diffraction data” technique [2][3] and the structural change by guest sorption and desorption processes were investigated. The powder X-ray diffraction pattern of the toluene inclusion crystal and the apohost crystal, which was obtained by heating of the inclusion crystal, are significantly different. However, even after the guest release, the apohost structure determined from the powder X-ray diffraction data was found to retain its crystal packing with one dimensional guest free channel. Interestingly, the apohost crystal easily absorbs the toluene and other aromatic molecules when the vapor was applied, and the crystal transforms into the guest inclusion crystal. Also, such vapor application is interesting method to switch the physicochemical property of the crystal. When naphthalene vapor was applied to the apohost crystal, naphthalene inclusion crystal was readily formed, and it became fluorescent crystal. This property was switched off by heating and guest desorption. In summary, the macrocyclic boronic ester is promising compound that forms inclusion supramolecular crystal, which can be utilized as guest storage / release, separate, protect, and other physicochemical functional material.