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Method Article
This article describes construction of a series of hydrogen-bonding supramolecular clusters in crystals using primary ammonium triphenylacetates, which are recrystallized from non-polar solvents. This selective construction of the supramolecular clusters leads to effective systematical symmetric studies about a correlation between the supramolecular clusters and their components.
Functions of clusters in nano or sub-nano scale significantly depend on not only kinds of their components but also arrangements, or symmetry, of their components. Therefore, the arrangements in the clusters have been precisely characterized, especially for metal complexes. Contrary to this, characterizations of molecular arrangements in supramolecular clusters composed of organic molecules are limited to a few cases. This is because construction of the supramolecular clusters, especially obtaining a series of the supramolecular clusters, is difficult due to low stability of non-covalent bonds compare to covalent bonds. From this viewpoint, utilization of organic salts is one of the most useful strategies. A series of the supramolecules could be constructed by combinations of a specific organic molecule with various counter ions. Especially, primary ammonium carboxylates are suitable as typical examples of supramolecules because various kinds of carboxylic acids and primary amines are commercially available, and it is easy to change their combinations. Previously, it was demonstrated that primary ammonium triphenylacetates using various kinds of primary amines specifically construct supramolecular clusters, which are composed of four ammoniums and four triphenylacetates assembled by charge-assisted hydrogen bonds, in crystals obtained from non-polar solvents. This study demonstrates an application of the specific construction of the supramolecular clusters as a strategy to conduct systematical symmetric study for clarification of correlations between molecular arrangements in supramolecules and kinds and numbers of their components. In the same way with binary salts composed of triphenylacetates and one kind of primary ammoniums, ternary organic salts composed of triphenylacetates and two kinds of ammoniums construct the supramolecular clusters, affording a series of the supramolecular clusters with various kinds and numbers of the components.
Supramolecules are fascinating and important research targets because of their unique functions, such as construction of supramolecular architectures, sensing of ions and/or molecules, and chiral separations, originated from their molecular recognition abilities using flexible non-covalent bonds1-11. In molecular recognitions, symmetry of supramolecular assemblies is one of the most important factors. Despite the importance, it is still difficult to design supramolecules with desired symmetries due to flexibility in numbers and kinds of the components as well as angles and distances of non-covalent bonds.
Clarification of correlations between symmetries of supramolecules and their components based on systematical studies is useful strategy to achieve construction of desired supramolecules. For this purpose, supramolecular clusters were selected as research targets because they are composed of limited number of components and are evaluable theoretically12-14. However, contrary to metal complexes, there are a limited number of reports constructing supramolecular clusters due to low stability of non-covalent bonds for sustaining the supramolecular structures15,16. This low stability also becomes a problem in obtaining a series of supramolecular assemblies which have the same kinds of structures. In this study, charge-assisted hydrogen bonds of organic salts, which are one of the most robust non-covalent bonds17-20, are mainly employed to construct specific supramolecular assemblies preferentially21-32. It is also noteworthy that organic salts are composed of acids and bases, and thus numerous kinds of organic salts are easily obtained just by mixing different combinations of acids and bases. Especially, organic salts are useful for systematic studies because combinations of a specific component with various kinds of counter ions result in the same types of supramolecular assemblies. Therefore, it is possible to compare structural differences of supramolecular assemblies based on kinds of counter ions.
In previous works, supramolecules with 0-dimensional (0-D), 1-dimensional (1-D), and 2-dimensional (2-D) hydrogen-bonding networks by primary ammonium carboxylates were confirmed and characterized from a viewpoint of chirality32. These multi-dimensional supramolecules are important research targets in hierarchical crystal design27 as well as applications exploiting their dimensionality. In addition, characterization of the hydrogen-bonding networks would give important knowledge about roles of biological molecules because all of amino acids have ammonium and carboxylic groups. Providing guidelines to obtain these supramolecules separately gives them further opportunities in applications. In these supramolecules, construction of supramolecular clusters with 0-D hydrogen-bonding networks is relatively difficult as demonstrated in statistical study28. However, after clarification of factors for constructing the supramolecular clusters, they were selectively constructed, and a series of the supramolecular clusters was obtained21-25,32. These works make it possible to conduct systematical symmetric study on the supramolecular clusters to clarify component-dependent symmetric characteristics of the supramolecular clusters. For this purpose, the supramolecular clusters of primary ammonium triphenylacetates have interesting features, that is, their topological variety in hydrogen-bonding networks24,32, which would reflect their symmetric features as well as chiral conformations of the component trityl groups (Figure 1a and 1b). Here methodologies for constructing a series of supramolecular clusters using primary ammonium triphenylacetates and for characterizing symmetric features of the supramolecular clusters are demonstrated. Keys for the construction of the supramolecular clusters are introduction of bulky trityl groups and recrystallization of the organic salts from non-polar solvents. Binary and ternary primary ammonium triphenylacetates were prepared for the construction of the supramolecular clusters. Crystallographic studies from viewpoints of topologies of the hydrogen-bonding networks24,32, topographies (conformations) of trityl groups33,34, and molecular arrangements as analogues of octacoordinated polyhedrons12 (Figure 1c) revealed component-dependent symmetric characteristics of the supramolecular clusters25.
1. Preparation of Single Crystals Composed of Primary Ammonium Triphenylacetates
2. Crystallographic Studies
Organic salt formation of TPAA and primary amines were confirmed by FT-IR measurements. Crystal structures of the organic salts were analyzed by single crystal X-ray diffraction measurements. As a result, the same kinds of the supramolecular clusters, which are composed of four ammoniums and four triphenylacetates by charge-assisted hydrogen bonds (Figure 1a), were confirmed in all of the single crystals of the organic salts regardless of kinds and numbers of the componen...
A series of supramolecular clusters with closed hydrogen-bonding networks was successfully constructed and characterized from viewpoints of chirality and polyhedral features using organic salts of TPAA, which has a trityl group, and various kinds and combinations of primary amines. In this method, the critical steps are introduction of a molecule with a bulky trityl group and recrystallization of organic salts composed of the molecule and counter ions from non-polar solvents. This is because the supramolecular cluster ha...
The authors have nothing to disclose.
This work was financially supported by Grant-in-Aid for Scientific Research B (24350072, 25288036) and Grant-in-Aid for Scientific Research on Innovative Areas (24108723) from MEXT and JSPS, Japan. T.S. acknowledges Grant-in-Aid for JSPS Fellows (25763), the GCOE Program of Osaka University and Grants for Excellent Graduate Schools, MEXT, Japan.
Name | Company | Catalog Number | Comments |
Triphenylacetic acid | Aldrich | T81205-10G | |
n-Butylamine | TCI | B0707 | |
Isobutylamine | TCI | I0095 | |
tert-Butylamine | TCI | B0709 | |
tert-Amylamine | TCI | A1002 | |
Methanol | Wako | 131-01826 | hazardous substance |
Toluene | Wako | 204-01866 | hazardous substance |
Hexane | Wako | 085-00416 | |
KBr | Wako | 165-17111 |
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