Anion-Coordination-Driven Assembly

Abstract

The assembly of discrete architectures has been an important subject in supramolecular chemistry because of their elegant structures and fascinating properties. During the last several decades, supramolecular chemists have developed manifold strategies for hierarchical assembly, which are normally classified by two main types of driving force: covalent and noncovalent interactions. Typical noncovalent interactions include metal coordination, hydrogen bonding, and other weak forces. These approaches have achieved great progress in the construction of various supramolecular structures, such as macrocycles, cages, polyhedra, and interlocked systems. Among these methods, metal-coordination-driven assembly is attractive due to the well-defined coordination properties of metal ions. Indeed, in terms of supramolecular chemistry, the concept of "coordination" has been expanded beyond transition metals. In particular, anion coordination chemistry, which was first proposed by Lehn in 1978 [Acc. Chem. Res. 1978 and then elucidated in detail by Bowman-James two decades later [Acc. Chem. Res. 2005, 38, 671], has grown up to a subfield of supramolecular chemistry. It is noticeable that anions also show "dual valencies" like transition metals, wherein the "primary valence" is the charge balance for anions by countercations while the "secondary valence", i.e., the coordination, refers to hydrogen bonding interactions where the electron flow is from the electron-rich anion (the coordination center) to hydrogen bonding donors (the ligands). Thus, anions also display certain coordination numbers and specific coordination geometries. Although such features are far less regular than those of transition metals, they are sufficient to allow anion coordination to serve as the driving force for assembling discrete supramolecular architectures. In this Account, the anion-coordination-driven assembly (ACDA), a new assembling strategy established by us during the past decade, will be presented. We summarize our work in the construction of a series of "aniono" supramolecular structures, especially triple helicates and tetrahedral cages, based on the coordination between oligourea ligands and anions (mostly phosphate). In particular, we will detail the considerations in the design of ligands, the assembling process including structural transformation, and functionalization of the systems toward guest inclusion, supramolecular catalysis, photoswitches, and molecular devices. These results demonstrate the great potential of ACDA in fabricating novel anion-based systems. Although the design concept was originally loaned from traditional coordination chemistry of transition metals, and structures of anion complexes bear some resemblance to metal complexes, there are significant differences of the aniono supramolecular assemblies from the metallo analogues. For example, these metal-free systems are held together by multiple hydrogen bonds (dozens to nearly 100), thus facilitating assembly/disassembly under mild conditions and relatively flexible s...

Gyroidal MOFs: anion-coordination effect enables inverse architecture and enhanced moisture stability

“…3c, the samples of GMOF-3 can maintain their structure at 68.9% RH. These results reveal that the anion coordination effect, which often plays an important role in the supramolecular assembling as well as the building of coordination cages, [29][30][31] may also be conducive to improving the moisture stability of MOFs. Thus, this work provides a new approach to strengthen the framework stability by tuning the coordination bond, which may give new insight into the construction of stable MOFs.…”

“Cage Walking” Synthetic Strategy for Unusual Unsymmetrical Supramolecular Cages

“…Over the past few decades, supramolecular assembly has received increasing attention − due to its powerful method for the construction of diverse metal-containing entities with designed geometries and symmetries. − The methods of supramolecular assembly that have been reported and thus far mainly include coordination-driven self-assembly, , folding assembly, and templated methods (covalent scaffolds, metal-ion templates, , anionic templates, , etc.). However, these methods are primarily suitable for constructing supramolecular cages with high symmetry. , For the construction of twisted supramolecular cages, these methods are often less suitable due to the requirement for multiple driving forces to work synergistically.…”

Directional Ionic Bonds

“…The spatial orientation of bonding orbitals imparts directionality to bonds with significant covalent character, − thereby enabling the three-dimensional structuring of complex molecules. − Next to the directionality of covalent bonds, the presence of directional noncovalent interactions is often essential for the structure of biological and synthetic organic matter. Most notably, the directionality of hydrogen bonds is relevant for the base pairing in DNA and for other molecular recognition phenomena in biology and chemistry and it is a ubiquitous design element for synthetic supramolecular chemistry. − In comparison with hydrogen bonds, the utility of other directional noncovalent interactions, of which halogen bonds are the main representative, is more limited. − Here, we focus on imparting directionality to ionic bonds in an attempt to provide a distinct directional noncovalent interaction for the three-dimensional structuring of matter. Ionic bonds are present in ionic solids, such as sodium chloride, and are relevant for the structure of organic materials, − synthetic supramolecular assemblies, , and biological molecules, including DNA and proteins .…”