Metal-ligand self-assembled capsules

Correct choice of ligands, metal ions, and conditions is essential for assembly of these capsules [4,6]. Two kinds of ligands (ie, one-dimensional and two-dimensional ligands) can be used for assembly of metal-ligand self-assembled capsules [7]. 

In the uncatalyzed reaction, the rate-determining step was electrocycUza-tion in the catalyzed reaction, water loss and electrocycUzation faced equal barriers. The observed rate enhancement was ascribed to the transition-state stabilization and enhanced basicity, which originated from encapsulation within the metal-ligand self-assembled capsule. 

Self-assembly can also be induced by metal-ligand interactions [33,34], In this case, supramolecular coordination complexes that are thermodynamically favored products can be obtained from spontaneous metal—ligand bond formation in a mixture of soluble metal and ligand precursors [19]. These self-assembled capsules, which are known as coordination-driven self-assembly, are more robust than hydrogen bond capsules [13,35]. 

There exist a vast number of discrete, polymetallic coordination clusters both of the metal-metal bonded type and linked by a tremendous variety of bridging ligands, notably carboxylates. Such compounds are not coordination polymers but oligomers and hence can be more soluble, more well-defined and easier to characterise than coordination polymers, for which they can serve as useful model systems. We will discuss discrete, self-assembled complexes of semi-protected metal ions that act as hosts in solution or as 3D capsules in the next chapter and we will not cover these systems in detail here except to note in passing a couple of examples that are of particular interest. One particularly prominent cluster is Muij-acetate, the mixed-valence compound... 

Since the nineties of the last century, research groups around the world have explored the application of nanocapsules as nanoreactors, i.e. reaction vessels for chemical transformations, and the influence of different cavity effects." In this chapter the focus will mainly be on recent developments concerned with synthetic nanoreactors that can be obtained in a selective and controlled manner through the use of self-assembly principles and rational design, and on their application as catalytically active capsules for respective chemical reactions. For the sake of clarity, each specific type of nanoreactor will be discussed in a separate section. Particular types of nanocapsules to be reviewed include assemblies held together by hydrogen bonding, metal-ligand interactions and hydrophobic... 

The efficiency of these hemicarcerand and polycavitand nanocapsule syntheses approaches those of self-assembly processes using hydrogen bonding or metal-ligand coordination and results from selection of the least strained capsule through the thermodynamic control and reversibility of the system. 

The Nazarov cyclization of pentamethylcyclopentadienol was catalyzed in the presence of a self-assembled metal-ligand supramolecular capsule [Ga4L6] at mild temperature [25]. Remarkable acceleration—2.1 million times comparable to that of enzymes—was observed. 

Fujita et al. developed self-assembly of predesigned organic ligands with transition metals having 90° coordination angles. They incorporated cw-protected square planar metals, [(L)M], into metal-organic frameworks such as cages, cones, capsules, and boxes (Fig. 9.19) [9]. 

These molecular capsules self-assemble through a range of forces such as metal-ligand interactions, hydrophobic interactions, hydrogen bonding, and dynamic covalent chemistry (Figure 8.2). Their lifetimes vary from milliseconds to days. 

In conclusion, self-assembled molecular capsules are held together by weak inter-molecular forces such as hydrogen bonds, CH-n interactions, van der Waals forces, strong metal-ligand binding, and even dynamic covalent chemistry. Capsule formation creates an inner cavity with a unique shape that can accommodate guests with complementary shapes and binding sites. The behaviors of molecules in dilute... 

Molecular arrangements where one or more molecules accommodate other molecules or ions within a confined space have received the name of molecular containers. To date molecular cOTitainers of various sizes and shapes have been reported, as have those containing interior recognition sites [73]. The nature of the molecular container can be permanent, in which the architecture of the container is constituted by covalent bonds, [74] or temporary with the structures being spontaneously self-assembled via weaker interactions, such as metal-ligand interactions [75], hydrogen bonds [76], or hydrophobic forces [77]. Containers that self-assemble through supramolecular interactions are typically referred to as capsules. 

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