Finite assembly

In Nature, self-assembly to form finite assemblies often involves the non-covalent organization of molecules containing not only amphiphilic character, but also specific information needed for additional intermolecular recognition processes to occur, e.g., hydrogen... 

Developments in supramolecular chemistry provide a number of very intriguing examples of rather complex, but nevertheless well-defined, hydrogen bonded superstructures which present new building blocks for conjugation with macromolecular systems. Figures 36a and b give two examples of finite assemblies based on... 

Discussing finite assemblies, one may not forget the nanogrids developed by Lehn. Complexation of multi-binding site ligands containing catenated bipyridine... 

The examples discussed above refer to finite assemblies or conjugates with linear macromolecules. Based on the pioneering work by Jean Marie Lehn a number of approaches have been developed in which secondary bonds are employed to build up chain molecules. Different supramolecular chain polymers, made up from hydrogen-... 

Supramolecular synthons, finite assemblies, and functional solids 112... 

It is with these ideas in mind that we focus here on the design and construction of finite molecular assemblies in the organic solid state. Our intention is to provide an overview of finite assemblies with emphasis on properties that such assemblies may provide solids. We will begin by outlining general criteria for constructing finite molecular assemblies in both the solid state and solution, and then describe assemblies isolated and characterized in the solid state to date. We will then use recent advances in our laboratory to illustrate how finite assemblies can be used to control solid-state reactivity and direct the synthesis of molecules. 

As the number of components that make up a finite molecular assembly increases so does the size and, generally, the complexity of the assembly. Thus, molecular assemblies with three, four, and five molecules as components may form 2D cyclic structures of increasing size in the form of trimers, tetramers, and pentamers, respectively (Scheme l).3a The components may also be arranged in three dimensions to form a cage. Notably, useful classifications of the structures of finite assemblies based on principles of plane (i.e. polygons) and solid geometry (i.e. polyhedra) have been recently discussed.4... 

Although a finite molecular assembly may form in either the liquid phase or the solid state, such an assembly will exhibit markedly different structural behavior in each medium. In the liquid phase, a molecular assembly will be in equilibrium with its parts, as well as possible undesired complexes.3a Such equilibria will reduce the structural integrity of an assembly and may require stronger forces to hold the parts together. It has been suggested that the sensitivity of multiple equilibria to subtle environmental factors in solution (e.g. solvent effects) has hindered the development of finite assemblies that exhibit function.3a In the solid state, the structural integrity... 

Although supramolecular synthons have been used to construct networks, it is important to note that such structural units may also be used to construct functional solids based on finite assemblies of molecules. That supramolecular synthons may be used to construct such solids stems from the fact that the synthetic strategy to... 

As stated, hydrogen bonds have been used to construct the majority of finite molecular assemblies. Thus, most synthons used to form finite assemblies in the solid state have been based on hydrogen bonds. Many such synthons have also been used to form networks.2 Examples include single-point hydrogen bonds based on phenols and imidazoles, as well as multi-point hydrogen bonds based on carboxylic acid dimers, pyridone dimers, urea dimers, cyanuric acid-melamine complexes, and pyridine-carboxylic acid complexes.2... 

Finite assemblies constructed owing to synthetic reasons have been used to either sequester3 assemblies from solution or develop new solid-state designs. Such assemblies have involved two components, as well as higher-order structures of ID, 2D, and 3D connectivity. 

The smallest number of molecules that may form a finite assembly is two.3,4 Thus, two molecules may assemble to form a finite structure in the form of either a homodimer or a heterodimer. Whereas single crystals of a homodimer are prepared via crystallization of the pure molecule, single crystals of a heterodimer are prepared via co-crystallization of the different individual components. 

Three or more molecules may assemble to form a finite assembly based on a ID geometry. Such an assembly will involve a central core capped by two mono-functional components. 

Three or more molecules may form a finite assembly with a 2D geometry. The connecting forces of such assemblies will be propagated within a plane. The components may assemble to adopt a cyclic geometry or branch from a central point. 

Three or more molecules may assemble in the solid state to form a finite assembly with connecting forces propagated in 3D. The components of such an assembly will typically form a polyhedral shell. The shell may accommodate chemical species as guests. The polyhedron may be based on a prism or antiprism, as well as one of the five Platonic (e.g. cube, tetrahedron) or 13 Archimedean (e.g. truncated tetrahedron) solids.4... 

Whereas the finite assemblies described above have been isolated owing largely to synthetic reasons, the assemblies that follow exhibit a particular function. Examples of such function include host guest behavior, chemical reactivity, and chirality. In doing so, the assembly affects or contributes to a property of a solid (e.g. inclusion, reactivity). Importantly, the functions arise owing to the geometric arrangement of the constituent components within the assemblies. 

As stated, a finite assembly with components arranged in 3D will possess a structure that conforms to a polyhedron. The simplest polyhedra are the prisms while poly-hedra of increasing complexity include Platonic and Archimedean solids. 

Resorcin[4]arenes that adopt a boat conformation have also been demonstrated to form finite assemblies in the solid state. In particular, members of a series of... 

A tetrameric capsule of ideal D2 symmetry has been reported by Rebek.65 In particular, a molecule with glycoluril and sulfamide functionality self-assembled in the solid state via 24 hydrogen bonds to form a finite assembly that accommodated 2,6-adamantanedione as a guest. The volume of the cavity of the capsule was determined to be approximately 184 A3 (Fig. 35). Each carbonyl oxygen atom of the... 

The molecular assemblies described above have inspired us, in recent years, to develop finite assemblies in the solid state that exhibit chemical reactivity. Specifically, we,69 and others,70 have been utilizing principles of molecular recognition and self-assembly to develop a method to direct the formation of covalent bonds in organic solids. The method builds on the work of Schmidt on the reactivity of cinnamic acids in the organic solid state.45 Specifically, Schmidt has described topochemical postulates that dictate geometry criteria for a [2 + 2] photodimerization to occur in a solid. The postulates state that two carbon-carbon double (C=C) bonds should be aligned in parallel and separated by a distance <4.2 A to react. 

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