Finite assemblies, solid state

Finite molecular assemblies in the organic solid state toward engineering properties of solids, 40, 109... 

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. 

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... 

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. 

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... 

An example of a molecule that self-assembles to give a finite 3D assembly is triphenylmethanol.32 The alcohol has been shown to self-assemble in the solid state, via O-H-O hydrogen bonds, to form a tetramer, with the point group C3 and a structure that conforms to a molecular tetrahedron (Fig. 14). The hydrogen bonds exhibited substantial dynamic disorder in the solid. Each molecule sits at the corner of the tetrahedron with the phenyl rings in propeller-like conformations. 

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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