Finite molecular assemblies

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. 

There are excellent reviews that address the structures of finite molecular assemblies.3 The literature principally involves molecular assemblies designed, constructed, and characterized in the liquid phase. This is unsurprising since interests in finite molecular assemblies largely originate from studies of molecular recognition and self-assembly phenomena in solution. 

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... 

A major impetus for the design and construction of a finite molecular assembly is to create function not realized by the individual components.3 The size, shape, and functionality of each component, which are achieved via methods of organic syntheses, are thus amplified within a final functional structure. The components may be synthesized, e.g., to give an assembly with cavities that host ions and/or molecules as guests.3 The components may also react to form covalent bonds.1 That a molecular assembly is, de facto, larger than a component molecule means that the components may be designed to assemble to form functional assemblies that reach nanometer-scale dimensions, and beyond.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... 

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... 

The application of linear templates to form reactive co-crystals relies on the ability of template molecules to juxtapose reactant olefins within finite molecular assemblies for reaction (Scheme 2.3.7) [42], By virtue of the finite nature of the molecular assembly, the reaction is expected to occur largely without disturbing the overall crystal structure. In that way, co-crystals involving linear templates can be expected to provide an environment suitable to achieve SCSC reactivity. The linear template... 

The ability of a SCSC reaction to occur within a finite molecular assembly based on a linear template was first demonstrated by MacGillivray and co-workers who utilized 1,8-naphthalenedicarboxylic acid as a template to assemble trans-1-(3-pyridyl)-2-(4-pyridyl)ethylene, via O-H - -N hydrogen bonds, fora regioconotrlled [2+2] photodimerization [55]. The SCSC reaction was found to proceed, upon UV-irradiation with 420 nm light, quantitatively to give the head-to-head photoproduct rctt-l,2-bis(3-pyridyl)-3,4-bis(4-pyridyl)cyclobutane. A comparison of the single-... 

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