Construction of Microporous Materials from Molecular Building Blocks

The considerable importance of microporous solids such as zeolites in shape-selective catalysis, ion-exchange processes, and molecular sieving applications has inspired the recent extensive efforts to produce analogous materials that are based on metal sulfide,1 metal-organic,2 and organic3 frameworks. A wide variety of molecular building units such as inorganic clusters, metal-organic complexes, porphyrin, organic macrocyles, and cyclic peptides have been linked by either metal ligation or hydrogen bonding to yield diverse extended assemblies having open frameworks. The voids present within these frameworks are usually in the form of channels or chambers where a guest molecule that was introduced during the synthesis resides. At least three issues pose a challenge to those embarking on the designed synthesis of crystalline microporous materials of this kind. First, the large open space within the constructed framework is often found to be occupied by other frameworks that are copies of the original to give an interpenetrating framework structure leaving either little or no voided space in the crystal. Second, since most of the assembly reactions are performed at or near room temperature, the formation of the ultimate product in crystalline form is usually found to be more an art than a science. This is due to the absence of a temperature gradient that allows for slow nucleation and consequently single crystal formation. Third, attempts to evaluate the porosity in these materials by exchanging or decomposing the guest species have in most cases resulted in destruction of the assembled frameworks leading to nonporous solids. This chapter describes our recent research efforts to address these issues. The molecular building block approach for the achievement of these materials will be outlined.