Scanning Electron Microscopy.

Abstract: The designed construction of extended porous frameworks from soluble molecular building blocks represents one of the most challenging issues facing synthetic chemistry today. Recently, intense research activities directed toward the development of this field have included the assembly of inorganic metal clusters,1 coordination complexes,2 and organic molecules3 of great diversity into extended motifs that are held together either by strong metal-ligand bonding or by weaker bonding forces such as hydrogen-bonding and π-π interactions. Materials that have been produced in this way are referred to as modular since they are assembled from discrete molecules which can be modified to have well-defined function.4 The fact that the integrity of the building blocks is preserved during the synthesis and ultimately translated into the resulting assembled network offers numerous opportunities for designing frameworks with desirable topologies and architectures, thus paving the way for establishing connections between molecular and solid properties. At least three challenges have emerged in this area that must be reckoned with in order for the ideas of rational and designed synthesis of porous materials to become a reality with routine utility. First, it is difficult to control the orientation and stereochemistry of the building blocks in the solid state in order to achieve a given target molecular topology and architecture. Second, in most cases, the products of such assembly reactions are obtained as poorly crystalline or amorphous solids, thus prohibiting their full characterization by single-crystal X-ray diffraction techniques. Third, access to the pores within open structuressan aspect that is so critical to their utility as porous materialssis often prevented by either selfinterpenetration as observed for very open frameworks or strong host-guest interactions that lead to the destruction of the host framework when removal or exchange of guests is attempted. To define and investigate the parameters contributing to the assembly of materials from molecular building blocks, we have established a program aimed at constructing modular porous networks by linking inorganic metal sulfide clusters and organic molecules with transition metal ions. Our work has focused primarily on studying the issues outlined above, and this Account presents our progress toward finding viable and general solutions to these challenges. This is illustrated by some representative examples chosen from the chemistry developed in our research effort for the three building blocks shown in a-c. Their functionality, shape, size, and