Porous Organic Coordination Solids: The ability to control the structure, topology and functionality of organic solids in molecular organic synthesis is one of the central issues in organic solid state chemistry. One fundamental component of such control is the interplay between molecular structure and function, and molecular packing in crystalline materials.
We have recently combined three ideas to connect molecular structure to crystal structure: the simplicity of highly symmetric and rigid organic molecules, the use of directional but reversible coordination bonds between molecules, and an understanding of inorganic packing principles using these principles to design organic porous solids with guest selective channels ranging from 15 to 35 Å in diameter. We have further demonstrated that both the functional character and the channel size can be modified without change to the overall topology of the backbone framework structure.
We are currently developing stronger porous solids in which all the host porous bonds are covalent in character. In our previously developed porous materials, the organic molecules were linked together via coordination bonds. Such reversible coordination bonds allow the formation of single crystals suitable for X-ray work, but severely limit the stability of the resultant phases. We have now prepared solvent channels in which the pore channels are rich in nucleophilic hydroxyl groups. We propose to introduce cross-linkable guests (such as di-isocyanides or disilyltriflates) which will react with these nucleophilic groups leading to a fully covalent organic porous solid. To date we have created crosslinked channels, but with insufficient crosslinking to generate a fully covalent host porous network.
Long range order in intermetallic phases: One of the most perplexing families of compounds with respect to their bonding patterns are intermetallic phases. Elements whose bonding motifs are quite simple when found in molecules, adopt a dizzying series of bonding patterns when involved in intermetallic compounds. Selenium (0) and (-I) which form, respectively, two bonds, and just one bond in many molecules, adopt local bonding environments based on square planar, square pyramidal and even distorted octahedral coordination to other selenium atoms in the solid state. One probe which provides a window into the bonding forces in these solids are the complicated superstructures adopted by these phases. For example we have recently examined the crystal structure of Dy132Se240. Amazingly this large unit cell of 372 atoms is a superstructure of a unit cell with just six atoms. Using a combined approach of X-ray crystallography, model Hamiltonians, and accurate quantum calculations, we have been able to create a simple chemical model which accounts for the complex superstructure. We find a combination of site occupancy waves and charge density waves are responsible for the large superstructure. As a further test for our models, we now propose to examine noble metal alloys where unit cell dimensions range from just a few, to almost 104 ?. We focus here on the large unit cells found in brass structures (Cu-Zn alloys). We propose that the large unit cells found in such systems are the result of two slightly different periodicities in the constituent atoms. This overall beat phenomena is driven by the Fermi surface of the alloy structure.
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