Calixarene and coordination
complex hosts for anions.

Abstract

My Ph.D research is mainly based on ‘calixarene and coordination complex hosts for anion’. The research is to make synthetic chloride ion channels for possible treatment of cystic fibrosis. Our approach is based on derivatives of larger calix[n]arenes. Calixarenes are chalice-shaped molecules that form readily under base catalyzed-condensation of tert-butyl phenol with formaldehyde. We have synthesized calixarenes which were tuned at the lower rim with bromo alkyl nitrile and reduced followed by reaction with a series of different isocyanates bearing fluorescent functional groups capable of binding and sensing chloride and nitrate.
We are also interested how the host ligand binds with metal anions in solution and solid state chemistry. We synthesised simple pyridyl urea ligands and isomorphs. Their anion binding studies are discussed in terms of solid state and solution chemistry. Solid state structures based on silver and copper metal forming contact ion pairs with series of anions like nitrate, chloride, bromide, trifluoroacetate and boron tetrafluoride. Their solution based anion binding studies were performed and control titration results show that these hosts bind anions in a one to one fashion which was confirmed with solid state structures. An interesting aspect is that 2-ureidopyridine
ligand exists in four different anhydrous crystal forms (I – IV0), which can be crystallized by several techniques. It was found that forms I and II only crystallize without another polymorph from methanol solution or in presence of inorganic salts. Thermoanalysis shows that all the modifications melt without previous transition and applying the heat of fusion rule proves this system to be completely monotropic. The crystal structures show that the molecule forms one intramolecular and two intermolecular hydrogen bonds to give S(6) and dimeric R (8) graph synthons in all four structures. However packing arrangements differ strikingly and since the hydrogen-bonding arrangement is the same for all the four forms, the differences in energy measured by thermoanalytical methods can only be caused by the different packing arrangements involving combinations of the weaker interactions, with the CH.S contacts being among the obvious differences. Thus, this system is a clear example in which the presence of robust, reproducible hydrogen bonded synthons does not lead to any control or predictability of polymorphic form.