Supramolecular Chemistry Of Nitrogen Dioxide
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Supramolecular chemistry has been defined as "chemistry beyond molecules", and involves investigating molecular systems held together reversibly by inter-molecular forces, not by covalent bonds. This dissertation discusses a supramolecular approach towards sensing, entrapment and utilization of NO2/N2O4 gases. Chapter 1 briefly discusses supramolecular chemistry and supramolecular chemistry of gases. In chapter 2, the interaction of NOx with metalloporphyrins is described. Specifically, ruthenium nitrosyl derivatives hold a special place in mimicking bio-relevant NO-metal interactions. A previously unnoticed reaction between NO2/N2O4 and a Ru(II) porphyrin is described. It causes disproportionation of N2O4 and leads to a stable nitrosyl nitrato complex. Our findings offer a new insight into the mechanism of sensing and fixation of NO2/N2O4 by metalloporphyrins. In chapter 3, the reaction between calixarenes and NO2/N2O4 gases was investigated. Exposure of tetra-O-alkylated cone or 1,3-alternate calixarenes to NO2/N2O4, both in chloroform solution and in the solid state, resulted in deeply colored calixarene-nitrosonium (NO+) complexes. In the presence of a Lewis acid, such as SnCl4, stable calixarene-NO+ complexes were isolated in a quantitative yield and fully characterized. NO+ is found encapsulated within the calixarene cavity, and forms a stable charge-transfer complex. The NO+ encapsulation was also demonstrated in titration experiments with calixarenes and NO+SbF6- salt in chloroform. The complexation process is reversible, and the complexes dissociate upon addition of water and alcohol, recovering the parent calixarenes. Chapter 4 describes the polymer supported calixarenes. Functionalized calixarenes were synthesized and attached to silica gels and polyethyleneglycol (PEG), which afforded solid materials capable of visual detection and entrapment of NO2/N2O4 both in the solid state and solution. The concept of encapsulated nitrosating reagent was introduced in chapter 5. Stable calixarene-NO+ complexes act as encapsulated nitrosating reagents; cavity effects control their reactivity and selectivity. They were effectively used for nitrosation of secondary amides. Unique size-shape selectivity was observed, allowing for favorable nitrosation of only less crowded N-Me amides. For robust, silica gel and PEG based calixarene materials, similar size-shape selectivity was observed. Enantiomerically pure encapsulating reagents were tested for nitrosation of racemic amide, showing modest but reproducible stereoselectivity.