Design, Synthesis, and Host-Guest Properties of Neutral Pd(II) Coordination Cages Supported by Imido-P(V) Anions

Abstract

Self-assembly encompasses the ability of a system to spontaneously organize itself into an ordered aggregate by a rational combination of complimentary components. Metal-organic cages (MOCs) are one such important class of compounds constructed from metal ions and organic ligands by a coordination-driven self-assembly process. These assemblies possess the advantages of exhibiting distinct portals and well-defined permanent intrinsic cavities, which makes them excellent candidates for exploring host-guest chemistry with high affinity and selectivity for the recognition of guest molecules. This thesis begins with a general introduction to metallo-supramolecular cage chemistry. Further, the various bonding techniques employed to synthesize these cages have been discussed, followed by a general concept of host-guest chemistry as a pronounced application of these supramolecular cages, especially neutral cage systems. In this regard, neutral polyhedral cages supported by Pd(II) ions, imido ligands, and carboxylate linkers are one of the emerging classes of supramolecular cages owing to their charge neutrality and excellent host-guest properties. Over the years our group has focused on the main group-based imido-P(V) anions as suitable co-ligands for the construction of polyhedral cages for Pd(II) ions in the presence of suitable organic linkers. Thereafter the following chapter reports a study of the mechanistic pathways and the intermediates behind the self-assembly of these cage assemblies, together with their structural and reactivity details. The guest encapsulation capabilities of these cages in solution and in the solid state, probed with a range of analytical techniques has been discussed in detail. The further chapter elaborates on the synthesis of cages with larger cavities and a preferential encapsulation of larger, regio-isomeric guest molecules where instead of traditional guest encapsulation, we employed an in-situ strategy to synthesize the cages in presence of the guest molecules. In the next chapter, a template-driven pathway to synthesize linker-free, neutral metallo-cube from a condensed linker-bound tetrameric precursor has been described. Here we successfully synthesized a new linker-free polyhedral cage assembly from a non-directing Cl- linker and have utilized the same towards effective encapsulation of aromatic polar guest molecules. These studies were accompanied by spectral, theoretical, and SCXRD structural analysis techniques. Finally, an investigation of the self-sorting behavior of these Pd(II) neutral cages has been performed wherein, we employed two types of imido-Pd3 based precursors: chiral and achiral, and observed their self-sorting behaviour when exposed to three different kinds of linkers with varies size, directionality and reactivity. The self-sorted chiral cages were further fully characterised and were eventually employed towards chiral recognition of three enantiomeric pairs of guests with different sizes and polarities.