Design and Synthesis of Discrete Coordination Driven Self-Assemblies Supported by P-N Scaffolds and their Functional Studies

Abstract

Coordination driven discrete self-assemblies are an important class of coordination compounds in supramolecular chemistry that has witnessed tremendous growth in the past last three decades. It comprises the spontaneous formation of directional metal-ligand coordination bonds in solution and in the solid-state with wide-ranging topologies in 2D macrocycles such as triangles, squares and hexagons, etc. and 3D assemblies such as cages, capsules and polyhedra, depending upon the directionality of the starting building units (ligands and Metal precursors). This thesis objects at the use of di- and tripodal pyridyl phosphoramide ligands P(V) and other similarly functionalized multidentate ligands for the construction of various metal-ligands discrete assemblies, study their structural chemistry and probe them for their applications in materials chemistry and as anticancer agents. Self-assembly reaction of ditopic metal ions and linear ligands can lead to the formation of several discrete 2D macrocycles. Among these structures, perhaps the simplest and well-studied ones are the self-assembled supramolecular squares. The formation of supramolecular trimer is feasible in solution if ligands backbone is flexible in nature, as they are entropically favored compared to supramolecular tetramer. Kinetically labile metal ligand coordination bonds provided the freedom to the metal-ligand self-assembled structures to self-correct themselves and form thermodynamically stable product in the solution. However, molecular trimers are less common in literatures, mainly because of the strain created in their structure. External factors such as solvents, temperature, and concentration, perhaps, can tune the possibility of the formation of structures in the solution. Here, using the 3-pyridyl functionalized phenyl phosphoric diamide ligands and tetrtopic Pd(II) acceptors, we have observed the preferential formation of entropically favored supramolecular trimer from the mixture of both supramolecular trimer and tetramer in solution. Most of the supramolecular coordination complexes (SCCs) are crystalline in nature; therefore, based on their symmetry in the solid state, researchers have utilized them in material chemistry. Recently, these cages were shown to exhibit ferroelectricity owing to their charge-separated structures supported by the toggling of the anions and their H-bonding interactions with the cage framework. Utilizing a Phosphoramide ligand containing 3-pyridyl functionalities in combination with 3D-transition metal ions [Ni (II), Zn (II)], we have synthesized non-centrosymmetric discrete charge-separated octahedral cages (cage1 and cage2) and explored their dielectric and piezoelectric properties. Polymer composites of these cages were prepared in combination with PDMS polymer and explored the piezoelectric properties and utilized them for the energy harvesting applications. Finally, we have investigated the octahedral Pd(II) cage (1) of this ligand (L1) and other similar cages (2 and 3) built from larger tritopic ligands (L2 and L2) for anti-cancer activities. While most of the known self-assembled coordination cages explored so far for these applications are based on Pt(II) and Pt(IV) ions, only recently other less expensive metal ions such as Ru(II) and Rh(III) are probed for this application. In fact, Pd(II) based systems are far less explored. The IC50 values obtained from the MTT assays of these new Pd(II) derived discrete assemblies (2 and 3) show excellent promise for their utility as anticancer agents.