Organic Transformations in the Confined Cavity of Self-Assembled Pd(II) Molecular Containers

Abstract

Biological systems utilize its extraordinary ability to develop complex and functional molecular assemblies employing reversible non-covalent interactions. These natural aesthetic examples motivated synthetic chemists in past few decades to develop synthetic protocol to produce myriad number of complex assemblies employing weak intermolecular forces. A number of such forces including hydrogen bonding, solvophobic effect, dynamic covalent interactions and metal-ligand coordination have been exploited to assemble the molecular building blocks and stitch them together to construct discrete ‘self-assembled’ architectures incorporated with desired functionalities. Metal-ligand coordination driven self-assembly certainly evolved as one of the most successful approaches for the construction of discrete supramolecular architectures during last two and half decades. The high directionality and reversible nature of certain metalligand bonds allow the pre-designing of sophisticated architectures which can be successfully obtained by ‘self-correcting’ mechanism via a thermodynamically controlled self-assembly process. However, coordination-driven self-assembly strategy allowed chemists to design molecular containers/vessels of high symmetry and well defined shapes and sizes. These selfassembled molecular containers have emerged as potential candidates because of their ability in encapsulating guest molecules, sensing, drug delivery, stabilizing reactive intermediates/transition states, as well as stereoselective product formation. Although various synthetic strategies have been exploited in the last two decades to acquire large number of 3D assemblies, the growing nature of this field demands architectures with complex topologies of specific shape and functionality. Primarily, symmetric and rigid building blocks have been employed so far to construct such molecular containers which solely preserves their geometrical coding throughout the self-assembly process and therefore the final assemblies are predetermined. Symmetrical pyridyl-based donors have been extensively used due to their specific biting angle to produce desired architectures, whereas, asymmetric pyridyl donors are of less interest as it may provide varieties of donor angles which may led to the formation of mixture of architectures. Moreover, most of the architectures have been constructed mainly via two-component self-assembly whereas multicomponent assemblies are scarce. The multi-component self-assembly offers one-pot synthesis of assemblies involving more than two components. The symmetry consideration along with the binding modes of the building blocks are the pivotal aspects in supramolecular design strategy as it installs the geometrical codes embedded in it which are responsible for the dimensionality, shapes and symmetry of the resulting assembly.

Unlike, mono-heterocyclic donors poly-heterocyclic donors are much more complex in terms of their symmetry, bite-angle and number of donor sites. While these multi-dentate donors offer less control over the productive design of discrete nano-assembly, on the other hand, it may provide unprecedented 3D molecular architectures. In addition, interesting symmetry aspects into the final assembly can be observed due to the presence of multiple donor atoms and biteangles, which otherwise not accessible through highly symmetric donors like pyridyl, where the final assembly possess multiple symmetry elements e.g. rotational axis and mirror plane. The physico-chemical properties of self-assembled coordination cages depend on the structures of the complexes. Presence of large internal cavity surrounded by aromatic core provides an excellent environment for the encapsulation of varieties of guest molecule or as nano-reactors for different organic transformations. The microenvironment offered by molecular vessels can be used for recognizing specific molecules or a specific conformation of that molecule. This phenomenon entirely depends on several factors e.g. size and shapes of the cavity, coulombic interaction between host and guest, hydrophobicity of the cavity and finally the symmetry elements of host and guest molecules. Although, host-guest encapsulation studies between 3D coordination cages and guest molecules are well explored, the possibility of host-guest formation in solid-solid phase has not been studied before because of the absence of any mobile phase (mainly solvent) and the restriction of molecular movements of both host and guest molecules in the solid phase does not allow any encapsulation to take place. However, by exposing to light or heat, molecules can gain required energy to allow itself to move inside the solid matrix and thereby a facile encapsulation might be possible.