Crystal Engineering

We are interested in structural investigation of Inclusion complexes

Coordination Complexes

Metal complexes, also known as coordination compounds, include all metal compounds, aside from metal vapours, plasmas and alloys. The study of "coordination chemistry" is the study of "inorganic chemistry" of all alkali and alkaline earth metals, transition metals, lanthanides actinides and metalloids. Thus, coordination chemistry is the chemistry of the majority of the periodic table.Coordination complexes were known since the beginning of chemistry, e.g. Prussian blue [Fe7(CN)18(H2O)x; x = 14–16] and copper vitriol (Cu(II)SO4). The key breakthrough occurred when Alfred Werner (1866–1919) proposed his revolutionary theory known as “Werner’s coordination theory”. He won the Noble Prize in Chemistry in 1913 for proposing the octahedral configuration of transition metal complexes. He resolved the first coordination complex into optical isomers, overthrowing the theory that chirality was necessarily associated with carbon compounds. The interest of the earlier workers was theoretical and structural aspects in coordination complexes. Later, the applications of coordination complexes have been developed and it eventually turned out to be the major aspect in chemistry. The major applications of coordination complexes are in electrochemistry, photography, catalysis, medicinal chemistry, biology, nuclear fuel cell, magnetic materials, porous materials, inclusion materials etc.

Inclusion compounds

Inclusion compounds are formed mainly because of inefficient packing of the host molecules. An inclusion compound can be defined as a complex in which one compound, the host forms a cavity within which the molecules of a second compound the guest are located. These complexes may contain two or more molecules or ions held together in unique structural relationships by various non–bonded interactions such as hydrogen–bonding, ions paring, Van der Walls force, – interactions etc. other than the normal covalent bonds. The inclusion of the guest molecules greatly depends on the supramolecular architecture of the host. The shapes of these molecules are such that they cannot pack efficiently in the three dimensions and thereby creating voids which are filled by small guest molecules during crystallization process. The voids, thus formed, can be of two types namely cage and channel. It can vary from discrete to extended channels formed between molecules in a crystal lattice in which guest molecules can fit. In molecular encapsulation a guest molecule is trapped inside another molecule. If the spaces in the host lattice are enclosed on all sides so that the guest species is “trapped” as in a cage, the resulting compounds are known as clathrates. (The word comes from the Greek klethra, meaning "bars".)

Design of inclusion compounds with desired and predictable supramolecular architecture requires thorough understanding of the various interactions that are responsible for the molecules to self-assemble in the solid state, and their exploitation. The foremost characteristic of a good building block is its rigidity. The basic building block must have less orientational degrees of freedom, high rigidity and molecular symmetry if one has to have significant amount of control over the target supramolecular network.

We are interested in Tetra–aryl porphyrins (TAPs) and tetra–aryl–metallo–porphyrins (TAMPs) and cryptand molecules, possess high rigidity and high molecular symmetry that are essential for the design of good building blocks.

Organic–Inorganic Hybrid Solids Based on Non–bonding Interaction

The control of inorganic structure by an organic component reveals an interactive structural hierarchy in materials. In organic–inorganic hybrid materials there is a mutual interaction between the organic and inorganic components. Since this interaction within these organic–inorganic hybrid materials has been derived from the nature of the interface between the organic component and the inorganic oxide, synthetic and structural studies of materials that exhibit such an interface might contribute to the development of structure–function relationships for these hybrid materials. Two types of interactions have been exploited in the construction of such supramolecular materials: 1) coordinate covalent bonds connecting metal centers and appropriate ligand types, and 2) hydrogen–bonds in organic solids.

Efforts have been made by various groups to create novel structures based on metal complex anions that accept hydrogen–bonds and organic cations with hydrogen–bond–donor capability. Recently, deliberate efforts have been made to construct intriguing supramolecular assemblies using metal bound halide based hydrogen–bonds. In these studies, protonated ring nitrogen N+–H (either aromatic or alicyclic) and perhalometalate (MX4; M = transition metals, X = halogen,mainly Cl) have been used as hydrogen–bond donor and acceptor respectively. We are interested in effect of numerous weak interactions such as C–H•••Cl in all these and their role in stabilizing the supramolecular network.


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