Synthesis and Structural Characterization of Main-Group Metal Coordination Networks

Abstract

Coordination Networks (CNs) or Metal-Organic Frameworks (MOFs) are crystalline materials, composed of infinite arrays of metal ions, connected by functionalized organic linkers, forming chains, layers or 3-D networks. The exploratory synthesis and characterization of novel MOFs or CNs is of current interest, because of their potential applications across a broad range of technologies, including gas storage, separation, catalysis, and luminescence. A wide range of metal centers and functionalized organic linkers are used to form CNs or MOFs under different synthetic conditions, giving rise to unprecedented structural diversity. First row transition metal centers are often chosen because of their well-known coordination behavior with carboxylate groups under hydro- and solvothermal conditions. On the other hand, CNs/MOFs based on s-block metal centers are relatively less studied. The ionic nature of M-O bonding in s-block CNs provides little room for prediction and control over coordination geometry. Despite difficulties in predicting the coordination geometry, the incorporation of s - block metal centers into CNs offers several advantages. The relatively high charge density and ionic nature of these metal ions leads to strong bonding interaction with carboxylate oxygen atoms. Porous networks prepared from early members of the s - block metal series could further provide gravimetric advantages for gas storage applications due to their low-atomic weight. Our aim was to understand the chemistry of s-block CNs using synthetic variables like temperature and solvents. To accomplish this, lithium based CNs were synthesized using a diverse range of aliphatic and aromatic polycarboxylates. Our study showed that the mutual orientation of the functional groups plays a pivotal role in determining the topologies of the networks. Electrochemical studies reveal the potential of these networks as Li-ion battery electrode. Solid state 6Li NMR was further applied to understand the desolvation-resolvation behavior of a Li-CN, which indicate rearrangement of the metal coordination sphere after solvent removal. The s-block metals CNs were further explored using magnesium and calcium as metal centers. These metal centers are inexpensive, non-toxic and essential in many biological processes. The structural chemistry of Mg2+ is similar to that of Zn2+, which forms many porous MOFs. A series of magnesium-3,5-pyridinedicarboxylates networks were synthesized, using common organic solvents in both pure and mixture forms as the synthetic variable. Networks of different dimensionalities were formed due to the variable coordination ability of solvent molecules with the metal center. Physical properties, such as thermal stability and gas-adsorption behavior of the synthesized networks, vary with the incorporation of different solvent molecules. Water molecules coordinate with the magnesium metal centers preferably over other polar solvents. A similar synthetic strategy was adopted to synthesize calcium based CNs using different solvent mixtures. The larger size of Ca2+ compared to Mg2+ leads to higher coordination numbers for the former, while the structural topologies of the networks formed are equally dependent on the chemical nature and geometry of the ligands and the synthetic variables.