Synthesis and Structural Characterization of Carboxylate-Based Metal-Organic Frameworks and Coordination Networks
The Graduate School, Stony Brook University: Stony Brook, NY.
Coordination networks (CNs) and metal-organic frameworks (MOFs) are crystalline materials composed of metal ions linked by multifunctional organic ligands. From these connections, infinite arrays of one-, two-, or three-dimensional networks can be formed. Exploratory synthesis and research of novel CNs and MOFs is of current interest because of their many possible industrial applications including gas storage, catalysis, magnetism, and luminescence. A variety of metal centers and organic ligands can be used to synthesize MOFs and CNs under a range of reaction conditions, leading to extraordinary structural diversity. The characteristics of the metals and linkers, such as properties and coordination preferences, play the biggest role in determining the structure and properties of the resulting network. Thus, the choice of metal and linker is dictated by the desired traits of the target network. The pervasive use of transition metal centers in MOF synthesis stems from their well-known coordination behavior with carboxylate-based linkers, thus facilitating design strategies. Conversely, CNs and MOFs based on s-block and lanthanide metals are less studied because each group presents unique challenges to structure prediction. Lanthanide metals have variable coordination spheres capable of accommodating up to twelve atoms, while the bonding in s-block metals takes on a mainly ionic character. In spite of these obstacles, lanthanide and s-block CNs are worthwhile synthetic targets because of their unique properties. Interesting photoluminescent and sensing materials can be developed using lanthanide metals, whereas low atomic weight s-block metals may afford an advantage in gravimetric advantages for gas storage applications. The aim of this research was to expand the current understanding of carboxylate-based CN and MOF synthesis by varying the metals, solvents, and temperatures used. To this end, magnesium-based CNs were examined using a variety of aromatic carboxylate linkers, solvents, and temperatures. Since the coordination chemistry of Zn2+ is similar to that of Mg2+, a zinc metal center is a reasonable proxy for magnesium CN design. Solvent and temperature proved to be the key factors in the synthesis as the topologies that formed depended on the amount of solvent incorporation and the temperature of the synthesis. Exploratory synthesis of magnesium CNs if often conducted with gas storage applications in mind, but the photoluminescence (PL) properties are rarely investigated because Mg2+ is a closed shell metal ion. However, since CN linkers can also contribute to the PL emission of the network, an appropriate choice in linker can lead to the development of a lightweight PL or sensing material. Fluorescence studies on the magnesium CNs illustrate that the PL activity of is not only dependent on the properties and makeup of the linker, but the overall structure and solvent effects as well. Carboxylate-based MOFs were further investigated with the use of lanthanide metal centers. The variable coordination spheres of lanthanide metals leads to a wide range structural topologies not possible with more common metal centers. Additionally, the network linkers used can also double as antenna ligands that further sensitize the metal centers and produce interesting photoluminescence (PL) properties. The photoactive ligand 2,5-thiophenedicarboxylate was used in this work to link four lanthanide MOFs using the metals Dy, Er, Nd, and Tb. Fluorescence studies show that the thiophene linker is a very good antenna ligand, as the Dy and Tb frameworks exhibit their characteristic lanthanide emissions. Finally, a series of transition metal formate MOFs were explored using ionothermal synthesis, a synthetic method that employs an ionic liquid solvent containing an anion and cation to template the desired network. This method proved to be another synthetic route to producing transition metal MOFs with magnetic properties. Anionic formate networks resulting from the ionthermal syntheses possess topologies dictated by the cationic species of the ionic liquid, which is incorporated to provide charge balance. Magnetic studies revealed that these transition metal formate MOFs behave similarly to related formate compounds in that they order as canted antiferromagnets.