Erlend Aunan

Research of zirconium-based metal-organic frameworks has for the past ten years steadily increased in popularity, and new methods for synthesis, modification and functionalization are published regularly. In the leading front of the research is the UiO-series[1] (UiO-66, UiO-67 and UiO-68). The series has have shown great thermal, chemical and physical stability, which partly is due to having each cluster fully coordinated to linkers (12 linkers per cluster, 6:1 ratio). Although the full coordination sphere increases their resistant properties, it leaves little room for functionalizing, leading to a necessity of introducing either missing linker defects in order to introduce new functionality to the MOF, or even direct chemical functionalization of the linkers themselves. MOF-808[2] is a MOF consisting of zirconium and trimesic acid (benzene-1,3,5-tricarboxcylic acid), and while having the same cluster as the UiO-series (Zr6O4(OH)4), only six linkers are coordinated per cluster (2:1 ratio), leaving another six coordination sites in an equatorial belt around the cluster uncoordinated. These six sites are often occupied by remnant anionic species from the synthesis (chloride, formate, acetate etc.), but these species can be exchanged for others in order to increase the functionalization of the MOF. In this work, an overview of the introduction of different functionalities into MOF-808 is shown.

Metal-organic frameworks (MOFs) are coordination compounds that exhibit a very high crystallinity and porosity. The structure is built using cationic metal clusters as cornerstones, and organic ligands (often dicarboxylic acids) bridging the cornerstones together, commonly referred to as inorganic clusters and organic linkers respectively. MOFs have been investigated for their potential in e.g. gas storage, gas adsorption, gas separation and heterogeneous catalysis.[1] The research on metal-organic frameworks has grown exponentially during the last couple of decades. The University of Oslo (UiO) has been participating actively in the research, and has since 2008 developed its own successful series of zirconium-based MOFs, namely UiO-66, UiO-67 (figure shown below) and UiO-68. The success behind the UiO MOF-series is based on their unprecedented stability to thermal, chemical and physical decomposition.[2] 3,3’-dialkoxy-1,1’-biphenyl-4,4’-dicarboxylic acids have the potential for being interesting UiO-67 linkers with their excellent tunability of hydrophobicity and steric bulk. This is made possible by adjusting the length of the alkyl chain. Three biphenyl linkers designed for UiO-67 metal-organic frameworks, two which are previously unreported, have been synthesized and fully characterized.