During the last decade, metal-organic frameworks have been intensively studied due to their applicability in the areas of gas storage and separation, catalysis, drug delivery, and sensing. These porous materials are constructed from inorganic metal cluster nodes and rigid organic linkers that connect those nodes; a wide array of organic linkers was used to date, yielding structures with various geometries and pore sizes. Our research aims to expand the "MOF toolbox" by creating tetragonal organic building blocks with 90 ° angles between the individual metal coordination sites.
Benzobisoxazole motif was chosen as the central core with the appropriate 90 ° geometry and two sets of substitution sites on the oxazole rings (X-axis in Figure) and the benzene ring (Y-axis in Figure). Starting with 2,5-diamino-3,6-dibromobenzoquinone, the X-axis sites were introduced first through the microwave-assisted condensation with various acyl chlorides bearing substituents that can serve as ligands to transition metals (protected –COOH or –CN). The introduction of Y-axis substituents was achieved by Sonogashira reactions. Final hydrolysis provided the desired tetracarboxylic acids. Synthesized compounds were characterized by 1H NMR, 13C NMR, and IR spectroscopy, and mass spectrometry. We have thus demonstrated that 90 ° tetravalent organic linkers can be derived from benzobisoxazoles and that we can independently modify the X- and the Y-axis sites in these systems.
Future work will focus on the reactions of carboxylic acids with transition metal salts (e.g. Zn(NO3)2 or Cu(NO3)2) and is expected to yield MOFs with paddle-wheel shaped (Figure) transition metal clusters. Crystallographic studies will be used to confirm the predicted geometries of ligands and frameworks.