Synthesis and Characterization of Uranyl Metal-Organic Frameworks Containing Selenophene and Tellurophene Linkers

Herein we describe the synthesis and characterization of uranyl metal-organic frameworks (MOFs) featuring 2,5-selenophene dicarboxylic acid (2,5-SeDC) and 2,5-tellurophene dicarboxylic acid (2,5-TeDC) linkers. All compounds were analyzed structurally using single-crystal and powder X-ray diffraction and spectroscopically characterized via Raman and infrared spectroscopies, thermogravimetric analysis, and scanning electron microscopy. Thermal analysis revealed that uranyl-2,5-SeDC and 2,5-TeDC based MOFs were stable to between 250-350 °C. These measurements also led to the discovery of an unprecedented ligand-induced redox transformation to uraninite (UO2) under inert atmosphere (N2), wherein heating of U(VI) hybrid materials featuring 2,5-SeDC and 2,5-TeDC did not lead to U(VI) oxides after framework degradation, rather U(IV) was formed at ambient pressure in the absence of chemical reductants. This redox transformation does not occur when hybrid materials are prepared with 2,5thiophenedicarboxylic acid (2,5-TDC) or 2,5-furandicarboxylic acid (2,5-FDC); however, physical mixing of 2,5-TeDC with uranyl-2,5-TDC or uranyl-2,5-FDC compounds in a 1:1 stoichiometric ratio does lead to the formation of UO2. Pre-and post-TGA SEM/EDS analyses revealed that tellurium content decreases to near-background levels after heating under N2, whereas selenium content is reduced but remains still detectable. PXRD analysis of the post-TGA residues further showed no evidence for crystalline seleniumor tellurium-containing phases, consistent with decomposition pathways involving volatile chalcogencontaining intermediates that facilitate spontaneous in situ U(VI) to U(IV) reduction under mild conditions.