Synthesis and Characterization of Three-Dimensional Uranyl Coordination Polymers Containing Selenophene and Tellurophene Linkers.

Herein, we describe the synthesis and characterization of three-dimensional uranyl coordination polymers (CPs) featuring 2,5-selenophenedicarboxylic acid (2,5-SeDC) and 2,5-tellurophenedicarboxylic acid (2,5-TeDC) linkers. All compounds were analyzed structurally using single-crystal and powder X-ray diffraction and were 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 CPs were stable between 250-350 °C. These measurements also led to the discovery of a ligand-induced redox transformation to a reduced uraninite-type (UO2/UO2+x) phase 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 2,5-thiophenedicarboxylic 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 a UO2/UO2+x phase. Pre- and post-TGA SEM/EDS analyses revealed that Se and Te content decrease to near-background levels after heating under N2, and these findings are consistent with decomposition pathways involving volatile chalcogen-containing intermediates that facilitate spontaneous in situ U(VI) to U(IV) reduction under mild conditions.