Tuning N-donor Coordination and Assembly Structures in Uranyl Compounds of Carboxylate Ligands with Divergent Vinyl Pyridine Groups

Abstract

Due to its relatively weak bonding with uranyl compared to O-donors, coordination ability of a single N-donor in hetero-donor ligands to uranyl without the aid of ortho-carboxyl groups is rarely explored. In this work, two hetero-donor ligands with divergent pyridine and carboxylate groups linked by a vinyl unit as the spacer and one N-methylated derivate were used to coordinate with uranyl hydrothermally, and six uranyl compounds (1-6) were synthesized. Molecular structures and physico-chemical properties of these compounds were investigated by single crystal X-ray diffraction and theoretical calculation, together with infrared (IR) spectroscopy, UV-vis absorption spectroscopy, and fluorescence spectroscopy. The results show that the pyridinyl N-donor in these compounds has the ability to participate in uranyl coordination, and its coordination behaviour can be regulated by structural modification or employing a secondary organic carboxylic acid as the competing ligand. Furthermore, 1 and 2 were used to assess their gas-phase iodine sorption capability, of which the maximum iodine sorption capacities were determined to be 122 mg·g-1 and 100 mg·g-1, respectively. X-ray photoelectron spectroscopy and Raman analysis confirmed iodine sorption mainly as triiodide (I3-), indicating electron transfer between guest iodine molecules and the electron-rich framework of 1 or 2. These results demonstrate the ability of single N-donors to coordinate with uranyl, and also highlight the potential of uranyl-based hybrid materials for environmental remediation applications, i.e. iodine capture.