Unveiling Structural Diversity of Uranyl Compounds of Aprotic 4,4′-Bipyridine N,N′-Dioxide Bearing O-Donors

As an aprotic O-donor ligand, 4,4 '-bipyridine N,N ' dioxide (DPO) shows good potential for the preparation of uranyl coordination compounds. In this work, by regulating reactant compositions and synthesis conditions, diverse coordination assembly between uranyl and DPO under different reaction conditions was achieved in the presence of other coexisting Odonors. A total of ten uranyl-DPO compounds, U-DPO-1 to UDPO-10, have been synthesized by evaporation or hydro/ solvothermal treatment, and the possible competition and cooperation of DPO with other O-donors for the formation of these uranyl-DPO compounds are discussed. Starting with an aqueous solution of uranyl nitrate, it is found that an anionic nitrate or hydroxyl group is involved in the coordination sphere of uranyl in (NO3)2(DPO)), and U-DPO-3 ((UO2)(DPO)(mu 2-OH)2), where DPO takes three different kinds of coordination modes, i.e. uncoordinated, monodentate, and biconnected. The utilization of UO2(CF3SO3)2 in acetonitrile, instead of an aqueous solution of uranyl nitrate, precludes the participation of nitrate and hydroxyl, and ensures the engagement of DPO ligands (4-5 DPO ligands for each uranyl) in a uranyl coordination sphere of U-DPO-4 ([(UO2)(CF3SO3)(DPO)2](CF3SO3)), U-DPO-5 ([UO2(H2O)(DPO)2](CF3SO3)2) and U-DPO-6 ([(UO2)(DPO)2.5](CF3SO3)2). Moreover, when combined with anionic carboxylate ligands, terephthalic acid (H2TPA), isophthalic acid (H2IPA), and succinic acid (H2SA), DPO works well with them to produce four mixedligand uranyl compounds with similar structures of two-dimensional (2D) networks or three-dimensional (3D) frameworks, UDPO-7 ((UO2)(TPA)(DPO)), U-DPO-8 ((UO2)2(DPO)(IPA)2 center dot 0.5H2O), U-DPO-9 ((UO2)(SA)(DPO)center dot H2O), and U-DPO-10 ((UO2)2(mu 2-OH)(SA)1.5(DPO)). Density functional theory (DFT) calculations conducted to probe the bonding features between uranyl ions and different O-donor ligands show that the bonding ability of DPO is better than that of anionic CF3SO3 -, nitrate, and a neutral H2O molecule and comparable to that of an anionic carboxylate group. Characterization of physicochemical properties of UDPO-7 and U-DPO-10 with high phase purity including infrared (IR) spectroscopy, thermogravimetric analysis (TGA), and luminescence properties is also provided.

Automated summary

As an aprotic O-donor ligand, 4,4'-bipyridine N,N'-dioxide (DPO) has been used in the preparation of various uranyl coordination compounds under different reaction conditions. The competition and cooperation between DPO and other O-donors in the formation of these compounds have been discussed. In addition, physicochemical characterization of UDPO-7 and U-DPO-10, including infrared spectroscopy, thermogravimetric analysis, and luminescence properties, has been provided.