Uranyl thiocyanate containing materials are of interest due to their potential applications in actinide separations, specifically liquid-liquid extraction techniques. A better understanding of assembly criteria within this class of materials could allow for advancement of these technologies, and more efficient separations moving forward. We report on two novel bimetallic uranyl/Co2+ thiocyanates, an example being (C4H12N)5[UO2(NCS)5][Co(NCS)4].H2O. These materials feature two discrete building units, or tectons, [UO2(NCS)5]3- and [Co(NCS)4]2-, held together by pseudohalogen-oxo interactions. To our knowledge, these are the first pseudohalogen-Oyl interactions to be observed, yet they are comparable to previously reported halogen-Oyl interactions. The corresponding homometallic phases were prepared in order to compare assembly modes between homometallic and heterometallic species, as well as to probe the effect of altering tecton diversity. Cations which interact in a purely electrostatic nature (e.g. Cs+ and tetramethylammonium) were utilized as ‘controls’ to avoid additional interactions. Materials were characterized via single crystal X-ray diffraction, and interrogated using spectroscopic techniques such as: UV-Vis, Raman, and fluorescence spectroscopy. Density functional theory (DFT) calculations were performed on each tecton in order to render electrostatic potential surfaces (ESPs) and rationalize trends in assembly. In homometallic materials, structure-directing non-covalent interactions occur between sulfur atoms on thiocyanate ligands of adjacent tectons to form favorable S…S interactions. In contrast, bimetallic phases tend to favor S…O=U interactions. This type of interaction is of note since the uranyl oxygen is nominally terminal, preventing its participation in further coordination. Analysis of the calculated ESPs suggest that the Co-NCS…O=U is the most significant interaction, arising from the two regions with largest potential differential. When introducing a cation with the ability to participate in non-covalent interactions, i.e. pyridinium, stronger hydrogen bonds outcompete the weaker interactions and only homometallic phases are seen. By comparing homometallic and heterometallic phases, we then track changes in structure and properties, delineate criteria for directed assembly, as well as propose a potential route to further unlocking interactions at the uranyl-Oyl by introducing tecton diversity, which are supported computationally.

Structural and computational investigation of pseudohalogen-oxo interactions in novel bimetallic uranyl/Co2+ thiocyanates

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.