Identification of a metastable uranium metal-organic framework isomer through non-equilibrium synthesis

Abstract

Since the structure of supramolecular isomers determines their performance, rational synthesis of a specific isomer hinges on understanding the energetic relationships between isomeric possibilities. To this end, we have systematically interrogated a pair of uranium-based metal-organic framework topological isomers both synthetically and through density functional theory (DFT) energetic calculations. Although synthetic and energetic data initially appeared to mismatch, we assigned this phenomenon to the appearance of a metastable isomer, driven by levers defined by Le Châtelier's principle. Identifying the relationship between structure and energetics in this study reveals how non-equilibrium synthetic conditions can be used as a strategy to target metastable MOFs. Additionally, this study demonstrates how defined MOF design rules may enable access to products within the energetic phase space which are more complex than conventional binary (e.g., kinetic vs. thermodynamic) products.

Scheme 1. Energy landscape of supramolecular products. Non-equilibrium non-dissipative syntheses produce metastable and kinetic products (top, pink), and equilibrium syntheses produce thermodynamic products (bottom, blue).

Fig. 1. Topological isomers NU-1305 and NU-1306. Augmented topological nets for (a) ctn and (d) bor resulting from the assembly of (c) a triangular node building block and a tetrahedral linker building block. Crystal structures of (b) NU-1305, in the ctn topology, and (e) NU-1306, in the bor topology. In (b) and (e), uranium is shown in yellow, oxygen in red, and carbon in blue. Hydrogen atoms are omitted for clarity.

Fig. 2. Isolated isomers through tuned synthetic conditions. PXRD data of reaction products from systematically (a) increasing modulator amount while holding temperature and reaction concentration constant, (b) increasing reaction temperature while holding modulator amount and reaction concentration constant, or (c) increasing reaction concentration while holding modulator and temperature constant. Diffraction patterns are normalized.

Fig. 3. Energetic analysis of isomer favorability. (a) Conversion of NU-1305 (left) to NU-1306 (right) and vice versa. Geometric analysis of (b) NU-1305 and (d) NU-1306 nodes. Only the immediately bound phenyl ring of one attached linker is shown for clarity (c) Standard deviation of linker dihedral angles for NU-1305 and NU-1306. In panels (a)–(d), uranium is shown in yellow, oxygen in red, and carbon in black. Hydrogen atoms are omitted for clarity. (e) Reaction coordinate diagram of NU-1305 and NU-1306 isomers. This panel is qualitative.

Fig. 4. Non-equilibrium conditions for NU-1306 synthesis. Photographs of (a) NU-1305 and (b) NU-1306 before (left) and after (right) the reaction. A black horizontal line drawn on the glass vial indicates the initial solvent line before heat was added. Reaction conditions are identical (0.9 FA : DMF, 0.8 mL DMF, 1.6 : 1 node:linker) except that (a) was heated at 120 °C for 24 hours while (b) was heated at 170 °C for 1 hour.