Synthesis of Denser Energetic Metal–Organic\nFrameworks via a Tandem Anion–Ligand Exchange Strategy

Abstract

High-density materials\nhave attracted extensive attention because of their broad applications.\nHowever, strategies for improving the densities of MOFs and preparing\ndenser MOFs remain almost unexplored. Herein, we propose a tandem\nanion–ligand exchange strategy for synthesizing denser MOFs\nby using three-dimensional cationic MOFs (3D CMOFs) with pillared\nlayered structures as precursors and high-density anions and small\nmonotopic ligands as exogenous guests. By means of this strategy,\nwe choose the high-density nitroformate ion [C­(NO₂)₃^–] as an exogenous anion and water as an\nexogenous ligand to successfully synthesize two layered CMOFs. Single-crystal\nX-ray diffraction showed that after this transformation, the extra-framework\nanions are replaced with the C­(NO₂)₃^– anions, and the distances between adjacent layers in the two-dimensional\n(2D) networks are more than 3.70 Å shorter than those of their\n3D precursors. The resultant materials exhibit higher densities, higher\nheats of detonation, higher nitrogen and oxygen contents, and lower\nmetal contents. In particular, the density of {Cu­(atrz)₂[C­(NO₂)₃]₂(H₂O)₂·atrz·2H₂O}_<i>n</i> (<b>2b</b>, ρ = 1.76 g cm^–3, atrz = 4,4′-azo-1,2,4-triazole)\nis increased by 0.12 g cm^–3 compared to its 3D precursor\n{<b>2a</b>, [Cu­(atrz)₃(NO₃)₂·2H₂O]_<i>n</i>, ρ = 1.64\ng cm^–3}, and its heat of detonation is also enhanced\nto more than 1900 kJ kg^–1. The resultant 2D layered\nCMOFs are also new potential high-energy density materials. This work\nmay provide new insights into the design and synthesis of high-density\nMOFs. Moreover, we anticipate that the approach reported here would\nbe useful for the preparation of new MOFs, in particular, which are\notherwise difficult or unfeasible through traditional synthetic routes.