Lattice Modification and Morphological Control of Halide-Substituted yqt-Type Zeolitic Imidazolate Frameworks Zn3mim5X, with X = F, Br, Cl, or OH

Zeolitic imidazolate frameworks (ZIFs) are a large subgroup of metal-organic frameworks (MOFs). The synthesis of ZIF phases showing a yqt-type framework by the reaction of methylimidazole (Hmim) with a zinc salt in an aqueous solution using different zinc salts like acetates, nitrates, chlorides, fluorides, bromides, iodides, and sulfates yields different morphologies and products: Zn(3)mim(5)XH(2)O center dot nH(2)O with X = Br, Cl and alpha-Zn(3)mim(5)X with X = F, OH. By thermal dehydration of the hydrated ZIF phases, a series of isostructural compounds, alpha-Zn(3)mim(5)X with X = Br, Cl, F, or OH, were obtained. The products were characterized by X-ray powder diffraction (XRPD) and vibrational (IR) and two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy. In all structures, zinc exhibits tetrahedral coordination and forms a yqt net. Inorganic anions are incorporated into this net as Zn-X-Zn bridges and show a major impact on the reversible de- and rehydration behavior, as well as on the thermal expansion. The latter property is mainly governed by bending of the Zn-X-Zn bridge. Within this series, only the chloride-containing phase, showing the strongest anisotropic thermal expansion, exhibits two polymorphs: alpha- and beta-Zn(3)mim(5)Cl, the alpha-form showing a bent and the beta-form showing a linear Zn-X-Zn bridge.

Automated summary

Zeolitic imidazolate frameworks (ZIFs) are an important subgroup of metal-organic frameworks (MOFs). Different morphologies and products of yqt-type ZIF phases can be obtained by reacting methylimidazole (Hmim) with zinc salts in aqueous solution. A series of isostructural compounds can be obtained by thermal dehydration of the hydrated ZIF phases. The products were characterized using X-ray powder diffraction (XRPD) and vibrational (IR) and two-dimensional (2D) solid-state nuclear magnetic resonance (NMR) spectroscopy. Zinc exhibits tetrahedral coordination and forms a yqt net in all structures. Inorganic anions are incorporated into this net as Zn-X-Zn bridges and have a significant impact on de- and rehydration behavior and thermal expansion.