Structural flexibility and intrinsic dynamics in the M2(2,6-ndc)2(dabco) (M = Ni, Cu, Co, Zn) metal–organic frameworks

Abstract

The synthesis and structural flexibility of the metal–organic frameworks M2(2,6-ndc)2(dabco) (DUT-8(M), M = Ni, Co, Cu, Zn; 2,6-ndc = 2,6-naphthalenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane) as well as their characterization by gas adsorption, 129Xe NMR and 13C MAS NMR spectroscopy are described. Depending on the integrated metal atom the compounds show reversible (DUT-8(Ni), DUT-8(Co)), non-reversible (DUT-8(Zn)) or no (DUT-8(Cu)) structural transformation upon solvent removal and/or physisorption of several gases. DUT-8(Co) exhibits a similar structural transformation by solvent removal and adsorption behavior as observed for DUT-8(Ni). DUT-8(Zn) undergoes an irreversible structural change caused by solvent removal. The non-flexible copper containing MOF reveals the best performance concerning porosity and gas storage capacities within the DUT-8 series. Xenon adsorption studies combined with 129Xe NMR spectroscopy are used to study the flexibility of the DUT-8 compounds. 129Xe chemical shift and line width strongly depend on the metal atom. Solid-state 13C NMR spectroscopy has been applied in order to further characterize the organic parts of the DUT-8 frameworks. While DUT-8(Ni) exhibits narrow, well-resolved lines in its “as made” state, the signals of DUT-8(Co) are broadened and shifted over an unusually wide chemical shift range (−72 to 717 ppm). No detectable signals are found in DUT-8(Cu) indicating significantly changed internal dynamics compared to DUT-8(Ni) and DUT-8(Co).