Framework flexibility (elasticity), i.e., the ability of a metal-organic framework (MOF) to considerably change its structure as a function of relevant parameters like pressure, temperature, and type of adsorbed molecules is only observed for some special compounds. The MOF compound Ni2(2,6-ndc)2(dabco) [2,6-ndc: 2,6-naphthalenedicarboxylate, dabco: 1,4-diazabicyclo[2.2.2]octane, further denoted as DUT-8(Ni)] can be synthesized in flexible and non-flexible (rigid) form of equal chemical composition just by controlling the particle size . The unit cell of the flexible form changes its volume by more than 100% during the reversible, adsorption-induced structure opening! Here, we comparatively study flexible and non-flexible DUT-8(Ni) in order to answer two questions: What are the structural differences between these two variants? And does framework flexibility influence adsorption selectivity from gas mixtures? To answer the first question, we selectively isotope-labeled promising framework positions with 13C and 2H. This allows to selectively detect carboxylate 13C atoms close by the Ni centers and to study the mobility of the organic linker molecules by 2H NMR spectroscopy. Extended solid-state NMR investigations encompassing 2H exchange spectroscopy (EXSY), 13C-1H heteronuclear correlation (HETCOR), and others revealed that the non-flexible form exhibits a higher fraction of defects and dynamically disordered linker molecules compared to the flexible variant. The second question is studied by quantitative high-pressure in situ 13C NMR spectroscopy of gas adsorption from mixtures containing 13C-enriched CO2 and CH4. Flexible DUT-8(Ni) indeeed exhibits a significantly higher selectivity for carbon dioxide adsorption from these mixtures than the rigid form . That means, framework flexibility seems to influence adsorption selectivity.
 N. Kavoosi, V. Bon, I. Senkovska, S. Krause, C. Atzori, F. Bonino, J. Pallmann, S. Paasch, E. Brunner, S. Kaskel, Dalton Trans. 2017, 46, 4685.
 M. Sin, N. Kavoosi, M. Rauche, J. Pallmann, S. Paasch, I. Senkovska, S. Kaskel, E. Brunner, Langmuir 2019, 35, 3162.