The dehydrated aluminum form of the metal−organic framework compound MIL-53 shows a temperature-driven phase transition with pronounced structural hysteresis as recently shown by neutron diffraction and scattering experiments (J. Am. Chem. Soc.200813011813). Thereby, the structure of the corner-sharing metal AlO4(OH)2 octahedra differs in terms of local symmetry for the dehydrated MIL-53(Al) material in its high- and low-temperature form with open or closed pore structure, respectively. In this work, some of the framework aluminum ions were exchanged by chromium(III) to introduce an electron spin resonance (ESR) active probe ion. The resulting material was investigated by means of ESR and electron nuclear double resonance (ENDOR) spectroscopy to verify the incorporation of chromium at the octahedral framework sites. In addition, variable-temperature ESR measurements were performed to analyze the temperature-dependent phase behavior of the doped MIL-53 material. The Cr(III) ions have an electron spin S = 3/2 which shows a characteristic fine structure interaction depending very sensitively on the local symmetry of the chromium site. Therefore, the fine structure splitting of the Cr(III) probe ions allows for a concise elucidation of changes in the local symmetry at the CrO4(OH)2 octahedra which occur upon structural transitions. In agreement with results from neutron experiments, the transformation from the open to the closed pore structure was found to occur in the temperature range between 150 and 60 K whereas the back transformation is taking place within a smaller temperature interval between 330 and 375 K.

The metal-organic framework (MOF) DUT-8(Ni) (DUT = Dresden University of Technology) shows a structural transformation from a non-porous to a porous phase during the adsorption of gases. A rigid derivative of this material has recently been synthesized, where this "gate pressure like" flexibility is completely absent. This rigid derivative of DUT-8(Ni) always stays in the porous phase even in the absence of any adsorbate. This motivates the present investigation of the adsorption of nitric oxide (NO) on the flexible and rigid forms of DUT-8(Ni) by continuous wave electron paramagnetic resonance (EPR) spectroscopy at Xband frequency. The EPR signal of desorbed NO is measured at moderate temperatures and the decrease of its intensity indicates the adsorption of this gas within the porous phase of DUT-8(Ni) at low temperatures. An adsorption and desorption related hysteresis loop of the intensity of this signal is observed for the flexible but not for the rigid . This difference might reflect the difference in the flexibility of both materials. Furthermore EPR signals with electron spin S = 1/2 are measured, which can likely be attributed to Ni 2+ -NO adsorption complexes at defective paddle wheel units within the porous phase of with the unpaired electron sitting at the Ni 2+ ion. The order of their g-tensor principle values allows a distinct characterisation of the ligand environment of these ions. Defects for which the EPR signals indicate, that at least one NDC (2,6-naphthalenedicarboxylate) ligand molecule does not coordinate to the paddle wheel, are only observed for the rigid but not for the flexible . Also the density of defective paddle wheel units with only one Ni 2+ ion or a missing dabco (1,4 -diazabicyclo[2.2.2]octane) ligand is indicated to be one order of magnitude larger in the rigid than in the flexible derivative of this MOF. The observed differences in the presence and amount of distinct defects might be related to the difference in the flexibility of both forms of the investigated material.

A highly porous metal-organic framework (DUT-8(Ni), DUT = Dresden University of Technology) is found to adopt a configurationally-degenerate family of disordered states that respond adaptively to specific guest stimuli. This disorder originates from non-linear carboxylate linkers arranging paddlewheels in closed loops of different local symmetries that in turn propagate as tilings of characteristic complex superstructures. Solvent exchange stimulates the formation of distinct disordered superstructures for specific guest molecules. Electron diffraction by desolvated DUT-8(Ni) nanoparticles demonstrates these superstructures to persist on the nanodomain level. Remarkably, guest exchange stimulates reversible and repeatable switching transitions between distinct disorder states. Deuterium NMR spectroscopy and in situ PXRD studies identify the transformation mechanism as an adaptive singular transformation event.

Controlling the adsorption behavior of switchable porous materials is essential to pave the way for their successful implementation in highly selective separation and sensing applications. The switchable MOF M 2 (2,6-ndc) 2 (dabco) (DUT-8(M), where DUT = Dresden University of Technology, 2,6-ndc = naphthalene dicarboxylate, dabco = 1,4-diazabicyclo-[2.2.2]-octane, and M = Ni and Co) shows distinct differences in gating adsorption behavior depending on the transition metal of the node. Both DUT-8(Ni) and DUT-8(Co) transform into the closed pore phase after solvent removal. The nickel-containing compound shows high responsivity and gate opening in response to nitrogen adsorption, p/p 0 = 0.1 (77 K), resulting in a huge pore volume change, while the Co compound remains in a closed pore phase and is completely nonresponsive to nitrogen at 77 K. Herein, we demonstrate the gradual tuning of the gate opening pressure in DUT-8(M), upon nitrogen adsorption, by partially substituting nickel with cobalt in a series of mixed metal MOFs. The substitution mechanism was analyzed by powder X-ray diffraction (PXRD), solid-state UV/vis spectroscopy, inductively coupled plasma−optical emission spectroscopy elemental analysis, and energy-dispersive X-ray spectroscopy. In particular, continuous wave electron paramagnetic resonance (EPR) spectroscopy demonstrated the coexistence of Ni/Ni, Co/Ni, and Co/Co paddle wheel (PW) units. The gradual substitution of Ni in DUT-8(Ni) with Co allows continuous tuning of the gate opening pressure from p/p 0 0.1 to 0.75 (75% Co). The integration of Ni/Co-PWs into this pillared layer MOF has enabled, for the first time, in situ monitoring of this gating phenomenon via parallelized adsorption of N 2 (71 K) and EPR spectroscopy. These observations can be compared directly with in situ PXRD data collected during N 2 adsorption at 77 K. These complementary techniques reveal unique mechanistic insights into the structural changes of the PWs during the gating process. In addition, the experimental observations are supported by computational methods using density functional theory.