EPR Insights into Switchable and Rigid Derivatives of the Metal–Organic Framework DUT-8(Ni) by NO Adsorption

The metal–organic framework (MOF) DUT-8­(Ni) (DUT = Dresden University of Technology) shows a structural transformation from a nonporous 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 X-band 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 DUT-8­(Ni). 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 Ni²⁺-NO adsorption complexes at defective paddle wheel units within the porous phase of DUT-8­(Ni) with the unpaired electron sitting at the Ni²⁺ ion. The order of their g-tensor principle values allows a distinct characterization 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 DUT-8­(Ni). In addition, the density of defective paddle wheel units with only one Ni²⁺ ion or a missing dabco (1,4-diazabicyclo[2.2.2]­octane) ligand is indicated to be 1 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.