P2: Monitoring adsorption induced structural transformations of MOFs at a molecular level by in situ EPR spectroscopy

Andreas Pöppl

One major challenge in the understanding and theoretical modelling of the flexibility, responsiveness, and gating processes of MOF materials is the identification of the related structural changes and their dynamics on a local molecular scale. The two fundamental questions to be addressed here are (I) which structural motifs provide the responsiveness/flexibility of the framework and (II) what is driving the structural transformation? Electron paramagnetic resonance (EPR) spectroscopy can be used to characterize the spin state of a paramagnetic framework ion and may provide detailed information about the local framework structure in the vicinity of the paramagnetic probe ion or a radical species. Therefore, continuous wave (cw) EPR spectroscopy is a powerful tool to explore such local changes of the framework structure and their kinetic properties in responsive MOF materials in dependence on external stimuli such as adsorbate gas pressure, type of adsorbate, and temperature. We employ EPR active local spin probes such as paramagnetic transition metal ions incorporated at framework metal ion sites, the paramagnetic gases NO and NO₂ as well as nitroxide radicals adsorbed in the pores or for the latter encapsulated within the pores during the MOF synthesis. In particular, the high spin states of Mn²⁺ and Cr³⁺ ions with their zero field splitting represent extremely sensitive magnetic probes to monitor even small structural changes at the framework ion sites of the flexible MOF systems (T1). Structural transformations in both gas and liquid phase adsorption processes can be studied by this approach. For gas phase adsorption experiments in situ cw EPR spectroscopy in a wide range of temperatures and adsorbate pressures is developed. Various gases such as Xe, Ar, N₂, CO₂, NO, NO₂ and volatile hydrocarbons are employed in such in situ cw EPR experiments. In that way information about the nature, mechanism, and kinetics of the phase transformations can be obtained on a molecular level complementary to those from e.g. diffraction methods (S2) and NMR spectroscopy (P1) as the three methods cover different time scales of dynamical processes. Adsorption experiments with paramagnetic NO and NO₂ probes leads to the formation of paramagnetic adsorption complexes at metal ion defect sites and provides unique information about the structure of such framework defects. In order to characterize the nature and structure of these defect sites advanced pulsed EPR methods (HYSCORE, ENDOR) are employed in addition to cw EPR spectroscopy.

© TUD/Ak Kaskel

B. Jee, M. Hartmann, A. Pöppl, “H₂ , D₂ and HD adsorption upon the metal-organic framework  [Cu_2.97Zn_0.03(btc)₂]_n studied by pulsed ENDOR and HYSCORE spectroscopy”, Mol. Phys., 2013, 111, 2950 – 2966.

B. Jee, P. St. Petkov, G. N. Vayssilov, T. Heine, M. Hartmann, A. Pöppl, “A Combined Pulsed Electron Paramagnetic Resonance Spectroscopic and DFT Analysis of the ¹³CO₂ and ¹³CO Adsorption on the Metal−Organic Framework Cu_2.97Zn_0.03(btc)₂ “, J. Phys. Chem. C, 2013, 117, 8231 – 8240.

M. Mendt, F. Gutt, N. Kavoosi, V. Bon, I. Senkovska, S. Kaskel, A. Pöppl, “EPR Insights into Switchable and Rigid Derivatives of the Metal−Organic Framework DUT-8(Ni) by NO Adsorption”, J. Phys. Chem. C, 2016,120, 14246–14259.

S. Friedländer, P. St. Petkov, F. Bolling, A. Kultaeva, W. Böhlmann, O. Ovchar, A. G. Belous, T. Heine, A. Pöppl, “Continuous-Wave Single-Crystal Electron Paramagnetic Resonance of Adsorption of Gases to Cupric Ions in the Zn(II)-Doped Porous Coordination Polymer Cu_2.965Zn_0.035(btc)₂”, J. Phys. Chem. C, 2016, 120, 27399−27411.

A. Kultaeva, T. Biktagirov, J. Bergmann, L. Hensel, H. Krautscheid, A. Pöppl, “A combined continuous wave electron paramagnetic resonance and DFT calculations of copper-doped  metal–organic framework”, Phys. Chem. Chm. Phys., 2017, 19, 310130 – 31038.

M. Mendt, B. Barth, M. Hartmann, A. Pöppl, “Low-temperature binding of NO adsorbed on MIL-100(Al)—A case study for the application of high resolution pulsed EPR methods and DFT calculations”, J. Chem. Phys., 2017, 147, 224701-1 − 224701-17.

A. Kultaeva, T. Biktagirov, P. Neugebauer, H. Bamberger, J. Bergmann, J. van Slageren, H. Krautscheid, A. Pöppl, “Multifrequency EPR, SQUID, and DFT Study of Cupric Ions and Their Magnetic Coupling in the Metal–Organic Framework Compound _∞³[Cu(prz–trz–ia)]”, J. Phys. Chem. 2018, 122, 26642 – 26651.

A. Kultaeva, V. Bon, M. S. Weiss, A. Pöppl, S. Kaskel, “Elucidating the Formation and Transformation Mechanisms of the Switchable Metal–Organic Framework ELM-11 by Powder and Single-Crystal EPR”, Inorg. Chem. 2018, 57, 11920 – 11929.

M. Mendt, P. Vervoorts, R. A. Fischer, A. Pöppl, “Probing Local Structural Changes at Cu²⁺ in a Flexible Mixed-Metal Metal-Organic Framework by in Situ Electron Paramagnetic Resonance during CO₂ Ad- and Desorption”, J. Phys. Chem. C,  2019, 122, 2940 – 2952.

M. Mendt, S. Ehrling, I. Senkovska, S. Kaskel, A. Pöppl, “Synthesis and Characterization of Cu−Ni Mixed Metal Paddlewheels Occurring in the Metal−Organic Framework DUT-8(Ni_0.98Cu_0.02) for Monitoring Open-Closed-Pore Phase Transitions by X-Band Continuous Wave Electron Paramagnetic Resonance Spectroscopy”, Inorg. Chem. 2019, DOI: 10.1021/acs.inorgchem.9b00123.