Monitoring the Local Structure and Magnetic Properties of the Dinuclear Cu₂-Paddle Wheel Nodes in the Mesoporous Metal–Organic Framework, DUT-49(Cu), upon Adsorption-Induced Breathing Transitions

Abstract

is a well-studied representative of flexible mesoporous frameworks, in particular, famous for long-lived overloaded metastable states in the presence of a variety of gases at defined temperatures, leading to "negative gas adsorption" transitions. Important mechanistic insights into these transitions in DUT-49 were obtained via in situ powder X-ray diffraction (PXRD) studies conducted in parallel to gas physisorption. However, for strongly X-ray absorbing probe molecules, such as xenon, such studies are not feasible, even if synchrotron radiation is used. Here we employ in situ electron paramagnetic resonance spectroscopy (EPR), PXRD, and adsorption isotherm measurements to explore the phase transformations in in the presence of xenon and compare its properties with the corresponding adsorption/desorption behavior of ethylene for this material. The antiferromagnetically coupled Cu II −Cu II dimers in the paddle-wheel (PW) units of this pillared layer MOF serve as local magnetic probes in the in situ EPR measurement. These experiments allowed us to monitor the op ↔ cp phase transformations during the xenon physisorption through the structural changes at the PW units encoded in the zero-field splitting parameters of the S = 1 state of the Cu II dimers. The EPR data indicates an expansion of the unit cell for the cp phase in the presence of xenon. This novel EPR-derived insight into the phase transformation phenomena of the xenon-loaded DUT-49(Cu) could be validated by combined in situ EPR, PRXD, and adsorption isotherm measurements for ethylene adsorption over the same MOF material in a comparable temperature range.

EPR and SQUID interrogations of Cr(III) trimer complexes in the MIL-101(Cr) and bimetallic MIL-100(Al/Cr) MOFs

“…Furthermore, the role played by the guest-framework intermolecular interactions, the influence of adsorption on the intramolecular interactions and the changes in the internal structure can be elucidated by tracking the paramagnetic species in the SBU [6,9,[15][16][17][18]. In this regard, EPR spectroscopy is one of the inevitable tools to understand the change in the local structure of MOFs due to guest molecular interaction during ex situ [16] and in situ [15,17,19] gas adsorption, liquid adsorption, light irradiation [20], post synthetically modified ion exchange, [2] and intermolecular magnetic couplings [2,[21][22][23] by monitoring the behavior of paramagnetic metal ions in the SBU.…”

Amplification of negative gas adsorption in a multivariate framework

“…Further filling of the structure leads to reopening to the open pore phase and can be characterized as breathing, previously described in MIL-53 structures. 11 The nature of the effect is now well understood from experimental 12 and theoretical standpoints, 13,14 indicating the importance of the mechanical properties of the ligand molecule, 15 crystal size effects, 16 nature of guest molecules and temperature. 17 The complete thermodynamic description of DUT-49 was charted by Evans and co-authors in the form of the free energy landscape describing the dependence of osmotic potential on the pressure and unit cell volume.…”

Selective adsorption of dihydrogen isotopes on DUT-8 (Ni,Co) monitored by in situ electron paramagnetic resonance

“…12 Even preceding that, the exotic in situ CW-EPR investigation at X-band frequency has been implemented as a versatile method to follow the gate opening phenomenon of several flexible MOFs triggered by the response toward guest molecule stimuli. 12–16 In the present work, we implement this in situ CW-EPR technique for the first time to monitor an isotope-selective phase transition of DUT-8(Ni 0.98 Co 0.02 ) towards dihydrogen isotopes.…”