Single Crystal Electron Paramagnetic Resonance with Dielectric Resonators of Mononuclear Cu²⁺ Ions in a Metal–Organic Framework Containing Cu₂ Paddle Wheel Units

Abstract

Dielectric resonator aided sensitivity-enhancing electron paramagnetic resonance was successfully applied to small single crystals of the previously reported metal-organicin a conventional X-band EPR spectrometer at 7 K sample temperature to reveal the nature of mononuclear Cu 2+ ion defect species. We found that these paramagnetic defects are not related to an impurity phase or extraframework species of the parent metal-organic framework material but are formed within the framework. Novel angular resolved single crystal continuous wave electron paramagnetic resonance supported by powder measurements and single crystal X-ray diffraction on this metal-organic framework compound identified defective copper paddle wheel units with one missing Cu 2+ ion as the observed mononuclear paramagnetic species in this compound. The sensitivity enhancement by an estimated factor of 8.6 for the single crystal electron paramagnetic resonance spectroscopy is required to efficiently record the Cu 2+ ion signals in single crystals of typical sizes of 200 × 50 × 50 µm 3 at X-band frequencies. The results demonstrate that conventional electron paramagnetic resonance spectrometers operating at X-band frequencies and equipped with dielectric resonators can successfully be used to perform single crystal studies of these porous, low density materials with very small volume samples at low temperatures.

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

“…A similar defect has recently been identified in a MOF material with Cu 2+ paddle wheels. 47 The ݀ ௭ మ orbital of the Ni 2+ and the ߨ ௬ * MO of the NO molecule will then form a bonding ݀ ௭ మ + ߨ ௬ * MO leading to an effective ൫݀ ௭ మ +…”

Adaptive Response of a Metal–organic Framework Through Reversible Disorder–disorder Transitions

“…37 Recently switchability in frameworks has led to the discovery of unexpected novel phenomena such as negative gas adsorption (NGA). 20,21,38,39 Switchability is a unique characteristic of a chosen few MOFs with the ability to adapt their pore size dynamically leading to extra-high selectivity in gas separation, ideal deliverable storage capacity in natural gas storage as well as promising sensing and catalytic functionality. [6][7][8][11][12][13][14][15][16][17][18]40 Hence, we were curious if switching between well-defined disordered configurations is achievable simply by repeated guest exchange.…”

“…We cannot fully exclude the assignment of the observed signals to monomeric low spin Co 2+ ions (S = 1/2) as they might form as defective broken paddle-wheel units with one missing metal ion. 21 However, all signals show significant loss in spectral resolution at a temperature of T = 71 K as shown in Figure S14 whereas for low spin Co 2+ species the spectral resolution is typically high up to temperatures of T = 130 K or above. [22][23][24][25] Such loss in spectral resolution at comparably low temperatures is rather typical for an electron species with a high spin Co 2+ ion (S = 3/2) in distorted octahedral coordination where the coupling between the electron spin and the not fully quenched orbital momentum can shorten the spin lattice relaxation time significantly.…”

“…The very simplest prerequisite for flexibility is bistability, which allows switching between two distinguishable states in response to external stimuli. Canonical examples are the narrow and large pore states of breathing MOFs such as MIL-53 19 or DUT-49 20,21 . More complex behaviour emerges for systems with more than one accessible state.…”

Extending electron paramagnetic resonance to nanoliter volume protein single crystals using a self-resonant microhelix

“…In practice, this has put a limit on the Λ ave obtainable to less than 1 mT/W 1/2 for X-band. Further EPR signal improvement is possible using dielectric resonators by increasing the dielectric permittivity, and dielectric resonators with permittivity up to 80 have been used for continuous-wave EPR experiments on crystals of porous materials and polymers ( 9 , 22 ). However, these resonators exhibit Q 0 -values over 2500 that make pulse experiments problematic.…”