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

Dielectric resonator aided sensitivity-enhancing electron paramagnetic resonance was successfully applied to small single crystals of the previously reported metal–organic framework compound _∞³[Cu₂^ICu₂^II(H₂O)₂L₂Cl₂] in a conventional X-band EPR spectrometer at 7 K sample temperature to reveal the nature of mononuclear Cu²⁺ 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 Cu²⁺ 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 Cu²⁺ ion signals in single crystals of typical sizes of 200 × 50 × 50 μm³ 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

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

Continuous-wave X-band electron paramagnetic resonance with dielectric resonators has successfully been applied to small single crystals of the metal–organic framework HKUST-1 and Cu_2.965Zn_0.035(btc)₂ to investigate the structure of paddle-wheel building blocks with pure Cu/Cu and mixed Cu/Zn pairs. The local paramagnetic Cu²⁺ ion probes were used to identify the magnetic <b>g</b> and <b>A</b> tensor orientations with respect to the crystal axes. We were able to monitor changes in these tensor orientations by EPR at gas adsorption on MOFs for the first time. We explored the spectral simulations of the spin Hamilton parameters of the single crystals and found results similar to those in previous studies of powder samples, but moreover, the tensor orientations are influenced upon gas adsorption, which is represented by a distinct line broadening effect in the angular resolved single-crystal EPR spectra. The as-synthesized, dehydrated, carbon dioxide-adsorbed, carbon monoxide-adsorbed, methanol-adsorbed, and reactivated states have been analyzed to reveal the magnetic tensor orientations, and the direct coordination of the adsorbed gas to the Cu²⁺ ions along with consistent, corresponding DFT calculations allows us to predict an improved model for the mixed paddle-wheel structure upon the adsorption of gases to a paddle-wheel based on perturbations of the <b>g</b> and <b>A</b> principal axis orientations. Additionally, we analyzed a reversibly occurring background signal observable not only in Cu_2.965Zn_0.035(btc)₂ but also in pure Cu₃(btc)₂ at very low temperatures