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 Cu2.965Zn0.035(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 g and A 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 g and A principal axis orientations. Additionally, we analyzed a reversibly occurring background signal observable not only in Cu2.965Zn0.035(btc)2 but also in pure Cu3(btc)2 at very low temperatures.
Bibliographical noteFunding Information:
The authors gratefully acknowledge financial support within the priority programs SPP 1362 (Poröse metallorganische Ger-üstverbindungen) and 1601 (Increasing Sensitivity with EPR) of the Deutsche Forschungsgemeinschaft (DFG) and support by the Center for Information Services and High Performance Computing (ZIH) at Technische Universitaẗ Dresden for the provided computational resources. The authors thank the group of Stefan Kaskel, Technische Universitaẗ Dresden, for providing and characterizing the samples.
All Science Journal Classification (ASJC) codes
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films