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

Abstract

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.

Mixed-metal metal–organic frameworks

“…In a couple of studies by Pöppl et al this antiferromagnetic coupling was exploited to obtain narrow-line EPR spectra of Cu 2+ ions in Cu3(BTC)2 by mixing in low concentrations (1%) of diamagnetic Zn 2+ ions. 67,68 At low temperature only the low concentration of mixed Cu-Zn paddlewheel SBU remained EPR active. EPR combined with ENDOR and HYSCORE then allowed to characterize the interaction of Cu 2+ in the mixed-metal paddlewheels with CH3OH 67 and HD gas.…”

Scrutinizing the Pore Chemistry and the Importance of Cu(I) Defects in TCNQ-Loaded Cu3(BTC)2 by a Multitechnique Spectroscopic Approach

“…The g- and 63 Cu hf-tensor principal values g z = 2.290(3) and A z = 0.0170(7) cm –1 of species A are typical for Cu­(II) ions in square planar to square pyramidal coordination as one would expect for Cu­(II) ions in mixed valence paddlewheel units. , Since Zn­(II) and Cu­(I) are both 3d 10 ions of similar size, one might expect similar spin Hamiltonian parameters for Cu­(II) ions in Cu­(II)–Cu­(I) and Cu­(II)–Zn­(II) paddlewheels. Friedländer et al determined spin Hamiltonian parameters g z = 2.281(1) and A z = 0.0187(2) cm –1 for Cu­(II) ions of mixed metal Cu­(II)–Zn­(II) paddlewheel units in activated Cu 2.965 Zn 0.035 BTC 2 . Even though they are of the same order of magnitude, they are still different to those of species A and allow no clear statement about their actual nature at this point.…”

“…Therefore, the use of different and more elaborate experimental techniques is necessary. One of these advanced tools is infrared (IR) spectroscopy of adsorbed probe molecules, which has shown great potential as a characterization technique, , especially for MOFs containing OMSs such as Cu 3 (BTC) 2 . , Furthermore, electron paramagnetic resonance (EPR) spectroscopy has proven to give important insights about the nature and electronic structure of magnetic species in Cu-based MOFs. − For example, the magnetic coupling and interactions of Cu­(II)–Cu­(II) paddlewheel units were characterized in detail by EPR for Cu containing MOFs. , In addition, EPR was used to study monomeric Cu­(II) defects in Cu paddlewheel-based MOFs, − as well as the adsorption of nonmagnetic or radical species on Zn-doped and pristine Cu 3 (BTC) 2 . , …”

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

“…This allows the reduction of the sample volume by one order of magnitude, from 200 to 20 μl using a loop-gap resonator (LGR) ( 8 ). Further reductions can be achieved by incorporating materials with a high dielectric constant in a standard resonator to reduce the active volume down to 1 μl ( 9 ). For protein single crystals, one must reduce the volume even further (less than 0.03 μl), which requires radical new approaches.…”

“…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.…”