RESUMEN
The flexibility of the MIL-53(M) metal-organic framework (MOF) has been elucidated through various characterization methodologies, particularly in gas and liquid adsorption processes. However, to the best of our knowledge, there has been no prior electron paramagnetic resonance (EPR) characterization of liquid-phase adsorption in the MOF MIL-53(M), which offers insights into local geometric changes at the oxygen octahedron containing the metal ions of the framework. In this study, we investigate, for the first time, the pore transformations within the MIL-53(Al0.99Cr0.01) framework during liquid-phase adsorption using EPR spectroscopy. Our investigation concentrates explicitly on the adsorption of pure solvents, including water, methanol, ethanol, isopropanol, pyridine, and mixed water/methanol phases. The EPR spectroscopy on the (Al0.99Cr0.01) MOF has allowed us to witness and comprehend the transitions between the narrow pore and large pore phases by examining changes in the zero-field splitting parameters of the S = 3/2 Cr(iii) species. Of all the solvents examined, a robust and distinct spectral feature observed during methanol adsorption unequivocally indicates the pore opening.
RESUMEN
The nature of the chemical bonding between NO and open-shell NiII ions docked in a metal-organic framework is fully characterized by EPR spectroscopy and computational methods. High-frequency EPR experiments reveal the presence of unsaturated NiII ions displaying five-fold coordination. Upon NO adsorption, in conjunction with advanced EPR methodologies and DFT/CASSCF modelling, the covalency of the metal-NO and metal-framework bonds is directly quantified. This enables unravelling the complex electronic structure of NiII-NO species and retrieving their microscopic structure.
RESUMEN
Metal-organic frameworks (MOFs) can respond to light in a number of interesting ways. Photochromism is observed when a structural change to the framework is induced by the absorption of light, which results in a color change. In this work, we show that introducing quinoxaline ligands to MUF-7 and MUF-77 (MUF = Massey University Framework) produces photochromic MOFs that change color from yellow to red upon the absorption of 405 nm light. This photochromism is observed only when the quinoxaline units are incorporated into the framework and not for the standalone ligands in the solid state. Electron paramagnetic resonance (EPR) spectroscopy shows that organic radicals form upon irradiation of the MOFs. The EPR signal intensities and longevity depend on the precise structural details of the ligand and framework. The photogenerated radicals are stable for long periods in the dark but can be switched back to the diamagnetic state by exposure to visible light. Single-crystal X-ray diffraction analysis reveals bond length changes upon irradiation that are consistent with electron transfer. The multicomponent nature of these frameworks allows the photochromism to emerge by allowing through-space electron transfer, precisely positioning the framework building blocks, and tolerating functional group modifications to the ligands.
RESUMEN
Metal-organic frameworks containing multiple metals distributed over crystallographically equivalent framework positions (mixed-metal MOFs) represent an interesting class of materials, since the close vicinity of isolated metal centers often gives rise to synergistic effects. However, appropriate characterization techniques for detailed investigations of these mixed-metal metal-organic framework materials, particularly addressing the distribution of metals within the lattice, are rarely available. The synthesis of mixed-metal FeCuBTC materials in direct syntheses proved to be difficult and only a thorough characterization using various techniques, like powder X-ray diffraction, X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy, unambiguously evidenced the formation of a mixed-metal FeCuBTC material with HKUST-1 structure, which contained bimetallic Fe-Cu paddlewheels as well as monometallic Cu-Cu and Fe-Fe units under optimized synthesis conditions. The in-depth characterization showed that other synthetic procedures led to impurities, which contained the majority of the applied iron and were impossible or difficult to identify using solely standard characterization techniques. Therefore, this study shows the necessity to characterize mixed-metal MOFs extensively to unambiguously prove the incorporation of both metals at the desired positions. The controlled positioning of metal centers in mixed-metal metal-organic framework materials and the thorough characterization thereof is particularly important to derive structure-property or structure-activity correlations.
