Analysis of a Cu-Doped Metal-Organic Framework, MFM-520(Zn_1-xCu_x), for NO₂ Adsorption

MFM-520(Zn) confines dimers of NO2 with a high adsorption of 4.52 mmol g(-1) at 1 bar at 298 K. The synthesis and the incommensurate structure of Cu-doped MFM-520(Zn) are reported. The introduction of paramagnetic Cu2+ sites allows investigation of the electronic and geometric structure of metal site by in situ electron paramagnetic resonance (EPR) spectroscopy upon adsorption of NO2. By combining continuous wave and electron-nuclear double resonance spectroscopy, an unusual reverse Berry distorted coordination geometry of the Cu2+ centers is observed. Interestingly, Cu-doped MFM-520(Zn0.95Cu0.05) shows enhanced adsorption of NO2 of 5.02 mmol g(-1) at 1 bar at 298 K. Whereas MFM-520(Zn) confines adsorbed NO2 as N2O4, the presence of monomeric NO2 at low temperature suggests that doping with Cu2+ centers into the framework plays an important role in tuning the dimerization of NO2 molecules in the pore via the formation of specific host-guest interactions.