Analysis of a Cu-Doped Metal-Organic Framework, MFM-520(Zn_1-x Cu_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(Zn_0.95 Cu_0.05 ) 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 N₂ O₄ , 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.