Analysis of a Cu-Doped Metal-Organic Framework, MFM-520(Zn 1-x Cu x ), for NO 2 Adsorption.

MFM-520(Zn) confines dimers of NO 2 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 Cu 2+ sites allows investigation of the electronic and geometric structure of metal site by in situ electron paramagnetic resonance (EPR) spectroscopy upon adsorption of NO 2 . By combining continuous wave and electron-nuclear double resonance spectroscopy, an unusual reverse Berry distorted coordination geometry of the Cu 2+ centers is observed. Interestingly, Cu-doped MFM-520(Zn 0.95 Cu 0.05 ) shows enhanced adsorption of NO 2 of 5.02 mmol g -1 at 1 bar at 298 K. Whereas MFM-520(Zn) confines adsorbed NO 2 as N 2 O 4 , the presence of monomeric NO 2 at low temperature suggests that doping with Cu 2+ centers into the framework plays an important role in tuning the dimerization of NO 2 molecules in the pore via the formation of specific host-guest interactions.