Deuterium NMR studies of framework and guest mobility in the metal-organic framework compound MOF-5, Zn4O(O2CC6H4CO2)3

Variable temperature wideline 2H NMR investigations of the mobility of aromatic rings in the framework of desolvated samples of the metal organic framework compound MOF-5 (Zn4O(O2CC6H4CO2)3) and the closely related solid MOCP-L have been performed. The aromatic rings in MOF-5 are stationary at temperatures lower than room temperature but perform 180° (π) flips at higher temperatures until all framework aromatic groups execute this motion at 373 K. The aromatic groups in MOCP-L demonstrate a higher degree of mobility with some motion even below 170 K. 2H NMR of perdeuterobenzene adsorbed onto MOF-5 at a loading of two molecules per Zn4O13 inorganic unit shows that upon cooling below 140 K the motion of benzene changes from isotropic reorientation to rotation about its six-fold axis. Computational simulation suggests that benzene occupies a site close to the Zn4O13 unit at low temperatures.