Anion-tuned sorption and catalytic properties of a soft metal-organic solid with polycatenated frameworks

A structurally flexible porous metal-organic framework, [Cu-II(bped)(2)(H2O)(2)(SiF6)] superset of 4H(2)O (Cu-MOF-SiF6) was prepared from CuSiF6 and meso-1,2-bis(4-pyridyl)-1,2-ethanediol (bped) in EtOH-H2O. Cu-MOF-SiF6 is a three-dimensional (3D) polycatenated framework consisting of two-dimensional (2D) (4,4) topological grid networks which display 2-fold inclined interpenetration in the crystal lattice. The SiF62- anions hosted in the one-dimensional channels of Cu-MOF-SiF6 can be readily exchanged with NO3- anions while keeping the framework intact, leading to isomorphous Cu-MOF-NO3. The Cu-MOFs show selective adsorption behaviors due to the host-guest interaction, which is dependent on the anions and the guest size. The MeOH uptake investigation indicates that Cu-MOF-SiF6 demonstrates a breathing effect, while Cu-MOF-NO3 exhibits distinctive stepwise MeOH sorption due to the smaller size of NO3- and the larger voids in the crystal lattice. Cu-MOF-NO3 also exhibits stepwise sorption for larger EtOH guests, while the uptake of EtOH is blocked in Cu-MOF-SiF6. In addition, the solids show anion-responsive catalytic properties. Both Cu-MOF-SiF6 and Cu-MOF-NO3 efficiently oxidize benzylic compounds to the corresponding carbonyl functionality under mild and convenient reaction conditions, but improved catalytic activity was observed for Cu-MOF-NO3. The catalysts can be reused with the framework left intact at least three times without losing of any activity.