Metal−Organic Frameworks from Silicon- and Germanium-Centered Tetrahedral Ligands

Metal−organic frameworks (MOFs) have been constructed from the reaction of Si(C6H4CO2H)4 and of Ge(C6H4CO2H)4 with Zn(II). The two structures are quite different from each other, and both are different from that of the known MOF of C(C6H4CO2H)4 with Zn(II). The zinc portion of the carbon system is a dinuclear zinc-oxo cluster with square-planar geometry, that of the silicon system consists of zinc-oxo chains containing two zinc atoms with different geometries, and that of the germanium system consists of linear zinc-oxo clusters containing three zinc atoms with two different geometries. For the silicon system, the secondary building unit around zinc is a distorted tetrahedron, whereas for the germanium system, the secondary building unit around zinc is a bipyramid with eight coordination. The differences in morphology may be due to the more open geometry provided as the M−C bond increases from carbon to germanium. Both MOFs have considerable vacancy in the crystal lattice, calculated to be ca. 42% for the silicon system and ca. 46% for the germanium system. Gas sorption experiments on the germanium system indicated significant uptake of both nitrogen and carbon dioxide.