First examples of aliphatic zirconium MOFs and the influence of inorganic anions on their crystal structures

Abstract

Utilizing the aliphatic linker molecule adipic acid (1,6-hexanedioic acid, HO2C-C4H8-CO2H) or 3-methyladipic acid (racemic mixture, HO2C-C4H7CH3-CO2H), the first crystalline zirconium adipates were synthesized under aqueous conditions. Their structures were deduced from powder X-ray diffraction data and were confirmed by Rietveld refinements. For all three compounds, the inorganic nodes are related to the well-known Zr6O4(OH)4 cluster frequently observed in aromatic zirconium MOFs. Employing ZrOCl2·8H2O and 3-methyladipic acid, a framework with bcu topology was obtained. Starting from adipic acid and Zr(SO4)2·4H2O, we observed the incorporation of sulfate into the crystal structure. Four sulfate anions are coordinated to each Zr-oxo cluster in a bidentate fashion. In this complex structure, square grids formed by Zr-oxo clusters and adipate anions and furthermore a hydrogen-bonded inorganic dia net can be observed. The third compound presented here is structurally related to the zirconium methyladipate. Using adipic acid and adding CrO42- under strongly acidic conditions leads to the incorporation of Cr2O72- into the bcu net. The dichromate anions are coordinated twofold to two different Zr-oxo clusters in a monodentate fashion and thus serve as inorganic connectors between the framework's nodes.