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.