Interesting fluorine anion water clusters [F−·(H2O)n] in metal complex crystals

Abstract

Three crystalline complexes containing fluorine anion water clusters, [Cd(Im)₆][F·H₂O]₂ (1), [Ni(Im)₆][F₂·(H₂O)₅] (2) and [Co(Im)₆][F·NO₃·(H₂O)₄] (3), were reported. The interesting fluorine anion water clusters are described in this paper. Single X-ray analysis shows that fluoride ions in the water clusters have different coordination environments, four-coordinated in a tetrahedral geometry for (1), five-coordinated in a trigonal bipyramidal geometry for (2), and three-coordinated in a planar trigonal geometry for (3). F⁻(H₂O)_n is assembled by strong H-bonding interactions [OH⋯F = 2.576(1)–2.813(1) Å], where a fluoride anion plays an important role in connecting an imidazole ligand of the host [M(Im)₆]²⁺. The strength of an ionic bond between the host and guest is strongly influenced by the F⁻(H₂O)_n water cluster adjacent to the ligand with NH⋯F distances of 2.607–2.755 Å. The TGA, IR and XRD studies for the above three compounds at different testing temperatures suggested that the fluorine anion water cluster, F⁻(H₂O)_n, was very stable. This result distinctly differentiates these compounds from the reported ones containing neutral water cluster molecules. This phenomenon indicated the stronger anionic hydrogen-bond interaction between F⁻ and H₂O, which also confirmed the intracluster proton transfer process F⁻·H₂O → HF·OH⁻. This result suggests that the fluorine anions and water molecules are H-bonded to each other in an alternating fashion within the fluoride−water hybrid cluster, where a fluorine anion plays the important role of connecting the host metal complex and guest water cluster.