Formation and structural characterization of the basic tin(II) fluoride, Sn9F13O(OH)(3)center dot 2H(2)O, containing the unprecedented, [Sn4O(OH)(3)](3+) cage-ion

Abstract

Single crystals of the title compound Sn9F13O(OH)center dot 2H(2)O, along with those of the oxofluoride Sn4OF6, 2, and the fluorophosphate Sn-3(PO4)F-3, 1, have been obtained in course of an experiment mimicking the reaction of the tooth paste additive SnF2 with Ca-3(PO4)(2) as dentin surrogate. All three compounds have been characterized by single crystal X-ray diffraction. While our data on the crystal structures 1 and 2 confirm previous results but improve bond lengths and angles precision significantly, 3 represents the first example of a basic tin(II) fluoride the structure of which was determined. Its crystal structure consists of bilayers held together through weak secondary Sn-F-bonds while their unsymmetrical, protic-aprotic monolayers are connected with each other on their protic sites via -OH center dot center dot center dot F-hydrogen bonds. The most prominent building block of the monolayers constitutes the unprecedented, stella octangula like [Sn4O(OH)](3+) cage-ion. The coordination spheres of all nine Sn(II) atoms are analyzed with respect to their constitution, geometry and bonding. The observed bond lengths and bond angles of the tin(II) atoms are interpretated in terms of p-p-bonding via 2e-2c bonds (first coordination sphere, SnX3, trigonal-pyramidal) and 4e-3c-bonds (second coordination sphere, SnX4/SnX5, seesaw/square-pyramidal). Bond valence calculations have been found to be of limited benefit to predict the oxidation state of Sn(II) especially when Sn-O bonds are present.