Formation and structural characterization of the basic tin(II) fluoride, Sn₉F₁₃O(OH)₃ ⋅ 2H₂O, containing the unprecedented [Sn₄O(OH)₃]³⁺ cage‐ion

Abstract

Single crystals of the title compound Sn9F13O(OH)3 ⋅ 2H2O, 3, along with those of the oxofluoride Sn4OF6, 2, and the fluorophosphate Sn3(PO4)F3, 1, have been obtained in course of an experiment mimicking the reaction of the tooth paste additive SnF2 with Ca3(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⋅⋅⋅F‐hydrogen bonds. The most prominent building block of the monolayers constitutes the unprecedented, stella octangula like [Sn4O(OH)3]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.

M6O4(OH)4 of M = Sn, Pb: Single Crystal Growth and Crystal Structure Determinations Far Away from Routine

“…Despite the larger size of lead in comparison to tin, most bond angles are very similar in both compounds, with the exception of the equatorial bond angles [96.6(1) • /96.2(1) • for Sn1/Sn2, 98.3(3) • /99.1(3) • for Pb1/Pb2] that are considerably larger for M = Pb. A simple but appropriate bonding concept for a {M II X 4 } ss coordination, taking into account the observed acute bond angles and the non-bonding electron pair of a bivalent group-14 element, has previously been described for M = Sn [39].…”

“…These values are of the same magnitude as the differences between the atom radii of tin and lead calculated from SCF functions[33].From the different parameters for bond valence calculations, BVS, available in literature[34][35][36][37][38], those of Sidey[38] for Sn-O [R 0 = 1.849, B = 0.5] and Krivovichev and Brown[37] for Pb-O [R 0 = 1.963, B = 0.49] give the best agreement with the oxidation state of +II for the metal atoms: BVS Sn1 = 1.99, BVS Sn2 = 1.92; BVS Pb1 = 1.98, BVS Pb2 = 1.98.Despite the larger size of lead in comparison to tin, most bond angles are very similar in both compounds, with the exception of the equatorial bond angles [96.6(1) • /96.2(1) • for Sn1/Sn2, 98.3(3) • /99.1(3) • for Pb1/Pb2] that are considerably larger for M = Pb. A simple but appropriate bonding concept for a {M II X 4 } ss coordination, taking into account the observed acute bond angles and the non-bonding electron pair of a bivalent group-14 element, has previously been described for M = Sn[39].…”