Reactions with oleum under harsh conditions: characterization of the unique [M(S2O7)3]2- ions (M=Si, Ge, Sn) in A2[M(S2O7)3] (A=NH4, Ag)

Abstract

The reactions of group 14 tetrachlorides MCl(4) (M=Si, Ge, Sn) with oleum (65% SO(3)) at elevated temperatures lead to the unique complex ions [M(S(2)O(7))(3)](2-), which show the central M atoms in coordination with three chelating S(2)O(7)(2-) groups. The mean distances M-O within the anions increase from 175.6(2)-177.5(2) pm (M=Si) to 186.4(4)-187.7(4) pm (M=Ge) to 201.9(2)-203.5(2) pm (M=Sn). These distances are reproduced well by DFT calculations. The same calculations show an increasing positive charge for the central M atom in the row Si, Ge, Sn, which can be interpreted as the decreasing covalency of the M-O bonds. For the silicon compound (NH(4))(2)[Si(S(2)O(7))(3)], (29)Si solid-state NMR measurements have been performed, with the results showing a signal at -215.5 ppm for (NH(4))(2)[Si(S(2)O(7))(3)], which is in very good agreement with theoretical estimations. In addition, the vibrational modes within the [MO(6)] skeleton have been monitored by Raman spectroscopy for selected examples, and are well reproduced by theory. The charge balance for the [M(S(2)O(7))(3)](2-) ions is achieved by monovalent A(+) counter ions (A=NH(4), Ag), which are implemented in the syntheses in the form of their sulfates. The sizes of the A(+) ions, that is, their coordination requirements, cause the crystallographic differences in the crystal structures, although the complex [M(S(2)O(7))(3)](2-) ions remain essentially unaffected with the different A(+) ions. Furthermore, the nature of the A(+) ions influences the thermal behavior of the compounds, which has been monitored for selected examples by thermogravimetric differential thermal analysis (DTA/TG) and XRD measurements.