Angew.Chem.Int.Ed.http://doi.org/fz43ft(2012)

Sulfate anions (SO42−), which adopt a sulfur-centred tetrahedral form in which each apex is an oxygen atom, do not often condense into oligosulfates — and when they do they typically form chain-like compounds. Henning Höppe and co-workers at the Universities of Augsburg and Freiburg in Germany have now prepared an unusual polysulfate anion by reacting together boric acid, potassium sulfate and sulfuric acid. The uncommon topology of the crystalline potassium borosulfate obtained, K5[B(SO4)4], was revealed by characterization through single-crystal and powder X-ray diffraction, infrared and Raman spectroscopy, and supported by computational calculations.

Credit: © 2012 WILEY

In the potassium borosulfate crystal obtained, sulfate anions do not undergo direct condensation. Instead, four sulfate tetrahedra coordinate to a central boron atom — an unusual environment for boron — each of them through a bridging oxygen atom. The resulting borosulfate moieties are discrete ionic B(SO4)45− assemblies that crystallize with potassium counter-cations in a polar lattice.

Such a topology has not been observed before with either sulfate or phosphate anions, which adopt the same tetrahedral morphology and have a similar size. Furthermore, it is also rare in nature; it has only been described for the tetrahedral silicate anions [Si(SiO4)4]12− in the mineral zunyite. In that case, however, the silicate species also connect to aluminate octahedra (AlO6) to form an extended network.

For each sulfate tetrahedron, the three terminal oxygen atoms — not involved in coordinating the borate atom — are slightly closer to the central sulfur atom than their bridging counterparts. This is consistent with a length discrepancy between bridging and terminal oxygen atoms previously observed for the disulfate anions of [Si(S2O7)3].

The force constants calculated for both types of S–O bonds and for B–O bonds are also in good agreement with those found for the disulfate (S2O72−) and tetrahydroxyborate [B(OH)4]− anions. The terminal oxygen atoms interact with the potassium counter-cations incorporated within the cavities of the borosulfate lattice. The electrostatic consistency of the structure of this borosulfate anion was supported by computational calculations.