Strong Lewis and Brønsted acidic sites in the borosulfate Mg${}_3$[H${}_2$O→B(SO${}_4$)${}_3$]${}_2$

Borosulfates provide fascinating structures and properties that go beyond a pure analogy to silicates. Mg${}_3$[H${}_2$O→B(SO${}_4$)${}_3$]${}_2$ is the first borosulfate featuring a boron atom solely coordinated by three tetrahedra. Thus, the free Lewis acidic site forms a Lewis acid-base adduct with a water molecule. This is unprecedented for borosulfate chemistry and even for borates. Quantum chemical calculations on water exchange reactions with BF${}_3$ and B(C${}_6$F${}_5$)${}_3$ revealed a higher Lewis acidity for the borosulfate anion. Moreover, proton exchange reactions showed a higher Brønsted acidity than comparable silicates or phosphates. Additionally, Mg${}_3$[H${}_2$O→B(SO${}_4$)${}_3$]${}_2$ was characterised by X-ray diffraction, infrared spectroscopy, thermogravimetric analysis, and density functional theory (DFT) calculations. (© 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH)