On silicate-analogous materials. Synthesis and characterisation of novel borosulfates

Borosulfates represent a relatively new class of silicate-analogous materials, whose potential goes beyond a simple analogy to silicates. In order to explore their capabilities, this work deals with the systematic expansion of borosulfates. Herein, twenty cations, such as the alkaline earth metals Mg and Sr, the transition metals Mn, Co, Ni, and Zn and the rare earth metals Y, La-Nd, Sm, Eu, Tb-Lu could be incorporated in borosulfates for the very first time. Accompanied by the elemental diversity, the structural diversity could be enriched by four new anion topologies, such as the B-O-B bridges containing anion [B${}_2$O(SO${}_4$)${}_6$]${}^{8-}$ in α-Mg${}_4$[B${}_2$O(SO${}_4$)${}_6$], the solely B-O-S bridges containing anion [B${}_2$(SO${}_4$)${}_4$]${}^{2-}$ in α-Mg[B${}_2$(SO${}_4$)${}_4$], the S-O-S bridges containing anion [B${}_2$(SO${}_4$)${}_3$(S${}_2$O${}_7$)]${}^{2-}$ in Sr[B${}_2$(SO${}_4$)${}_3$(S${}_2$O${}_7$)] and the Lewis acid-base adduct [H${}_2$O→B(SO${}_4$)${}_3$]${}^{3-}$ in Mg${}_3$[H${}_2$O→B(SO${}_4$)${}_3$]${}_2$. The latter one represents a novel connection pattern, featuring a boron atom solely coordinated by three tetrahedra, and thus, provides a Lewis acidic site that forms an adduct with a water molecule. In addition, due to this tremendous progress, the first-ever UV-vis spectra could be recorded on the borosulfates of the open-shell transition metals Co${}^{2+}$ and Ni${}^{2+}$, and the first-ever fluorescence spectra could be measured on the borosulfates of Ce${}^{3+}$, Eu${}^{3+}$ and Tb${}^{3+}$, providing highly interesting insights into the coordination behaviour for borosulfates. Besides, the decisive progress that was achieved concerning the synthesis of borosulfates made it possible for the first time to selectively address a desired connection pattern of the anion with regard to S-O-S, B-O-S and B-O-B bridges in the quaternary systems M-B-O-S (M = Sr, Ba). The consequent thermal transformation of the anion from S-O-S to B-O-S to B-O-B bridges deepened the understanding of the stabilities of the respective connection pattern. Finally, computational investigations on Mg${}_3$[H${}_2$O→B(SO${}_4$)${}_3$]${}_2$ revealed high Lewis and Brønsted acidity. The knowledge gained in this work unravels the synthesis and stability of borosulfates, significantly advances the elemental and structural diversity of borosulfates, showed first insights into the properties and thus opens new horizons for borosulfate chemistry.