High-temperature synthesis, single-crystal X-ray structure determination and solid-state NMR investigations of Ba7[SiO4][BO3]3CN and Sr7[SiO4][BO3]3CN

The novel alkaline earth silicate borate cyanides Ba₇SiO₄]BO₃]₃CN and Sr₇SiO₄]BO₃]₃CN have been obtained by the reaction of the respective alkaline earth metals M=Sr, Ba, the carbonates M^IICO₃, BN, and SiO₂ using a radiofrequency furnace at a maximum reaction temperature of 1350°C and 1450°C, respectively. The crystal structures of the isotypic compounds M^II₇SiO₄]BO₃]₃CN have been determined by single-crystal X-ray crystallography (P6₃mc (no. 186), Z=2, a=1129.9(1) pm, c=733.4(2) pm, R₁=0.0336, wR₂=0.0743 for M^II=Ba and a=1081.3(1) pm, c=695.2(1) pm, R₁=0.0457, wR₂=0.0838 for M^II=Sr). Both ionic compounds represent a new structure type, and they are the first examples of silicate borate cyanides. The cyanide ions are disordered and they are surrounded by Ba²⁺/Sr²⁺ octahedra, respectively. These octahedra share common faces building chains along 001]. The BO₃]³⁻ ions are arranged around these chains. The SiO₄]⁴⁻ units are surrounded by Ba²⁺/Sr²⁺ tetrahedra, respectively. The title compounds additionally have been investigated by ¹¹B, ¹³C, ²⁹Si, and ¹H MAS-NMR as well as IR and Raman spectroscopy confirming the presence of SiO₄]⁴⁻, BO₃]³⁻, and CN⁻ ions.