M₆[Cd₂(CO₃)₂(B₁₂O₁₈)(OH)₆] (M = K, Rb): Borate–Carbonates with Two CdCO₃ Embedded in a Cyclic Oxoboron Anion

Two metal borate–carbonates, M6[Cd2(CO3)2(B12O18)(OH)6] [M = K (1), Rb (2)], were obtained under surfactant-thermal conditions. In 1 and 2, each cyclic [(B12O18)(OH)6]6– anion captures two CdCO3 in two sides of the rings and finally forms the unusual (CdCO3)2@[(B12O18)(OH)6] cluster. Both 1 and 2 show moderate birefringence. Density functional theory calculations indicate that carbonate groups have a major contribution to electron-related optical transition.