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.