M 6 [Cd 2 (CO 3 ) 2 (B 12 O 18 )(OH) 6 ] (M = K, Rb): Borate-Carbonates with Two CdCO 3 Embedded in a Cyclic Oxoboron Anion.

Two metal borate-carbonates, M 6 [Cd 2 (CO 3 ) 2 (B 12 O 18 )(OH) 6 ] [M = K ( 1 ), Rb ( 2 )], were obtained under surfactant-thermal conditions. In 1 and 2 , each cyclic [(B 12 O 18 )(OH) 6 ] 6- anion captures two CdCO 3 in two sides of the rings and finally forms the unusual (CdCO 3 ) 2 @[(B 12 O 18 )(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.