Na8MB21O36 (M = Rb and Cs): Noncentrosymmetric Borates with Unprecedented [B21O36]9- Fundamental Building Blocks.

Two noncentrosymmetric borates, Na8MB21O36 (M = Rb and Cs), have been synthesized, structurally analyzed, and characterized for the first time. Their fundamental building block (FBB) is determined as the unprecedented [B21O36]9- units, which is cotemplated by four-connected [BO4]5- and five-connected [B5O11]7- clusters. To the best of our knowledge, the [B21O36]9- FBB has never been reported in borates. Besides, the structure comparisons among all the available anhydrous borates with high-polymeric FBBs were carried out to enlighten the investigation of B-rich borates.