K2[B4O5(OH)4]·H2O and K2[B4O5(OH)4]: two new hydrated potassium borates with isolated [B4O5(OH)4]2− units and different structural frameworks†
Two new hydrated potassium borates, K2[B4O5(OH)4]·H2O (I) and K2[B4O5(OH)4] (II), have been synthesized by using a mild flux method. The compounds crystallize in the space groups P (No. 2) and Pbcn (No. 60), respectively, and consist of fundamental building blocks (FBBs) of isolated [B4O5(OH)4]2− units forming the final three-dimensional anionic network through hydrogen bonds. In the absence of lattice water, the K–O groups show evolution from the [K2O10]∞ double-chain in I to the [KO4]∞ corrugated-layer in II. Specifically, the assembly types of hydrous [B4O5(OH)4]2− and anhydrous [B4O9]6− FBBs in a series of borates have been discussed. In addition, first-principles calculations were implemented to analyze the relationship between the electronic structures and optical properties of the title compounds.