Three mixed-alkaline-metal borates with {Li@B12Ox(OH)24−x} (x = 18, 22) clusters: from isolated oxoboron cluster to unusual layer

Three new mixed-alkaline-metal borates, namely KNa8[Li@B12O18(OH)6](CO3)2 (1), Cs10[Li@B12O20(OH)2]2·6H2O (2) and Cs5[Li@B12O20(OH)2]·3H2O (3), have been synthesized under hydro/solvothermal conditions. Compound 1 is a borate carbonate with the isolated [Li@B12O18(OH)6]5− clusters and interesting layers formed by Na+ and CO32− ions, forming a 2D supramolecular structure with a 3,6-connecting binodal system. After changing the synthetic conditions, the isolated [Li@B12O18(OH)6]5− clusters are successfully extended to the layers in 2 and 3 by condensation reactions with the elimination of water molecules between oxoboron clusters. Each new fundamental building block [Li@B12O22(OH)2]9− in 2 and 3 connects with four neighboring units to form a 2D layer via B–O–B bonds for the first time, and 14-membered rings are also observed. Such a layer structure is first observed in dodecaborates. Since compound 3 crystallizes in the non-centrosymmetric space group, the second harmonic generation measurement and the dipole moments calculation have been carried out.