Two Mixed Alkali-Metal Borates Templated from Cations to Clusters

Two mixed alkali-metal borates, K0.5Li[B6O10]·0.5H3O (1) and [(μ5-OH)@(Na4Li)]0.5[B6O10]·0.5B(OH)3 (2), have been prepared under solvothermal conditions. Both of them are obtained in the same synthetic system and contain a B6O138- cluster as the structural-building unit (SBU) but exhibit quite different structural features. 1 is a centric three-dimensional (3D) oxoboron (B-O) framework, where templated mixed K+ and Li+ cations occupied the cavities of the structure. 2 crystallizes in an acentric space group under the templating effect of a unique acentric alkali-metal cluster [(μ5-OH)@(Na4Li)]4+. The SBU of 2 is also the B6O138- cluster, which acts as six-connected nodes linked together to form a 3D B-O framework, showing different characters from 1 because of two types of templates; the acentric [(μ5-OH)@(Na4Li)]4+ clusters and the electroneutral B (OH)3 groups fill in two different cages in the framework and further connect each other via Na-O-B bonds to build a novel two-dimensional (2D) wavy bricklike network, resulting in a 3D B-O framework interpenetrated by a 2D [(μ5-OH)@(Na4Li)]-B(OH)3 network. As a crystal material with an acentric space group, 2 shows a good second harmonic generation response of about 2.8 times that of KDP (KH2PO4) and has a cutoff edge below 190 nm, which suggests that 2 is a potential deep-UV NLO material.