Two Alkaline Metal Borates: from Layer to Layer-Pillared Framework

Two borates, Na₂K[B₅O₈(OH)₂]·H₂O (1) and Na₄K₂[{B₅O₈}₂{B₃O₅}{BO₂(OH)₂}]·2H₂O (2) have been designed and made under solvothermal conditions. Compound 1 exhibits a 2D fluctuant layer based on the [B₅O₁₀(OH)₂]^7- clusters, containing two types of 9-membered ring (MR) channels and showing a four-connected sql topology net. By modifying the reactants and reaction temperature, compound 2 was obtained from compound 1. The addition of boron-containing components not only increased their concentration but also enriched the B-O fragment while higher temperatures promoted greater condensation of the anionic clusters. Compound 2 crystallizes in the acentric space group Pc and is composed of two types of [B₅O₁₁]^7-clusters, [B₃O₇]^5- clusters and [BO₂(OH)₂]^3- tetrahedra. The two kinds of [B₅O₁₁]^7- clusters form a 2D layer similar to 1, which is further connected by the chain built from the [B₃O₇]^5- clusters and decorated by [BO₂(OH)₂]^3- groups to produce a 3D layer-pillared framework. Notably, compound 2 is the first layer-pillared borate containing three types of B-O clusters and six types of channels. Second harmonic generation (SHG) measurements reveal that compound 2 displays a moderate SHG response of about 1.1 times that of KH₂PO₄ (KDP).