pH-dependent assembly of two polyoxometalate host–guest structural isomers based on Keggin polyoxoanion templates

Abstract

Two interesting polyoxometalate-templated host–guest compounds 1 and 2 with the same chemical formulae of [Ag₆(tpt)₄(SiW₁₂O₄₀)Cl₂]·2H₂O have been obtained by hydrothermal reactions of monolacunary Keggin-type K₈[α-SiW₁₁O₃₉]·13H₂O with CH₃COOAg and tpt ligands under different pH conditions. Compounds 1 and 2 are structural isomers. In 1, the novel chair-like tetranuclear [Ag₄(μ₃-Cl)₂]²⁺ clusters and additional Ag⁺ ions are linked by tpt ligands to form a 2D double-layered structure featured with two different types of voids in the layer, and the double-layers are stacked into a 3D host supramolecular framework with 1D channels filled with Keggin [SiW₁₂O₄₀]⁴⁻ polyoxoanions and water molecules as guests. In 2, Ag⁺ ions are linked by tpt ligands into a 3D framework of 10³-ths topology. The most outstanding structural feature of 2 is that upon six-fold interpenetration among 10³-ths nets there are still nanosized channels along the a axis to accommodate Keggin polyoxoanions and water molecules as guests. Compound 2 represents the first example of six-fold interpenetrating cationic networks templated with polyoxoanions. In this work, the obvious differences in structure between 1 and 2 show that hydrochloric acid solution conditions, which offer not only H⁺ to regulate the pH value but also Cl⁻ to coordinate with Ag⁺ ions, play an important role in the formation of host frameworks templated by Keggin-type polyoxoanions. The photoluminescence properties of 1 and 2 have been investigated in the solid state.