Polyoxometalate-directed assembly and various structures of inorganic–organic hybrid compounds based on a semi-rigid bis-pyridyl-bis-amide

Abstract

Through tuning different kinds of polyoxometalates (POMs), four POM-based inorganic–organic hybrid compounds with different dimensionalities, namely [(H₂L)₂(Mo₈O₂₆)] (1), [Cu^II₂L₆(P₂W₂₄O₇₉)·4H₂O]·7H₂O (2), {Cu^II₂L₂(μ₂-OH)[CrMo₆(OH)₆O₁₈]·4H₂O}·4H₂O (3), and {Cu^I₂Cu^II₂L₂(μ₃-OH)₂[CrMo₆(OH)₅O₁₉]·6H₂O}·4H₂O (4) [L = 1,4-bis(3-pyridinecarboxamido)benzene], have been synthesized under hydrothermal conditions and characterized by IR spectroscopy, TG analysis, powder XRD and single-crystal X-ray diffraction. Single-crystal X-ray analyses reveal that compound 1 exhibits a two-dimensional (2D) supramolecular network, which is constructed from protonated L ligands and [Mo₈O₂₆]⁴⁻ anions through hydrogen bonding interactions. Compound 2 is a 1D hybrid chain based on an uncommon inorganic [P₂W₂₄O₇₉]⁴⁻ chain and “Y”-shaped (CuL₃)²⁺ subunits. Compound 3 shows a 2D network constructed from 1D inorganic Cu₂–CrMo₆ chains and bidentate L ligands, which is obtained at pH = 4.1. When the pH was adjusted to 4.8, compound 4 is obtained which exhibits a 4-connected {6⁶} 3D framework. The CrMo₆ anions are connected by adjacent [Cu^I₂Cu^II₂(μ₃-OH)₂(H₂O)₆]⁴⁺ subunits to form 1D Cu₄–CrMo₆ inorganic chains, which are further extended by L ligands to construct a 3D network. The effect of different types of polyanions and pH values on the assembly and various structures of the title compounds has been discussed. Furthermore, the electrochemical properties of 1 and 2, and the photocatalytic properties of 2 and 3 under different conditions have been investigated.