pH-Dependent syntheses of copper–quinoxaline–polyoxotungatate hybrids: variable role of Keggin-type polyanion in different pH conditions

Abstract

Four new hybrids based on the Keggin-type polyoxometalate, formulated as (Hbm)₃(PW₁₂O₄₀)·4H₂O (1) (bm = benzimidazole, which is synthesized in situ by quinoxaline), [Cu₆(qx)₉(PW₁₂O₄₀)₂] (2), [Cu₃(qx)₅(PW₁₂O₄₀)(H₂O)] (3) and [Cu₄(qx)₄(HPCu^IIW₁₁O₃₉)]·1.5H₂O (4) (qx = quinoxaline), were hydrothermally synthesized and structurally characterized by routine techniques and single-crystal X-ray diffraction. In 1, the [PW₁₂O₄₀]³⁻ (PW12) clusters only act as counteranions and combine with the protonated organic ligand by electrostatic interactions. In 2, a 6³ topological 2D layer is formed with the PW12 as the template. In 3, the PW12 anions link with four metal–organic chains via Cu–O weak interactions to construct a 3D framework. In 4, the Cu(II)-substituted Keggin unit as a hexadentate inorganic ligand coordinates with four Cu(I) ions from metal–organic chains and two O atoms from neighboring POMs to form a sandwich-like 2D layer. The structure difference of compounds 1–4 reveals that the self-assembly process is pH-dependent and the Keggin-type POM plays different role in different pH conditions.