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(HPCuIIW11O39)]·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.