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

Four new hybrids based on the Keggin-type polyoxometalate, formulated as (Hbm) ₃(PW ₁₂O ₄₀)·4H ₂O (1) (bm = benzimidazole, which is synthesized in situ by quinoxaline), [Cu ₆(qx) ₉(PW ₁₂O ₄₀) ₂] (2), [Cu ₃(qx) ₅(PW ₁₂O ₄₀)(H ₂O)] (3) and [Cu ₄(qx) ₄(HPCu ^IIW ₁₁O ₃₉)]·1.5H ₂O (4) (qx = quinoxaline), were hydrothermally synthesized and structurally characterized by routine techniques and single-crystal X-ray diffraction. In 1, the [PW ₁₂O ₄₀] ^3- (PW12) clusters only act as counteranions and combine with the protonated organic ligand by electrostatic interactions. In 2, a 6 ³ 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.