Self-assembly of polyoxometalate-based hybrid aggregates: from a monomer to dimers by changing the pH value of reaction systems

Abstract

Three new compounds have been synthesized at different pHs under identical hydrothermal conditions, namely, (H₂bimb)[Cu(bimb)₃(H₂O)₂(H₂P₂W₁₈O₆₂)]·4H₂O (1), (H₂bimb)₃[Cu₂(bimb)₅(H₂O)₄(HP₂W₁₈O₆₂)₂]·9H₂O (2), and (H₂bimb)₅[Cu(bimb)₂(H₂O)₂(P₂W₁₈O₆₂)₂]·8H₂O (3) (bimb = 4-bis(imidazol-1-yl)benzene). Their structures were determined using single crystal X-ray diffraction analyses and characterized using routine methods. When the pH was adjusted to 2.0–2.5, compound 1 was obtained, which is a monomer. Compounds 2 and 3 were obtained at pH values of 2.5–3.2 and 3.2–4.0, which are two kinds of distinct dimers (I and II), respectively. The pH of the reaction plays a key role in the assembly of POM-based hybrid aggregates. Electrochemical experiments indicate that a 1-based carbon paste electrode (1-CPE) exhibits high catalytic efficiency and high stability towards reduction of NO₂⁻. Namely, the electrocatalytic efficiency towards the reduction of NO₂⁻ is ca. 235% in 1 M H₂SO₄ containing 50 mM NO₂⁻, and the current signal after 100 cycles exhibits almost no loss for the 1-CPE.