Controllable self-assembly of four new metal–organic frameworks based on different phosphomolybdate clusters by altering the molar ratio of H3PO4 and Na2MoO4

Abstract

By adjusting the molar ratio of the reactants H₃PO₄ and Na₂MoO₄, four new metal–organic frameworks, namely, CoH(bix)₄[PMo^VI₈V^V₄O₄₀(V^IVO)₂] (1), (H₂bix)₂(NaHP₂Mo₅O₂₃)·2H₂O (2), H₂(bix)₄[Cd(H₂O)₄][Cd(HPO₄)₄(H₂PO₄)₄(MoO₂)₁₂ (OH)₆]·10H₂O (3) and (H₂en)₃(Co₃P₄Mo₄O₂₈) (4) (bix = 1,4-bis(imidazol-1-ylmethyl)benzene, en = 1,2-ethylenediamine), have been hydrothermally prepared and characterized by elemental analyses, IR spectroscopy, TG analyses, and single-crystal X-ray diffraction. Compound 1 consists of straight chains based on pseudo-Keggin [PMo^VI₈V^V₄O₄₀(V^IVO)₂]³⁻polyanions and [CoH(bix)₄]³⁺ complex fragments, which are further connected into a three-dimensional (3D) open framework by hydrogen bonding interactions between polyanions and organic bix ligands . Compound 2 shows 3D supramolecular networks constructed from weak interactions between free biprotonated bix, water and oxygen atoms of polyanions [P₂Mo₅O₂₃]⁶⁻. In compound 3, [Cd(HPO₄)₄(H₂PO₄)₄(MoO₂)₁₂(OH)₆]⁴⁻polyanions are linked by [Cd(H₂O)₄]²⁺ cations to build one-dimensional (1D) chains. Then weak interactions between free bix ligands, water and oxygen atoms of the polyanions extend these 1D chains to 3D supramolecular frameworks. Compound 4 exhibits open-framework structures. Interestingly, it also shows 1D left and right-handed helical chains and 2D (6,3)-layers. The molar ratio of the reactants H₃PO₄ and Na₂MoO₄ influence the transformation of the phosphomolybdate clusters in compounds 1–4. The electrochemistry and photocatalysis properties of these compounds have also been investigated in this paper.