Polyoxometalate-directed assembly of various multinuclear metal–organic complexes with 4-amino-1,2,4-triazole and selective photocatalysis for organic dye degradation

Abstract

A series of polyoxometalate (POM)-based metal–organic complexes containing multinuclear Cu(II) clusters with a 4-amino-1,2,4-triazole (4-atrz) ligand, namely, [Cu₃(4-atrz)₈(PMo₁₂O₄₀)₂(H₂O)₂]·2H₂O (1), [Cu₂(4-atrz)₆(SiW₁₂O₄₀)(H₂O)]·6H₂O (2), [Cu₂(4-atrz)₄(μ₂-OH)(CrMo₆(OH)₆O₁₈)]·3H₂O (3), [Cu₃(4-atrz)₃(Mo₈O₂₇)(H₂O)₄]·6H₂O (4) and [Cu₃(4-atrz)₃(V₁₀O₃₀)_0.5(μ₃-OH)(H₂O)]·H₂O (5) have been synthesized by selectively adding additional citric acid or boric acid under hydrothermal conditions and have been structurally characterized by single-crystal X-ray diffraction and powder X-ray diffraction. Compound 1 has a zero-dimensional (0D) architecture, which is constructed from a linear trinuclear cluster [Cu₃(4-atrz)₈(H₂O)₂]⁶⁺ and two Keggin PMo₁₂O₄₀³⁻ anions. Compound 2 shows a 1D zigzag chain, in which the binuclear [Cu₂(4-atrz)₆(H₂O)]⁴⁺ clusters and Keggin SiW₁₂O₄₀⁴⁻ anions connect to each other. Compound 3 is a 1D linear chain based on linear [Cu₃(4-atrz)₆]⁶⁺ clusters; the [CrMo₆(OH)₆O₁₈]³⁻ anions hang on two sides of the 1D chain. Compound 4 has a 2D layer constructed from trigonal [Cu₃(4-atrz)₃(H₂O)₄]⁶⁺ clusters and rare infinite [Mo₈O₂₇]_n⁶ⁿ⁻ chains. The [Mo₈O₂₇]⁶⁻ anion is transformed from the Anderson-type [CoMo₆(OH)₆O₁₈]³⁻ anion, which is also rare in the POM-based reaction system. Compound 5 has a 3D framework constructed from trigonal trinuclear [Cu₃(4-atrz)₃(OH)]⁵⁺ clusters and rare [V₁₀O₃₀]¹⁰⁻ polyanions, which represents the first example of a V₁₀O₃₀-based 3D metal–organic complex. Structural analyses indicate that different POMs show great effect on the various structures of 1–5 and the additional acids play an important role in the formation of 1–5. Photocatalytic experiments of 1–5 on degradation of three organic dyes (methylene blue, Rhodamine B and methyl orange) manifest that compounds 1–3 are good candidates for the photocatalytic degradation of methylene blue, and compound 1 is a good photocatalyst for the degradation of Rhodamine B.