Application of flexible bis-pyrazine–bis-amide ligands to construct various polyoxometalate-based metal–organic complexes

Abstract

By introducing the flexible bis-pyrazine–bis-amide ligands into a polyoxometalates (POMs) system, five POM-based metal–organic complexes, {Cu₃(L¹)₂[CrMo₆(OH)₆O₁₈]₂(H₂O)₂}·10H₂O (1), [CuL¹(Mo₈O₂₆)_0.5]·H₂O (2), [Cu₂(L¹)₂(HPMo^VI₁₀Mo^V₂O₄₀) (H₂O)₂]·2H₂O (3), [Cu₂(L¹)₂(SiMo₁₂O₄₀) (H₂O)₂]·2H₂O (4), [Cu₂(L²)₂(SiMo₁₂O₄₀)]·2H₂O (5) [L¹ = N,N′-bis(2-pyrazinecarboxamide)-1,3-propane, L² = N,N′-bis(2-pyrizinecarboxamide)-1,6-hexane], were synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction analyses, elemental analyses, IR spectra, powder X-ray diffraction (PXRD) and thermal gravimetric analyses (TG). Single-crystal X-ray analysis reveals that compound 1 is a 1D infinite ribbon-like chain with Anderson type polyanions [CrMo₆(OH)₆O₁₈]³⁻ as building blocks. Compound 2 is a 2D layer constructed from 1D helical [Cu-L¹]_n²ⁿ⁺ chains and [Mo₈O₂₆]⁴⁻ anions, in which the octamolybdates also act as inorganic building blocks. Compounds 3 and 4 are isostructural, and display 2D supramolecular networks based on the 1D [Cu-L¹]_n²ⁿ⁺ chains, in which the Keggin polyanions serve as noncoordinated templates. Compound 5 is a 3D supramolecular framework constructed from the 2D [Cu-L²]_n²ⁿ⁺ layers and the Keggin [SiMo₁₂O₄₀]⁴⁻ polyanion templates. In compounds 1–4, the L¹ ligands display the same symmetric coordination mode with two chelating sites. When lengthening the spacer of the ligand, L² exhibits an asymmetric coordination mode in compound 5. The structural diversities of 1–5 show that the different polyoxoanions and spacer lengths of the ligands play key roles in the construction of various architectures. The title compounds represent the first examples of introducing flexible bis-pyrazine–bis-amide ligands into the POMs system. In addition, the electrochemical properties of compounds 2–5 and the photocatalytic activities of the title compounds on the degradation of methylene blue (MB) under UV, visible light and sunlight irradiation have been investigated in detail.