Solvent-controlled synthesis of various Anderson-type polyoxometalate-based metal–organic complexes with excellent capacity for the chromatographic separation of dyes

Abstract

By changing solvent systems, five new Anderson-type polyoxometalates (POMs)-based metal–organic complexes, namely, {(H₂PCAP′)₂[CrMo₆(OH)₅O₁₉]}·H₂O (1), {Cu₃(PCAP)₂[CrMo₆(OH)₅O₁₉](H₂O)₃(DMF)₂}·4H₂O·2HCHO (2), {Cu₃(PCAP)₂[CrMo₆(OH)₆O₁₈]Cl(H₂O)₅}·10H₂O (3), {Co₃(PCAP)₂[CrMo₆(OH)₅O₁₉](H₂O)₆}·9H₂O·HCHO (4) and {Co₃(HPCAP)₂[CrMo₆(OH)₆O₁₈]₂(H₂O)₁₀}·7H₂O·2CH₃CH₂OH (5) (HPCAP = 3-(2-pyridinecarboxylic acid amido)pyridine, PCAP′ = 3-(pyridinecarboxylic acid)amido pyridine), were successfully synthesized and structurally characterized by single-crystal X-ray diffraction, IR spectra, powder X-ray diffraction (PXRD) and thermogravimetric analyses (TGA). Complex 1 was obtained with H₂O as the solvent and exhibited a 3D supramolecular network based on CrMo₆ anions and protonated H₂PCAP′ molecules via hydrogen bonds. It should be noted that the PCAP′ was in situ transformed from HPCAP. Complexes 2–5 were synthesized with different mixed solvents. In complex 2, the CrMo₆ anions acted as an inorganic bridging ligand to link the 1D [Cu₃(PCAP)₂]_n⁴ⁿ⁺ chains to construct a 2D layer. Complexes 3 and 4 showed similar 3D frameworks. Two orientations of metal–organic loops [M₂(PCAP)₂] (M = Cu for 3, Co for 4) could be observed, which directly extended the 1D M–CrMo₆ chains into 3D networks with a 3,3,4,4-connected {4²·12³·14}{4²·6}₂ topology. In 5, the 1D Co–CrMo₆ inorganic chain, [Co₂(PCAP)₂] binuclear loop unit and discrete CrMo₆ polyanion were connected with each other by hydrogen-bonding interactions to form a 1D ladder-like supramolecular dual chain. The structural diversities showed that the solvents play key roles in the construction of various architectures and in the in situ transformation of HPCAP. The adsorption behaviours of the title complexes for organic dyes were investigated in detail. All of the title complexes showed an efficient adsorption capacity for the cationic dyes gentian violet (GV) and methylene blue (MB). In particular, complex 4 could selectively separate GV from the mixture of GV&RhB and GV&MO within 5–10 min, which can be used as a chromatography column for dye removal. In addition, the electrochemical properties of the title complexes were also studied.