Hydrothermal synthesis and characterization of POMs-based compounds based on binuclear [M2(3-bpo)2] (M = Co, Cu) unit and its derivatives

Abstract

Four novel polyoxometalates (POMs) based inorganic–organic hybrid compounds, [(SiMo12O40)Cu4(3-bpo)4]·2H2O (1), [(SiMo12O40)Cu2(3-bpo)2(NA)2]·2H2O (2), [(H2P2W18O62)Cu2(3-bpo)4(H2O)2]·2H2O (3), and [(H2P2W18O62)Co2(3-bpo)4(H2O)2]·9H2O (4) (NA=nicotinic acid, C5H4NCOOH; 3-bpo=2,5-bis(3-pyridyl)-1,3,4-oxadiazole, C12H8N4O) have been synthesized under hydrothermal condition. Single crystal X-ray diffraction study reveals that all compounds contain the same basic binuclear {M2(3-bpo)2} (M=Co, Cu) moieties as building blocks. In compound 1, the [Cu2(3-bpo)2]2+ unit formed from the CuI ions and the 3-bpo molecules connect with tetradentate [SiMo12O40]4− polyanions to form 1D chains which further link each other to construct 2D layer structure. In compound 2, the [Cu2(3-bpo)2]4+ units are modified by two monodentate nicotinic acid molecules to form a [Cu2(3-bpo)2(NA)2]4+ moiety. The [Cu2(3-bpo)2(NA)2]4+ moiety is linked by tetradentate [SiMo12O40]4− polyanions to construct 1D linear chains and the adjacent 1D chains stack into the 3D structure through π–π interactions. In compounds 3 and 4, the binuclear {M2(3-bpo)2} units are further coordinated by two monodentate 3-bpo molecules and two water molecules to form [M2(3-bpo)4(H2O)2]4+ (M=Co, Cu) moieties which are further linked by tetradentate {P2W18} polyanions to construct 1D wave-like chains. The influences of POMs type and pH value on the formation of the four compounds have been discussed. Furthermore, the electrochemical performances of the compounds 1–4 have been investigated by cyclic voltammetry.