Novel polyoxometalate-based cobalt complexes based on rigid pyridyl-triazole-tetrazole and pyridyl-bis(triazole) ligands

Abstract

Three novel polyoxometalate (POM)-based metal–organic complexes (MOCs) constructed from rigid pyridyl-triazole-tetrazole and pyridyl-bis(triazole) ligands have been hydrothermally synthesized and structurally characterized: [Co₃(Htyptz)₂(γ-Mo₈O₂₆)(H₂O)₆]·4H₂O (1), H₂[Co₃(Htyptz)₂(TeMo₆O₂₄)(H₂O)₈]·6H₂O (2), [Co₃(Hpyttz)₂(γ-Mo₈O₂₆)(H₂O)₈]·2H₂O (3) (H₂typtz = 3-(tetrazol-5-yl)-5-(pyrid-3-yl)-1,2,4-triazolyl, H₂pyttz = 3-(pyrid-4-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl). Single-crystal X-ray diffraction analyses reveal that complex 1 is a 3D metal–organic framework. Both 2 and 3 are 2D metal–organic architectures. In complex 1, the adjacent [γ-Mo₈O₂₆]⁴⁻ anions are linked by the Co^II ions to form a 1D Co-γ-Mo₈O₂₆ inorganic chain. Then the 1D Co-γ-Mo₈O₂₆ inorganic chains are linked together by the 1D metal–organic Co-Htyptz chains to form a 3D framework. In complex 2, the adjacent Co-Htyptz metal–organic chains are connected by TeMo₆ polyoxoanions to form a 2D architecture. In complex 3, [γ-Mo₈O₂₆]⁴⁻ anions are bridged by the Co^II ions to give similar 1D Co-γ-Mo₈O₂₆ inorganic chains to those in 1, which are connected through [Co(Hpyttz)₂] units to form a 2D network. 1 and 2 represent the first examples of POM-based MOCs derived from H₂typtz. The effects of the POM type and different ligands on the structures of the title complexes were discussed. The adsorption activities for organic dyes methylene blue (MB), gentian violet (GV), methyl orange (MO), rhodamine B (RhB), neutral red (NR) and safranin T (ST) have been investigated in detail. Furthermore, the electrochemical properties of 1 and 2 have been studied, which display electrocatalytic activities toward the reduction of nitrite and bromate. 1-CPE shows pH-dependent electrochemical behaviors and may be used as a potential sensor.