A series of new polyoxometalate-based metal-organic complexes with different rigid pyridyl-bis(triazole) ligands: assembly, structures and electrochemical properties.

Five new polyoxometalate (POM)-based metal-organic complexes (MOCs) with different rigid pyridyl-bis(triazole) ligands, namely, H{Co(Hpyttz-I)(HO)[CrMo(OH)O]}·8HO (1), {Co(Hpyttz-I)(HO)[TeMoO]}[Co(HO)]·3HO (2), {Co(Hpyttz-II)(HO)[γ-MoO]}·10HO (3), {Ni(Hpyttz-II)(HO)[γ-MoO]}·10HO (4), {Ni(Hpyttz-III)(HO)[γ-MoO]}·10HO (5) (Hpyttz-I = 3-(pyrid-2-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl, Hpyttz-II = 3-(pyrid-3-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl, Hpyttz -III = 3-(pyrid-4-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl), 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 is a two-dimensional (2D) supramolecular network based on the binuclear complex unit: [Co(Hpyttz-I)(HO) [CrMo(OH)O]]. Complex 2 is a 1D supramolecular chain derived from the binuclear cobalt complex: {Co(Hpyttz-I)(HO)[TeMoO]}, the discrete [Co(HO)] units act as counter cations. Complexes 3 and 4 are isostructural with different center metals (M = Co or Ni), the adjacent γ-MoO anions are linked by the M ions to form a 1D M-γ-MoO inorganic chain. Then 1D M-γ-MoO inorganic chains are linked together by the 1D metal-organic M-(Hpyttz-II) chains to form a 3D framework. In complex 5, γ-MoO anions are bridged by the Ni ions to give a 1D Ni-γ-MoO inorganic chain, the adjacent 1D Ni-γ-MoO chains are connected through [Ni(Hpyttz-III)] units to form a 2D layer. The effect of POM type and coordination site of the ligands on the structures of the title complexes were discussed. The title complexes 1, 2 and 5 exhibit excellent bifunctional electrocatalytic activities toward the reduction of bromate/hydrogen peroxide and the oxidation of ascorbic acid. In addition, the redox potentials of complexes 1, 2 and 5 are highly sensitive to pH and may be used as a kind of potential pH sensor.