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

Abstract

Five new polyoxometalate (POM)-based metal–organic complexes (MOCs) with different rigid pyridyl-bis(triazole) ligands, namely, H{Co₂(Hpyttz-I)₂(H₂O)₆[CrMo₆(OH)₆O₁₈]}·8H₂O (1), {Co₂(H₂pyttz-I)₂(H₂O)₄[TeMo₆O₂₄]}[Co(H₂O)₆]·3H₂O (2), {Co₃(Hpyttz-II)₂(H₂O)₆[γ-Mo₈O₂₆]}·10H₂O (3), {Ni₃(Hpyttz-II)₂(H₂O)₆[γ-Mo₈O₂₆]}·10H₂O (4), {Ni₃(Hpyttz-III)₂(H₂O)₈[γ-Mo₈O₂₆]}·10H₂O (5) (H₂pyttz-I = 3-(pyrid-2-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl, H₂pyttz-II = 3-(pyrid-3-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl, H₂pyttz -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: [Co₂(Hpyttz-I)₂(H₂O)₆ [CrMo₆(OH)₆O₁₈]]. Complex 2 is a 1D supramolecular chain derived from the binuclear cobalt complex: {Co₂(H₂pyttz-I)₂(H₂O)₄[TeMo₆O₂₄]}²⁻, the discrete [Co(H₂O)₆]²⁺ units act as counter cations. Complexes 3 and 4 are isostructural with different center metals (M = Co or Ni), the adjacent γ-Mo₈O₂₆⁴⁻ anions are linked by the M^II ions to form a 1D M-γ-Mo₈O₂₆ inorganic chain. Then 1D M-γ-Mo₈O₂₆ inorganic chains are linked together by the 1D metal–organic M-(Hpyttz-II) chains to form a 3D framework. In complex 5, γ-Mo₈O₂₆⁴⁻ anions are bridged by the Ni^II ions to give a 1D Ni-γ-Mo₈O₂₆ inorganic chain, the adjacent 1D Ni-γ-Mo₈O₂₆ chains are connected through [Ni(Hpyttz-III)₂] 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.