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(2)(Hpyttz-I)(2)(H(2)O)(6)[CrMo(6)(OH)(6)O(18)]}·8H(2)O (1), {Co(2)(H(2)pyttz-I)(2)(H(2)O)(4)[TeMo(6)O(24)]}[Co(H(2)O)(6)]·3H(2)O (2), {Co(3)(Hpyttz-II)(2)(H(2)O)(6)[γ-Mo(8)O(26)]}·10H(2)O (3), {Ni(3)(Hpyttz-II)(2)(H(2)O)(6)[γ-Mo(8)O(26)]}·10H(2)O (4), {Ni(3)(Hpyttz-III)(2)(H(2)O)(8)[γ-Mo(8)O(26)]}·10H(2)O (5) (H(2)pyttz-I = 3-(pyrid-2-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl, H(2)pyttz-II = 3-(pyrid-3-yl)-5-(1H-1,2,4-triazol-3-yl)-1,2,4-triazolyl, H(2)pyttz -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(2)(Hpyttz-I)(2)(H(2)O)(6) [CrMo(6)(OH)(6)O(18)]]. Complex 2 is a 1D supramolecular chain derived from the binuclear cobalt complex: {Co(2)(H(2)pyttz-I)(2)(H(2)O)(4)[TeMo(6)O(24)]}(2−), the discrete [Co(H(2)O)(6)](2+) units act as counter cations. Complexes 3 and 4 are isostructural with different center metals (M = Co or Ni), the adjacent γ-Mo(8)O(26)(4−) anions are linked by the M(II) ions to form a 1D M-γ-Mo(8)O(26) inorganic chain. Then 1D M-γ-Mo(8)O(26) inorganic chains are linked together by the 1D metal–organic M-(Hpyttz-II) chains to form a 3D framework. In complex 5, γ-Mo(8)O(26)(4−) anions are bridged by the Ni(II) ions to give a 1D Ni-γ-Mo(8)O(26) inorganic chain, the adjacent 1D Ni-γ-Mo(8)O(26) 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.