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.