A series of Anderson-type polyoxometalate-based metal-organic complexes: their pH-dependent electrochemical behaviour, and as electrocatalysts and photocatalysts.

Abstract

Seven polyoxometalate-based (POM) metal-organic complexes (MOCs) with different pyridyl-amide ligands were hydrothermally synthesized and structurally characterized. In 1, the [CrMo6(OH)5O19](4-) (CrMo6) polyoxoanions bridge the Cu(II) ions to generate a 1D Cu-CrMo6 inorganic double chain, which is further consolidated by the μ2-bridging 2-pdya ligands. Complex 2 exhibits a 2D layer based on [γ-Mo8O26]n(4n-) chains. In complex 3, the β-Mo8O26 anions link the metal-organic units [Cu(4-Hdpyp)2](4+) to construct a 1D fishbone-like chain. Complex 4 shows a 3D (6,6)-connected framework constructed by the 2D inorganic network [Cu4(μ3-OH)2(H2O)4(γ-Mo8O27)] and 3-dpyh bridging ligands. In 2-4, all the [CoMo6(OH)6O18](3-) (CoMo6) anions were in situ transformed to Mo8O26(4-) or Mo8O27(6-) anions. Complexes 5 and 7 are isostructural, each [TeMo6O24](6-) (TeMo6) polyoxoanion coordinates to two Cu(II) ions to generate a discrete copper complex [Cu2(4-Hdpye)2(TeMo6O24)(H2O)6] and [Cu2(4-Hdpyb)2(TeMo6O24)(H2O)6], respectively. In complex 6, the TeMo6 polyoxoanions bridge the Cu(II) ions to generate a 2D [Cu3(TeMo6)]n inorganic layer, which is further linked by the μ2-bridging 3-dpyb ligands to form a 3D metal-organic framework. The effects of POM types and their various coordination modes, as well as the pyridyl-amide ligands on the structures of the title complexes have been discussed. Their electrochemical behavior reveals characteristic multi-electron redox processes related to Mo(VI) centers. The electrocatalytic reduction performance toward hydrogen peroxide and bromate was fully measured and discussed; both complexes exhibit excellent electrocatalytic activity towards the reduction of bromate and hydrogen peroxide. In addition, the redox potentials of complexes 5-7 are highly pH sensitive and may be used as a kind of potential pH sensor. The photocatalytic activities of the title complexes are also investigated in detail.