A series of Anderson-type polyoxometalate-based metal–organic complexes: their pH-dependent electrochemical behaviour, and as electrocatalysts and photocatalysts
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 CuII 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]n4n− 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 Mo8O264− or Mo8O276− anions. Complexes 5 and 7 are isostructural, each [TeMo6O24]6− (TeMo6) polyoxoanion coordinates to two CuII 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 CuII 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 MoVI 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.