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]_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 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.