A series of polyoxometalate-based hybrid complexes constructed by a tripodal ligand containing mixed N/O donors

By changing the transition metals (TMs) and polyanions, six polyoxometalate (POM)-based complexes 1–6 were synthesized by the hydrothermal method, namely [Cd^II(L₁)₂(HPMo₁₂O₄₀)]·2H₂O (1), {M^II₂(H₂O)₄(L₁)₂[H₂(β-Mo₈O₂₆)]}_n·xH₂O (n = 1, x = 1, M = Co (2), Cu (3), Fe (4); n = 0.5, x = 0, M = Ni (5), Zn (6)). The organic component 3,5-bis-(2,5-dihydro-imidazole-1-yl)benzoic acid (L₁) is a tripodal ligand containing two imidazole and one carboxylic acid group. The N donors in the imidazole groups and the O donors in the carboxylic acid group of L₁ can coordinate with different transition metal atoms to construct POM-based complexes. In complex 1, the Keggin anions are linked by [Cd(L₁)₂]²⁺ subunits to construct a chain. Complexes 2–6 are isostructural with [β-Mo₈O₂₆]⁴⁻ anions linked by double metal–organic chains to build a 2D layer. The electrochemical behaviour and photocatalytic properties of these complexes were studied. These complexes can be used as amperometric sensors for the amperometric detection of Cr(VI), Fe(III) and H₂O₂. Moreover, these complexes can be used for fluorescence selective sensing of Cr³⁺.