Two novel 3D Zn(II)/Cd(II) MOFs with tri- or di-nuclear cluster units for multifunctional sensing applications in aqueous media

Abstract

With flexible 2-methylenesuccinate acid (H2MSA) and rigid 4, 4′-bipyridine (bpy) as mixed ligands, two three-dimensional (3D) metal−organic frameworks (MOFs), {[Zn3(μ2-OH)2(MSA)2(bpy)2]·4H2O}n (1) and {[Cd(MSA)(bpy)]·2H2O}n (2), were obtained solvothermally. Complexes 1 and 2 both have a 3D framework structures, which include respectively a linear trinuclear [Zn3] or a rare dinuclear [Cd2] cluster node that consisting of two seven-coordinated Cd(II) ions. The adjacent [Zn3] or [Cd2] nodes in 1 and 2 are firstly linked together by the MSA2− ligand to form a 1D chain or a 2D layer unit respectively, which are further bridged by the bpy linkers to produce the 3D framework structures. If treating the [Zn3] and [Cd2] cluster as 4- and 6-connected nodes, the 3D frameworks of 1 and 2 can be simplified into a CdSO4 topological network with a Schläfli symbol of (65.8) and a rob network with a Schläfli symbol of (48.66.8), respectively. The PXRD patterns suggest that 1 and 2 have good stability when soaked in water for at least 48 h. The fluorescence sensing properties of 1 and 2 toward different metal inorganic ions were investigated, and two complexes both could sense Fe3+, MnO4− and Cr2O72− in aqueous solution with high sensitivity and selectivity (the LODs were 3.20 μM, 1.54 μM and 1.95 μM for 1, 1.15 μM, 0.48 μM and 1.23 μM for 2, respectively) as well as good recircularity. In addition, the mechanism of the selective luminescence quenching response for Fe3+, MnO4− and Cr2O72− could be attributed to the energy competition between the excitation of complex 1/2 and the ultraviolet absorption of responsive substances.