Four new metal–organic frameworks based on diverse secondary building units: sensing and magnetic properties

Abstract

Four new metal–organic frameworks (MOFs), {[Zn₃(L)(OH)(H₂O)₅]·NMP·2H₂O}_n (1), {[H₂N(Me)₂][Zn₂(L)(H₂O)]·DMF·H₂O}_n (2), {[Co₅(L)₂(H₂O)₁₁]·2H₂O}_n (3) and {[Mn₅(L)₂(H₂O)₁₂]·6H₂O}_n (4), were assembled employing a symmetrical V-shaped rigid multicarboxylic acid ligand H₅L (H₅L = 2,4-di(3′,5′-dicarboxylphenyl)benzoic acid) with different metal ions, resulting in versatile frameworks as well as various types of coordination modes of H₅L. 1 forms a three-dimensional (3D) 4-connected sra net based on trinuclear [Zn₃(μ₃-OH)(μ₂-COO)(μ₁-COO)₄] clusters, while 2 displays a 3D (4,6)-connected net based on two types of binuclear [Zn₂(μ₂-COO)₂(μ₁-COO)₄] and [Zn₂(μ₂-COO)₄] clusters. 3 and 4 contain similar [M₃(μ₂-COO)₄(μ₁-COO)₂] (3, M = Co; 4, M = Mn) clusters but result in different 4-connected 3D and 2D frameworks, respectively. 1 and 2 show solid-state luminescence properties at ambient temperature. Meanwhile, 1 shows high selectivity and sensitivity for not only Fe³⁺ cations but also for CrO₄²⁻, Cr₂O₇²⁻ and MnO₄⁻ anions via a luminescence quenching effect with a low detection limit, which thus means that it could be a potential crystalline material for detecting these anions. The mechanisms of the quenching effect and sensing properties of 1 are discussed in detail. In addition, 3 and 4 have the presence of antiferromagnetic interactions between the metal ions.