Assembly of metal–organic frameworks based on 4-connected 3,3′,5,5′-azobenzenetetracarboxylic acid: structures, magnetic properties, and sensing of Fe3+ ions

Abstract

Three metal–organic frameworks, {[Co₂(ao_0.34btc)(DMF)₃]·3DMF}_n (1), {(Me₂NH₂)₂[Mn₁₂(OH)₄(abtc)₆(DMA)₆(H₂O)₆][Mn(H₂O)₄]·36H₂O}_n (2), and {[Zn₃(abtc)_1.5(H₂O)_2.5(DMA)_0.5]·5DMA·2CH₃OH}_n (3), were successfully designed and synthesized by using H₄abtc ligand under solvothermal conditions (H₄abtc = 3,3′,5,5′-azobenzenetetracarboxylic acid, DMF = N,N-dimethyformamide, and DMA = N,N-dimethylacetamide). In complex 1, the ao_0.34btc⁴⁻ ligands connect [Co₂(CO₂)₄] second building units (SBUs) to generate a three-dimensional (3D) crystal structure, showing the “PtS”-type topology. The [Mn₃OH(CO₂)₆] SBUs connected by abtc⁴⁻ produce the 3D “soc”-type cage in complex 2. Complex 3 consists of two categories of [Zn₂(CO₂)₄] SBUs, resulting in the “NbO”-type cage with the assistance of the abtc⁴⁻ ligands. Magnetic measurements reveal that antiferromagnetic exchange interactions exist in complexes 1 and 2. Besides, the luminescence investigations of complex 3 show its excellent sensing ability toward Fe³⁺ ions by fluorescence quenching.