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

Three metal-organic frameworks, {[Co-2(ao(0.34)btc)(DMF)(3)]3DMF}(n) (1), {(Me2NH2)(2)[Mn-12(OH)(4)(abtc)(6)(DMA)(6)(H2O)(6)][Mn(H2O)(4)]36H(2)O}(n) (2), and {[Zn-3(abtc)(1.5)(H2O)(2.5)(DMA)(0.5)]5DMA2CH(3)OH}(n) (3), were successfully designed and synthesized by using H(4)abtc ligand under solvothermal conditions (H(4)abtc = 3,3,5,5-azobenzenetetracarboxylic acid, DMF = N,N-dimethyformamide, and DMA = N,N-dimethylacetamide). In complex 1, the ao(0.34)btc(4-) ligands connect [Co-2(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 abtc(4-) produce the 3D soc-type cage in complex 2. Complex 3 consists of two categories of [Zn-2(CO2)(4)] SBUs, resulting in the NbO-type cage with the assistance of the abtc(4-) 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.