Synthesis, structure and adsorption of coordination polymers constructed from 3,3′,5,5′-azobenzenetetracarboxylic acid and Zn ions

Abstract

3,3′,5,5′-Azobenzenetetracarboxylic acid (H4abtc) was synthesized by reduction of 5-nitroisophthalic acid in basic aqueous/ethanol solution in the presence of Zn powder. Three novel coordination polymers {[Zn2(η6-ao2btc)(η2-2,2′-bpy)2(H2O)2]·2H2O}1n (1), {[Zn2(η8-aobtc)(η2-phen)(H2O)]·DMF}3n (2), and {(Hap)2[Zn3(η9-aobtc)2]·2H2O}3n (3) (2,2′-bpy = 2,2′-bipyridine, phen = 1,10-phenanthroline, ap = 4-aminopyridine, ao2btc4− and aobtc4− are two oxidized forms of H4abtc ligand) were prepared under hydrothermal conditions. The structures of 1–3 were characterized by single-crystal X-ray diffraction. Complex 1 is a 1D chain polymer, while 2 and 3 are porous 3D metal–organic frameworks with a cavity size of 9 A diameter and 7 × 15 A rectangular cavities, respectively. In 1, ao2btc4− links four mononuclear ZnO6 chromophores. Ligand aobtc4− in 2 links four dinuclear Zn2(CO2)4(H2O)N2, while aobtc4− in 3 bridges four trinuclear Zn3(CO2)8. From a topology point of view, ao2btc4− or aobtc4− are all 4-connected linkers in 1–3, while the mononuclear ZnO6 in 1, dinuclear Zn2(CO2)4(H2O)N2 in 2 and trinuclear Zn3(CO2)8 in 3 are 2, 4 and 8 connected nodes, respectively. Zn(II) in complex 2 can be replaced by Cu(II), Ni(II) and Co(II) with simultaneous loss of crystallinity. The metal ion exchange rate decrease in the order Co(II) < Ni(II) < Cu(II). 2 can encapsulate iodine (I2) in cyclohexane solution to form 2⊃0.1I2. The encapsulated I2 can be released completely in ethanol. The Hap+ in 3 can be replaced by methylene blue in aqueous solution to form 3⊃0.1methylene blue. The insignificant replacement is an indication that the guest molecule in the cavity channel is immobile, which prevents further substitution. With a bis-oxo group in the azo moiety, the framework of 1 is so unstable that it will decompose at ∼150 °C with simultaneous release of NO. Complexes 2 and 3 are stable at 330 °C. IR and fluorescence spectra were also discussed.