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}¹_n (1), {[Zn₂(η₈-aobtc)(η₂-phen)(H₂O)]·DMF}³_n (2), and {(Hap)₂[Zn₃(η₉-aobtc)₂]·2H₂O}³_n (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 Å diameter and 7 × 15 Å 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.