Fe(III) and Cu(II) coordination polymers assembled from di/tris-phosphonic acid and auxiliary ligands：Structure, SOD-like activity and magnetic behavior

Abstract

Hydrothermal reactions of (benzylazanediyl)bis(methylene)-diphosphonic acid (H4L1) or (benzene-1,3,5-triyltris(methylene))triphosphonic acid (H6L2) with Fe(III)/Cu(II) nitrate and 4, 4′-bipy or 2, 2′-bipy result in five new phosphonates, namely, [FeHL1(H2O)]n (1), {[Cu2L1(4, 4′-bipy)0.5(H2O)]·0.25H2O}n (2), [Cu2H2L2(4, 4′-bipy)(H2O)2]n (3), [CuH4L2(2, 2′-bipy)]n (4) and [Cu2(H4L2)2(2, 2′-bipy)2(H2O)4] (5), respectively. X-ray structural analysis indicates that 2 and 3 feature 3D framework structures with 3, 3, 3, 4-connected and 4, 5-connected topology; 1 and 4 display 1D chain structures, while 5 has a dimer structure. The di- or trisphosphonate ligands (H4L1 and H6L2) in the five complexes adopt different coordination modes and auxiliary ligands (4, 4′-bipy and 2, 2′-bipy) control the dimensionality of the final complexes. The superoxide dismutase (SOD) activities of 2–5 were assessed by modified nitrobluetetrazolium (NBT) assay. The IC50 values required to yield 50% inhibition of the reduction of NBT are 1.84, 2.02, 6.95 and 1.70 μM for 2–5, respectively, demonstrating better SOD-like activity than most of the Cu-SOD mimic enzymes reported in literatures. Among them, 2 and 3 have good stability and reusability under the simulative physiological conditions. The dismutation of 4 and 5 toward superoxide anion radicals can be attributed to the synergistic effect of the decomposition products of 4 and 5 containing Cu(II) and [H4L2]2- with or without 2, 2′-bipy or Cu(II) and 2, 2′-bipy, which were identified by liquid mass spectrometry (LC/MS). The magnetic properties of complexes 1–4 were also investigated, indicating the presence of magnetic exchange between metal ions. The results show that 1 and 3 exhibit antiferromagnetic interactions, while 2 and 4 show ferromagnetic interactions between metal centres.