Synthesis and characterization of the tetranuclear iron(III) complex of a new asymmetric multidentate ligand. A structural model for purple acid phosphatases

The ligand, 2-((2-hydroxy-5-methyl-3-((pyridin-2-ylmethylamino)methyl)benzyl)(2-hydroxybenzyl)amino)acetic acid (H3HPBA), which contains a donor atom set that mimics that of the active site of purple acid phosphatase is described. Reaction of H3HPBA with iron(III) or iron(II) salts results in formation of the tetranuclear complex, [Fe4(HPBA)2(OAc)2(µ-O)(µ-OH)(OH2)2]ClO4·5H2O. X-Ray structural analysis reveals the cation consists of four iron(III) ions, two HPBA3− ligands, two bridging acetate ligands, a bridging oxide ion and a bridging hydroxide ion. Each binucleating HPBA3− ligand coordinates two structurally distinct hexacoordinate iron(III) ions. The two metal ions coordinated to a HPBA3− ligand are linked to the two iron(III) metal ions of a second, similar binuclear unit by intramolecular oxide and hydroxide bridging moieties to form a tetramer. The complex has been further characterised by elemental analysis, mass spectrometry, UV-vis and MCD spectroscopy, X-ray crystallography, magnetic susceptibility measurements and variable-temperature Mössbauer spectroscopy