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.