Tetramethyl Carboxylic Acids Derived from <i>o</i>-Phenylenediamines as Sequestering Agents for Iron(III): Thermodynamic Studies. X-ray Crystal Structure of Sodium Aqua(4-chloro-1,2-phenylenediamine-<i>N,N,N</i>‘<i>,N</i>‘-tetraacetato)ferrate(III)−Water (1/1.5)

Abstract

The coordinating ability of the ligands 3,4-toluenediamine-N,N,N‘,N‘-tetraacetic (3,4-TDTA), ortho-phenylenediamine-N,N,N‘,N‘-tetraacetic (o-PhDTA), and 4-chloro-1,2-phenylenediamine-N,N,N‘,N‘-tetraacetic (4-Cl-o-PhDTA) acids, H4L, with Cu(II) and Fe(III) is described. The stability constants and the partial molar enthalpies of the complexes formed (25 °C, I = 0.5 mol dm-3 in KNO3) were respectively determined by means of emf and calorimetry measurements. For the system 3,4-TDTA−Fe(III) (25 °C, I = 0.5 mol dm-3 in NaClO4), the stability constants of the complexes FeL- and [FeL(OH)]2- were also determined spectrophotometrically. The calorimetric measurements show that the complex formation is entropy driven, exothermic for Cu(II) and endothermic for Fe(III). The partial molar entropy of complexation (ΔS°) is much the same as that of the EDTA complexes. X-ray diffraction structural analysis of Na[Fe(4-Cl-o-PhDTA)(H2O)]·1.5H2O revealed that, in [FeL(OH2)]- (monoclinic C2/c, a = 10.693(3) Å, b = 13.931(3) Å, c = 24.686(6) Å, β = 94.21(2)°, V= 3667(2) Å3, Z = 8), the iron(III) is seven coordinated, with one water molecule filling the seventh position, similar to the iron(III) anionic chelates of 3,4-TDTA, o-PhDTA, and EDTA. The presence of the Cl atom in the aromatic ring produces an asymmetry in the Fe−N bond distances.