Kinetics and Mechanism of the Formation of (1,5)bis(2-hydroxybenzamido)3-azapentaneiron(III) and Its Reactions with thiocyanate, azide, acetate, sulfur(IV) and Ascorbic Acid in Solution, and the Synthesis and Characterization of (nitrato)bis(2-hydroxybenzamido)3-azapentaneiron(III). The Role of phenol–amide–amine Coordination

Abstract

The complexation of iron(III) with (1,5)bis(2-hydroxybenzamido)3-azapentane (H2L) under varying [H+]T (0.01–0.1 mol dm−3) and [FeIII]T (3.0 × 10−4–1.7 × 10−2, [L]T =(0.5 - 1.0) × 10-4 mol dm−3) (I=0.3 mol dm−3, 10% v/v, MeOH + H2O, 25.0 °C) was reversible and displayed monophasic kinetics; the dominant path involved FeOH2+ and H3L+. The mechanism is essentially a dissociative interchange (Id) and dissociation of the aqua ligand from the encounter complex, [Fe(OH2)5OH2+, H3L+] is rate-limiting. Equilibrium measurements indicated that the ligand binds iron(III) in a bidentate, tetradentate and pentadentate fashion under varying pH conditions. Iron(III) promoted deprotonation of the phenol moieties, and sec-NH2+ of the dien unit are in tune with this proposition. The octahedral coordination of [Fe(HL/L})(OH2)]2+/+ is further supported by the aqua ligand substitution by AcO−, NCS−, N3-/N3H, SO32-/HSO3-. However, marked pK perturbation of the bound ascorbate in [Fe(L)(HAsc/Asc)]0/− (ΔpK{[Fe(L)(HAsc)] − HAsc−}=6) is compelling evidence for chelation of HAsc−/Asc2− leading to unusual hepta coordination of iron(III) in the ascorbate complexes. Despite the multidentate nature of the ligand, its iron(III) complexes remain sensitive to reduction by SIV and ascorbic acid. The complex (nitrato){(1,5)bis(2-hydroxybenzamido)3-azapentane}iron(III) has been synthesised and characterised by elemental analysis, i.r. and u.v.–vis spectral measurements. The room temperature magnetic moment (μeff=4.2 BM) conforms to the intermediate spin state of iron(III) (S=3/2) which is further supported by e.s.r. measurements (77 K, g=4.2, 8.1) and the 57Fe Mossbauer spectrum (δ=0.41 mm s−1; ΔEQ=0.78 mm/s). The cyclic voltametry (MeOH, TEAP as background electrolyte) display only one quasi-reversible peak in the −0.254 to −0.4 V range (vs. SCE), the irreversibility being due to the formation of an iron(II) complex which dissociates under the experimental conditions.