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 [Fe^III]_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 (I_d) 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-NH 2 ⁺ 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, SO₃ ²⁻ /HSO₃⁻ . However, marked pK perturbation of the bound ascorbate in [Fe(L)(HAsc/Asc)]^0/- (Δ_pK{[Fe(L)(HAsc)] – HAsc⁻}=6) is compelling evidence for chelation of HAsc^-/Asc^2- 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 S^IV 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 (μ_eff=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 Mössbauer spectrum (δ=0.41 mm s⁻¹; ΔE Q=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.