Phenol-amide chelates of iron(III). Kinetics and mechanism of reversible formation of (diaqua) (1,3) bis (2-hydroxybenzamido)propaneiron(III) and its reactions with thiocyanate, azide, imidazole, sulphur(IV) and ascorbic acid in aqueous medium

Abstract

(1,3) bis (2-hydroxybenzamido)propane (H²L) forms mononuclear iron(III) complex, Fe(HL)(OH₂)₂²⁺. The kinetics and mechanism of its formation has been investigated at 25.0 ≤ t/°C ≤ 40.0 (I = 0.3 mol dm⁻³, 10% (v/v) MeOH + H₂O). For the reaction, Fe(OH₂)₅(OH)²⁺ + H₂L Fe(HL)(OH₂)₂²⁺, k₂ (25°C) = (2.14 ± 0.15) × 10³ dm³ mol^-1 s^-1, ΔH^≠ = 39.7 ± 1.3 kJ mol^-1, ΔS^≠=-48 ± 4 J K^-1 mol^-1; k_-2 (25°C) = 0.168 ± 0.0014 s^-1, ΔH^≠ = 76.0 ± 1.5 kJ mol^-1, ΔS^≠ =-4 ± 5 J K^-1 mol^-1 where k₂ and k_-2 denote the forward and reverse rate constants respectively. Fe(HL) (OH₂)₂²⁺ is a moderate acid (pK = 2.25 ± 0.05, 25.0°C, 1 = 0.5 mol dm^-3, 20% (v/v) MeOH + H₂O). ΔpK (HL^--Fe(HL)²⁺) = 8 is in conformity with the fact that both the phenolic groups are coordinated to iron(III). The metal ion promoted ionisation of the amide function also points to the N-coordination of this tetradentate ligand in Fe(HL)²⁺/FeL⁺. The high value of the stability constant of FeL⁺ (ca log K_stab ≈ 18), which is further enhanced by the amide deprotonation, is favourable for this ligand to be an efficient iron(III) sequestering agent. Fe(L) (OH₂)₂⁺ undergoes facile aqua ligand substitution forming Fe(L)(X)(OH₂)^(n-1)-(X^n- = N₃^-, NCS^-, imidazole, SO₃^2-); with ascorbate anion (HAsc^-) replacement of both H₂O occurs leading to Fe(L)(HAsc)/Fe(L)(Asc)^- in which HAsc^-/Asc^2- is chelated (ΔpK(HAsc--Fe(L)(HAsc)) = 6). No significant labilising action of the coordinated amide (even after N-H deprotonation) and phenoxide/phenol functions is evident. Fe(L)(SO₃)^-, Fe(L)(HAsc)/Fe(L)(Asc)^- undergo facile internal redox despite the stabilising action of the multidentate ligand.