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

Abstract

(1,3) <i>Bis </i>(2-hydroxy benzamido)propane (H₂L) forms mononuclear iron(III) complex, Fe(HL)(OH₂)₂²⁺. The kinetics and mechanism of its formation has been investigated at 25.0≤ <i>t/°</i>C<i> </i>≤ 40.0 (<i>I </i>= 0.3 mol dm^-3, 10% (v/v)<i> </i>MeOH + H₂O). For the reaction, Fe(OH₂)₅(OH)²⁺ + H₂L<img src='http://www.niscair.res.in/jinfo/exchangearrow.gif' border=0> Fe(HL)(OH₂)₂²⁺, <i>k</i>₂<i> </i>(25°C) = (2.14 ± 0.15)×10³ dm³ mol^-1 s^-1, Δ<i>H^≠</i>= 39.7 ± 1.3 kJ mol^-1,Δ<i>S</i>^≠<i> </i>= -48 ± 4 J K^-1 mol^-1; <i>k </i>_-2 (25°C) = 0.168 ± 0.0014 s^-1, Δ<i>H^≠</i>= 76.0 ± 1.5 kJ mol^-1, Δ<i>S</i>^≠= -4 ± 5 J K^-1 mol^-1where <i>k</i>₂<i> </i>and <i>k </i>_-2 denote the forward and reverse rate constants respectively. Fe(HL) (OH₂)₂²⁺ is a moderate acid <i>(pK </i>= 2.25 ± 0.05, 25.0°C, <i>1= </i>0.5 mol dm^-3, 20% (v/v)<i> </i>MeOH + H₂O).Δ<i>pK </i>(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⁺ (<i>ca </i>log <i>K</i>_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-<i> </i>is chelated (Δ<i>pK</i>(HAsc‾-Fe(L)(HAsc)) = 6). No significant labilising action of the coordinated amide (even after <i>N-H </i>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.