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|Title:||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.|
Dash, Anadi C
|Abstract:||(1,3) Bis (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≤ 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 Kstab≈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)- (Xn- = N3‾, NCS‾,imidazole, SO32-); with ascorbate anion (HAsc‾) replacement of both H2O occurs leading to Fe(L)(HAsc)/Fe(L)(Asc)‾ in which HAsc‾/Asc2- 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.|
|Appears in Collections:||IJC-A Vol.42A(09) [September 2003]|
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