The complexation of iron(III) with (1,5)bis(2-hydroxybenzamido)3-azapentane (H 2 L) under varying [H + ] T (0.01-0.1 mol dm )3 ) and [Fe III ] T (3.0 · 10 )4 -1.7 · 10 )2 , ½L T ¼ ð0:5 À 1:0Þ Â 10 À4 mol dm )3 ) (I=0.3 mol dm )3 , 10% v/v, MeOH + H 2 O, 25.0°C) was reversible and displayed monophasic kinetics; the dominant path involved FeOH 2+ and H 3 L + . The mechanism is essentially a dissociative interchange (I d ) and dissociation of the aqua ligand from the encounter complex, [Fe(OH 2 ) 5 OH 2+ , H 3 L + ] 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 þ 2 of the dien unit are in tune with this proposition. The octahedral coordination of ½FeðHL=LÞðOH 2 Þ 2þ=þ is further supported by the aqua ligand substitution by AcO ) , NCS ) , N À 3 =N 3 H; SO 2À 3 =HSO À 3 . However, marked pK perturbation of the bound ascorbate in [Fe(L)(HAsc/Asc)] 0/) (DpK {[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 (l 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 57 Fe Mo¨ssbauer spectrum (d=0.41 mm s )1 ; D E Q ¼ 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.
