Kinetics and mechanism of the formation of (1,8)bis(2-hydroxybenzamido)3,6-diazaoctaneiron(III) and its reactions with thiocyanate, azide, acetate, sulfur(IV) and ascorbic acid in solution, and the synthesis and characterization of a novel oxo bridged diiron(III) complex.: The role of phenol-amide-amine coordination

The interaction of (1,8)bis(2-hydroxybenzamido)3,6-diazaoctane (LH2) with iron(III) in acidic medium resulted in the formation of a mononuclear complex, Fe(LH3)(4+) which further yielded, [Fe(LH2)](3+), [Fe(LH)](2+), and [FeL](+) due to protolytic equilibria. The formation of [Fe(LH3)](4+) was investigated under varying [H+](T) (0.01-0.10 mol dm(-3)) and [Fe3+](T) (1.00 x 10(-3)-1.70 x 10(-2), [L](T) = 1.0 x 10(-4) mol dm(-3)) (I = 0.3 mol dm(-3), 10% MeOH + H2O, 25.0 degrees C). The reaction was reversible and displayed monophasic kinetics; the dominant path involved Fe(OH)(OH2) (5) (2+) and LH (2+)(4). The mechanism is essentially a dissociative interchange (I (d)) and the dissociation of the aqua ligand from the encounter complex, [Fe(OH2)(5)OH2+, H4L2+] is rate limiting. The ligand binds iron(III) in a bidentate ([Fe(H3L)](4+)), tetradentate ([Fe(H2L)](3+)), pentadentate ([Fe(HL)](2+) and hexadentate fashion ([FeL]+) under varying pH conditions. Iron(III) promoted deprotonation of the amide and phenol moieties and chelation driven deprotonation of the sec-NH2+ of the trien spacer unit are in tune with the above proposition. The mixed ligand complexes, [Fe-III (LH)(X)] (X = N (3) (-) , NCS-, ACO(-)) are also reversibly formed in solution thus indicating that there is a replaceable aqua ligand in the complex conforming to its octahedral coordination, [Fe(LH)(OH2)](2+), the bound ligand is protonated at the sec-NH site. Despite the multidentate nature of the ligand the Fe-III complexes are prone to reduction by sulfur(IV) and ascorbic acid. The redox reactions of different iron(III) species, Fe-III(LHi) which involved ternary complex formation with the reductants have been investigated kinetically as a function of pH, [S-IV](T) and [ascorbic acid](T). The substantial pK perturbation of the bound ascorbate in [Fe(LH)(HAsc/Asc)](+/0) (Delta pK ({[Fe(LH)(HAsc)] - HAsc - }) > 6) is considered to be compelling evidence for chelation of HAsc(-)/Asc(2-) leading to hepta coordination of iron(III) in the ascorbate complexes. A novel binuclear complex with composition, [Fe-III 2C20N4H35O11 (NO3)] has been synthesized and characterized by i.r., u.v.-vis, e.s.r., e.s.i.-Mass, Fe-57 Mossbauer spectroscopy and magnetic moment measurements. The complex was isolated as a mixture of two forms C (1) and C (2) with 75.3 and 24.7%, respectively as computed from Mossbauer data. The isomer shift (delta) (quadrupole splitting, Delta E (Q)) are 0.32 mm s(-1) (0.75 mm s(-1)) and 0.19 mm s(-1) (0.68 mm s(-1)) for C (1) and C (2), respectively. The variable temperature magnetic moment measurements (10-300 K) of the sample showed that C (1) is an oxo dimer exhibiting antiferromagnetic interaction between the iron(III) atoms (S (1) = S (2) = 5/2, J = - 120 cm(-1)) while the dimer C (2) is a high spin species (S (1) = S (2) = 5/2) and exhibits normal paramagnetism obeying the Curie law. The cyclic voltametry response of the sample (DMF, [TEAP] = 0.1 mol dm(-3)) displayed quasi-reversible responses at - 0.577 V and - 1.451 V (versus SCE). This is in tune with the fact that the C (2) species reverts rapidly in solution to the relatively more stable oxo-bridged dimer (C (1)) which is reduced in two sequential steps: C-1 + e(-) -> [FeL](+) + Fe-II; [FeL](+) + e(-) -> Fe-II L, the high labilility of the Fe-II complex is attributed to the irreversibility. The X-band e.s.r. spectrum of the polycrystalline sample at room temperature displayed a weak (unresolved) band at g = 4.2 and a strong band at g = 2.0 with hyperfine splitting due to the coordinated nitrogen (I = 1). At 77 K the band at g = 4.2 is intensified while that at g = 2 is broadened to the extent of near disappearance in agreement with the presence of the exchange coupled iron(III) centres.