Phenol amide chelates of iron(III). Interaction of 1,5-bis-(2-hydroxybenzamido)- 3-aza-pentane with iron(III) in aqueous methanol medium
Anadi C. Dash, Rohit K. Rath · INDIAN JOURNAL OF CHEMISTRY- SECTION A · 2004
The complexation equilibrium of 1.5-bis-(2-hydroxybenza mido)-3 -azapentanc with Fe(OH2)6'+ has been investigated at 25.0°C and 1= 0.5 mo l dm-' in aqueous methanol [2.5 % MeOH + H20 (v/~ )l. The formation of only the mono nuclear complex has been o bserved which led to the iron( llI) promoted amide deprotonation in mild acidic (pK1 (cO-NII) = 5.2) and mild alkaline (pK2(cO.NH) = 9.2) medium. The ligand is a very good iron(lll ) sequestering agent with log K5Iab = 23.3 (Fe 3 + + L 2 - = FeL +, KSl ab) . ]n the recent years increasing attention is being paid to the carboxamido complexes of metal ions 1-9. The coordination chemistry of the carboxamido complexes of iron(IIIIII) has attracted special attention as it has biological relevance. For example, nitrile hydratase, lipoxigenase and isopenicillin synthase are ion containing enzymes which have the carboxamide function as the donor site 'o . . Recently, a few mononuclear iron(III) complexes with (N-H) deprotonated carboxamido ligands have been synthesized and structurally characterized 1.1 2. However, the solution state complex equilibria of iron(III) with multidentate carboxamide ligands have been rarely investigated 13 and this has been our current research interest 14. Recently, Kiss et at. 5 have reported an elegant study of the complex equilibria of oxovanadium(lV) with salicylglycine wherein the metal ion promoted peptide-amide deprotonation in aqueous solution was reported. Herein we report the chelate equilibria of iron(lII) with a diphenol-diamide ligand, 1,5-bis-(2-hydroxybenzamido)-3-aza-pentane (L) in aqueous methanol (2.5 % v/v MeOH + H20) medium. This linear multidentate ligand possessing a sec-NH, two amide functions and two phenolic mOIetIes can undergo several stages of protonation/deprotonation and can offer opportunities to examine its complexing action under different stages of ionization generating the iron(III) complexes of varying structure and charge types. o