Insight into structural aspects and study of reaction kinetics of model [oxo(salen)iron(IV)] complexes with dipeptides

Abstract

Iron(III)-salen complexes (salen = N,N’-bis(salicylidene)ethylenediaminato) react with H2O2 in aqueous CH3CN medium to form short lived intermediate [oxo(salen)iron(IV)] radical cation as an active oxidant. The present study devotes to establish the nature and reactivity of this active oxidant by spectral (UV–vis and Mössbauer), electrochemical (Cyclic voltammetric) and density functional theory (DFT) method. Mössbauer spectral study gives a clue to the formation of [oxo(salen)iron(IV)] radical cation with doublet (S = 2) spin multiplicity as an intermediate. Geometry optimization of oxo-iron species using DFT method, Mulliken charges and spin densities uphold that the doublet (S = 2) state is more stable and less in energy than the quartet (S = 4) and sextet (S = 6) spin states. The calculated average FeO bond distance for all the [oxo(salen)iron(IV)] complexes is 1.63 Å which is in good agreement with X-ray crystallography value of 1.65 Å. The CV study shows a clear oxidation wave at 1.18 (±0.02) V with increase in the oxidation peak current for FeIII/FeIV redox couple. Kinetics of oxygenation of four dipeptides (eg. Met-Gly, Met-Ala, Met-Leu, Met-Ser) with [oxo(salen)iron(IV)] radical cation into the corresponding sulfone follow Michaelis-Menten kinetics. The reaction rate is sensitive to the nature of the substituent present in the phenolic part of salen ligand and side chain present in the peptides. To account for spectral, kinetic and DFT results a suitable mechanism has been proposed.