Kinetics and intermediates in the autoxidation of (cyclidene)iron(II) dioxygen carriers in a variety of solvent systems

Abstract

The autoxidation of [Fe II(Me,Me,m-xyl)Cl] + was studied in a variety of solvent systems, with the two extremes being aprotic acetonitrile containing the powerful axial ligand N-methylimidazole ( MeCN MIM ) and protic methanol with the weak axial ligand chloride ( MeOH LiCl ), and the results are compared with those from a previous investigation for the solvent system 3:1:1 acetone : pyridine : water (APW). The differences in the solvent systems are mainly manifested in the product distribution, whereas the rate law maintains its algebraic form and the previously proposed general mechanism of autoxidation (electron transfer operating in parallel with the dioxygen adduct equilibrium) requires only slight modification in order to account for the behavior of the full range of systems. Systems in which chloride serves as the axial ligand, yield only the product of the electron transfer step, [Fe III(Me,Me,m-xyl)Cl] 2+, and its hydrolysis products, [Fe III(Me,Me,m-xyl)OH] 2+, and [{Fe III(Me,Me,m-xyl)} 2O] 4+. In contrast, systems in which the nitrogenous bases pyridine or 1-methylimidazole replace chloride as the axial ligand also produce a (peroxo)iron(III) species, in addition to [Fe III(Me,Me,m-xyl)B] 3+, where the axial ligand depends on the medium. The formation of Fe III(Me,Me,m-xyl)OH] 2+ and [{Fe III(Me,Me,m-xyl)} 2O] 4+ depends, as expected, on the water content of the system. The effects of substituents, at constant bridging group and cavity size, on the autoxidation of [Fe II(R 3,R 2,m-xyl)Cl] + with R 3 = Me, Ph and R 2 = Me, Bz in MeOH LiCI are dramatic. Remarkably, the combined steric and/or electron-withdrawing effect of replacing methyl groups at both the R 2 and R 3 positions appears to be multiplicative rather than additive.