Synthesis and Study of the Binuclear μ‐Oxodiiron(III) Complexes of 5‐Monoaza‐ and 5,15‐Diaza‐Substituted β‐Octaalkylporphyrins

Abstract

AbstractBinuclear μ‐oxo‐bridged FeIII complexes of 2,3,7,8,12,18‐hexamethyl‐13,17‐dibutyl‐5‐monoazaporphine [μ‐O(FeMAP)2], and 3,7,13,17‐tetramethyl‐2,8,12,18‐tetrabutyl‐5,15‐diazaporphine [μ‐O(FeDAP)2] have been prepared and characterised using UV/Vis, IR and 1H NMR spectroscopies. The stretching vibrations of the μ‐oxo‐bridge νas(Fe–O–Fe) are observed at 880 cm–1 for μ–O(FeMAP)2 and at 871 cm–1 for μ‐O(FeDAP)2. The structure of μ‐O(FeDAP)2 has been confirmed by the single‐crystal X‐ray diffraction study of its monobenzene solvate. The Fe1 and Fe2 atoms are displaced from the mean planes formed by the coordinating N‐atoms by 0.572 and 0.565 Å, respectively, and are connected to one another through the μ‐oxo‐bridge. The average Fe1–N and Fe2–N bond lengths are 2.053 and 2.050 Å, and the Fe1–O and Fe2–O bond lengths are 1.777 and 1.776 Å, respectively. The Fe1–O–Fe2 angle is 152.1°, which leads to a non‐coplanar arrangement of the two adjacent macrocyclic units (tilt angle 26.3°) allowing for the close approach of the solvating benzene molecule to the bridging oxygen atom with a μ‐O···H(benzene) distance of 2.584 Å. Addition of acid HX leads to dissociation of the binuclear μ‐oxodiiron complexes and formation of the mononuclear pentacoordinate FeIII complexes (X)FeMAP and (X)FeDAP (X = AcOH, HCl, etc). Analysis of the kinetic results obtained for the reactions of the μ‐oxodiiron complexes of (aza)porphyrins with acetic acid in benzene solution indicates that the stability of the μ‐oxo bridge towards acid dissociation is primarily determined by its steric accessibility; for species with flexible alkyl chains in β‐pyrrolic positions the stability is much lower than in the presence of shielding meso‐ or β‐phenyl groups.