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

Binuclear mu-oxo-bridged Fe(III) complexes of 2,3,7,8,12,18-hexamethyl-13,17-dibutyl-5-monoazaporphine [mu-O(FeMAP)(2)], and 3,7,13,17-tetramethyl-2,8,12,18-tetrabutyl-5,15-diazaporphine [mu-O(FeDAP)(2)] have been prepared and characterised using UV/Vis, IR and (1)H NMR spectroscopies. The stretching vibrations of the mu-oxo-bridge nu(as)(Fe-O-Fe) are observed at 880 cm(-1) for mu-O(FeMAP)(2) and at 871 cm(-1) for mu-O(FeDAP)(2). The structure of mu-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 angstrom, respectively, and are connected to one another through the mu-oxo-bridge. The average Fe1-N and Fe2-N bond lengths are 2.053 and 2.050 angstrom, and the Fe1-O and Fe2-O bond lengths are 1.777 and 1.776 angstrom, respectively. The Fe1-O-Fe2 angle is 152.1 degrees, which leads to a non-coplanar arrangement of the two adjacent macrocyclic units (tilt angle 26.3 degrees) allowing for the close approach of the solvating benzene molecule to the bridging oxygen atom with a mu-O center dot center dot center dot H(benzene) distance of 2.584 angstrom. Addition of acid HX leads to dissociation of the binuclear mu-oxodiiron complexes and formation of the mononuclear pentacoordinate Fe(III) complexes (X)FeMAP and (X)FeDAP (X = AcOH, HCl, etc). Analysis of the kinetic results obtained for the reactions of the mu-oxodiiron complexes of (aza)porphyrins with acetic acid in benzene solution indicates that the stability of the mu-oxo bridge towards acid dissociation is primarily determined by its steric accessibility; for species with flexible alkyl chains in beta-pyrrolic positions the stability is much lower than in the presence of shielding meso- or beta-phenyl groups.