Synthesis and Characterization ofanti-bisFe(III) Porphyrins,syn-bisFe(III)-μ-oxo Porphyrin, andsyn-bisFe(III)-μ-oxo Porphyrin Cation Radical

Abstract

BisFe(III) porphyrins bridged by a highly flexible ethane linker are reported in which the bisporphyrin platform “opens” and “closes” its binding pockets leading to facile syn-anti conformational switching with very high vertical flexibility in a single molecular framework. With axial ligand X (X: Cl, Br, I, ClO4), the anti-forms of the molecule are stabilized. The X-ray structure of anti bis perchloratoFe(III)porphyrin is reported, and the molecule is found to be high-spin in nature. In sharp contrast, all other Fe(III) porphyrins with ClO4 as axial ligands are in an either intermediate or admixed spin state. The very strong Fe−OClO3 bond and relatively weaker Fe−N(p) bonds are responsible for the high-spin nature of the molecule. Upon treatment with base, bis Fe(III)-μ-oxo porphyrin is generated in which the Fe−O−Fe unit is remarkably bent (with a 147.9(1)° angle) and two porphyrin rings in a molecule are so close that at least six carbon atoms from each of the porphyrin macrocycles are driven to be essentially closer than the van der Waals contact distance. Upon manipulating the acidity/basicity of the solution, the facile syn-anti conformational switching takes place that is also reversible in nature. The complex catalyzes the rapid photoinduced oxygenation of phosphites under mild condition using aerial O2. Electrochemical data reveals that bis Fe(III)-μ-oxo porphyrin undergoes four one electron oxidations and one electron reduction. However, oxidations become easier in syn Fe(III)-μ-oxo dimer rather than the anti-form of the molecule (with axial ligand X). The presence of two porphyrin macrocycles within a short distance in the syn form makes the porphyrin core highly nonplanar and more electron rich, and that might be responsible for easier oxidations compared to [Fe(OEP)]2O. Oxidations of the μ-oxo complex are performed using both chemical and electrochemical methods. The addition of 1.00 mol equiv of iodine−silver perchlorate generates the singly oxidized product that shows large isotropic shifts in 1H NMR with eight diastereotopic methylene proton signals (at 295 K) spreading over 8 to 16 ppm, and two meso resonances occurs at −10.2 and −13.7 ppm in 2:1 ratio. However, the addition of less than 1 equiv of oxidizing agent gives an average 1H NMR signal. This indicates that intra- and intermolecular electron transfer is rapid in the NMR time scale, and no porphyrin structural modification has occurred during oxidation.