The Effect of Steric Crowding on Porphyrin Conformation and Ring Orientations in a Series of Iron(III) μ-Oxo Dimers Containingmeso-Nitrooctaethylporphyrins

Abstract

We have examined the electronic and geometrical effects of the progressive addition of bulky electron-withdrawing NO2 groups at the adjacent meso positions of FeIII-μ-oxo octaethylporphyrin dimers. The successive addition of mono-, di-, tri-, and tetranitro substituents at the meso positions increases the steric congestion at the periphery, which leads to distortion of the porphyrin macrocycle. The X-ray structures of four such molecules [Fe(din-OEP)]2O, [Fe(trn-OEP)]2O, [Fe(tn-OEP)]2O (triclinic), and [Fe(tn-OEP)]2O (monoclinic) have been determined; [Fe(tn-OEP)]2O authenticates the first structurally characterized oxo-bridged dimer with a dodecasubstituted and most-distorted porphyrin reported so far and also shows a significant increase in the Fe–Np and Fe–O distances and the displacement of Fe from mean porphyrin plane. The most important structural feature of the molecules is the very different torsional angles. As the number of nitro group increases, the torsional angles also increase from [Fe(OEP)]2O to [Fe(din-OEP)]2O and to [Fe(tn-OEP)]2O. However, in the case of [Fe(trn-OEP)]2O, the torsional angle is zero, while the angles are very different for both molecules present in the asymmetric unit of [Fe(tn-OEP)]2O (triclinic). Our study suggests that ruffling accommodates steric congestion at the periphery and thus reduces inter-ring interactions, which leads to smaller torsional angles. Spectroscopic studies are indicative of strong antiferromagnetic coupling between two high-spin iron(III) centers. The addition of NO2 groups in the porphyrin macrocycle serves both to protect the reactive meso positions from rapid oxidative attack and also to stabilize both porphyrin and metal against any oxidative degradation during catalysis. [Fe(tn-OEP)]2O therefore ranks amongst the most-effective catalyst in the oxygenation process. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)