The Effect of Steric Crowding on Porphyrin Conformation and Ring Orientations in a Series of Iron(III) μ‐Oxo Dimers Containing meso‐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)

Nonplanar porphyrins: synthesis, properties, and unique functionalities

“…Nonplanar conformations are found for most of the limited number of undecasubstituted 94,184 and b 17 ,b 18 -H 2 -decasubstituted 6,185 examples which have been reported; exceptions are the NO 2 partially meso-substituted species, which are mostly planar. 186 2,3,5,7,8,12,13,15,17,18-Decasubstituted porphyrins, those with two free meso positions, are generally planar species which demonstrate in-plane core-elongation (B 2g ) as the primary mode of distortion, excepting the influence of, e.g., Ni metal centers discussed below. 184b,187,188 The degree of nonplanarity accessed is generally dependent principally on the degree of peripheral substitution (N non-H atoms) of the macrocycle -that the mean deviation of the 24 atoms of the porphyrin core from planarity (D 24 ) for (N = 12) 4 D 24 (N = 11) 4 D 24 (N = 10).…”

Reactivity of a Bis(amidinato)iron(II) Complex [Fe(MesC(NPri)2)2] toward Some Oxidizing Reagents

“…Single-crystal X-ray diffraction study has shown the dinuclear complex 5 possesses a μ 2 -oxo bridge that connects two (MesC(NPr i ) 2 ) 2 Fe III fragments with the Fe–O–Fe angle of 176.5(1) deg (Figure ). The bond distances and angles of the (MesC(NPr i ) 2 ) 2 Fe units are close to each other, and accord well with those of the aforementioned high-spin bis(amidinato)iron(III) complexes. , Similar to many of the known oxo-bridged dinuclear iron(III) complexes, 5 is diamagnetic and its 1 H NMR spectrum features resonances in the range 1.0 to 8.0 ppm suggesting strong antiferromagnetic coupling between the two iron(III) centers …”

Heterogeneous catalytic properties of unprecedented μ-O-[FeTCPP]2dimers (H2TCPP = meso-tetra(4-carboxyphenyl)porphyrin): an unusual superhyperfine EPR structure

“…These distances and angles are typical for high-spin iron(III) μ-oxo dimers. 28,30,[56][57][58][59]64,69,[83][84][85][86][87] The macrocyclic rings are essentially parallel to each other, the angle between the two central N 4 planes being 0.31°. The relative orientations of the two porphyrin rings make an average N p -Fe-Fe′-N′ p dihedral angle (torsional angle) of 33.62°(Fig.…”

“…22 In this sense, a large number of model systems have been explored, [23][24][25][26] and the use of highly distorted di-iron oxo dimers containing electron withdrawing groups ranks amongst the most effective catalyst models attracting considerable attention in the last few years. [27][28][29] Some μ-oxo dimers of FeTPP (TPP = meso-tetra(4-phenyl)porphine) have been used as model compounds of cytochromes P-450 and exhibit greater catalytic activity than the FeTPP monomers. 30 In order to achieve heterogeneous catalysts, there are successful approaches consisting of anchoring the catalyst into the cavities of porous coordination networks, [31][32][33] doping the network with the catalyst 34 or post functionalizing the network.…”

“…50,54,55 On the other hand, most of the reported μ-oxo dimers are of the FeTPP type [56][57][58][59][60][61][62][63] (H 2 TPP = meso-tetra(4phenyl)porphyrin or octaethylporphyrins (OEP)). 28,[64][65][66][67][68][69][70][71][72][73][74][75][76][77] However, as far as we know, no dimers of the μ-oxo FeTCPP type have been reported so far.…”