The meso Reactivity of Porphyrins and Related Compounds. I. Nitration

Bonnett, Raymond; Stephenson, G. F.

doi:10.1021/jo01019a070

Cited by 90 publications

(45 citation statements)

References 6 publications

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“…Chromatographic silica gel and alumina were dried at 200'C and 0.01 mm for 20 hr. Octaethylporphyrin, H2(OEP) (16), meso-formyloctaethylporphyrin, H2(OEP-CHO) (17), meso-nitrooctaethylporphyrin, H2(OEP-NO2) (18), a,y-dimethyletioporphyrin-II, H2(DMEtio) (19), and MoCl2(CO)4 (20) …”

Section: Methodsmentioning

confidence: 99%

Rotational barrier of a molybdenum-molybdenum quadruple bond

Collman

Woo

1984

Proc. Natl. Acad. Sci. U.S.A.

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The synthesis and characterization of molybdenum(II) porphyrin dimers containing unbridged metal-metal quadruple bonds are presented. Variable temperature 300 MHz 'H NMR studies of meso-substituted derivatives provide novel solution evidence for both the existence of quadruple bonds and for a barrier to rotation about the metal-metal bonds in these complexes. The activation energy for this rotational process is 10.1 + 0.5 kcal molF'. Fig. 1A.Based on molecular orbital theory (2, 9), the o, and pair of degenerate -r bonds place no angular constraint on the orientation of the two ML4 fragments. Only the 8 bond should be sensitive to rotation about the metal-metal axis. The overlap of the dy orbitals that gives rise to this bond is at a maximum when the two ML4 fragments are eclipsed. Rotation of the ML4 fragments about the metal-metal bond to form the staggered complex decreases this overlap. At a dihedral angle of 450, the dy orbitals effectively become nonbonding with respect to the metal-metal interaction as shown in Fig. 1B. Consequently, a quadruple bond should exhibit an electronic barrier to rotation about the metal-metal axis. The observation of such a barrier has not been previously reported.Recently, it was suggested that rotation about a quadruple bond should not be observable on the NMR time scale due to the presence of the 8 bond (10). This hypothesis may derive from early estimates based on electronic absorption spectroscopy, which place the strength of the 8 bond in the range of 60-90 kcal mol1 (7,8). However, structural evidence (11, 12) suggests that the 8 bond provides only a minor contribution to the overall quadruple bond strength. Spectroscopic measurements by Trogler and Gray (4) support this latter view and place an upper limit of 20 kcal-mol-on the 8 bond strength. It is interesting to note that one theoretical study predicts a rotational barrier of less than 1 kcal mol-1 for a quadruple bond (13) and, in another case (14), the staggered geometry is predicted to be more stable than the eclipsed form by 4 kcal molF'! Herein we report the synthesis and characterization of molybdenum(II) porphyrin dimers containing quadruple bonds and our investigation of the rotational barrier about the metal-metal bonds in these complexes.

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Section: Methodsmentioning

confidence: 99%

Rotational barrier of a molybdenum-molybdenum quadruple bond

Collman

Woo

1984

Proc. Natl. Acad. Sci. U.S.A.

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“…and Stephenson (4). A well-characterized example of this phenomenon was shown by the X-ray crystal studies of both protonated and distorted meso-tetraphenyl and meso-tetra-4-pyridylporphyrins (2 1).…”

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confidence: 91%

“…Under more forcing conditions (HN03/H2S04), 20% of the 5,10,15-trinitrooctaethylporphyrin (4) can be formed but isolable amounts of the 5,10,15,20-tetranitroporphyrin (5) are not observed (4). NO, H H NO, NO2 H NO, H NO2 NO, Nor NO?…”

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confidence: 99%

Nitrooctaethylporphyrins: synthesis, optical and redox properties

Gong¹,

Dolphin²

1985

Can. J. Chem.

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LIANG-CHU GONG and DAVID DOLPHIN. Can. J. Chem. 63, 401 ( 1 985). The reaction of zinc octaethylporphyrin with N z 0 4 in dichloromethane gives, in a stepwise reaction, the zinc complexes of mono-, di-, tri-, and tetra-nitrooctaethylporphyrins. Demetallation, under acidic conditions, gives the corresponding free base porphyrins. The meso-nitro groups exert both steric and electronic effects on the porphyrin macrocycle. Thus N-protonation of the nitrated species causes a distortion of the ring and gives an optical spectrum similar to that of protonated meso-arylsubstituted porphyrins. In addition, the nitro groups make the oxidation of the porphyrin ring more difficult and at the same time facilitate the ring reductions.LIANG-CHU GONG et DAVID DOLPHIN. Can. J. Chem. 63, 401 ( I 985). La rkaction du sel de zinc de I'octaCthylporphyrine avec le N204 dans le dichloromCthane conduit, par I'intermCdiaire d'une sCrie de rCactions, aux complexes de zinc des mono-, di-, tri-et tCtra-nitrooctaCthylporphyrines. La dCmCtalation, en utilisant des conditions acides, conduit aux porphyrines correspondantes qui existent sous forme de bases libres. Les groupes nitro en positions mPso exercent des effets la fois stCriques et Clectroniques sur le macrocycle de porphyrine. La N-protonation des espkces nitrCes provoque donc une distortion du cycle et fournit un spectre optique semblable a celui des porphyrines protontes portant des substituants aryles en position meso. De plus, les groupes nitro rendent le cycle porphyrine moins susceptible a I'oxydation et ils facilitent les reductions des cycles.[Traduit par le journal]Porphyrins, like most other aromatic systems, undergo electrophilic substitution. However, as with all heteroaromatic systems, the competition between the basicity of the heteroatom and the nucleophilicity of the aromatic a-system can prevent many substitution reactions from occurring. A striking example of this is the faster rate of meso-substitution of octaethylporphyrin (OEP) in CH3COzD compared to CF3C02D (I)! ~, Electrophilic substitution at the porphyrin periphery has been recently reviewed (2) and, while a wide variety of reactions have been described, few high-yield reactions have been reported. The usually large effort that must be expended to prepare any porphyrin, other than the trivially easy reactions to generate tneso-arylporphyrins (3), requires that further manipulations should be as efficient as possible. We report here on the convenient preparation of the nitrooctaethylporphyrins and the effect the electron-withdrawing groups have on the redox properties of the free bases and some metal complexes.The direct nitration of OEP in fuming nitric acidlacetic acid gives good yields of 5-nitrooctaethylporphyrin (5-(N02)OEP (1)) with considerably diminished yields of the dinitroporphyrins 5,10-(N0)z0EP (2) and 5,15-(NOz)OEP (3). Under more forcing conditions (HN03/H2S04), 20% of the 5,10,15-trinitrooctaethylporphyrin (4) can be formed but isolable amounts of the 5,10,15,20-tetranitroporphyrin (5) are not ob...

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“…Successive addition of mono-, di-, tri-, and tetranitro substituents at adjacent meso positions of octaethylporphyrins has a marked effect on the structures of the Fe III -µ-oxo dimers. The free ligands 5-mononitrooctaethylporphyrins (mn-H 2 OEP), 5,10-dinitrooctaethylporphyrin (din-H 2 OEP), and 5,10,15-trinitrooctaethylporphyrins (trn-H 2 OEP) were prepared by the reported method of Bonnett and Stephenson, [14] while 5,10,15,20-tetranitrooctaethylporphyrins (tn-H 2 OEP) were prepared by the method of Gong and Dolphin. [15] Iron metal is inserted into the cavity by using the standard procedure of Adler and co-workers [16] under nitrogen to form Fe(por)Cl in excellent yields.…”

Section: Resultsmentioning

confidence: 99%

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

et al. 2009

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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)

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The meso Reactivity of Porphyrins and Related Compounds. I. Nitration

Cited by 90 publications

References 6 publications

Rotational barrier of a molybdenum-molybdenum quadruple bond

Rotational barrier of a molybdenum-molybdenum quadruple bond

Nitrooctaethylporphyrins: synthesis, optical and redox properties

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

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