Porphyrins electronic effects

Electronic effects on the reactions of [Rh(Por)h dimers and hydrides were probed by varying the porphyrin macrocycle. OEP and TPP vary considerably in their properties, with OEP being one of the strongest and TPP one of the weakest (7-donors among porphyrin derivatives. However. Rh(Por)]2, Rh(Por)H, and Rh(Por)r showed the same reactivity in a variety of reactions for both OEP and TPP, indicating that electronic effects relating to the porphyrin ligand have... 

The consequences of polychlorination of porphyrins on redox properties of complexes has been investigated.1404 The highly chlorinated porphyrin 3-octachloro-/ /c.vo-tetrakis(3,5-dichloro-2,6-dimethoxyphenyl)porphyrin exhibits a substantial anodic shift for reduction of over 0.5 V and a smaller shift for oxidation versus the unchlorinated precursor. Contrastingly, small potential shifts for the octabromo-substituted 5,10,15,20-tetraphenylporphyrinate arise from the dominance of macrocycle ruffling over electronic effects. In the polychloro complex, distortion does not compensate fully for electron-withdrawing effects of the Cl substituents. 

An important class of porphyrins is that constituted by confor-mationally distorted porphyrins, which mimic the non-planar geometry of the porphyrins present in photosynthetic systems.89 Obtainment of such non-planar distortions is associated with the introduction into the macrocyclic frame of proper crowding substituents, which therefore not only cause structural distortion but also affect, through their electronic effects, the redox potentials. A typical case is that constituted by [Cun(OETPP)] (OETPP = 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin), the saddle-distorted molecular structure of which is illustrated in Figure 56.102... 

The porphyrin ligand is sufficiently flexible to adopt nonplanar conformations in response to steric or electronic effects induced by the central metal, the axial ligands, or substitution at the porphyrin periphery. Coordination of the small, nonmetallic Si(IV), Ge(IV), P(V), and As(V) ions to porphyrins offer an ideal opportunity to study not only the direct effect on this phenomenon of... 

To study the possibility of electronic effects on the photodissociation localized to the porphyrin ring, we examined -CO, a compound similar to 1 -CO except for the substitution of ethyl groups in place of vinyl groups on the outside of the porphyrin ring. Earlier bmolecular kinetic studies (11,15) had shown that this change in the electron distribution of the porphyrin had no effect on the reaction of CO with the heme. Our results also indicate that this perturbation has no apparent effect on the picosecond photodissociation. 

Diffuse reflectance spectroscopy (DRS) of VO-porphyrins on reduced and sulfided catalysts exhibit shifts in the porphyrinic electronic spectra (Soret, a, (3 bands) to higher frequencies. Adsorption results in modification of the delocalized electronic resonance structure not observed on the oxide form of the catalyst. X-ray photoelectron spectroscopy reveals shifts to higher Mo binding energies on reduced and sulfided catalysts following VO-porphyrin adsorption, consistent with transfer of electrons from Mo electron donor sites to the V02+ ion. Interaction at the electron donor sites is stronger than interaction at electron acceptor sites typical of the oxide catalyst. This gives rise to the possibility of lower VO-porphyrin diffusion rates on sulfided catalysts, but this effect has not been experimentally demonstrated. 

Gregory A. M. and F. A. Walker. Electronic Effects in Transition Metal Porphyrins. 5. Thermodynamics of Axial Ligand Addition and Spectroscopic Trends of a Series of Symmetrical and Unsymmetrical Derivatives of Tetrephenylporphinatozinc(II). Inorganica chimica Acta. 91 (1984) pp. 95-102. 

According to theoretical calculations (280,281), electronegative substituents on the phenyl ring stabilize the porphyrin ring against oxidation, by diminishing the electronic density on it (259, 282, 283). Similar effects are expected for the supramolecular m o-pyridylporphyrins however, in addition, the catalytic activity also can be modulated by the electronic effects induced by the peripheral ruthenium complexes, particularly in the case of Mn(4-TRPyP). The electronic influence from the peripheral complexes should be smaller for the Mn(3-TRPyP)... 

Another question is which one, the bis coordination of O2 to two CoP centers or the electronic effects induced by the peripherally coordinated Ru(III)Ru(III) Ru(II), would be responsible for the cobalt porphyrin activation in the multielectron-transfer process. In fact, considering that the Co(4-TCPyP) films are formed by stacks of supermolecules, it is not possible a priori to rule out the first hypothesis. This problem was solved by repeating the experiments using the electrostatic assembled Co(4-TCPyP)/ZnTPPS films in which the bis coordination of dioxygen to two cobalt porphyrin centers is not possible (176). The catalytic behavior is reproduced in the ion pair film, confirming that the activation is mainly due to electronic effects induced by the peripheral ruthenium clusters, in synergistic association with their normal role as intramolecular multielectron source. 

The activation of the cobalt porphyrin center was assigned to synergistic and induced electronic effects (170,176). A reduced metalloporphyrin center, probably with the character of M P, is responsible for the electrocatalytic reduction of sulfite and nitrite ions by M(3-TRPy P), where M = Ni, Co, and Mn, since the free-base and... 

Significant differences in the equilibrium constants for carbon monoxide binding to cytochromes P450 from bacterial, liver microsomal, and adrenal cortex microsomal sources, different isozymes of the liver microsomal proteins, and for substrate-free and substrate-bound enzymes, have been observed and have been related to similar factors that affect O2 and CO binding in oxygen transport and storage heme proteins. The importance of the cis and tmns effects, that is electronic effects associated with the porphyrin... 

The use of a synthetic model system has provided valuable mechanistic insights into the molecular catalytic mechanism of P-450. Groves et al. [34]. were the first to report cytochrome P-450-type activity in a model system comprising iron meso-tetraphenylporphyrin chloride [(TPP)FeCl] and iodosylbenzene (PhIO) as an oxidant which can oxidize the Fe porphyrin directly to [(TPP)Fe =0] + in a shunt pathway. Thus, (TPP)FeCl and other metalloporphyrins can catalyze the monooxygenation of a variety of substrates by PhIO [35-40], hypochlorite salts [41, 42], p-cyano-A, A -dimethylanihne A -oxide [43-46], percarboxylic acids [47-50] and hydroperoxides [51, 52]. Catalytic activity was, however, rapidly reduced because of the destruction of the metalloporphyrin during the catalytic cycle [34-52]. When (TPP)FeCl was immobilized on the surface of silica or silica-alumina, catalytic reactivity and catalytic lifetime both increased significantly [53]. There have been several reports of supported catalysts based on such metalloporphyrins adsorbed or covalently bound to polymers [54-56]. Catalyst lifetime was also significantly improved by use of iron porphyrins such as mew-tetramesitylporphyrin chloride [(TMP)FeCl] and iron mcA o-tetrakis(2,3,4,5,6-pentafluorophenyl)por-phyrin chloride [(TPFPP)FeCl], which resist oxidative destruction, because of steric and electronic effects and thereby act as efficient catalysts of P-450 type reactions [57-65]. 

Porphyrin molecules provide a good example of this effect.144 In Table 7, a clear connection between the size of the ortho substituents and the chain-transfer constant, Cc, is observed. The electronic effects of the substituents are less important because methyl and chloro have similar Cc but significantly different electronic effects. According to X-ray data, porphyrins such as 80 are so restricted in rotation of the phenyl rings that four atropo-isomers can be easily separated by preparative chromatography. 

Y. M. Goh, W. Nam, Significant electronic effect of porphyrin ligand on the reactivities of high-valent iron(rV) oxo porphyrin cation radical complexes, Inorg. Chem. 38 (1999) 914. 

The Fe lICb/HOOH/MeCN system is able to catalyze a variety of substrate transformations analogous to those of cytochrome P-450. The simplicity of the system allows the mechanism of the various reactions to be examined in the absence of the complications that are introduced by the electronic effects of the porphyrin ring and the axial thiolate group. 

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