Metalloporphyrins chemical properties

The use of models that mimic a protein active site is normally prompted by the desire to eliminate any influence of the polypeptide backbone surrounding the active site in real biological molecules, which may obscure its physico-chemical properties. The first attempts to synthesize metal complexes similar to the active site of haemoproteins, through the use of simple metalloporphyrin derivatives, failed. The failure was due to the fact that these complexes react irreversibly with dioxygen as a consequence of side autooxidative reactions of the type ... 

Beilstein Handbook of Organic Chemistry, E III/IV, Vol. 26, Springer Verlag, Heidelberg 1978, pp. 3243-3246 Stoll, A., Wiedemann, E. Fortschr. Chem. Org. Naturst. I, 1938, 159-253 Fischer, H., Stern, A. Die Chemie des Pyrrols, Akad. Verlagsges., Leipzig, 1940 Smith, K.M. (Ed.) Porphyrins and Metalloporphyrins, Elsevier, Amsterdam, 1975 (physical and chemical properties)... 

The electron transfer proceeded with the participation of the triplet excited state of Chi. The discovery of the reversible reaction of chlorophyll photoreduction served as a stimulus for starting systematic research on photochemical redox reactions of chlorophyll and its synthetic analogs, i.e, various metalloporphyrins (MP). For a review of the physical and chemical properties of MP, see, for example, Ref. [62]. 

P. Peretz, D. Solomon, D. Weintraub, and M. Faraggi, Chemical Properties of Water-Soluble Porphyrins. 3. The Reaction of Superoxide Radicals with Some Metalloporphyrins, Int. J. Radiat. Biol., 42 (1982) 449. 

Tetrapyrrolic macrocycles, such as porphyrin, consisted of four pyrrole units bonded by different bridges, for example methene in the case of porphyrins and aza-methene in the case of phthalocyanines [1]. These ligands can complex metal ion transition and the synthesized metallocomplexes are extremely stable [2, 3]. While some metalloporphyrins constitute the redox center of naturally occurring proteins, like heme in hemoglobin, metallophthalocyanines are purely synthetic molecules. The (electro) chemical properties of MN4 complexes have been widely studied and have been particularly used for the catalysis of several electrochemical reactions in homogeneous solutions [4]. It was shown that the electrochemical properties of a... 

The synthesis, chemical properties, and electrochemistry of metallo-porphyrins with metal-metal and metal-carbon bonds have recently been described in two extensive reviews Porphyrins are known to coordinate with most metallic and pseudo-metallic elements and, in theory, synthesis of numerous organometallic porphyrins is possible. However, to date, the synthesis of metal-carbon a-bonded complexes has been limited to metallo-porphyrins with the following central metals Fe, Ru, Co, Rh, Ir, Ti, Al, Ga, In, Tl, Si, Ge, Sn and Zn. These organometallic complexes are of importance as model compounds for understanding the functions and relationships of several biologically important macromolecules, as well as for their chemical reactivity. The insertion of small molecules between the metal ion and the carbon atom of a metalloporphyrin may result in activation of the inserted molecule or may generate new monomeric or polymeric materials. In addition, metal-carbon o-bonded porphyrins can act as precursors in the synthesis of metal-metal bonded derivatives. 

To select the metal to be incorporated into the substrate porphyrin unit, the following basic properties of metalloporphyrins should be considered. The stability constant of MgPor is too small to achieve the usual oligomeric reactions and purification by silica gel chromatography. The starting material (Ru3(CO)i2) for Ru (CO)Por is expensive and the yield of the corresponding metalation reaction is low. Furthermore, the removal of rutheniirm is difficult, and it is likewise difficult to remove the template from the obtained ruthenium CPOs. Therefore, ZnPor is frequently used as a substrate in this template reaction, because of the low prices of zinc sources (zinc acetate and/or zinc chloride), the high yield in the metalation reaction, the sufficient chemical stability of the ZnPor under con-... 

In summary, the four chemical systems described in this paper demonstrate the versatility and selectivity of electrochemical methods for synthesis and characterization of metal-carbon a-bonded metalloporphyrins. The described rhodium and cobalt systems demonstrate significant differences with respect to their formation, stability and to some extend, reactivity of the low valent species. On the other hand, properties of the electroche-mically generated mono-alkyl or mono-aryl germanium and silicon systems are similar to each other. 

Although relatively expensive and difficult to use, CASSCF/ CASPT2 methods often provide meaningful chemical descriptions. This, coupled with the continuing increase in computational power, suggests that the study of properties and reactions involving metalloporphyrins should be pursued with these accurate methods. 

The studies on these Hangman porphyrins and other macrocyclic Hangman platforms [205] clearly demonstrate that exceptional catalysis may be achieved when redox and PT properties of a cofactor are controlled independently. A key requirement is that the PT distance is kept short, which may be accomplished by orthogonalizing ET and PT coordinates. The benefits of incorporating PT functionality into redox catalysis can only be realized when a suitable geometry is established. Moreover, the Hangman platforms show that a multifunctional activity of a single metalloporphyrin-based scaffold is achieved by the addition of proton control to a redox platform. This observation is evocative of natural heme-de-pendent proteins that employ a conserved protoporphyrin IX cofactor to affect a myriad of chemical reactivities. 

Membrane-s )Anmn have been prepared. Tetraphenylpor-phyrins having jc-carboxy and jc-amino functionalities have been polymerized interfacially to produce chemically asymmetric membrane films.Mercaptoporphyrins have also been polymerized to form ultrathin films of a porphyrin homopolymer based upon 5,10,15,20-tetrakis-(Q -mercapto-j9-tolyl) porphyrin,and thiol-derivatized porphyrins for attachment to electroactive surfaces have been prepared and examined as means of information storage,including those having a ferrocenyl moiety appended in several ways. Various means of linking porphyrins to form arrays have been reviewed. Preparation and investigation of the physical properties of cyclic tetra-, hexa-, hepta-porphyrin and star-shaped multiporphyrin or star-shaped tetra- or octa-porphyrin, or monophthalocyanine arrays have been described and reviewed. As part of this work, methods of forming porphyrins in the presence of acid-labile metalloporphyrins that provide a new route to mixed-metal multiporphyrin arrays have been reported. ... 

Many chemical model systems based on metalloporphyrin catalysts and mimicking cytochrome P450-dependent monooxygenases have been described during these last decade. Several review articles have been devoted to these systems 2-10. in that context, very recent results about the preparation and catalytic properties of new homogeneous and supported catalysts will be described in a first chapter. In the second chapter, some preliminary results showing that the oxidation of alkanes by a dioxygenase-like mechanism could occur in the presence of iron porphyrin catalysts activated either photochemically or thermally, will be reported. 

The biological and chemical importance of metalloporphyrins has caused much interest in the nature of the metal-ligand linkage in such complexes. Fleischer (1970) has discussed certain structural properties of the porphyrin molecule necessary for understanding such physical properties as its solubility, magnetic susceptibility, and visible absorption, electron spin resonance (ESR), and NMR spectra. He has also... 

In all cases, the films were obtained by oxidative electropolymerization of the cited substituted complexes from organic or aqueous solutions. The mechanism of metalloporphyrin Him formation was suggested to be a radical-cation induced polymerization of the substituents on the periphery of the macrocycle. As it was reported for the case of polypyrrole-based materials ", cyclic voltammetry and UV-visible spectroscopy with optically transparent electrodes were extensively used to provide information on the polymeric films (electroactivity, photometric properties, chemical stability, conductivity, etc.). Based on the available data, it appears that the electrochemical polymerization of the substituted complexes leads to well-structured multilayer films. It also appears that the low conductivity of the formed films, combined with the cross-linking effects due to the steric hindrance induced by the macrocyclic Ugand, confers to these materials a certain number of limitations such as the limited continuous growth of the polymers due to the absence of electronic conductivity of the films. Indeed, the charge transport in many of these films acts only by electron-hopping process between porphyrin sites. 

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