Metalloporphyrin dioxygen complexes

Karlin, K. D. Zuberbuhler, A. D. Formation, structure, and reactivity of copper dioxygen complexes, Bioinorganic Catalysis , 2nd edn. (Revised and Expanded) Eds. Reedijk, J. Bouwman, E. Marcel Dekker New York, 1999, pp. 469-534. Fukuzumi, S. Imahori, H. Biomimetic electron-transfer chemistry of porphyrins and metalloporphyrins, Electron Transfer in Chemistry , Vol. 2 Ed. Balzani, V. Wiley-VCH Verlag GmbH Weinheim, 2001, pp. 927-975. 

No reports have appeared on oxidation of substrates using metalloporphyrins under conditions at which dioxygen complexes are known to exist at ambient temperatures, for example, the 02 adducts of specially designed picket fence (5, 29, 132) and polymer-supported metalloporphyrins (133), (py)Cr(7TP)02 (134), and... 

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 ... 

In the most important series of polymers of this type, the metallotetraphenylporphyrins, a metalloporphyrin ring bears four substituted phenylene groups X, as is shown in 7.19. The metals M in the structure are typically iron, cobalt, or nickel cations, and the substituents on the phenylene groups include -NH2, -NR2, and -OH. These polymers are generally insoluble. Some have been prepared by electro-oxidative polymerizations in the form of electroactive films on electrode surfaces.79 The cobalt-metallated polymer is of particular interest since it is an electrocatalyst for the reduction of dioxygen. Films of poly(trisbipyridine)-metal complexes also have interesting electrochemical properties, in particular electrochromism and electrical conductivity.78 The closely related polymer, poly(2-vinylpyridine), also forms metal complexes, for example with copper(II) chloride.80... 

Usually, a large excess of the reducing agent(s) is necessary. Representative examples are (1) porphyrin systems metalloporphyrin or metallophthalocyanin -I- reductant (H2, NaBH4, etc) -I- O2 and (2) Gif systems iron catalyst -I- reductant (eg, Zn) -)- O2. Another example is to combine dioxygen and aldehyde, which has recently attracted attention as an effective oxidizing system when combined with various metal complexes (eq. (13)) (23). 

Of course, the current direction is to synthesize complexes which will more closely mimic hemoglobin and myoglobin and to develop oxygenactivating species in general. In 1973 and 1974, several model systems were devised. We will thus be concerned here with the interaction of molecular oxygen (dioxygen) and carbon monoxide with metalloporphyrin complexes as models of biologically important hemoproteins. 

Biochemists have been interested in the unique mechanism of the reductive dioxygen activation by monooxygenase since such a unique type of oxidation has never been found in the field of chemistry [3]. However, the biological system is so complicated as to clarify the mechanism in detail. The elucidation of the mechanisms has also attracted attention of physical organic and inorganic chemists. Chemists have applied model studies using synthetic metal complexes such as metalloporphyrins and nonheme metal complexes, and have obtained an insight into the enzymatic reaction mechanisms and the structures of the active species in the catalytic cycles [4-8]. 

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