Metalloporphyrin reactivity

Metalloporphyrin reactivity and catalytic properties can also be controlled by modifications of the porphyrin core. Ni coordination chemistry of core-modified porphyrins has been reviewed.1791,1792 Several X-ray structures of four-, five-, and six-coordinate Ni11 complexes with oxa-, thia-, and dioxatetraarylporphyrins are known. 

Kadish K (1984) Redox tuning of metalloporphyrin reactivity. J Electroanal Chem 168 (l-2) 261-274... 

The high stability of porphyrins and metalloporphyrins is based on their aromaticity, so that porphyrins are not only most widespread in biological systems but also are found as geoporphyrins in sediments and have even been detected in interstellar space. The stability of the porphyrin ring system can be demonstrated by treatment with strong acids, which leave the macrocycle untouched. The instability of porphyrins occurs in reduction and oxidation reactions especially in the presence of light. The most common chemical reactivity of the porphyrin nucleus is electrophilic substitution which is typical for aromatic compounds. 

Reactivity of compounds 1 and 2c with O2 is remarkably different from that of 5c, 5cCo, and 8 (Fig. 18.18). Metalloporphyrin 2c binds O2 reversibly at room temperature under anhydrous conditions to produce a diamagnetic ferric-superoxo/Cu adduct as demonstrated by resonance Raman spectroscopy using 2 and 2... 

Metal-oxenoid (oxo metal) species and metal-nitrenoid (imino metal) species are isoelectronic and show similar reactivity both species can add to olefins and be inserted into C—H bonds. Naturally, the study of nitrene transfer reactions began with metalloporphyrins, which were originally used as the catalysts for oxene transfer reactions. 

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. 

Various theoretical methods and approaches have been used to model properties and reactivities of metalloporphyrins. They range from the early use of qualitative molecular orbital diagrams (24,25), linear combination of atomic orbitals to yield molecular orbitals (LCAO-MO) calculations (26-30), molecular mechanics (31,32) and semi-empirical methods (33-35), and self-consistent field method (SCF) calculations (36-43) to the methods commonly used nowadays (molecular dynamic simulations (31,44,45), density functional theory (DFT) (35,46-49), Moller-Plesset perturbation theory ( ) (50-53), configuration interaction (Cl) (35,42,54-56), coupled cluster (CC) (57,58), and CASSCF/CASPT2 (59-63)). 

A large number of different types of DFT calculation have appeared over the years. Probably nearly all available DFT functionals have now been used to describe the properties and reactivity of metalloporphyrins, with the B3LYP (64,65) functional being the most popular choice. 

First of all, I would like to express my gratitude to the institutions that facilitated this work. Studies related to the formation, properties, and reactivity of metalloporphyrins were supported by grants from the Alexander von Humboldt Stiftung (fellowship 2005-2006), by the Deutsche Forschungsgemeinschaft through SFB 583, Redox-Active Metal Complexes Control of Reactivity via Molecular Architectures and by a grant of computer time on the Hochstleistungsrechner in Bayern II (HLRB II). 

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. 

The metalloporphyrin-PhIO system catalyzes the oxidation of alkanes mainly to alcohols under mild conditions. High selectivity for the hydroxylation at the tertiary carbon is observed.132 Yields up to 40% based on the oxidant consumed are obtained. Acyclic alkanes usually exhibit very poor reactivity. A large isotope effect and retention of configuration in the oxidation of ds-decalin are additional important characteristics of the process. 

The extremely hindered manganese(III) tetra(2,4,6-triphenylphenyl)porphyrin acetate [Mn(TTPPP)OAc], in contrast, gives the less hindered 3,3-dimethylbutane-l-ol with exceptionally high selectivity. Hindered metalloporphyrins also exhibit an increasing reactivity in the oxidation of the -1 CH2 site in straight-chain alkanes.148... 

Porphyrins with metals other than iron (and cobalt) are not of particular direct relevance to the present section, although it should be noted that porphyrins have been extracted from coal and oil shales with metals such as gallium and the early transition metals coordinated. The biological significance of these observations is questionable. Nevertheless, detailed studies on a range of metalloporphyrins have contributed substantially to our appreciation of the bonding in such molecules, and the influence of cis and trans ligands on their structure and reactivity.599 609... 

Hydrodemetallation pathways for Ni-etioporphyrin and Ni-tetra(3-methylphenyl)porphyrin are shown in Fig. 20. Both are characterized by a sequential hydrogenation-hydrogenolysis global mechanism, but important differences are apparent. Ware and Wei (1985a) rationalized the differences in porphyrin reactivity on the basis of porphyrin molecular structure. Structural differences on the periphery of the metalloporphyrin, in particular the substituent groups at the /3-pyrrolic and methine bridge... 

Rate coefficients reported in the literature indicate that pure metalloporphyrins in clean systems are up to 10 times more reactive than naturally... 

Reduction potentials of a large number of peripherally substituted metal-free porphyrins have been measured, and substituent partial potentials have been derived the reduction potentials can also be correlated with porphyrin basicity (p/ ) and reactivity (80MI30701). For a review of electrochemistry as applied to porphyrin and metalloporphyrin systems,... 

Groves, J. T. (2000) Reactivity and mechanisms of metalloporphyrin-catalyzed oxidations, Journal of Porphyrins and Phthalocyanines 4, 350-352. 

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