System metalloporphyrin

This leads us finally to a brief discussion of our recent work on metalloproteins, and their model systems, metalloporphyrins. Here, the basic longterm objectives are to obtain a better understanding of how CO and 02, and their isoelectronic counterparts the isocyanides (RNC) and nitrosoalkanes/nitrosoarenes (RNO), bind to Fe, as well as to probe the structure of cytochrome c, a small protein involved not only in electron-transfer, but in apoptosis, or programmed cell death (44). 

The second example of research being funded by DOE involves a model system, metalloporphyrin, which looks at excited-state evolution using time-resolved X-rays. This research sets the groundwork for future research that will be conducted on much shorter time scales than the femtosecond domain. 

Metal-free porphyrins can undergo several steps of reduction and oxidation at the macrocyclic ring Tr-system. Metalloporphyrins may undergo reduction and oxidation reactions both at the porphyrin Tr-system and at the central metal ion. The site and rate of such redox reactions strongly depend on the porphyrin structure, the nature of the central metal ion, and the environment. Many of the fundamental reactions of porphyrins and metalloporphyrins have been studied by radiation chemical methods these studies are reviewed in this chapter. 

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). 

Photoinduced electron transfer reactions in supramolecularmodel systems based on metalloporphyrins 97YGK557. 

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. 

Recent developments in the studies of titanium and vanadium porphyrins with special emphasis on oxygen adducts, low valent metalloporphyrins and related systems of sulphur and selenium. R. Gui-lard and C. Lecomte, Coord. Chem. Rev., 1985, 65, 87 (76). 

Much effort has also been directed toward mimicking electron transfer on natural photosynthetic systems. Recently, the group of Harada has been able to prepare monoclonal antibodies against metallo porphyrins and show that the biological edifice can control photoinduced electron transfer from the porphyrin to organic acceptor molecules in solution. As it was important to design a biomolecule able to accommodate not only the metalloporphyrin unit but also organic substrates, Harada recently used a hexacoordinated phosphorus... 

When the metalloporphyrin bears a donor group on its periphery, it can behave as a self-complementary ditopic unit capable of metal-ligand induced dimerization. Many systems have been synthesized using different metals, ligands, and spacers. The length and geometry of the spacer groups determine the stoichiometry of the assembly process. 

A polymeric structure can be generated by intermolecular coordination of a metalloporphyrin equipped with a suitable ligand. Fleischer (18,90) solved the crystal structure of a zinc porphyrin with one 4-pyridyl group attached at the meso position. In the solid state, a coordination polymer is formed (75, Fig. 30). The authors reported that the open polymer persists in solution, but the association constant of 3 x 104 M 1 is rather high, and it seems more likely, in the light of later work on closed macrocycles (see above), that this system forms a cyclic tetramer at 10-3 M concentrations in solution (71,73). 

The chemically catalyzed oxidation of carotenoids by metalloporphyrins has also been described in the literature. In 2000, French et al. described a central cleavage mimic system (ruthenium porphyrin linked to cyclodextrins) that exhibited a 15,1 S -regiosclectivity of about 40% in the oxidative cleavage of [3-carotene by tert-butyl hydroperoxide in a biphasic system (French et al. 2000). 

A prime example of this is the crucial ease of such oxidation in the magnesium(II) complex of the chlorin system present in chlorophyll during photosynthesis. Table 8 shows the half-wave potentials for the first ring oxidation of some metalloporphyrins, the examples chosen being based on 5,10,15,20-tetraphenyl porphyrin, TPP (2) and on 1,2,7,8,12,13,17,18-octaethylporphyrin, OEP (16). 

Metalloporphyrin systems have been used to transport peripherally coordinated U1ln for tumor localization purposes (Section 9.22.8.1).146 The mIn complex of HpD has been described.364... 

Ligand substitution reactions of NO leading to metal-nitrosyl bond formation were first quantitatively studied for metalloporphyrins, (M(Por)), and heme proteins a few decades ago (20), and have been the subject of a recent review (20d). Despite the large volume of work, systematic mechanistic studies have been limited. As with the Rum(salen) complexes discussed above, photoexcitation of met allop or phyr in nitrosyls results in labilization of NO. In such studies, laser flash photolysis is used to labilize NO from a M(Por)(NO) precursor, and subsequent relaxation of the non-steady state system back to equilibrium (Eq. (9)) is monitored spectroscopically. 

An interesting, pH-dependent mechanistic changeover was reported in the H2A-02-[Fem(TPPS)] (TPPS = 5,10,15,20-tetrakis(p-sulfonatophe-nyl)porphyrinate) system in aqueous solution (23). This water-soluble metalloporphyrin exists as a monomer under slightly acidic conditions and reacts with ascorbic acid on the time-scale of several hours to produce [Fen(TPPS)] in the absence of dioxygen. The formation of the [Fem(TPPS +)] radical was also reported at pH 5, but it is not clear how... 

The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

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. 

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