Group-VIII Porphyrins

Work on the redox chemistry of iron porphyrins and their protein adducts deals mainly with two problems, namely the electron transfer, which is mediated by the FeII/Fe111 couple in cytochromes, and the partial reduction of the oxygen molecule to the superoxide anion or to a peroxo bridge between two iron porphyrins. The work on isolated iron porphyrins will be summarized in this section, whereas the hemoproteins will be covered in Section VI. 

The reduction of (FeTPP) 2O in tetrahydrofuran by sodium amalgam yielded a light-red solution which produced a magnetic susceptibility [iett of 5.2 BM at room temperature (S= ) and an esr spectrum at 77 °K, which is indicative of an S= Fe system (g, =1.93 gr = 2.30). The visible spectrum of this FeI(TPP) anion contains peaks at 540 nm (12,900) and 605 nm (sh, 4,200) [Cohen [35a)]. 

An interesting aspect of the redox activity of Fe11 porphyrins is their ability to reduce alkyl halides to hydrocarbon radicals, which then undergo various secondary reactions [Wade (189)]  

This indicates the highly nucleophilic character of the central electron-rich Fe11 ion, similar to the Co1 case (see IV.6). 

The electronic spectra of high- and low-spin ferric complexes are usually of the hematin type and have been reviewed recently [Willi- 

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Dioxygen binds to metal porphyrins in the three expected modes, i.e. u-superoxo, peroxo and bridging peroxo. In contrast to die simple complexes discussed previously, dioxygen coordination occurs with a wide range of transition metals from titanium and niobium through to the Group VIII metals. 

Hypso-type spectra look similar to normal spectra but the B and Q bands are blue shifted (i.e. hypsochromically). This type of spectrum is shown by d-block elements with unfilled d-orbitals of the type d -d , which includes all the metals from groups VIII to IB. Here, d-electrons may be donated into the porphyrin s empty iT -orbitals (i.e. metal-to-ring charge transfer), thus raising their energy. This increases the energy of the porphyrin ir-ir transition, with respect to the metal-free porphyrin, leading to a blue... 

Dceda, O., H. Fukuda, and H. Tamura (1986). The effect of heat treatment on group VIII B porphyrins as electrocatalysts in the cathodic reduction of oxygen. J. Chem. 

A general investigation of the Vlllth group metals led to the discovery of much improved and often unusual efficiences (up to 100%) and selectivities with palladium(II) and rhodium(II) carboxylates. Some metal carbonyls (Rh6(C0)16 and Ru3(C0)12, VKC0)6, Mo(C0)6) [6] catalyze typical carbene reactions. More recently, new catalytic systems,(Rh(III) (porphyrin) VII [7], Co(III) (oxime) VIII [8]. ..) have been described. Their application in synthesis is however limited for different reasons such as the cost, the difficult synthesis of the complex or their lack of versatility. 

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