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Ruthenium Porphyrins David Dolphin 5 Heme ( 1 ) , the iron complex of protoporphyrin, when bound to a number of proteins such as hemoglobin, myoglobin and cytochrome P-450, reversibly binds molecular oxygen when the iron is in the ferrous oxidation state. When, however, the heme is not bound to the protein it undergoes irreversible oxidation to the ferric stage which no longer binds oxygen. A number of approaches, by ColIman, Traylor and others, which involve modifications of the porphyrin or low temperature, have been used to stabilise the iron-porphyrin oxygen adducts. More recently another approach, by Farrell, Dolphin and James, using simple ruthenium porphyrins, has provided systems which reversibly bind molecular oxygen to give oxygen complexes which are stable at room temperature. Complexes ( 2 ) (where L=L1=CH3CN) were prepared from both octaethyl- (OEP) and meso_-tetrapheny!porphyrins by photolyses of the corresponding complexes ( 2 ; where L=CO and L1 = CH3CH2OH) in acetonitrile. The course of a photolysis is shown in Fig. 1 and the isosbestic points indicate the smoothness of the reaction. Photolysis in other solvents including pyrrole, dimethylformamide (DMF), dimethylacetamide (DMA) and a variety of nitrogenous bases gives rise to similar bis-solvated adducts. When simple ferrous-iron porphyrins are reacted with molecular oxygen an initial 1:1 complex ( 3 ) is formed. By itself 3 does not undergo irreversible oxidation since oxygen is a two electron oxidising agent and ferrous iron a one-electron reductant. However, if a second iron porphyrin coordinates to the oxygen to give 4 (this can be prevented by sterically encumbering the porphyrin, which is the case when heme is bound to a protein) an irreversible reaction occurs to give the bridging peroxide ( 5 ) . When RU(II)(OEP)(CH3CN)2 in toluene reacts with molecular oxygen a slow irreversible oxidation occurs to give a ruthenium(III) complex. However, solutions of RU(II)(OEP)(CH3CN)2 in DMF, DMA, or pyrrole reversibly absorb 1.0 mole of O2 per ruthenium at room temperature. Figure 2 shows the spectral changes when the reaction is carried out in pyrrole. The rate of oxygenation depends upon the axially coordinated solvent, at 1 atm pressure of O2 the reactions are pseudo-first-order with the reaction being almost instantaneous in DMA but having t½=30 min in DMF. The reverse deoxygenation is much slower and requires pumping for 24 h. Addition of CO to the oxygenated complex generates Ru(II)(OEP)(CO)L with no change in the gaseous volume. All of the above chemistry is consistent with that shown in Scheme 1. The stability of the oxygen adducts is probably due to the slow off-rate for loss of oxygen, which is to be expected from substitution inert ruthenium(II) complexes, and the correspondingly slow formation of the dimeric peroxide species analogous to 5 . Apo myoglobin has been reconstituted with both Ru(II) and Ru(III) mesoporphyrin (D. Paulson, A.W. Addison, D. Dolphin and B.R. James, unpublished results). Ru(II)-mesoporphyrin and reduction of the Ru(III)-Containing complex gives ruthenomyoglobin which reacts with carbon monoxide, Fig. 3, to give the CO-adduct similar to that of native myoglobin. At the present time addition of oxygen to ruthenomyoglobin appears to bring about an irreversible oxidation of the metal. In a manner analogous to complexes of type 2 , ruthenomyoglobin also reversibly binds molecular nitrogen. Oxidation of ruthenium(II)-containing porphyrins follows two paths. Brown et al. have shown that oxi- dation of 2 (where L=L1=pyridine) results in oxidation of the metal to give the ruthenium(III) species. When CO is axially bonded its strong -rr-accepting properties raise the potential for oxidation of the metal to.a value higher than that of the porphyrin ring which is instead oxidised to the porphyrin p-cation radical. In those cases where the axial ligands of 2 are ones other than CO (including CH3CN, DNIF, DMA) oxidation initially occurs at the metal. Bibliography J.P. Collman, Acc Chem. Res., 10, 265 (1976). T.G. Traylor, in "Bioorganic Chemistry, Vol. IV, Electron transfer and energy conversion; cofactors, probed," ed. E.E. van Tamelen, Academic Press, New York, N.Y., 1978 N.Farrell, D.H. Dol phin, and B.R. James, Soc., 100, 324 (1978). J. Am. Chem. G.M. Brown, F.R. Hopf, J.A. Ferguson, T.J. Meyer, and D.G. Whitten, J. Am. Chem. Soc., 95, 5939 (1973). Figure 3 . Spectral changes during the conversion of Ru(III)-myoglobin, curve 1, to carbonmonoxyruthenomyoglobin by sodium dithi onite and CO, curve 2 . Fig 2 Fig 3