Porphyrins dioxygen carriers

Figure 52 Some synthetic iron(II) porphyrins which function as models for dioxygen carriers...

The most common dioxygen carriers of nature, the heme proteins hemoglobin and myoglobin, bind Oj in a characteristic structure. The Oj molecule binds to the metal atom through one of its atoms, and the M—O—O unit is angular in shape. Studies on small molecules that bind O2 in this way have concentrated on two classes of ligands, porphyrins and Schiff bases.In the first case, bulky superstructures are usually appended to the porphyrin ligand this superstructure inhibits autoxidation of the metal atom in the... 

Sauer-Masarwa, A., N. Herron, C.M. Fendrick, and D.H. Busch (1993). Kinetics and intermediates in the autoxidation of synthetic, non-porphyrin iron(II) dioxygen carriers. Inorg. Chem. 32, 1086-1094. 

Gerothanassis, I. R, Momenteau, M. (1987). NMR spectroscopy as a tool for studying synthetic oxygen carriers related to biological systems Application to a synthetic single-face hindered iron porphyrin-dioxygen complex in solution, J. Am. Chem. Soc., 109 6944. 

The cyclidene ligand family (Fig. lOE-G and 12) was chosen for investigations dedicated to finding non-porphyrin O2 carriers because their iron(II) complexes exhibit Fe /Fe electrode potentials similar to those of the natural dioxygen carriers hemoglobin and myoglobin (Fig. 11). A second feature of their chemistry that favored the cyclidenes is the ease with which structural variants can be produced (Fig. 

This review is particularly concerned with the bonding modes of the dioxygen ligand, and the factors affecting which of the various possible orientations it assumes in any particular complex. We shall therefore consider all previous work, from the earliest work on synthetic oxygen carriers, right up to the most recent studies on picket-fence and other synthetic porphyrins. All work concerned with an irreversible system will illustrate some principle appertaining to reversibility. On this basis we shall then attempt to provide a unified rationale for ... 

The protein chain in hemoglobin may place restraints on the iron-to-proxi-mal histidine bond. On the other side of the heme, the distal histidine and occluded water molecules may hydrogen-bond to the coordinated dioxygen and force ligands to adopt geometries that are different from those observed in the absence of steric hindrances. The conformation of the porphyrin skeleton may also be perturbed by the protein chain. Clearly, it is the protein chain that bestows the property of cooperativity on oligomeric oxygen carriers. 

Newton and Hall [175] performed ab initio calculations on dioxygen complexes of Co and Fe porphyrins, which are models for natural oxygen carriers. The Co-O system is best described as a superoxocobalt(III)... 

Achiral manganese porphyrins Mn(TPP) and (TPP)Cl included into a carrier protein (bovine serum albumin) effect the stereoselective dioxygenation of L(+)- and D(-)-AA [228]. 

Epoxy alcohols are the normal products of the [VO(acac)2]+(Me2C(CN)N a -catalysed oxidation of cyclic olefins by dioxygen however, cyclo-octene is oxidized exclusively to cyclo-octene oxide. The oxidation of sulphides and alkenes by peroxides with a [V(0)(acac)2] catalyst have been compared and the nature of the monoperoxovanadium(v) intermediate investigated. Complexation of a Cr(CO)3 unit to aromatic hydrocarbons enhances the benzylic positions towards attack by superoxide ion, e.g., diphenylmethane is readily converted into benzophenone. Metal porphyrin complexes ML4 continue to attract attention both as reversible oxygen-carriers (M = Fe) and oxidation catalysts (M = Mn, Fe, or Co ). For example [Mn (=0 IPh)(TPP)Cl] is believed to be involved in the oxidation of cyclohexene to cyclohexanol by PhIO in the presence of [Mn(TPP)]+ and a ferryl intermediate [Fe (0)L4] has been proposed in the oxygenation of triphenylphosphine with iron(ii) porphyrin. [M(TPP)]X (M=Mn, X = OAc M=Fe, X=C1 M = Co, X=Br) catalyses the epoxidation of styrene and cyclohexene with NaOCl under phase-transfer conditions. ... 

---