Picket-oxygen

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 ... 

Originally, this temperature variation of AEq was attributed to the dynamic distribution of the terminal oxygen of the Fe02-moiety, as suggested by X-ray structural results for picket-fence porphyrins [26,27]. This view is now supported by NFS studies which provide more information on dynamic processes in iron-containing molecules. 

In order to prepare and isolate solid-state, crystalline, oxygenated iron-heme model complexes, chemists learned to synthesize (by self-assembly methods) and oxygenate many types of hindered porphyrins. For instance, capped porphyrins were synthesized by direct condensation of a suitable tetraaldehyde with four pyrrole molecules.37 Picket-fence porphyrins such as I e(TPP)((V-MeIm) (where TPP = meso-tetraphenylporphyrin and /V-Melm = (V-methylimidazole)... 

These strategies have been successful and have led to the X-ray crystallographic structure of the oxygenated picket-fence porphyrin described in Section 4.7. 

Synthetic models of myoglobin and hemoglobin are complex molecules that mimic the stereochemical properties of the protein active center [24] and have oxygen affinities similar to those measured for the protein [25-27]. The first heme model that reversibly binds oxygen (i.e. the picket-fence-oxygen complex Fe(TpivPP)(l,2-Melm)(02), shown in Fig. 3.3) was obtained in the early nine-teen-seventies by Collman and coworkers (TpivPP = tetrapivalami-nophenyl porphyrin 2-meIm = 2-methylimidazole) [18]. Research on synthetic models of the protein has led to a deeper understand-... 

Fig. 3.3 The myoglobin biomimetic molecule Fe(TpivPP)(2-Melm)(02), known as picket-fence-oxygen [18d]. 

The Picket-fence-oxygen Biomimetic Complex 3.3.2.1 Interplay Structure/Electronic State... 

Tab. 3.2 Main parameters defining the optimized structure of the picket-fence-oxygen molecule, FeTpivPP(2-melm)02, in comparison with the FeP(lm)02 model. Distances are given in A and angles in degrees.

Figure 9 A few schematic examples of mononuclear Fe(II) -porphyrin derivatives used to prevent the irreversible oxidation to oxygen-bridged binuclear Fe(III) derivatives. (a) Picket-fence complexes (b) strapped complexes (c) capped complexes (d) picnic-basket complexes (e) pocket complexes...

Particularly important in affirming this kind of investigation has been the Collman s picket-fence complex illustrated in Figure 10a, in that the molecular structure of its oxygenated form has been solved, Figure 10b.14... 

Recently, the Anson group sought to find the elusive Co—O2 adduct that is supposedly formed as an intermediate for the electron-transfer reaction [100]. A 1-methylimidazole-substituted picket fence porphyrin was chosen due to its high affinity for molecular oxygen. Unfortunately, the desired adduct was not observed instead, the four-molecule picket appended to one side of the porphyrin ring actually hindered the catalytic reduction of molecular oxygen. 

Fig. 19.4 Perspective view of picket-fence dioxygen udduci. The apparent presence of four different 02 atoms results from a four-way statistical disorder of the oxygen atoms on different molecules responding to Ihe X-ray diffraction [From Collman. J. P. Gagne. R. R. Reed. C. A. Robinson, W. T.t Rodlcy. G. A. Proc. Natl. Acad. Sci. V S. A. 1974. 71. 1326-1329. Reproduced with permission.]...

Figure 10.13 Perspective view of a picket fence model of the porphyrin ring system of hemoglobin. Note that the oxygen molecule is bound at an angle (there is a four-fold statistical distribution of the terminal O because of rotation about the Fe—O bond). [From J. P. Collman, R. R. Gagne, C. A. Reed, W. T. Robinson, and G. A. Rodley, Proc. Natl. Acad. Sci. USA 71,1326 (1974).]...

Iron(II) meso-tetrakis(2,4,6-trimethoxyphenyl)porphyrin and the triethoxy analogue have been synthesized by Vaska. These are different to the capped and picket fence porphyrins in that both sides of the porphyrin are sterically hindered. Dioxygen uptake by these iron(II) porphyrins at 25 °C in solution was found to be partially reversible. La Mar and co-workers166 have shown that the porphyrins Fe[T(2,4,6-OMe)3PP], Fe[T(2,4,6-OEt)3PP] and Fe[TpivPP] take up oxygen at low temperatures, in the absence of a base such as Melm or pyridine, to give dioxygen adducts of the... 

Collman, J. P., Gagne, R. R., Halbert, T. R., Marchon, J. C., Reed, C. A., Reversible oxygen adduct formation in ferrous complexes derived from a picket fence porphyrin - model for oxymyoglobin. J. Am. Chem. Soc. 1973, 95, 7868-7870. 

Collman JP et al (1973) Reversible oxygen adduct formation in ferrous complexes derived from a picket fence porphyrin - model for oxymyoglobin. J Am Chem Soc 95 7868-7870... 

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