Porphyrin, capped

The kinetics and mechanism for oxygen transfer between 4-cyano-V,V,-dimethylaniline V-oxide and a C2-capped mexo-tetraphenylporphyrinatoiron(III) and mc5 o-tetrakis(pentafiuorophenyl)-porphyrinatoiron(III) have been established. Addition of a copper(II) porphyrin cap to an iron(II)-porphyrin complex has the expected effect of reducing both the affinities and rate constants for addition of dioxygen or carbon monoxide. These systems were studied for tetradecyl-substituted derivatives solubilized by surfactants such as poly(ethylene oxide) octaphenyl ether. ... 

To study this discrimination, generally expressed as an M value, the ratio of the O2 and CO equilibrium constant values, researchers have used sterically hindered porphyrins (capped, pocket and hybrid porphyrins) with a view to providing an environment which would permit normal binding of dioxygen whilst hindering the binding of carbon monoxide. Kinetically, the major effect is a decrease in CO association rates, consistent with steric blocking in productlike transition states. 

Copper(I)-threaded pseudorotaxanes can be capped by such large moieties as porphyrins [228] or fullerenes [229] - indeed there is a tris-Cu pseudorotaxane which can be deemed a rotaxane if the two terminal copper-bonded centres are considered as caps for the central unit [179]. Porphyrin-capping is of particular interest as provi ng a means to have an electron donor and electron acceptor... 

By direct condensation of suitable tetraaldehydes with pyrolle, workers 236) have succeeded in synthesizing capped porphyrins a particular one is shown in Fig. 16. 

In a further study the capped porphyrin (314) has also been synthesized (Almog, Baldwin, Dyer Peters, 1975). The cavity in this case is large enough to incorporate molecular oxygen but will exclude other small molecules (such as solvent). 

In a typical experiment, the Fe(n) derivative of (314) rapidly binds dioxygen in pyridine subsequent deoxygenation may be effected by freeze-thawing. Several such cycles can be performed with little deterioration of the system. Moreover, the Oz adduct in pyridine has a half-life of about 20 hours. Following this initial success, the 02-binding properties of a number of other related capped porphyrin derivatives have been investigated (Baldwin Perlmutter, 1984). 

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)... 

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...

Not only is carbon monoxide produced during catabolism of hemes, but it is also necessary for maintenance of neuronal function and cell signaling in the vasculature and therefore is present normally in the vicinity of Mb and Hb. For synthetic systems of picket-fence, pocket, or capped porphyrins, CO affinity exceeds that for O2 sometimes by many orders of magnitude. The in... 

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