Transition metal oxygen-binding complexes

An obvious choice for the construction of artificial oxygen binding mimics is the use of oxidisible transition metal (particularly iron) complexes of porphyrin ligands such as 12.23, as in the natural system. In such cases, the O2 affinity is governed essentially by the reduction potential of the metal centre (Ej) as in the reaction + e = M +. Iron(ll) and cobalt(ll) are particularly favourable in... 

PINO generation being a key step of the overall process, in most cases, the use of NHPI was proposed in combination with different cocatalysts or initiators. Several examples report the beneficial effect of transition metal salts and complexes for this purpose [4]. In this context, the main role of metal salts (including Mn, Co, Cu, V, and Fe salts) is not only to accelerate classical autoxidation, by promoting the decomposition of the intermediate hydroperoxides (Scheme 16.3a), but also to bind oxygen (Scheme 16.3b), leading to the formation of PINO radical without requiring thermal treatment (Scheme 16.3c). 

The nature of the donor atom also plays an important role in stabilizing complexation. Substituting one or more sulfur atoms for oxygen in [18]crown-6 (1) causes a decrease in Kt for K+ complexation and an enhanced binding of soft cations such as Ag+. A similar substitution by nitrogen atoms results in a smaller reduction in K+ binding but an increased Ks for transition metal complexation. 

Carbon monoxide has 14 electrons, which pair to give a net spin of zero. Carbon monoxide complexes of transition metals, like oxygen complexes, cannot convert an even electron system to an odd electron system. In the case of iron, CO usually binds only to ferrous ions, which have six 3d electrons. As a consequence, CO complexes and O2 complexes with iron-containing proteins are generally not detectable by EPR. 

Metalloporphyrins have been used for epoxidation and hydroxylation [5.53] and a phosphine-rhodium complex for isomerization and hydrogenation [5.54]. Cytochrome P-450 model systems are represented by a porphyrin-bridged cyclophane [5.55a], macrobicyclic transition metal cyclidenes [5.55b] or /3-cyclodextrin-linked porphyrin complexes [5.55c] that may bind substrates and perform oxygenation reactions on them. A cyclodextrin connected to a coenzyme B12 unit forms a potential enzyme-coenzyme mimic [5.56]. Recognition directed, specific DNA cleavage... 

Electronic properties of intrazeolitic complexes of transition metal ions with oxygen are of Interest for elucidation of oxygen binding and its activation for oxidation reactions. In an earlier study, dioxygen and monooxygen chromium species were reported to be formed by specific interactions between the oxygen molecules and the Cr ions planted in the Type A zeolite (1). 

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