Redox inert metallic center

Our goal is to motivate our experimental colleges to search the right combination of redox-active hgand with a redox inert metallic center, such as Ca(II), that can catalyze the C-H functionalization. 

Such pentacarbonyl species can be further decarbonylated when the sample is heated to 373 K under an inert gas stream and under reduced pressure. This slow decarbonylation process provides the surface Mo(CO)3 species depicted in Figure 9.4, which is stable up to 473 K [14]. In contrast with the relevant chemical behavior in solution (9.1 and 9.2), in the solid state, where the species are somewhat diluted and present low mobility, no dimeric species have been identified as resulting from penta- or tricarbonyl species. Heating to 673 K gives rise to the evolution of H2, CO, CO2 and CH4, due to redox reactions between the metal center and the OH surface groups. The resulting oxidation states, as determined by XPS measurements, are mainly II and IV, besides some Mo(0) species ]20]. It is worth underHn-... 

Consecutive oxidation and reduction processes would make the metal center M shuttle back and forth, between A and B, along a determined route. The rate of the translocation process should depend on the nature of the coordinative interactions between M and receptors A and B, whether labile or inert, and on the feasibility of the stereochemical rearrangement which may accompany the metal displacement. Examples of redox-driven metal ion translocation within a two-component system have been recently investigated, and refer to the Fe(III)/Fe(II) and Cu(II)/Cu(I) couples. 

An alternative pathway for the participation of transition metal complexes in photosynthetic transformations involves activation of the substrates by their direct coordination to the metal center. In this respect the metal center acts as an active site, in addition to its redox functions. Numerous transition metal complexes activating inert molecules (i.e., carbon dioxide or nitrogen) by coordination to the metal center are known [132,133]. 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

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