Dihydrogen photochemical reductive

Most reductive elimination reactions are induced thermally, but some important reductive elimination reactions are induced photochemically. Most examples of such reductive eliminations form dihydrogen from metal-dihydride complexes. Two examples of the photochemical reductive elimination of dihydrogen to generate intermediates that add alkanes are shown in Equations 8.6 and 8.7. In these examples, the photochemical irradiation provides the energy to induce the reductive elimination process. 

Transition-metal catalyzed photochemical reactions for hydrogen generation from water have recently been investigated in detail. The reaction system is composed of three major components such as a photosensitizer (PS), a water reduction catalyst (WRC), and a sacrificial reagent (SR). Although noble-metal complexes as WRC have been used [214—230], examples for iron complexes are quite rare. It is well known that a hydride as well as a dihydrogen (or dihydride) complex plays important roles in this reaction. 

For example Kurihara and Fendler [258] succeeded in forming colloid platinum particles, Ptin, inside the vesicle cavities. An analogous catalyst was proposed also by Maier and Shafirovich [164, 259-261]. The latter catalyst was prepared via sonification of the lipid in the solution of a platinum complex. During the formation of the vesicles platinum was reduced and the tiny particles of metal platinum were adsorbed onto the membranes. Electron microscopy has shown a size of 10-20 A for these particles. With the Ptin-catalyst the most suitable reductant proved to be a Rh(bpy)3+ complex generated photochemically in the inner cavity of the vesicle (see Fig. 8a). With this reductant the quantum yield for H2 evolution of 3% was achieved. Addition of the oxidant Fe(CN), in the bulk solution outside vesicles has practically no effect on the rate of dihydrogen evolution in the system. Note that the redox potential of the bulk solution remains positive during the H2 evolution in the vesicle inner cavities, i.e. the inner redox reaction does not depend on the redox potential of the environment. Thus redox processes in the inner cavities of the vesicles can proceed independently of the redox potential in the bulk solution. 

Reductive elimination of dihydrogen from [Pt2H2(//-HX/i-dppm)2]+ may also be induced photochemically in acetonitrile or pyridine solution. 

Elimination of H2, RH, or HX from a metal hydride complex may occur when the metal atom can adopt a stable oxidation state two units below that in the parent complex. Reductive elimination of dihydrogen can readily be achieved by thermal or photochemical methods, and is promoted by the presence of a donor ligand capable of reacting with the product. For example, the following reactions proceed photochemically ... 

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