Transition metal complexes photocatalysts

Scheme 7 illustrates the transformations of a bicycloheptadiene in the presence of transition metal complexes as photocatalysts or photogenerated catalysts [11] ... 

TRANSITION METAL COMPLEXES AS SOLAR PHOTOCATALYSTS IN THE ENVIRONMENT A SHORT REVIEW OF RECENT DEVELOPMENT... 

The role of photocatalysis by transition metal complexes in the environment is reviewed, and its influence on composition of the environmental compartments, transport between them, and activation of the environmental self-cleaning behavior is characterized. In description of atmospheric processes, the attention is paid to coordination compounds as photocatalysts of the transfer and redox reactions of nitrogen oxides. In the case of hydrosphere and soils, various mechanisms of organic pollutant photodegradations are presented in which the iron, copper, and chromium complexes play... 

Solar photocatalysts. See Transition metal complexes Spectral sensitization aspects, 248... 

The prototypical photochemical system for CO2 reduction contains a photosensitizer (or photocatalyst) to capture the photon energy, an electron relay catalyst (that might be the same species as the photosensitizer) to couple the photon energy to the chemical reduction, an oxidizable species to complete the redox cycle and CO2 as the substrate. Figure 1 shows a cartoon of the photochemical CO2 reduction system. An effective photocatalyst must absorb a significant part of the solar spectrum, have a long-lived excited state and promote the activation of small molecules. Both organic dyes and transition metal complexes have been used as photocatalysts for CO2 reduction. In this chapter, CO2 reduction systems mediated by cobalt and nickel macrocycles and rhenium complexes will be discussed. 

Several types of transition metal complexes have been used as photocatalysts for C02 reduction,63,67 but the ones most studied are ruthenium (II) and rhenium (I) complexes with polypyridine ligands. Thus, Ru(bpy)32+ can be both photosensitizer and catalyst or another metal complex may serve as catalyst. Alternatively, Re(bpy)(CO)3X-type complexes may serve as... 

Transition Metal Complexes as Solar Photocatalysts in the Environment ... 

In this review, we classify the reported photocatalysts for CO2 reduction into two categories, i.e., homogeneous and heterogeneous systems. Typically, the former are transition metal complexes and the latter are semiconductors. 

In homogeneous systems, some photocatalysts using transition metal complexes show outstanding performance, such as high absorbance in the visible region, high quantum yields, and high product selectivities. However, no metal complex which can use water as the reductant for CO2 reduction has been reported so far. 

While several of the oxidative techniques listed previously utilize transition metal complexes as promoters, the use of a completely organic photocatalyst for the oxidative synthesis of a-aminophosphonates has been reported (Scheme 4.108 and Example 4.27)... 

To this category belong, e.g., homogeneous photocatalytic systems based on soluble metal complexes or organic dyes as photocatalysts. Instructive examples are photoreactions assisted by heteropolyacids (HPAs), transition meal complexes with carbonyl, phosphine or some other ligands, and metal porphyrins. 

Surface doping of oxide colloids and nanostructured electrodes with transition metal ions and complexes is of great interest for improving efficiency and selectivity of photocatalysts and photoelectrodes. Such surface ions as electron donors or acceptors play an important role as catalytic active centers, in charge transfer and in adsorption. There were many publications on this subject and we will try to bring forward the most... 

Dyes such as erythrosin B [172], eosin [173-177], rose bengal [178,179], rhodamines [180-185], cresyl violet [186-191], thionine [192], chlorophyll a and b [193-198], chlorophyllin [197,199], anthracene-9-carboxylate [200,201], perylene [202,203] 8-hydroxyquinoline [204], porphyrins [205], phthalocyanines [206,207], transition metal cyanides [208,209], Ru(bpy)32+ and its analogs [83,170,210-218], cyanines [169,219-226], squaraines [55,227-230], and phe-nylfluorone [231] which have high extinction coefficients in the visible, are often employed to extend the photoresponse of the semiconductor in photoelectro-chemical systems. Visible light sensitization of platinized Ti02 photocatalyst by surface-coated polymers derivatized with ruthenium tris(bipyridyl) complex has also been attempted [232,233]. Because the singlet excited state of these dyes is short lived it becomes essential to adsorb them on the semiconductor surface with... 

The first popular photocatalysts in homogeneous media were transition metal carbonyls (eg M(CO)6, where M=Cr, Mo, W Fe(CO)5, Fe3(CO)i2, Ru3(CO)i2, Mn2(CO)10) and their derivatives [28], Recently, other systems have been intensively studied, eg iron(III) complexes, especially in the photo-Fenton reactions [63-78, 139-142],polyoxometallates,porphyrins, metalloporphyrins,and other porphyrinoid sensitizers. 

The effects of other transition metal ions are qualitatively similar, although their extent is decidedly smaller. Complexes of copper, chromium, and manganese with central atoms in their higher oxidation states are fairly good candidates to play the role of an environmental photocatalyst [20],... 

As an example, Yoon [34] has studied the inter- and intramolecular 2+2 cycloaddition of unsaturated ketones (an example in Scheme 8.19). A chiral version of the reaction cannot be obtained by simply adding a chiral catalyst, because electronic excitation promotes the addition of the aromatic ketone also when not complexed, as indeed it is often the case with this kind of reactions. However, he was successful when he used two catalysts, viz., a visible-absorbing transition metal photocatalyst and a stereocontrolling Lewis acid cocatalyst. Under these conditions, the excited molecule could react only with a molecule complexed with a chiral ligand (Scheme 8.20). 

While the majority of transition metal photocatalysts are based upon ruthenium and iridium complexes, similar intermolecular cyanation reactions can also be accomplished using gold-based transition metal photocatalysts. 

Due to the presence of two double bonds in norbornadiene and its derivatives, these compounds readily form 7t-complexes with transition metal salts and coordination compounds. The latter may serve as photosensitizers S for the N/Q transformation and can also be designated as photocatalysts.Photoexcitation of the stable ground state adducts [S—N], results in the formation of quadricyclanes ... 

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