Sacrificial reagents

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. 

Galinska, A., Walendziewski, J. 2005. Photocatalytic water splitting over Pt TiO, in the presence of sacrificial reagents. Energy Fuels 19 1143-1147. 

Reduction methods based on the use of a sacrificial reagent such as an alcohol (usually isopropanol) together with a metal or transition metal, or other... 

Semiconductors doped Band gap (uV) Sacrificial reagent Rate of gas evolution (fimol/h) Ref... 

A Pd(OAc)2/phenanthroline catalytic system was reported to catalyze the benzene-to-phenol conversion in the presence of CO as a sacrificial reagent in an autoclave at 180 °C [149]. Similarly, phenol was produced from benzene with air (10-15 atm) in the presence of CO (10 atm) as a sacrificial reagent by using molybdovanadopho-phoric acids as catalysts in a liquid phase, involving acetic acid at 90 °C, while no reaction occurred in the absence of CO [150]. 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

In spite of the remarkable improvement upon previously existing methodology, there is one disadvantage that remains. For the synthesis of an olefin, a second olefin has to be sacrificed. It is obvious that a process that would enable dehydrogenation to occur in the absence of sacrificial reagents would be highly desirable. Moreover, the selectivities that can be obtained at high turnovers are still too low for practical applications. Neither turnover frequencies nor turnover numbers of the catalysis are sufficient to be useful for industrial processes. These limitations are less of an issue in total synthesis, provided that the quality of the metal-mediated reaction justifies the use of stoichiometric processes. 

It must be borne in mind that in all the cases above where the band gap of the semiconductor was effectively shrunk to values in the 2.0-2.5 eV range, overall water splitting (that is, evolution of H2 and O2 with no sacrificial reagents) was not observed under visible light irradiation. This contrasts with the P-GesN4 case where, however, UV radiation had to be used. 

Figure 6 Schematic diagram of photocatalytic water splitting in the presence of redox (sacrificial) reagents (a) reducing reagent (Red) for H2 evolution and (b) oxidizing reagent (Ox) for O2 evolution (Maeda and Domen, 2007).

Photocatalysts (reference) Band-gap energy (eV) Cocatalyst Sacrificial reagent Activity gmol h H2 g O2 ... 

Liu, H., Yuan, J., and Shangguan, W., Photochemical Reduction and Oxidation of Water Including Sacrificial Reagents and Pt/Ti02 Catalyst, Energy Fuels, 20(6), 2289-92, 2006. 

Anatase is much more active than rutile in the liquid-phase photooxidation of 2-propanol, and the observed reactivity compared well with that of gaseous alcohol on platinized titania [106a]. Because most early studies employed the alcohol as a sacrificial reagent for the photochemical production of hydrogen, the organic product was often not analyzed and little effort was devoted to the selective activation of alcohols in the presence of other functional groups. In the oxidation of car-... 

In Scheme 4, Sens stands for sensitizer, S for substrate, S for transformed substrate, Sac for sacrificial reagent (because 1 mol of Sac is consumed for 1 mol of S —> P transformation). Case 4.1 corresponds to an energy transfer induced transformation of the substrate into product(s). Cases 4.2 and 4.3 correspond to a transformation resulting from an electron transfer between the sensitizer in its excited state and the substrate. 

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