Carbonyl complexes, electron-transfer reactions

Remarkable positive shifts of the °red values of the singlet excited states of the metal ion-carbonyl complexes as compared to those of the triplet excited states of uncomplexed carbonyl compounds (Table 2) result in a significant increase in the redox reactivity of the Lewis acid complexes versus uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. For example, photoaddition of benzyltrimethylsilane with naphthaldehydes and acetonaphthones proceeds efficiently in the presence of Mg(C104)2 in MeCN, although... 

Trimethylsilyl triflate (McsSiOTf) acts as an even stronger Lewis acid than Sc(OTf)3 in the photoinduced electron-transfer reactions of AcrCO in dichloro-methane. In general, such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C—C bond formation between organosilanes and aromatic carbonyl compounds via the Lewis-acid-catalyzed photoinduced electron transfer. Formation of the radical ion pair in photoinduced electron transfer from PhCHiSiMes to the (l-NA) -Mg(C104)2 complex (Scheme 11) and the AcrCO -Sc(OTf)3 complex (Scheme 12) was confirmed by the laser flash experiments [113]. 

The prototypically zero oxidation state complexes of the group are the binary hexacarbonyls M(CO)6. Early studies of the electrochemistry of these 18-electron closed-shell systems in nonaqueous electrolytes has perhaps been seminal in understanding the electron-transfer reactions of more substituted systems and of metal carbonyls in general. 

The catalytic effect of metal ions such as Mg2+ and Zn2+ on the reduction of carbonyl compounds has extensively been studied in connection with the involvement of metal ions in the oxidation-reduction reactions of nicotinamide coenzymes [144-149]. Acceleration effects of Mg2+ on hydride transfer from NADH model compounds to carbonyl compounds have been shown to be ascribed to the catalysis on the initial electron transfer process, which is the rate-determining step of the overall hydride transfer reactions [16,87,149]. The Mg2+ ion has also been shown to accelerate electron transfer from cis-dialkylcobalt(III) complexes to p-ben-zoquinone derivatives [150,151]. In this context, a remarkable catalytic effect of Mg2+ was also found on photoinduced electron transfer reactions from various electron donors to flavin analogs in 1984 [152], The Mg2+ (or Zn2+) ion forms complexes with a flavin analog la and 5-deazaflavins 2a-c with a 1 1 stoichiometry in dry MeCN at 298 K [153] ... 

As demonstrated in this review, photoinduced electron transfer reactions are accelerated by appropriate third components acting as catalysts when the products of electron transfer form complexes with the catalysts. Such catalysis on electron transfer processes is particularly important to control the redox reactions in which the photoinduced electron transfer processes are involved as the rate-determining steps followed by facile follow-up steps involving cleavage and formation of chemical bonds. Once the thermodynamic properties of the complexation of adds and metal ions are obtained, we can predict the kinetic formulation on the catalytic activity. We have recently found that various metal ions, in particular rare-earth metal ions, act as very effident catalysts in electron transfer reactions of carbonyl compounds [216]. When one thinks about only two-electron reduction of a substrate (A), the reduction and protonation give 9 spedes at different oxidation and protonation states, as shown in Scheme 29. Each species can... 

The carbon dioxide anion radical was used for one-electron reductions of nitrobenzene diazonium cations, nitrobenzene itself, quinones, aliphatic nitro compounds, acetaldehyde, acetone and other carbonyl compounds, maleimide, riboflavin, and certain dyes (Morkovnik Okhlobystin 1979). This anion radical reduces organic complexes of Com and Rum into appropriate complexes of the metals in the valence 2 state (Morkovnik Okhlobystin 1979). In the case of the pentammino-p-nitrobenzoato-cobalt(III) complex, the electron-transfer reaction passes a stage of the formation of the Co(III) complex with the p-nitrophenyl anion radical fragment. This intermediate complex transforms into the final Co(II) complex with the p-nitrobenzoate ligand as a result of an intramolecular electron transfer. Scheme 1-89 illustrates this sequence of transformations ... 

This article is intended to review the published work on the photochemistry and photophysics of osmium complexes that has appeared in the literature over the past several years. We have attempted to cover, albeit somewhat selectively, literature dating back to the year 2000. A variety of reviews pertaining to particular aspects of osmium photophysics and photochemistry were published prior to 2000. A few reviews discuss the photophysical behavior of primarily monometallic Os complexes in solution [1,2]. Several earlier reviews discuss light induced energy and electron transfer reactions involving osmium complexes in much of this work the Os complex is not the chro-mophore [3-6]. Finally, one review exists discussing the photochemistry of Os carbonyl complexes [7]. 

The formation of a hydrogen bond between the amide proton and one carbonyl oxygen of NQ was indicated in the Ec + —NQ /M" complex to stabilize the complex (see above). Electron-transfer reactions were believed to be regulated through such noncovalent interactions that play an important role in biological ET systems, where electron donors and acceptors are usually bound to proteins at a fixed distance (123-127). Eor example, in the bacterial photosynthetic reaction center (bRC) from Rhodobacter Rb) sphaeroides, an electron is transferred from... 

Carbene Complexes Carbonyl Complexes ofthe Transition Metals Cyanide Complexes of the Transition Metals Dinuclear Organometallic Cluster Complexes Electron Transfer in Coordination Compounds Electron Transfer Reactions Theory Electronic Structure of Organometallic Compounds Luminescence Nucleic Acid-Metal Ion Interactions Photochemistry of Transition Metal Complexes Photochemistry of Transition Metal Complexes Theory Polynuclear Organometallic Cluster Complexes. 

In addition to standard methods of monitoring the reaction progress by UV and visible spectroscopies, other detection methods also can be used. For example, electron-transfer reactions between monomeric and dimeric metal carbonyl complexes in Eq. 11 have been studied by infrared stopped-flow spectroscopy utilizing a tunable CO laser as a source of infrared radiation and a HgCdGe detector [12]. 

Although dinitrogen complexes such as [M(N2)2(dppe)2] are not strictly organometallic, they closely resemble the carbonyls described in Section II,C,6. This analogy as well as the discovery of the syntheses of organo-nitrogen compounds via electron-transfer reactions have led us to include a brief description of the most recent developments. 

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