Rhodium complexes, electron-transfer reactions

Rh(phi)2(phen)]3+ is a particularly suitable luminescence quencher for our investigations of electron-transfer reactions on DNA. Its electronic properties are favorable for electron transfer, and this rhodium complex is primarily sequence neutral, so that nearly random binding of the donor and acceptor is expected. Moreover, the photocleavage reaction actually allows us to identify the positions of binding of the acceptor to the DNA double helix. 

Rhodium complexes related to 48 and 49 also undergo electron-transfer reactions. Thus, [ Rh(/i-NO)Cp 2] (50) is reduced in a reversible one-electron step ( = -1.17 V) to a stable monoanion. Surprisingly, since the dication [ Rh(/u,-NO)(i7-C5Me5) 2p can be isolated from [Rh(CO)2(7 CsMes)] and [NOJpFe], the oxidation of 50 is totally irreversible. 

The excited states of dinuclear platinum, rhodium, and iridium complexes with a variety of bridging ligands exhibit unusually diverse reactivity. These types of compound in their lowest triplet state engage in oxidative and reductive electron transfer reactions, and exciplex formation [56], They can also engage in atom transfer reactions i.e. they can abstract hydrogen atoms from a wide range of substrates as well as halogen atoms from alkyl and aryl halides. 

The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

Reaction of this lO-S-3 [279] tetraazapentalene derivative with [Pd(PPhj) ] or [Rh(PPh3)3)Cl] results in the formal substitution of sulfur by the transition metal accompanied by a redox reaction (see Figure 4.93) [280], The endocyclic sulfur atom is transferred to a PPhj ligand (oxidation of phosphorus to PhjP=S). At the same time the transition metal is oxidis (palladium from 0 to +11 rhodium from +1 to +III), which leaves sulfur to be reduced by four electrons (it is -II in Ph I S and thus must have been +II in the tr-sulfurane starting material). It follows from this electron transfer analysis that the rt-sulfurane is indeed better desaibed as the sulfur complex of a doubly amide functionahsed NHC ligand. 

It is clear from the examples reported that carbon monoxide, when coordinated to a metal in a neutral complex, is not sufficiently activated to react with organic nitro compounds under mild conditions. More precisely, the first act of this reaction is the electron transfer from the metal to the nitro group to give a radical couple and this requires a very basic metal. This explains why basic ligands usually activate transition metal carbonyls in these catalytic reactions. Moreover, basic ligands such as Bipy favor the in-situ formation of the [Rh(CO)4] species from rhodium clusters. The effect of co-catalysts such as halide anions is more subtle, but even the action of these might, at least in part, be directed toward an increase of the electron density of the metal. 

Pti-x ZXjc supported on carbon or alumina, Kt/Kb is proportional to x, suggesting electron transfer from platinum to zirconium, as predicted by the Engel-Brewer theory, and (2) chemisorption of sulfur on platinum has been shown to decrease electron density of the surface, while carbon has the opposite effect. The ratio Kt/Kb was very large for ruthenium, about 10 for rhodium and about unity for palladium, which may help to explain their different activities in these and other reactions. An extensive kinetic study of the hydrogenation of mixtures of benzene and toluene on NiA zeolite has however revealed a situation of some complexity, and it is not certain that the original simple concept is totally valid. 

The rhodium(II) radical [Rh(dmgH)2PPh3], formed by laser photolysis of the corresponding dimer, is oxidized by a range of cobalt(II) complexes which have coordinated halides. " Electron transfer is believed to occur through a ligand-mediated pathway and the reaction rate is dependent upon the choice of halide. In an examination of the Ti(III)-induced cyclization of epoxyolefins, a mechanism has been invoked which involves a metal-centered radical. ... 

---