Redox Reactions 1 Rhodium Complexes

Though thermally stable, rhodium ammines are light sensitive and irradiation of such a complex at the frequency of a ligand-field absorption band causes substitution reactions to occur (Figure 2.47) [97]. The charge-transfer transitions occur at much higher energy, so that redox reactions do not compete. 

Several approaches have been undertaken to construct redox active polymermodified electrodes containing such rhodium complexes as mediators. Beley [70] and Cosnier [71] used the electropolymerization of pyrrole-linked rhodium complexes for their fixation at the electrode surface. An effective system for the formation of 1,4-NADH from NAD+ applied a poly-Rh(terpy-py)2 + (terpy = terpyridine py = pyrrole) modified reticulated vitreous carbon electrode [70]. In the presence of liver alcohol dehydrogenase as production enzyme, cyclohexanone was transformed to cyclohexanol with a turnover number of 113 in 31 h. However, the current efficiency was rather small. The films which are obtained by electropolymerization of the pyrrole-linked rhodium complexes do not swell. Therefore, the reaction between the substrate, for example NAD+, and the reduced redox catalyst mostly takes place at the film/solution interface. To obtain a water-swellable film, which allows the easy penetration of the substrate into the film and thus renders the reaction layer larger, we used a different approach. Water-soluble copolymers of substituted vinylbipyridine rhodium complexes with N-vinylpyrrolidone, like 11 and 12, were synthesized chemically and then fixed to the surface of a graphite electrode by /-irradiation. The polymer films obtained swell very well in aqueous... 

Electrochemical redox reactions have been reported for triazenide complexes of iron 214), cobalt 214), and rhodium 43). 

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. 

The remaining results are concerned with redox reactions /t-superoxo-/<-peroxo redox reactions have been reported for rhodium complexes The complex [(NC)sCo( -02)Mo(0)(OH2)(CN)5] decomposes on standing in oxygen to [(>7 02)Mo(0)(CN)4] and isotopic labelling studies show that the dioxygen in this complex does not originate from [(NC)5Co(u-02)Mo(0)(OH2)(CN)5] °). The details of this reaction are not clear but the final product could conceivably arise from the formation of a Mo(IV) complex on breakdown of the rj complex followed by addition of dioxygen. 

Chemical or photochemical oxidation of a nucleic acid is accomplished very efficiently by a variety of metal complexes. In the presence of hydrogen peroxide and thiol, bis(phenanthroline) cuprous ion very efficiently cleaves DNA (26). Tris(phenanthroline) complexes of cobalt(IIl) or rhodium(III) promote redox reactions in their excited states (27, 28). These photoac-tivated probes bind to the DNA helix in a fashion comparable to the spectroscopic probes described above and then, upon photoactivation, promote DNA strand cleavage. 

As the next example of soft catalysis, we shall discuss the dimerization of ethylene to 1-butene, which is catalyzed by rhodium complexes in a redox cycle (Scheme 2-6). The active Rh catalyst A undergoes oxidative addition of HCl and insertion of ethylene into the Rh-H bond to give the Rh alkyl complex B. The following ethylene insertion reaction is the rate-determining step and is favored by the medium-hard Rh center. The resulting Rh butyl complex C has a hard-soft dis-... 

A number of rhodium(III) complexes can be used effectively in place of viologens as relays. Thus photolysis of a solution containing Ru(bpy)32+ as the photosensitizer, ascorbate as the electron donor and [Rh(dpm)3Cl]3 (dpm = diphenylphosphinobenzene-m-sulfonate) as the electron relay leads to nett formation of hydrido-rhodium species via a reductive quenching cycle. The hydrido-rhodium product acts a two-electron carrier for the reduction of NAD-i- to NADH. In place of NADH, synthetic nicotinamide analogues such as N-benzyl nicotinamide or N-alkylnicotinamides can be similarly reduced in the photosystem [68]. The sequence of cyclic redox reactions can be extended by the addition of an enzyme. In the presence of... 

The photochemistries of the mixed ligand cf complexes Rh(NH3)5X (X = Cl, Br, I), Rh(NH3)5L (L=CH3CN, py) and Ru(NH3)5L (L=N2, py, H2O) have been widely studied. In contrast to the photochemistry of cobalt(III) complexes where both substitution and redox reactions are observed, the photochemistry of the analogous rhodium(III) complexes results in substitution ... 

Coordinated phosphines can undergo intramolecular redox reactions. For example, the hydroperoxorhodium complex, [RhCl(OOH)(acac)(PPh3)2l, was obtained from a deoxygenation reaction carried out in chloroform in the presence of excess triphenylphosphine. The hydrooxorhodium intermediate, (Rh(OH)Cl(acac)(PPh3)2l, reacts with CHCI3 to form the rhodium dichloride product. Scheme 9.14 ... 

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