Rhodium redox reactions

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. 

Rearrangements of clusters, i.e. changes of cluster shape and increase and decrease of the number of cluster metal atoms, have already been mentioned with pyrolysis reactions and heterometallic cluster synthesis in chapter 2.4. Furthermore, cluster rearrangements can occur under conditions which are similar to those used to form simple clusters, e.g. simple redox reactions interconvert four to fifteen atom rhodium clusters (12,14, 280). Hard-base-induced disproportionation reactions lead to many atom clusters of rhenium (17), ruthenium and osmium (233), iron (108), rhodium (22, 88, 277), and iridium (28). And the interaction of metal carbonyl anions and clusters produces bigger clusters of iron (102, 367), ruthenium, and osmium (249). 

Hydrogen, preadsorbed on noble metals, is commonly used to prepare bimetallic catalysts by redox reaction. This requires the parent metal to chemisorb hydrogen (Pt, Pd, Rh, Ru, etc.) and to introduce a modifier that is reducible by hydrogen (Cu, Re, Ir, Rh, Pd, Pt, Au, etc.). All combinations of these metals have been prepared and characterized. For example, this technique has been used to prepare model Pt-Re reforming catalysts. Also, Pt-Rh and Pd-Rh were preformed to examine the interaction between platinum and rhodium in exhaust gas catalysts [8-10, 15-20, 21]. 

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. 

S. Baral, P. Hambright, A. Harriman, and P. Neta, Radiolytic Studies of the Redox Reactions and Alkylation of Rhodium Tetrakis(4-sulfonatophenyl)porphyrin in Aqueous Solutions, J. Phys. Chem., 89 (1985) 2037. 

The adsorption of probe molecules followed by different techniques allows one to prove the metal-metal interaction for catalysts prepared by redox reactions. For example, by chemisorption measurements, a decrease in the total amount of adsorbed H2 was observed with an increasing germanium content introduced by catalytic reduction on parent rhodium catalysts [81]. As TEM characterization showed a comparable mean particle size for all the catalysts, such evolution suggests that Ge covers the Rh surface [41]. These results were consistent with those... 

In fuel cells, well known catalyst is produced from carbon black-supported Pt particles (Pt/C) for hydrogen and oxygen redox reactions which occurs at anode and cathode but conventional Pt/C catalyst has low durability and can be easily poisoned by carbon monoxide. Electrospun Pt/ruthenium, Pt/rhodium, and Pt nanowires have been produced and compared with Pt/C showing better performance in a proton exchange membrane fuel cell (PEMFC). 

Scheme 52 Rhodium-catalyzed intramolecular redox reaction of alkynyl ethers...

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 ... 

Annulation reactions between unactivated alkynes and a range of heteroatom-containing substrates have facilitated the preparation of a host of heterocycles. For example, the preparation of isoquinolones was achieved through a rhodium-catalyzed annulation of internal acetylenes with 0-methyl hydroxamates (Scheme 3.78 and Example 3.12) [81]. The reaction conditions were quite mild, and most reactions were complete within 16 h. The overall process was redox neutral and used a catalytic amount of cesium acetate to promote the reaction. Additionally, the process was not appreciably sensitive to the electronic composition of the 0-methyl hydroxamates. One of the more attractive aspects of this chemistry was the observation that the rhodium-catalyzed reaction... 

---