Rhodium aquo complexes

Fig. 17. Electroenzymatic reduction of 4-phenyI-2-butanone catalyzed by HLADH with in-situ indirect electrochemical regeneration of NADH using a Cp (2,2 -bipyridyl)aquo rhodium(III) complex as mediator...

Chloro(aquo) rhodium(III) octaethylporphyrin reacts with ethyl vinyl ether in the presence of ethanol to give a rhodium(III) porphyrin which is readily hydrolysed to produce a (formylmethyl) rhodium(III) complex (Scheme 4) . 

Preliminary results of the reaction between vanadium(iii)-tetrasulpho-phthalocyanine complex with oxygen have been reported these data were compared with those obtained for the corresponding reaction of the hexa-aquo complex ion. The oxidation of methyl ethyl ketone by oxygen in the presence of Mn"-phenanthroline complexes has been studied Mn " complexes were detected as intermediates in the reaction and the enolic form of the ketone hydroperoxide decomposed in a free-radical mechanism. In the oxidation of 1,3,5-trimethylcyclohexane, transition-metal [Cu", Co", Ni", and Fe"] laurates act as catalysts and whereas in the absence of these complexes there is pronounced hydroperoxide formation, this falls to a low stationary concentration in the presence of these species, the assumption being made that a metal-hydroperoxide complex is the initiator in the radical reaction. In the case of nickel, the presence of such hydroperoxides is considered to stabilise the Ni"02 complex. Ruthenium(i) chloride complexes in dimethylacetamide are active hydrogenation catalysts for olefinic substrates but in the presence of oxygen, the metal ion is oxidised to ruthenium(m), the reaction proceeding stoicheiometrically. Rhodium(i) carbonyl halides have also been shown to catalyse the oxidation of carbon monoxide to carbon dioxide under acidic conditions ... 

Rhodium(III) tetra(/ -sulfonatophenyl)porphyrin [(TSPP)Rh] aquo and hydroxo complexes react with a series of alkenes in water to form /3-hydroxyalkyl coordination compounds. Addition reactions of (TSPP)Rh-OH to unactivated terminal alkenes CH2=CHR invariably occur with both kinetic and thermodynamic preferences to place rhodium on the terminal carbon to form (TSPP)Rh-CH2CH(OH)R complexes. Acrylic and styrenic alkenes initially react to place rhodium on the terminal carbon to form [Rh]-CH2CH(OH)R as the kinetically preferred isomer, but subsequently proceed to an equilibrium distribution of regioisomers where [Rh]-CH(CH2OH)R is the predominant thermodynamic product. Equilibrium constants for reactions of the diaquo rhodium(III) compound [(TSPP)Rhm(H20)2]3 in water with a series of terminal alkenes that form /9-hydroxyalkyl complexes were directly evaluated and used in deriving thermodynamic values for addition of the Rh-OH unit to alkenes. The AG° for reactions of the Rh-OH unit with alkenes in water was found to be approximately 3 kcalmol-1 less favourable than the comparable Rh-H reactions in water.100... 

Steckhan et al. demonstrated convincingly that systems that fulfil these conditions are tris(2,2 -bipyridyl)rhodium complexes [57] and, more effectively, substituted or unsubstituted (2,2 -bipyridyl(pentamethylcyclopentadienyl)chloro or aquo rhodium complexes [58] (see Scheme 1). Electrochemical reduction of these complexes at potentials between —680 mV and —840 mV vs. SCE leads to the formation of hydrido rhodium complexes. Strong catalytic effects observed in cyclic voltammetry and preparative electrolysis indicate a very fast hydride transfer from the hydrido complex to NAD(P) under formation of only 1,4-NAD(P)H and the starting complex as shown in the following reaction scheme [58] ... 

Aqueous chemistry. This chemistry is almost exclusively that of complex compounds. Aquo ions of Ru11, Rum, Rh111 and Pd11 exist, but complex ions are formed in presence of anions other than CIO4, BFJ, or -toluene-sulfonate, etc. The precise nature of many supposedly simple solutions, e.g., of rhodium sulfate, is complicated and often unknown. 

Ruthenium, osmium, rhodium, and iridium form octahedral MClj" chloro complexes which, in aqueous solution, usually exist as chloro-aquo mixed species of the general composition [MCl6 ,(H20)J ( c = 1-6). Ruthenium and osmium also form complexes of the type [MCl6 t(H2P),] (jr = 1-6) and oxygen-bridged complexes such as [(RuCl5)20] . In many cases, complexes of the PGM are kinetically inert [4]. 

More recently, systems based on polypyridine coordination compounds of ruthenium(II) [46-49], rhodium(I) [50a] and iridium(I) [50] have been shown to efficiently catalyse the thermal WGSR. An important effect of the substituent ortho to the nitrogen atom of the ligand has been demonstrated in the case of Ir(I) leading to one of the most efficient catalysts known today [50b]. [Ru(bpy)2(CO)Cl] has also been studied and all of the possible intermediates within the catalytic cycle (hydrocarbonyl complex, metal hydride, aquo species) have been isolated and characterized [48]. 

The [Rh(NH3)5(OH)] cation, and its chromium(III) analog, react very rapidly with sulfur dioxide in aqueous solution, to give O-sulfito complexes, which lose sulfur dioxide on addition of acid. Rate constants for reaction of the rhodium(III), chromium(III), and cobalt(III) complexes [M(NH3)5(OH)] with sulfur dioxide are 1.8 x 10, 2.9 x 10, and 4.7 x 10 dm mol" s", respectively. This great similarity of rates contrasts with the situation in respect of formation of carbonato complexes, where formation rate constants show a marked dependence on the p a of the aquo form of the hydroxo substrate. 

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