Rhodium hydroxide

The proposed mechanism (Scheme 54) indicated that transmetallation of the aryl silanediol to the rhodium hydroxide catalyst followed by 1,4-addition and hydrolysis of the O-bound enolate generated the addition product and regenerated the Rh-OH catalyst. 

The base of the series, chloro-pentammino-rhodium hydroxide,... 

Rh (NHo) 5C1] (OH) 2, is produced by mixing the chloride with moist silver oxide. Silver chloride is precipitated, and a strongly alkaline liquid produced containing the hydroxide. The base absorbs carbon dioxide from the air, removes ammonia from ammonium salts, and precipitates metallic hydroxides from solutions of the metallic salts. It is only known in solution, and on evaporation of the liquid it is slowly transformed in the cold, more rapidly on heating, into a mixture of aquo-pentammino-rhodium chloride and aquo-pentammino-rhodium hydroxide, thus ... 

Iodo-pentammino-rhodium Iodide, [Rh(NH3)jI]I2, is best prepared by warming aquo-pentammino-rhodium hydroxide at 100° C. with excess of hydriodic acid. It forms rhombic crystals wiiich are bright orange-yellow in colour, sparingly soluble in water and insoluble in hydriodic acid and in alcohol.2... 

Aquo-pentammino-rhodium Nitrate, [Rh(NH3)5H20](N03)3, is prepared by the addition of nitric acid to a concentrated solution of aquo-pentammino-rhodium hydroxide, when the salt is precipitated in microscopic quadratic plates. It is easily soluble in water, and the dry salt loses water at 100° C., being converted into nitrato-pentammino-rhodium nitrate thus ... 

The bromide, [Rh(NH3)5H20]Br3, is formed when concentrated hydrobromic acid is added to a solution of aquo-pentammino-rhodium hydroxide. It crystallises in small oetahedra or six-sided plates. At 100° C. it loses water and passes into bromo-pentammino-rhodium bromide. It is easily soluble in water. 

Dichloro-tetrapyridino-rhodium Hydroxide, [Rh py4Cl2](OH), may be obtained from the chloride by grinding it with freshly precipitated silver oxide. The liquid so produced absorbs carbon dioxide from the air and liberates ammonia from ammonium salts. 

Alkalies yield a yellowish brown precipitate of rhodium hydroxide, Rh203.3H20, on boiling. This precipitate is soluble in excess of the reagent (contrast ruthenium). 

Rhodium hydroxide is also effective in ring saturation of the hydrogenolysis-sensitive hydroquinone dimethyl or diphenylether . Use of Rh-on-carbon as catalyst permits hydrogenation of naphthols to decalols [equation (d)] and of tetralones to decalols [equation (e)] without hydrogenolysis of alkoxy substituents. ... 

Aromatic nitriles hydrogenate to primary amine over palladium-on-carbon in ethanol-HCl or acetic acid" finely divided Ni (Raney Ni) in ethanol/NH3 [equation (f)]. Rhodium hydroxide and (7 3) rhodium-platinum oxide inhibited by LiOH convert ben-zonitrile to benzylamine, and isophthalonitrile or terephthalonitrile to the corresponding diamines [equation (g)]. ... 

Aromatic nitriles can give mixtures of the primary amine and dibenzylamines (via the condensation reaction), but this depends on the catalyst. Rhodium hydroxide [Rh(OH)3] is very effective for production... 

According to Krauss and Umbach, attempts to prepare rhodiiun sulfate from rhodium hydroxide and sulfuric acid lead to two different products, depending on the conditions these are yellow rhodium sulfate Rh3(S04)3 15 HgO and red rhodium sulfate Rhs(S04)3 4 HgO. 

The yellow sulfate is produced on solution of moist hydrated rhodium (III) oxide (rhodium hydroxide) in dilute (1 10) sulfuric acid at temperatures not exceeding 50 °C. Then the hydrated Rh (in) oxide is precipitated from the cold solution with KOH (avoiding an excess of the latter) and washed on a membrane filter until the colloidal hydroxide passes through. Suction is then applied and as much water as possible is removed the residue is dissolved without heating in dilute sulfuric acid. 

Similar 7i-allyl intermediates were proposed in Rh-catalyzed conjugate addition of phosphido groups to enones [87-89]. SUylphosphine substrates were suggested to generate Rh-phosphido intermediates from a Rh-OH catalyst precursor. Nucleophilic attack at the alkene, accompanied by formation of a Rh-7t-oxyallyl group, could be followed by protonolysis to yield the phosphine-ketone and regenerate a rhodium hydroxide. To differentiate this mechanism, in which only the Rh(l) oxidation state is involved, from one in which P-Si oxidative addition to yield Rh... 

Rh supported on silica is prepared by the cationic exchange technique, using an aqueous solution of chloropentamine rhodium hydroxide RhCl(NH3)5(OH)2, following the procedure already described [93]. 

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