Rhodium preparation

Metallic rhodium prepared by heating its compounds in hydrogen must be allowed to cool in an inert atmosphere to prevent catalytic ignition of the sorbed hydrogen on exposure to air. 

Active po vder can been produced from molten zinc, less than 100 pm in size, at a rate of 1 kg/h under sonication [74], Dispersed palladium, platinum and rhodium prepared by sonochemical reduction of aqueous salts also exhibit a higher reactivity, assigned to a surface area increase [75],... 

Catalytic hydrogenation is an obvious method for the conversion of cyclobutenes into cyclobutanes. Thus, hydrogenation of a solution of unstable spiro[3.4]octa-l,5,7-triene in pentane at — 40 °C over rhodium [prepared in situ from rhodium(III) chloride by reduction with sodium borohydride24] gave spiro[3.4]octane (9) as the major volatile product in over 90% yield.14... 

In the case of rhodium, however, it was demonstrated early that in the synthesis of [Rh6C(CO)l5]2 the encapsulated carbon atom originated as chloroform, which had reacted with the rhodium carbonyl anion [Rh7(CO)l6]3- (59). In the cobalt analog, [Co6C(CO)l5]2-, the carbon atom is derived indirectly from carbon tetrachloride [via Co3(CO)9CCl] (60) Both these syntheses are performed under mild conditions, and there are apparently no examples of carbidocarbonyl clusters of cobalt or rhodium prepared directly from the metal carbonyls under pyrolysis conditions. 

Occurrence and History of Rhodium—Preparation—Proportion- Colloidal Rhodium—Rhodium Black—Uses—Atoraio Weight —Alloys. 

Stable suspensions of eolloidal rhodium prepared in the presence of hydroxyalkylammo-nium bromide surfactants can be recycled after the hydrogenation of various aromatic derivatives, without loss of activity [74]. 

Water soluble phosphines such as the sodium salt of sulfonated triphenylphos-phine, (m-QH4S03Na)3P, function as stabilizers for hydrosols of colloidal rhodium, prepared by hydrogen reduction of Rha3 3H20 in the presence of the surfactant. [67] The colloidal material has been described as a polyhydroxylated rhodium particle, which implies considerable oxidation of, at least, the rhodium surface to Rh(I). 

Asymmetric hydrogenation has been achieved with dissolved Wilkinson type catalysts (A. J. Birch, 1976 D. Valentine, Jr., 1978 H.B. Kagan, 1978). The (R)- and (S)-[l,l -binaph-thalene]-2,2 -diylblsCdiphenylphosphine] (= binap ) complexes of ruthenium (A. Miyashita, 1980) and rhodium (A. Miyashita, 1984 R. Noyori, 1987) have been prepared as pure atrop-isomers and used for the stereoselective Noyori hydrogenation of a-(acylamino) acrylic acids and, more significantly, -keto carboxylic esters. In the latter reaction enantiomeric excesses of more than 99% are often achieved (see also M. Nakatsuka, 1990, p. 5586). 

MMA and MAA can be produced from ethylene [74-85-1/ as a feedstock via propanol, propionic acid, or methyl propionate as intermediates. Propanal may be prepared by hydroformylation of ethylene over cobalt or rhodium catalysts. The propanal then reacts in the Hquid phase with formaldehyde in the... 

Low Pressure Syntheses. The majority of metal carbonyls are synthesized under high pressures of CO. Early preparations of carbonyls were made under superpressures of 1 GPa (ca 10,000 atm). Numerous reports have appeared in the Hterature concerning low pressure syntheses of metal carbonyls, but the reactions have been restricted primarily to the carbonyls of the transition metals of Groups 8—10 (VIII). A procedure for preparing Mn2(CO)2Q, however, from commercially available methylcyclopentadienyknanganese tricarbonyl [12108-13-3] and atmospheric pressures of CO has been reported (117). The carbonyls of mthenium (118,119), rhodium (120,121), and iridium (122,123) have been synthesized in good yields employing low pressure techniques. In all three cases, very low or even atmospheric pressures of CO effect carbonylation. Examples of successful low pressure syntheses are... 

There are currentiy no commercial producers of C-19 dicarboxyhc acids. During the 1970s BASF and Union Camp Corporation offered developmental products, but they were never commercialized (78). The Northern Regional Research Laboratory (NRRL) carried out extensive studies on preparing C-19 dicarboxyhc acids via hydroformylation using both cobalt catalyst and rhodium complexes as catalysts (78). In addition, the NRRL developed a simplified method to prepare 9-(10)-carboxystearic acid in high yields using a palladium catalyst (79). 

Rhodium. Rhodium is the most commonly plated platinum-group metal. In addition to its decorative uses, rhodium has useful properties for engineering appHcations. It has good corrosion resistance, stable electtical contact resistance, wear resistance, heat resistance, and good reflectivity. The use of rhodium for engineering purposes is covered by an ASTM specification (128). Typical formulas are shown in Table 15. The metal content is obtained from prepared solutions available from proptietary plating supply companies. Replenishment is requited because anodes are not soluble. Rhodium for decorative use may be 0.05—0.13 p.m thick for industtial use, it maybe 0.50—5.0 p.m thick. 

The 17-ethylene ketal of androsta-l,4-diene-3,17-dione is reduced to the 17-ethylene ketal of androst-4-en-3,17-dione in about 75% yield (66% if the product is recrystallized) under the conditions of Procedure 8a (section V). However, metal-ammonia reduction probably is no longer the method of choice for converting 1,4-dien-3-ones to 4-en-3-ones or for preparing 5-en-3-ones (from 4,6-dien-3-ones). The reduction of 1,4-dien-3-ones to 4-en-3-ones appears to be effected most conveniently by hydrogenation in the presence of triphenylphosphine rhodium halide catalysts. Steroidal 5-en-3-ones are best prepared by base catalyzed deconjugation of 4-en-3-ones. ... 

Reduction of the A" -double bond with the rhodium complex is a very slow reaction, but it has been accomplished in 17)S-hydroxyandrost-4-en-3-one (140)d The product, 4a, 5a-d2-androstan-17j3-ol-3-one (141), is a further example of the preferential a-side deuteration in homogeneous solution as contrasted with the )S-face attack with heterogeneous catalysts. [For a more convenient preparation of compound (141) see section V-C.]... 

Trifluoromethyl-substituted diazonium betaines [176]. Synthetic routes to trifluoromethyl-substituted diazo alkanes, such as 2,2,2-trifluorodiazoethane [ 177, 7 78, 179] and alkyl 3,3,3-trifluoro-2-diazopropionates [24], have been developed Rhodium-catalyzed decomposition of 3,3,3-tnfluoro-2-diazopropionates offers a simple preparative route to highly reactive carbene complexes, which have an enormous synthetic potential [24] [3-1-2] Cycloaddition reactions were observed on reaction with nitnles to give 5-alkoxy-4-tnfluoromethyloxazoles [750] (equation 41)... 

The anionic cluster [Ir6(CO)i5] is octahedral and an increasing number of Ir clusters have been reported recently though their preparations are more difficult and yields usually smaller than for rhodium. [Iri4(CO)27] has the highest nuclearity so far and is obtained as black crystals by oxidizing [Ir6(CO)i5] with ferricinium ion (Fig 26.9b). 

Cases of the S-coordinated rhodium and iridium are quite scarce. To complete the picture, we next consider the possibilities of S-coordination using complicated derivatives of thiophene. 2,5-[Bis(2-diphenylphosphino)ethyl]thiophene is known to contain three potential donor sites, two phosphorus atoms and the sulfur heteroatom, the latter being a rather nucleophilic center (93IC5652). A more typical situation is coordination via the phosphorus sites. It is also observed in the product of the reaction of 2,5-bis[3-(diphenylphosphino)propyl]thiophene (L) with the species obtained after treatment of [(cod)Rh(acac)] with perchloric acid (95IC365). Carbonylation of [Rh(cod)L][C104]) thus prepared yields 237. Decarbonylation of 237 gives a mixture of 238 and the S-coordinated species 239. Complete decarbonylation gives 240, where the heterocycle is -coordinated. The cycle of carbonylation decarbonylation is reversible. 

Complex [(CXI )Ir(/j,-pz)(/i,-SBu )(/j,-Ph2PCH2PPh2)Ir(CO)] reacts with iodine to form 202 (X = I) as the typical iridium(II)-iridium(II) symmetrical species [90ICA(178)179]. The terminal iodide ligands can be readily displaced in reactions with silversalts. Thus, 202 (X = I), upon reaction with silver nitrate, produces 202 (X = ONO2). Complex [(OC)Ir(/i,-pz )(/z-SBu )(/i-Ph2PCH2PPh2)Ir(CO)] reacts with mercury dichloride to form 203, traditionally interpreted as the product of oxidative addition to one iridium atom and simultaneous Lewis acid-base interaction with the other. The rhodium /i-pyrazolato derivative is prepared in a similar way. Unexpectedly, the iridium /z-pyrazolato analog in similar conditions produces mercury(I) chloride and forms the dinuclear complex 204. 

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