Rhodium with oxides

The pattern of iridium halides resembles rhodium, with the higher oxidation states only represented by fluorides. The instability of iridium(IV) halides, compared with stable complexes IrCl4L2 and the ions IrX (X = Cl, Br, I), though unexpected, finds parallels with other metals, such as plutonium. Preparations of the halides include [19]... 

Until recently, well-authenticated cases of the rhodium(II) oxidation state were rare, with the exception of the dinuclear carboxylates. They fall into two main classes, although there are other rhodium(II) complexes ... 

For rhodium the oxidation state + 3 for the metal is normal, although complexes with the metal in the +A and even the -l- 5 state are reported. 

Reduction of aromatic nitro group takes preference to the reduction of the aromatic ring. Under certain conditions, however, even the benzene ring was reduced. Hydrogenation of nitrobenzene over platinum oxide or rhodium-platinum oxide in ethanol yielded aniline while in acetic acid cyclohexylamine was produced [55]. Heating of nitrobenzene with formic acid in the presence of copper at 200° gave a 100% yield of aniline, whereas similar treatment in the presence of nickel afforded 67% of cyclohexylamine [71]. 

Padwa has shown that rhodium-catalyzed oxidation of indolyl carbamate 67 employing either Phl(OAc)2 or Phl=0 follows a path similar to that of the D-aUal carbamate (Scheme 17.26) [95]. In principle, indole attack of the putative rhodium-nitrene generates zwitterion 68, which is trapped subsequently by an exogenous nucleophile. Spiro-oxazolidinone products (for example, 69) are isolated as single diastereomers in yields ranging from 50 to 85%. As an intriguing aside, Padwa has found that certain carbamates react with Phl=0 in the absence of any metal catalyst to furnish oxazoHdinone products. This result may have implications for the mechanism of the rhodium-catalyzed process, although it should be noted that control experiments by Espino and Du Bois confirm the essential role of the metal catalyst for C-H amination [57]. 

Mitsubishi Kasei introduced a process to manufacture isononyl alcohol, an important PVC (polyvinyl chloride) plasticizer, via the hydroformylation of octenes (a mixture of isomers produced by dimerization of the C4 cut of naphtha cracker or FCC processes).95 First a nonmodified rhodium complex exhibiting high activity and selectivity in the formation of the branched aldehyde is used. After the oxo reaction, before separation of the catalyst, triphenylphosphine is added to the reaction mixture and the recovered rhodium-triphenylphosphine is oxidized under controlled conditions. The resulting rhodium-triphenylphosphine oxide with an activity and selectivity similar to those of the original complex, is recycled and used again to produce isononanal. 

Rh compounds exhibit valences of 2, 3, 4, and 6. The tnvalent form is by far the most stable. When Rh is heated in air, it becomes coated with a film of oxide. Rhodium(III) oxide, Rh Os, can be prepared by heating the finely divided metal or its nitrate in air or O2. The rhodium IV) oxide is also known. Rhodium trihydroxide may be precipitated as a yellow compound by adding the stoichiometric amount of KOH to a solution of RhCb. The hydroxide is soluble in adds and excess base. When the freshly precipitated Rh(OH) is dissolved in HC1 at a controlled pH, a yellow solution is first obtained in which the aquochloro complex of Rh behaves as a cation. The hexachlororhodatetHI) anion is formed when the solution is boiled for 1 hour with excess HC1. The solution chemistry of RI1CI3 is often very complex. Two trichlorides of Rh aie known The trichloride formed by high-temperature combination of the elements is a red, crystalline, nonvolatile compound, insoluble in all aads. When Rh is heated in molten NaCl and treated with Clo, Na RJiClg is formed, a soluble salt that forms a hydrate in solution. Rhodium(III) iodide is formed by the addition of KI to a hot solution of tnvalent Rh. 

This possibility of intimate association of rhodium with the aromatic ring suggests further experiments. A logical extension of asymmetric syntheses involving prochir-al reactants is a kinetic resolution with related chiral reactants under similar conditions. In the one case of hydroboration-amination where this has been applied, it has proved to be very effective. The reactant was prepared directly by a Heck reaction on 1,2-dihydronaphthalene, and under the standard conditions of catalytic hydrobora-tion gave >45% of both enantiomerically pure recovered alkene with (after oxidative work-up) the alcohol of opposite hand, mainly as the trans-isomer. This procedure forms a simple and potentially useful route to pharmacologically active substances, demonstrated by the racemic synthesis shown [105] (Scheme 34). 

For propane oxidation, sulfation with SO2 induces an inhibiting effect on monometallic pfatinum catalysts which increases with the amount of sulfur accumulated on the catalyst (Fig 3b). On the other hand, sulfur storage enhances the activity of coimpregnated platinum-rhodium catalysts oxidized before hydrocarbon oxidation. However, it seems that an optimum sulfur storage exists since catalyst activity decreases as the amount of sulfur stored on the sample increases (Fig 3b). We examined also the effect of sulfation on catalyst activity for... 

Zymalkowski and Strippel hydrogenated dihydroxybenzenes with only slight hydrogenolysis over rhodium-platinum oxide in acetic acid at room temperature and 10 MPa H2 and obtained the corresponding cyclohexanediols in 92-96% yields (eq. 11.17).97 The hydrogenations at atmospheric pressure required much longer reaction times, and the yield of cyclohexanediol decreased to 86% with resorcinol. The proportions of cis isomer in the cyclohexanediols obtained at 10 MPa H2 were 81% from catechol, 68% from resorcinol, and 68% from hydroquinone and always greater than those obtained at atmospheric pressure (75,49, and 52%, respectively). 

Metallic iridium is thus obtained together with oxide of iron. The whole is heated to redness with potassium hydrogen sulphate, which removes the iron and any remaining traces of rhodium. The residue is well washed with water, then with chlorine water to remove any traces of gold, and finally with hydrochloric acid to take out any silica which may have accidentally been introduced with the alkalies or have come from the vessels employed. The resulting iridium is calcined with charcoal and melted into an ingot. 

However, the same author obtained tetrahydrofurfuryl alcohol in nearly quantitative yield, with a rhodium-platinum oxide catalyst in ethanol, and also in glacial acetic acid, i.e., without hydrogeno-lysis.207 Nickel skeleton catalysts also yield mainly alcohols as hydrogenolysis products at 175° in a continuous system ... 

The reduction of neutral carbonyls with alkali metal hydroxides or carbonates is an important route to HNCC dianions. In a few instances reduction of rhodium clusters has been reported to proceed further with oxidation of another CO. Examples are the formation of [Rh6(CO)i4] from [Rh6(CO)i5] 170) (Scheme 16) and the reduction of the dicarbide [Rhi2(C)2(CO)24] to give [Rhi2(C)2C023] 213). In the latter case, the unstable paramagnetic intermediate [Rh 12(0)2(00)23] has also been isolated upon slight alteration of the reaction conditions [Eqs. (32) and (33)]. 

A very dangerous fire and moderate explosion hazard when exposed to heat or flame can react vigorously with oxidizing materials. Warning pyrophoric in air. Mixtures with nitrogen oxide explode above 50°C. Violent reaction with zinc + transition metal halides (e.g., cobalt halides, rhodium halides, ruthenium halides). Mixtures with acetic acid + water produce a pyrophoric powder. To fight fire, use water, foam, CO2, dr " chemical. See also CARBONYLS and IRON COMPOUNDS. 

The oxidation of alumina-supported rhodium by oxygen in a temperature range between 280 and 870 K has been studied using calorimetry. The heat of dioxygen adsorption was found to vary only sHghtly with the dispersion of rhodium, with a value of 294 6 kJ moT 294 6 kJ mol [78]. 

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