Rhodium oxides hydrated

Meanwhile, Wacker Chemie developed the palladium-copper-catalyzed oxidative hydration of ethylene to acetaldehyde. In 1965 BASF described a high-pressure process for the carbonylation of methanol to acetic acid using an iodide-promoted cobalt catalyst (/, 2), and then in 1968, Paulik and Roth of Monsanto Company announced the discovery of a low-pressure carbonylation of methanol using an iodide-promoted rhodium or iridium catalyst (J). In 1970 Monsanto started up a large plant based on the rhodium catalyst. 

Hydrated rhodium oxide (Rh203.5H20) decomposes [48] in vacuum or in air to yield Rh20j.H20, while in water vapour it seems that the dihydrate is formed. Each of these compounds reacts further to produce the anhydrous oxide, which may be decomposed to the metal at higher temperatures. 

The ubiquitous RhCl3-3H20 appears to contain a variety of aquachloro species, as do solutions prepared by aquation of [RhClg] " or hydrous rhodium oxide . Solutions of hydrated RhCls , treated with base, perchloric acid and then with various amounts of LiCl exhibited Rh NMR signals which can be assigned to the 10 possible isomers of [Rh(H20) Cl4 ] ( = 0-6). ... 

Since 1960, the Hquid-phase oxidation of ethylene has been the process of choice for the manufacture of acetaldehyde. There is, however, stiU some commercial production by the partial oxidation of ethyl alcohol and hydration of acetylene. The economics of the various processes are strongly dependent on the prices of the feedstocks. Acetaldehyde is also formed as a coproduct in the high temperature oxidation of butane. A more recently developed rhodium catalyzed process produces acetaldehyde from synthesis gas as a coproduct with ethyl alcohol and acetic acid (83—94). 

Oxidations of pyridopyrimidines are rare, but the covalent hydrates of the parent compounds undergo oxidation with hydrogen peroxide to yield the corresponding pyridopyrimidin-4(3 T)-ones. Dehydrogenation of dihydropyrido[2,3-(i]pyrimidines by means of palladized charcoal, rhodium on alumina, or 2,3-diehloro-5,6-dicyano-p-benzo-quinone (DDQ) to yield the aromatic derivatives have been reported. Thus, 7-amino-5,6-dihydro-1,3-diethylpyrido[2,3-d]-pyri-midine-2,4(lif,3f/)-dione (177) is aromatized (178) when treated with palladized charcoal in refluxing toluene for 24 hours. 

Elemental composition (Rh203) Rh 81.09%, 0 18.91%. The oxide may be solubilized by treatment with alkali to form hydrated oxide, which may be dissolved in acid and diluted for analysis of rhodium metal by AA or ICP. The oxide may be characterized by x-ray diffraction, physical properties, and reaction with strong alkali to form yellow precipitate of pentahydrate, and in excess alkali a black precipitate of the trihydrate. 

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. 

The reduction of metal ions in higher oxidation states by CO and H20 has been known for many years. Work on the reduction of Hg2+, Ag+, Ni2+, Cu2 +, and Pd2+ has been summarized recently (4). The reduction of these metal ions does not proceed via a stable intermediate carbonyl. Since a metal carbonyl must be an intermediate in this reaction, however, the coordinated carbonyl must be very susceptible to attack by water, reacting as soon as it is formed. The ability of a metal in a higher oxidation state to activate a coordinated carbonyl to attack by as weak a nucleophile as water was noted previously in the description of the work by James et al., on the reduction of rhodium(III) by carbon monoxide and water (62). Here a stable rhodium(III) carbonyl, Rh(CO)Cl2-, can be observed as the initial product of reaction of RhCl3 3HzO with CO. The Rh(III) is then efficiently reduced to the rhodium(I) anion [RhCl2(CO)2], even in nonaqueous solvents such as dimethylacetamide, where the only water available for reaction is the water of hydration of the starting rhodium chloride. 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

In this process (Fig. 1), the reactor contains a rhodium-platinum catalyst (2 to 10% rhodium) as wire gauzes in layers of 10 to 30 sheets at 750 to 920°C, 100 psi, and a contact time of 3 X 10"4 second. After cooling, the product gas enters the absorption tower with water and more air to oxidize the nitric oxide and hydrate it to nitric acid in water. Waste gases contain nitric oxide or nitrogen dioxide, and these are reduced with hydrogen or methane to ammonia or nitrogen gas. Traces of nitrogen oxides can be... 

Hence the rhodium III solvated by lattice oxide ions and presumably extra framework oxide ions or hydroxo ligands (depending on the dehydration state) could be carboxylated reductively to rhodium I dicarbonyl according to one of the following reaction scheme depending on the hydration state... 

The dihydrido complex [RhH2Cl(PPh3)2] is a very important intermediate in the homogeneous catalytic hydrogenation of alkenes.20 The monohydrido complexes (Table 63) can be made by the oxidative addition of HY species to rhodium(I) complexes (equation 187). Similar complexes can be obtained when bulky tertiary phosphines are allowed to react with alcoholic solutions of hydrated rhodium trichloride.268 269... 

The rhodium(III) complexes can be prepared either by oxidative addition to the corresponding rhodium(I) complexes or by direct reaction of the ligands with rhodium(ITI) salts. Normally the reducing properties of tertiary polyphosphines ensure that rhodium(I) complexes are formed hence the rhodium(III) complexes of these ligands have been prepared via oxidative addition reactions. However, the sterically hindered ligand (105) fails to reduce hydrated rhodium trichloride even when allowed to react with the latter in refluxing ethanol (equation 260).235... 

There are three types of rhodium(II) complex. By far the most common are the dimeric carboxylatorhodium(II) species. Octahedral complexes may also be generated by the radiolysis of aqueous solutions of classic rhodium(lll) complexes. Square-planar complexes containing bulky tertiary phosphine ligands can be produced by carehil reduction of hydrated rhodium trichloride. The chemistry of rhodium(ll) differs very considerably from the well-known monomeric octahedral or tetrahedral cobalt(II) species because cobalt(ll) complexes are high-spin (f species while rhodium(II) complexes are all low spin. No spin reorientation is required upon oxidation to rhodium(lll), so monomeric rhodium(II) complexes are excellent reducing agents. 

The tridentate ligands (38)-(40) form rhodium(I) complexes. The complexes of the first two ligands readily undergo oxidative addition to form rhodium(III) complexes. The complex [RhCl(38)] also adds either SO2 or BF3 to form pentacoordinate rhodium(I) complexes. The tetraden-tate ligands (41) (Z = P, As) and (42) and the hexadentate ligand (43) form both rhodium(I) and (III) complexes. By contrast, the tri(tertiary arsine) ligand (44) fails to reduce hydrated rhodium trichloride and forms both fac- and mer-trihalorhodium(in) complexes. 

Binary rhodium(lV) compounds are confined to the purple red tetrafluoride and the black dioxide. The hydrated dioxide may be prepared by oxidizing rhodium(IIl) compounds, either with chlorine or electrochemically. Attempts to dehydrate this material lead to decomposition. No cationic rhodium(IV) complexes have been characterized unambiguously, but both [RhCle] -and [RhFe] are well established. The alkali metal salts of the hexafluororhodate(lV) ion are all isomorphous with their platinate(fV) analogs. 

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