Rhodium trichloride hydrate

Di-/i-chloro-bis(7)4-1,5-cyclooctadiene)dirhodium(l), [RhCl(l,5-C8Hi2)]2, has been prepared in 60% yield by reducing rhodium trichloride hydrate in the presence of excess olefin in aqueous ethanol.1 In the present preparation the yield has been greatly increased (to 94%). Two related complexes, [RhCl(l,5-C6Hio)]22 and [RhCl(C6H12)2]2, are similarly prepared in high yield from 1,5-hexadiene and 2,3-dimethyl-2-butene, respectively. 

Dimethylformamide (99.8%), 2-methylpropanethiol (99%), triphenylphosphine (99%), and triphenylphosphite (99- -%) were purchased from Aldrich and used as received. Rhodium trichloride hydrate was a generous loan from Engelhard-CLAL, and iridium iodide was purchased from Johnson Matthey as a mixture of M3 and M4 of several formula M3 4. The trisodium salt of tris(3-sulfonatophe-nyl)phospine (TPPTS) was a gift from Hoechst. 

One of the most carefully studied hydrogenations is the one catalyzed by the Rh(I) complex RhCl(PPh3)3, usually known as the Wilkinson catalyst. It was discovered in 1965 and is easily prepared by the reduction of rhodium trichloride hydrate in the presence of triphenylphosphine. 

Raney nickel, 13, 54, 76, 109, 119, 172, 337,401,502-503, 651,667 Reductive acetylation, 4 Reductive methylation, 528 Reformatsky reaction, 293, 674-675 Retro Diels-Aldet reaction, 379 Retro Michael cleavage, 284 Rhodium on alumina, 503 Rhodium on carbon, 503 Rhodium carbonyl, 504 Rhodium trichloride Silica, 504 Rhodium trichloride hydrate, 504 Ring expansion, 162-163, 194, 252-253,... 

Apparently, no colloids are formed when polyethylene glycol is added to rhodium trichloride hydrate to obtain a water-soluble rhodium polyethylene glycolate highly active in the hydroformylation of heavy alkenes such as dodec-l-ene [75]. Such a system does not require the presence of other ligands and works efficiently at 70-120 bar. Besides a hypothesis in which it is envisioned a migration of the catalyst into the organic phase, a model where the reaction takes place in the interphase, due to the presence of polyethylene glycol, is favored. 

Rhodium trichloride trihydrate (1.00 g, 3.80 mmol) was dissolved in water (5.0 ml) with heating (70°C). A solution of triphenylphosphine (1.95 g, 7.43 mmol) in acetone (25.0 ml) was then added under a nitrogen atmosphere in the course of 20 min. After 10 min hydrazine hydrate (1.90 ml 39.09 mmol) was added with stirring and the mixture was heated at reflux temperature for 3 hours, then kept at 45°C for a further 1 hour. The crystalline solid was filtered off under nitrogen and washed with a little acetone and then with diethyl ether. 1.05 g of an orange-coloured solid were obtained. 

Under an atmosphere of argon, a mixture of 7.5 mg of rhodium trichloride, 30.0 mg of tris-(hexylphenyl)-phosphine, 3 ml of acetone and 15 ml of hydrazine hydrate is heated with stirring and reflux cooling for 4 hours. 

Soluble Rhodium Trichloride, RhCls. HaO, may be prepared by dissolving the hydrated sesquioxide in concentrated hydrochloric acid and evaporating. The product is not quite pure on account of the presence of alkali in the sesquioxide. Consequently it is advisable to extract with alcohol, which dissolves the rhodium salt, filter, evaporate, and recrystallise from water. 

The product is hydrated rhodium trichloride, which, according to Claus 6 contains eight molecules of water. Leidie,1 on the other hand, concluded that the amount of water varies and does not correspond to any definite hydrate. It is an amorphous, briek-red, deliquescent substance which, on heating to 90-95° C., still retains four to five molecules of water and two of hydrogen chloride. At 100° C. it loses water and hydrogen chloride simultaneously, and at 175-180° C. it is completely dehydrated. At 360° C. it becomes insoluble in water, hut it is most... 

Finally several complexes can be prepared by utilizing the ligand to reduce hydrated rhodium trichloride in homogeneous solution (equation 25).71,73 75,s3,84... 

The yellow diethyl phenylphosphonite complex is also known. It can be prepared directly from hydrated rhodium trichloride (equation 71).195... 

The second complex of this type has been prepared directly from hydrated rhodium trichloride (equation 76).216 Later work has shown that the addition of formaldehyde is superfluous.217 An alternative preparation uses [BH4]- as the reducing agent (equation 77).218... 

Many large tertiary phosphines fail to reduce hydrated rhodium trichloride and form rhodium(I) complexes. Under mild conditions they usually reduce the salt to paramagnetic rhodium(II) complexes. This was first discovered when P(o-C6H4Me)3 was employed, as in equation (92).26S... 

Emerald green crystals of rrans-[RhCl2(PR3)2] can be obtained from the tertiary phosphine and hydrated rhodium trichloride at room temperature. The NMR spectrum is degraded and multiple peaks from vRh. c, in the far IR spectrum suggest that the product is a trans rhodium(II) complex.272... 

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... 

When hydrated rhodium trichloride is allowed to react with (78 Y = OMe, NMe Z = As), complexes of empirical formula [RhCl3(78)2] are obtained. The orange-red dimethylaminophos-phine complex has been shown to have the meridional structure (79), 957-959 However, one chloro ligand is labile and can be replaced in the inner coordination sphere by the previously uncoordinated nitrogen atom of the second neutral ligand. This process is enhanced by allowing the complex to react with metal salts whose anions are of low coordinating power (Scheme 37).957... 

Complexes of this type can be prepared in three principal ways. The most obvious is by allowing hydrated rhodium trichloride to react with the bidentate ligand (equation 231).229 Unfortunately this method gives rise to both cis and trans products. However, the reactions between rhodium(III) halides and l,2-bis(diphenylphosphino)benzene (90) yield the trans product in the case of the chloride, whilst both the bromide and iodide form the cis product.1037... 

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... 

If nitrosyl bromide is allowed to react with hydrated rhodium trichloride and PPh3 in ethanol, the mixed chlorobromo complex is obtained (equation 301).13 3 When [RhCl(PPh3)3] is the substrate the dibromo complex is formed (equation 302). This complex can also be obtained from rhodium tribromide, JV-methyl-JV-nitrosotoluene-p-sulfonamide and PPh3. Replacement of PPh3 by other tertiary phosphines permits analogous dibromo complexes to be prepared (cf. equation 299). Another source of the triphenylphosphine complex is [Rh(NO)2Br] . The diiodo complex can be obtained from a similar reaction mixture containing Lil (equation 304).1293... 

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. 

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