Rhodium(III) chloride

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Rhodium(III) chloride
Other names

Rhodium trichloride

Identifiers
CAS number 10049-07-7 YesY, 13569-65-8 (trihydrate) YesY[ICSC]
PubChem 24872
ChemSpider 8096382 YesY
EC number 233-165-4
RTECS number VI9290000
Jmol-3D images {{#if:[Rh+3].[Cl-].[Cl-].[Cl-]|Image 1
Properties
Molecular formula RhCl3
Molar mass 209.26 g/mol
Appearance dark red solid
deliquescent
Density 5.38 g/cm3, solid
Melting point ca. 450 °C
Boiling point 717 °C
Solubility in water insoluble
Solubility soluble in hydroxide and cyanide solutions, also soluble in aqua regia
Acidity (pKa) acidic in solution
Structure
Crystal structure Monoclinic, mS16
Space group C12/m1, No. 12
Coordination
geometry		 octahedral
Thermochemistry
Std enthalpy of
formation ΔfHo298		 −234 kJ/mol
Hazards
EU Index Not listed
Flash point Non-flammable
Related compounds
Other anions Rhodium(III) fluoride
Rhodium(III) bromide
Rhodium(III) iodide
Other cations Cobalt(II) chloride
Iridium(III) chloride
Related compounds Ruthenium(III) chloride
Palladium(II) chloride
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Rhodium(III) chloride refers to inorganic compounds with the formula RhCl3(H2O)n, where n varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh(III) centres. Depending on the value of n, the material is either a dense brown solid or a soluble reddish salt. The soluble salt is widely used to prepare compounds used in homogeneous catalysis.[1]

Structures

Hydrated rhodium trichloride usually refers to compound with the approximate formula RhCl3(H2O)3.[2] 103Rh NMR spectroscopy shows that solutions of this material consist of several species, the proportions of which change with time and depend on the concentration of chloride. The relative distribution of these species determines the colour of the solutions, which can range from yellow (the hexaaquo ion) to "raspberry-red". Some of these species are [Rh(H2O)6]3+, [RhCl(H2O)5]2+, cis- and trans-[RhCl2(H2O)4]+, and [RhCl3(H2O)3].[3] Individual ions have been separated by ion exchange chromatography.[4]

Anhydrous rhodium chloride crystallises with a monoclinic crystal structure, same as in YCl3 or AlCl3.[5] It is a dense brown solid that is insoluble in common solvents and of little value in the laboratory.

Preparation

RhCl3(H2O)3 is produced from salts such as Na3RhCl6, the latter being obtained in the purification of rhodium from the other platinum group metals such as platinum and iridium. The sodium salt is converted to H3RhCl6 by ion exchange chromatography. Recrystallization of this acidic salt from water affords the hydrated trichloride, sometimes called "soluble rhodium trichloride."[6] Anhydrous RhCl3 is prepared by reaction of chlorine with rhodium sponge at 200–300 °C.[7] Above 800 °C, the anhydrous chloride reverts to Rh metal and chlorine.[6]

Coordination complexes

RhCl3(H2O)3 is the precursor to a wide variety of complexes, some of which are commercially useful. Complexes of Rh(III) are generally kinetically inert with octahedral geometry. Rh(I) derivatives tend to adopt square planar molecular geometry or be five-coordinate.

Amines and pyridine

Solutions of RhCl3(H2O)3 react with ammonia to give salts of the pentammine chloride [RhCl(NH3)5]2+. Zinc reduction of this cation followed by the addition of sulfate gives the colourless hydride complex [RhH(NH3)5]SO4.[8]

Upon boiling in a mixture of ethanol and pyridine (py), hydrated rhodium trichloride converts to trans-[RhCl2(py)4)]Cl. The reducing influence of the ethanol is apparent because the corresponding reaction in water affords fac-RhCl3(pyridine)3, analogous to the thioether derivatives. Oxidation of aqueous ethanolic solution of pyridine and RhCl3(H2O)3 by air affords blue paramagnetic[Cl(py)4Rh-O2-Rh(py)4Cl]5+.[9]

Thioethers and tertiary phosphines

Ethanolic solutions of hydrated rhodium trichloride react with dialkyl sulfides.

RhCl3(H2O)3 + 3 SR2 → RhCl3(SR2)3 + 3 H2O

Both fac and mer stereoisomers of such compounds have been isolated.[5]

Reaction of RhCl3(H2O)3 under mild conditions with tertiary phosphines affords adducts akin to the aforementioned thioether complexes. When these reactions are conducted in boiling ethanol solution, reduction of Rh occurs, leading to Rh(I) derivatives such as RhCl(PPh3)3, Wilkinson's catalyst:[10]

RhCl3(H2O)3 + 3 PPh3 + CH3CH2OH → RhCl(PPh3)3 + CH3CHO + 2 HCl + 3 H2O

Alkenes and carbon monoxide

Reaction of RhCl3(H2O)3 with olefins affords compounds of the type Rh2Cl2(alkene)4. With 1,5-cyclooctadiene, RhCl3(H2O)3 react in ethanol to give cyclooctadiene rhodium chloride dimer.[11]

RhCl3(H2O)3 reacts with 1 bar of carbon monoxide to produce the dicarbonyldichlororhodate(I) anion, [RhCl2(CO)2]−. Treatment of solid RhCl3(H2O)3 with flowing CO gives [RhCl(CO)2]2.[12]

Structure of cyclooctadiene rhodium chloride dimer, which is prepared from hydrated rhodium trichloride.

Numerous Rh-CO-PR3 (R = organic group) compounds have been prepared in the course of extensive investigations on hydroformylation catalysis. RhCl(PPh3)3 reacts with CO to give trans-RhCl(CO)(PPh3)2, stoichiometrically analogous to but less reactive than Vaska's complex. Trans-RhCl(CO)(PPh3)2 reacts with a mixture of NaBH4 and PPh3 to give RhH(CO)(PPh3)3, a highly active catalyst for hydroformylation.[13]

Catalysis

Beginning especially in the 1960s, RhCl3(H2O)3 was demonstrated to be catalytically active for a variety of reactions involving CO, H2, and alkenes.[14] For example, RhCl3(H2O)3 was shown to dimerise ethene to a mixture of cis and trans 2-butene:

2 C2H4 → CH3-CH=CH-CH3

Unfortunately this reaction fails for higher alkenes.

Ethylene dimerization was shown to involve catalysis by Rh2Cl2(C2H4)4. This and many related discoveries nurtured the then young field of homogeneous catalysis, wherein the catalysts are dissolved in the medium with the substrate. Previous to this era, most metal catalysts were "heterogeneous", i.e. the catalysts were solids and the substrates were either liquid or gases. Another advance in homogeneous catalysis was the finding that PPh3-derived complexes were active catalytically as well as soluble in organic solvents.[13] Best known of the phosphine-supported catalysts is RhCl(PPh3)3,:[14] which catalyzes the hydrogenation and isomerization of alkenes. The hydroformylation of alkenes is catalyzed by the related RhH(CO)(PPh3)3. Catalysis by rhodium is so efficient that it has significantly displaced the previous technology based on less expensive cobalt catalysts.

Safety

Rhodium(III) chloride is not listed under Annex I of Directive 67/548/EEC, but is usually classified as harmful, R22: Harmful if swallowed. Some Rh compounds have been investigated as anti-cancer drugs. It is listed in the inventory of the Toxic Substances Control Act (TSCA).

References

  1. ↑ Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Ed. ed.). Oxford: Butterworth-Heinemann. ISBN 0-7506-3365-4. 
  2. ↑ Canterford, J. H.; & Colton, R. (1968). Halides of the Second and Third Row Transition Metals. London: Wiley-Interscience. 
  3. ↑ Christopher Carr, Julius Glaser, Magnus Sandström "103Rh NMR chemical shifts of all ten [RhCln(OH2)6−n]3−n complexes in aqueous solution" Inorganica Chimica Acta, 1987, vol. 131, pp. 153–156. doi:10.1016/S0020-1693(00)96016-X
  4. ↑ W. C. Wolsey, C. A. Reynolds, J. Kleinberg "Complexes in the Rhodium(II1)-Chloride System in Acid Solution" Inorg. Chem. 1963, vol. 2, p. 463. doi:10.1021/ic50007a009
  5. ↑ 5.0 5.1 Cotton, S. A. (1997). Chemistry of the Precious Metals. Chapman&Hall. ISBN 0-7514-0413-6. 
  6. ↑ 6.0 6.1 Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1587.
  7. ↑ Hermann Renner et al. "Platinum Group Metals and Compounds" in Ullmann's Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a21_075
  8. ↑ J. A. Osborn, K. Thomas, G. Wilkinson "Pentaamminechlororhodium (III) Dichloride and Pentaamminehydridorhodium (III) Sulfate" Inorganic Syntheses, 1972, Volume 13, pp. 213–215. doi:10.1002/9780470132449.ch43
  9. ↑ R. D. Gillard, G. Wilkinson, "trans-Dichlorotetra(pyridine)rhodium (III) Salts" Inorganic Syntheses, 1967, Volume 10, pp. 64–67. doi:10.1002/9780470132418.ch11
  10. ↑ J. A. Osborn, G. Wilkinson "Tris(Triphenylphosphine) Halorhodium (I)" Inorganic Syntheses 1967 vol. 10, p. 67–71. doi:10.1002/9780470132418.ch12
  11. ↑ G. Giordano, R. H. Crabtree, "Di-μ-chloro-bis(η4-1,5-cyclooctadiene)dirhodium(I)" Inorganic Syntheses, 1990, vol. 28, pp. 88–90. doi:10.1002/9780470132500.ch50.
  12. ↑ J. A. McCleverty, G. Wilkinson, "Dichlorotetracarbonyldirhodium: (Rhodium Carbonyl Chloride)" Inorganic Syntheses, 1966, Volume 8, p. 211–214.doi:10.1002/9780470132395.ch56
  13. ↑ 13.0 13.1 Hartwig, J. F. Organotransition Metal Chemistry, from Bonding to Catalysis; University Science Books: New York, 2010. ISBN 1-891389-53-X
  14. ↑ 14.0 14.1 Bennett, M. A.; & Longstaff, P. A. (1965). "Complexes of Rhodium(I) with Triphenylphosphine". Chem. Ind. (London) 846.

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