Rhodium(III) chloride

Rhodium(III) chloride usually refers to hydrated rhodium trichloride, a molecular compound with the formula RhCl₃(H₂O)₃. Another prominent rhodium chloride is RhCl₃, a polymeric solid with the AlCl₃ structure. Inexperienced workers sometimes confuse the two rhodium chlorides, but their behavior is completely different. Most chemistry ascribed to "rhodium trichloride" requires the use of the hydrated form. Some procedures calling for a rhodium chloride imply the use of Na₃RhCl₆, which is also a molecular, hence reactive, form of Rh(III).

Rhodium(III) chlorides are the products of the separation of rhodium from the other platinum group metals.

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Properties

RhCl₃(H₂O)_x exists as dark red diamagnetic crystals. It is mildly hygroscopic. It is soluble in water to give reddish solutions that, depending on the age of the solution, contain varying proportions of RhCl₃(H₂O)₃, [RhCl₂(H₂O)₄]⁺, and [RhCl(H₂O)₅]²⁺.

Preparation

RhCl₃(H₂O)₃ is produced by the action of hydrochloric acid on hydrated rhodium(III) oxide. RhCl₃(H₂O)₃ can be crystallized from a solution in concentrated hydrochloric acid. This method helps to remove nitrogen-containing impurities.

RhCl₃ is prepared by reaction of chlorine with rhodium sponge at 200-300°C. The corresponding reaction in molten sodium chloride affords Na₃RhCl₆.

Coordination complexes

RhCl₃(H₂O)₃ is used to prepare a variety of complexes, as illustrated below. The Rh(III) complexes are generally kinetically inert with octahedral geometry. Rh(I) derivatives tend to be square planar.

Ammines

Ethanol solutions of RhCl₃(H₂O)₃ react with ammonia to give the pentammine chloride [RhCl(NH₃)₅]²⁺. Zinc reduction of this cation followed by the addition of sulfate gives the colourless hydride[[RhH(NH₃)₅]SO₄. Note that complexes of NH₃ are generally referred to as "ammines".

Thioethers

Ethanolic solutions of RhCl₃(H₂O)₃ 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.

Tertiary phosphines

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, one obtains Rh(I) derivatives such as RhCl(PPh₃)₃, Wilkinson's catalyst. In this case, ethanol probably serves as the reducing agent, affording acetaldehyde.

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

Alternatively, PPh₃/H₂O could be the reductant, affording OPPh₃ and HCl.

Pyridine

Upon boiling in a mixture of ethanol and pyridine (py), RhCl₃(H₂O)₃ gives 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-O₂Rh(py)₄Cl]⁵⁺.

Alkenes

Reaction of RhCl₃(H₂O)₃ with olefins affords compounds of the type Rh₂Cl₂(alkene)₄. Most commonly, dialkenes are employed in this reaction, such as norbornadiene and 1,5-cyclooctadiene. Illustrative of its high reactivity of its alkene complexes, when 1,3-cyclooctadiene is treated with RhCl₃(H₂O)₃ in ethanol, one obtains the 1,5-cyclooctadiene complex. The diolefin ligands can be removed by decomplexation using cyanide.

Carbon monoxide

Stirring a methanol solution of RhCl₃(H₂O)₃ under 1 bar of carbon monoxide produces the dicarbonyldichlororhodate(I) anion, [RhCl₂(CO)₂]⁻. Treatment of solid RhCl₃(H₂O)₃ with flowing CO gives [RhCl(CO)₂]₂, a red solid which in turn dissolves in alcohols to in the presence of chloride to give the aforementioned dichloride.

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. This same compound can be prepared using formaldehyde in place of CO. Trans-RhCl(CO)(PPh₃)₂ reacts with a mixture of NaBH₄ and PPh₃ gives RhH(CO)(PPh₃)₃.

Rhodium and catalysis

Beginning especially in the 1960's, RhCl₃(H₂O)₃ was demonstrated to be catalytically active for a variety of reactions involving CO, H₂, and alkene. 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).

Over the following decades, however, rhodium-based catalysis has emphasized reactions in organic solvents using organic ligands in place of H₂O. Thus, 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.

A significant advance in homogeneous catalysis was the finding that PPh₃-derived complexes were particularly active catalytically as well as soluble in organic solvents. Best known of the phosphine-supported catalysts is RhCl(PPh₃)₃,:^[1] 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. Rhodium is not an essential element.

It is listed in the inventory of the Toxic Substances Control Act (TSCA).

Bibliography

• Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements, 2nd Ed., Oxford: Butterworth-Heinemann. ISBN 0-7506-3365-4. 
• Canterford, J. H.; & Colton, R. (1968). Halides of the Second and Third Row Transition Metals. London: Wiley-Interscience. 
• Cotton, S. A. (1997). Chemistry of the Precious Metals. Chapman&Hall. ISBN 0-7514-0413-6. 

References

1. ^ Bennet, M. A.; & Longstaff, P. A. (1965). Complexes of Rhodium(I) with Triphenylphosphine. Chem. Ind. (London) 846. Collman, J. P.; Hegedus, L.S.; Norton, J. R.; & Finke, R. G. (1987). Principles and Applications of Organotransition Metal Chemistry. Mill Valley (CA):University Science Books. ISBN 0-935702-51-2.