Rhodium Chloride

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 trihydrated (n = 3) salt is widely used to prepare compounds used in homogeneous catalysis, notably for the industrial production of acetic acid and hydroformylation.

Structures

Aqueous solutions of RhCl₃(H₂O)₃ have been characterized by ¹⁰³Rh NMR spectroscopy, which shows the presence 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 h(H₂O)₆sup>3+, hCl(H₂O)₅sup>2+, ''cis''- and ''trans''- hCl₂(H₂O)₄sup>+, and hCl₃(H₂O)₃ Individual ions have been separated by ion exchange chromatography.

Anhydrous rhodium chloride crystallises in the YCl₃ and AlCl₃ motif (see image in upper right). The metal centres are octahedral, and the halides are doubly bridging. 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." Anhydrous RhCl₃ is prepared by reaction of chlorine with rhodium sponge metal at 200–300 °C. Above 800 °C, the anhydrous chloride reverts to Rh metal and chlorine.

Various rhodium chloride complexes are intermediates in the purification of rhodium from its ores.

Coordination complexes

RhCl₃(H₂O)₃ is the precursor to a wide variety of complexes, some of which are commercially useful. It reacts with acetylacetone to give rhodium acetylacetonate.

Amines and pyridine

Solutions of RhCl₃(H₂O)₃ react with ammonia in the presence of alcohol to give the salt pentamminerhodium chloride, hCl(NH₃)₅l₂. Zinc reduction of this cation followed by the addition of sulfate gives the colourless hydride complex Rh(NH₃)₅O₄.

Upon boiling in a mixture of ethanol and pyridine (py), hydrated rhodium trichloride converts to ''trans''- hCl₂(py)₄)l. In the absence of a reductant, the reaction affords '' fac''- hCl₃(py)₃ analogous to the thioether derivatives. Oxidation of aqueous ethanolic solution of pyridine and RhCl₃(H₂O)₃ by air affords a blue paramagnetic oxygen-bridged compound,

Thioethers and tertiary phosphines

Ethanolic solutions of hydrated rhodium trichloride react with dialkyl sulfides.

:RhCl₃(H₂O)₃   +   3 SR₂   -> ce/chem>   RhCl₃(SR₂)₃   +   3 H₂O

Both ''fac'' and ''mer'' stereoisomers of such compounds have been isolated.

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 rhodium(III) occurs, leading to rhodium(I) derivatives such as hCl(PPh₃)₃ Wilkinson's catalyst, with oxidation of the solvent or more commonly with an excess of the phosphine:

::RhCl₃(H₂O)₃   +   3 PPh₃   +   CH₃CH₂OH   -> ce \text/chem>   RhCl(PPh₃)₃   +   CH₃CHO   +   2 HCl   +   3 H₂O

::RhCl₃(H₂O)₃   +   4 PPh₃   -> ce \text/chem>   RhCl(PPh₃)₃   +   OPPh₃   +   2 HCl   +   2 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.

RhCl₃(H₂O)₃ in methanol reacts with carbon monoxide (1 bar) to produce H hCl₂(CO)₂ which contains the dicarbonyldichloridorhodate(I) anion; further carbonylation in the presence of sodium citrate leads to the formation of tetrarhodium dodecacarbonyl, Rh₄(CO)₁₂, a rhodium(0) cluster compound. Treatment of solid RhCl₃(H₂O)₃ with flowing CO gives the dimeric rhodium(I) compoundrhodium carbonyl chloride, CO)₂Rh(μ-Cl)sub>2.

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 HRh(CO)(PPh₃)₃, a highly active catalyst for hydroformylation of alkenes.

When treated with cyclopentadienes or its derivatives, organometallic half sandwich compounds can be produced. For example, reacting the trihydrate with pentamethylcyclopentadiene (Cp*H) in hot methanol leads to the precipitation of the pentamethylcyclopentadienyl rhodium dichloride dimer, p*RhCl₂sub>2:

:2 Cp*H   +   2 RhCl₃(H₂O)₃   -> ce \text/chem>   p*RhCl₂sub>2   +   2 HCl   +   6 H₂O

This compound was first prepared from hexamethyl Dewar benzene and RhCl₃(H₂O)₃. The hydrohalic acid necessary for the ring-contraction rearrangement is generated ''in situ'' in methanolic solutions of the rhodium salt, and the second step has been carried out separately, confirming this mechanistic description. The reaction occurs with the formation of 1,1-dimethoxyethane, CH₃CH(OCH₃)₂, and hexamethylbenzene is produced by a side reaction. This rhodium(III) dimer can be reduced with zinc in the presence of CO to produce the rhodium(I) complex p*Rh(CO)₂

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. For example, RhCl₃(H₂O)₃ was shown to dimerise ethene to a mixture of ''cis'' and ''trans'' 2-butene:
:2 C₂H₄ -> ce/chem> CH₃-CH=CH-CH₃
Unfortunately this reaction fails for higher alkenes.

Ethylene dimerization was shown to involve catalysis by the dimeric rhodium(I) compound [(η²-C₂H₄)₂Rh(μ-Cl)₂Rh(η²-C₂H₄)₂]. 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 triphenylphosphine, PPh₃-derived complexes were active catalytically as well as soluble in organic solvents, the best known such catalyst being Wilkinson's catalyst that 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

External links

International Chemical Safety Card 0746

{{Chlorides

Chlorides
Platinum group halides
Rhodium(III) compounds