Rhodium(III) chloride
Rhodium chloride refers to inorganic compounds with the formula RhCl3n, where n varies from 0 to 3. These are diamagnetic solids featuring octahedral Rh centres. Depending on the value of n, the material is either a dense brown solid or a soluble reddish salt. The soluble trihydrated 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 RhCl33 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 to "raspberry-red". Some of these species are 3+, 2+, cis- and trans-+, and . Individual ions have been separated by ion exchange chromatography.Anhydrous rhodium chloride crystallises in the YCl3 and AlCl3 motif. 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
RhCl33 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
RhCl33 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 RhCl33 react with ammonia in the presence of alcohol to give the salt pentamminerhodium chloride, Cl2. Zinc reduction of this cation followed by the addition of sulfate gives the colourless hydride complex SO4.Upon boiling in a mixture of ethanol and pyridine, hydrated rhodium trichloride converts to Dichlorotetrakisrhodium chloride|trans-Cl. In the absence of a reductant, the reaction affords fac-, analogous to the thioether derivatives. Oxidation of aqueous ethanolic solution of pyridine and RhCl33 by air affords a blue paramagnetic oxygen-bridged compound,
Thioethers and tertiary phosphines
ic solutions of hydrated rhodium trichloride react with dialkyl sulfides.Both fac and mer stereoisomers of such compounds have been isolated.
Reaction of RhCl33 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 occurs, leading to rhodium derivatives such as , Wilkinson's catalyst, with oxidation of the solvent or more commonly with an excess of the phosphine:
Alkenes and carbon monoxide
Reaction of RhCl33 with olefins affords compounds of the type Rh2Cl24. With 1,5-cyclooctadiene, RhCl33 react in ethanol to give cyclooctadiene rhodium chloride dimer.RhCl33 in methanol reacts with carbon monoxide to produce H, which contains the dicarbonyldichloridorhodate anion; further carbonylation in the presence of sodium citrate leads to the formation of tetrarhodium dodecacarbonyl, Rh412, a rhodium cluster compound. Treatment of solid RhCl33 with flowing CO gives the dimeric rhodium compoundrhodium carbonyl chloride, 2.
Numerous Rh-CO-PR3 compounds have been prepared in the course of extensive investigations on hydroformylation catalysis. RhCl3 reacts with CO to give trans-RhCl2, stoichiometrically analogous to but less reactive than Vaska's complex. Trans-RhCl2 reacts with a mixture of NaBH4 and PPh3 to give HRh3, 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 in hot methanol leads to the precipitation of the pentamethylcyclopentadienyl rhodium dichloride dimer, 2:
This compound was first prepared from hexamethyl Dewar benzene and RhCl33. 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, CH3CH2, and hexamethylbenzene is produce by a side reaction. This rhodium dimer can be reduced with zinc in the presence of CO to produce the rhodium complex .
Image:Hexamethyl Dewar benzene reacting with rhodium chloride under acidic conditions.PNG|700px|frameless|center|Synthesis of the rhodium dimer 2 from hexamethyl Dewar benzene
Catalysis
Beginning especially in the 1960s, RhCl33 was demonstrated to be catalytically active for a variety of reactions involving CO, H2, and alkenes. For example, RhCl33 was shown to dimerise ethene to a mixture of cis and trans 2-butene:Unfortunately this reaction fails for higher alkenes.
Ethylene dimerization was shown to involve catalysis by the dimeric rhodium compound chlorobisrhodium dimer|. 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, 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 RhH3. Catalysis by rhodium is so efficient that it has significantly displaced the previous technology based on less expensive cobalt catalysts.