Wilkinson’s catalyst is a burgundy-colored solid featuring a central rhodium(I) in a 16-electron, square-planar coordination environment. First reported by Osborn and Wilkinson in 1966,[1] the complex became a centerpiece of their seminal studies on catalytic hydrogenation and functionalization of olefins between 1965 and 1990. Since then, the complex has become a staple catalyst in the hydrogenation of alkenes, dienes, and alkynes (both internal and terminal)[2] at least in part due to its facile synthesis.
Procedure:[3] In a 500mL two-neck round bottom flask with stir bar, RhCl3 trihydrate (2.00g, 7.60mmol) was dissolved in absolute ethanol (70mL). The setup was attached to a reflux condenser on the Schlenk line and purged/sparged with argon to remove headspace/dissolved oxygen. A solution of triphenylphosphine (12.0g, 45.8mmol) in 350mL of hot ethanol (sparged with argon) was added via cannula, and the mixture was refluxed for 2h. The hot mixture was then transferred from the reflux condenser onto a swivel-frit apparatus under argon flow, and subsequently filtered. Solvent was removed in vacuo and the solid was washed with anhydrous ether (50mL). After drying in vacuo, the product was obtained as a burgundy red solid (reported yield: 6.25g, 88%). Product was then transferred into the glovebox and stored in the freezer.
Notes:
- RhCl(PPh3)3 is air sensitive in solution, and should thus be handled appropriately. Some reports say that it degrades slowly in the solid state, other say it is indefinitely air stable in the solid state - Unless you have a few mg to test around, assume air-sensitive. It is soluble in DCM and chloroform, partially soluble in toluene and benzene, insoluble in hexanes and cyclohexane.[2] It reacts with pyridine, DMSO, and acetonitrile to afford RhCl(PPh3)2(L) adducts.
- RhCl(PPh3)3 is known to crystallize in two polymorphs: The classic burgundy red form and the orange form. The latter can form if the reaction is carried out under at higher concentrations and shorter reaction time.[3,4]
- Heating solutions/suspensions of Wilkinson’s catalyst in less polar solvents such as benzene or toluene can lead to precipitation of a pink solid.[3] This is the chloride-bridged dimer [RhCl(PPh3)2]2, which is in equilibrium with the active catalyst (see below). It shouldn’t significantly influence its reactivity.
- Wilkinson’s catalyst is actually the precatalyst, which is believed to lose a phosphine in solution and form a reactive 14-electron species as the active catalyst. The active catalyst and precatalyst exist in equilibrium in solution. A detailed mechanistic investigation has been computationally carried out by Prof. Henry Rzepa and published in his blog.
- 31P NMR of the complex displays two phosphorus signals (dd and dt). A signal for unbound PPh3 may also be found at ca. -5ppm due to ligand association/dissociation equilibrium.
- Wilkinson’s catalyst was allegedly synthesized by accident when Fred Jardine, the doctoral student working under Wilkinson, tried to make a Rh(III)-phosphine adduct by mixing ethanolic solutions of RhCl3 trihydrate and excess triphenylphosphine together.[5] The extra triphenylphosphine acted as a sacrificial reducing agent, precipitating RhCl(PPh3)3, and forming triphenylphosphine oxide in the process. The excess PPh3 is thought to form Ph3PCl2 upon reducing rhodium, subsequently decomposing to Ph3PO after solvolysis. Interestingly, if less bulky trialkylphosphines (PMe3, PEt3) are employed, RhCl3(PR3)3 complexes (fac- and mer-isomers) are obtained.[5] Tributylphosphine gives solely mer-RhCl3(PBu3)3. Using P(o-tolyl)3, PCy3, PtBu2R affords trans-RhCl2(PR3)2. I wonder if PPh3 gave Wilkinson’s catalyst because of its higher crystallinity and lower solubility, which may act as a driving force.
- After the synthesis and early studies on RhCl(PPh3)3, several groups went on to explore derivatives, none of which could be made via the original procedure - PPh3 seems to be the magical ligand. Some accounts are shown below:
- RhCl(PMe3)3: Obtained by first doing a ligand exchange from RhCl(PPh3)3 and excess PMe3 to [Rh(PMe3)4]Cl. This intermediate complex was then heated in toluene for 48h to form the target.[6]
- RhCl(PCy3)3: Neither the target, nor the dimer were formed. However, RhCl(PCy3)2 could be transiently generated in solution.[7]
- RhCl(PR3)3: A report outlining syntheses for Rh complexes with R = Et, Pr, iPr was published. However, its procedures are vague and sound dubious.[8]
- It seems that Wilkinson’s catalyst produced via this protocol contains trace amounts of a paramagnetic impurity, discovered by EPR spectroscopy. It is believed to be rhodium(II) complex, RhCl2(PPh3)2.[1,9]
- Reference 1 is a must-read for comprehensive information on RhCl(PPh3)3. It also describes a protocol for synthesizing RhBr(PPh3)3, which was produced via the standard method followed by halogen exchange with LiBr.[1]
[1] Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. Chem. Soc. A, 1966, 1711-1732.
[2] a) Burgess, K.; van der Donk, W. A.; Jun, C.-H.; Park, Y. J. Chlorotris(triphenylphosphine)-rhodium(I). EROS, 2006. DOI: 10.1002/047084289X.rc162s.pub2. b) Cotton, S. A. Rhodium and iridium. In: Chemistry of Precious Metals. Springer, Dordrecht. 1997. DOI: 10.1007/978-94-009-1463-6_2.
[3] Osborn, J. A.; Wilkinson, G. Mrowca, J. J. Chlorotris(Triphenylphosphine)Rhodium(I)(Wilkinson’s Catalyst). Inorg. Synth. 1990, 28, 77-79.
[4] Cotton, S. Wilkinson’s catalyst. Molecule of the Month July 2013. (accessed 2024-03-30). DOI: 10.6084/m9.figshare.5258596.
[5] Bennett, M. J.; Donaldson, P. B. Inorg. Chem. 1977, 16, 3, 655-660.
[6] Jones, R. A.; Mayor Real, F.; Wilkinson, G.; Galas, A. M. R.; Hursthouse, M. B.; Abdul Malik, K. M. J. Chem. Soc., Dalton Trans. 1980, 511-518.
[7] van Gaal, H. L. M.; Moers, F. G.; Steggerda, J. J. J. Organomet. Chem. 1974, 65, C43-C45.
[8] Intille, G. M. Tertiary Phosphine Complexes of Rhodium(I) and Rhodium(III) Chlorides. Inorg. Chem. 1972, 11, 4, 695-702.
[9] Osborn Dunbar, K. R.; Haefner, S. C. Inorg. Chem. 1992, 31, 3676-3679.