Wilkinson's catalyst (chloridotris(triphenylphosphine)rhodium(I)) is a coordination complex of rhodium with the formula [RhCl(PPh3)], where 'Ph' denotes a phenyl group. It is a red-brown colored solid that is soluble in hydrocarbon solvents such as benzene, and more so in tetrahydrofuran or chlorinated solvents such as dichloromethane. The compound is widely used as a catalyst for hydrogenation of alkenes. It is named after chemist and Nobel laureate Sir Geoffrey Wilkinson, who first popularized its use.
Historically, Wilkinson's catalyst has been a paradigm in catalytic studies leading to several advances in the field such as the implementation of some of the first heteronuclear magnetic resonance studies for its structural elucidation in solution (31P),[3]parahydrogen-induced polarization spectroscopy to determine the nature of transient reactive species,[4] or one of the first detailed kinetic investigation by Halpern to elucidate the mechanism.[5] Furthermore, the catalytic and organometallic studies on Wilkinson's catalyst also played a significant role on the subsequent development of cationic Rh- and Ru-based asymmetric hydrogenation transfer catalysts which set the foundations for modern asymmetric catalysis.[6]
Structure and basic properties
According to single crystal X-ray diffraction the compound adopts a slightly distorted square planar structure.[7]
In analyzing the bonding, it is a complex of Rh(I), a d8transition metal ion. From the perspective of the 18-electron rule, the four ligands each provides two electrons, for a total of 16-electrons. As such the compound is coordinatively unsaturated, i.e. susceptible to binding substrates (alkenes and H2). In contrast, IrCl(PPh3)3 undergoes cyclometallation to give HIrCl(PPh3)2(PPh2C6H4), a coordinatively saturated Ir(III) complex that is not catalytically active.[8]
Synthesis
Wilkinson's catalyst is usually obtained by treating rhodium(III) chloride hydrate with an excess of triphenylphosphine in refluxing ethanol.[9][10][1] Triphenylphosphine serves as both a ligand and a two-electron reducing agent that oxidizes itself from oxidation state (III) to (V). In the synthesis, three equivalents of triphenylphosphine become ligands in the product, while the fourth reduces rhodium(III) to rhodium(I).
Wilkinson's catalyst is best known for catalyzing the hydrogenation of olefins with molecular hydrogen.[11][12] The mechanism of this reaction involves the initial dissociation of one or two triphenylphosphine ligands to give 14- or 12-electron complexes, respectively, followed by oxidative addition of H2 to the metal. Subsequent π-complexation of alkene, migratory insertion (intramolecular hydride transfer or olefin insertion), and reductive elimination complete the formation of the alkane product, e.g.:[13]
In terms of their rates of hydrogenation, the degree of substitution on the olefin substrate is the key factor, since the rate-limiting step in the mechanism is the insertion into the olefin which is limited by the severe steric hindrance around the metal center. In practice, terminal and disubstituted alkenes are good substrates, but more hindered alkenes are slower to hydrogenate. The hydrogenation of alkynes is troublesome to control since alkynes tend to be reduced to alkanes, via intermediacy of the cis-alkene.[14] Ethylene reacts with Wilkinson's catalyst to give RhCl(C2H4)(PPh3)2, but it is not a substrate for hydrogenation.[10]
In the presence of strong base and hydrogen, Wilkinson's catalyst forms reactive Rh(I) species with superior catalytic activities on the hydrogenation of internal alkynes and functionalized tri-substituted alkenes.[17]
Upon stirring in benzene solution, RhCl(PPh3)3 converts to the poorly soluble red-colored dimer [RhCl(PPh3)2]2. This conversion further demonstrates the lability of the triphenylphosphine ligands.
In the presence of base, H2, and additional triphenylphosphine, Wilkinson's complex converts to hydridotetrakis(triphenylphosphine)rhodium(I), HRh(PPh3)4. This 18e complex is also an active hydrogenation catalyst.[18]
^Meakin, P.; Jesson, J. P.; Tolman, C. A. (1 May 1972). "Nature of chlorotris(triphenylphosphene)rhodium in solution and its reaction with hydrogen". Journal of the American Chemical Society. 94 (9): 3240–3242. doi:10.1021/ja00764a061. ISSN0002-7863.
^Duckett, Simon B.; Newell, Connie L.; Eisenberg, Richard (1994). "Observation of New Intermediates in Hydrogenation Catalyzed by Wilkinson's Catalyst, RhCl(PPh3)3, Using Parahydrogen-Induced Polarization". Journal of the American Chemical Society. 116 (23): 10548–10556. doi:10.1021/ja00102a023.
^Halpern, Jack (1 January 1981). "Mechanistic aspects of homogeneous catalytic hydrogenation and related processes". Inorganica Chimica Acta. 50: 11–19. doi:10.1016/S0020-1693(00)83716-0.
^Hartwig, John F. (2010). Organotransition metal chemistry- From bonding to Catalysis. University Science Books. ISBN978-1-891389-53-5.
^Bennett, Michael J.; Donaldson, Peter B. (1977). "Crystal and molecular structure of the orange and red allotropes of chlorotris(triphenylphosphine)rhodium(I)". Inorganic Chemistry. 16 (3): 655–660. doi:10.1021/ic50169a033.
^Bennett, M. A.; Milner, D. L. (1969). "Chlorotris(triphenylphosphine)iridium(I) and Related Complexes. Oxidative Addition Reactions and Hydrogen Abstraction from the Coordinated Ligand". Journal of the American Chemical Society. 91 (25): 6983–6994. doi:10.1021/ja01053a016.
^Bennett, M. A.; Longstaff, P. A. (1965). "Complexes of Rhodium(I) with Triphenylphosphine". Chem. Ind. (London). 1965: 846.
^ abOsborn, J. A.; Jardine, F. H.; Young, J. F.; Geoffrey Wilkinson (1966). "The Preparation and Properties of Tris(triphenylphosphine)halogenorhodium(I) and Some Reactions Thereof Including Catalytic Homogeneous Hydrogenation of Olefins and Acetylenes and Their Derivatives". Journal of the Chemical Society A. 1966: 1711–1732. doi:10.1039/J19660001711.
^Arthur Birch; Williamson, D. H. (1976). "Homogeneous Hydrogenation Catalysts in Organic Synthesis". Organic Reactions. 24: 1.
^James, B. R. (1973). Homogeneous Hydrogenation. New York, NY: John Wiley & Sons.
^Evans, D. A.; Fu, G. C.; Hoveyda, A. H. (1988). "Rhodium(I)-catalyzed hydroboration of olefins. The documentation of regio- and stereochemical control in cyclic and acyclic systems". J. Am. Chem. Soc.110 (20): 6917–6918. doi:10.1021/ja00228a068.
^Ojima, I.; Kogure, T. (1972). "Selective reduction of α,β-unsaturated terpene carbonyl compounds using hydrosilane-rhodium(I) complex combinations". Tetrahedron Lett.13 (49): 5035–5038. doi:10.1016/S0040-4039(01)85162-5.
^Perea Buceta, Jesus E.; Fernández, Israel; Heikkinen, Sami; Axenov, Kirill; King, Alistair W. T.; Niemi, Teemu; Nieger, Martin; Leskelä, Markku; Repo, Timo (23 November 2015) [2015]. "Diverting Hydrogenations with Wilkinson's Catalyst towards Highly Reactive Rhodium(I) Species". Angewandte Chemie International Edition. 54 (48): 14321–14325. doi:10.1002/anie.201506216. ISSN1521-3773. PMID26437764.
^Eduardo Peña-Cabrera "Hydridotetrakis(triphenylphosphine)rhodium" Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons. doi:10.1002/047084289X.rh030m
