Wilkinson's catalyst (chloridotris(triphenylphosphine)rhodium(I)) is a

coordination complex A coordination complex is a chemical compound consisting of a central atom or ion, which is usually metallic and is called the ''coordination centre'', and a surrounding array of chemical bond, bound molecules or ions, that are in turn known as ' ...

rhodium Rhodium is a chemical element; it has symbol Rh and atomic number 45. It is a very rare, silvery-white, hard, corrosion-resistant transition metal. It is a noble metal and a member of the platinum group. It has only one naturally occurring isot ...

with the formula hCl(PPh₃)₃ 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 Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...

or chlorinated solvents such as dichloromethane. The compound is widely used as a catalyst for hydrogenation of

alkene In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...

s. 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), parahydrogen-induced polarization spectroscopy to determine the nature of transient reactive species, or one of the first detailed kinetic investigation by Halpern to elucidate the mechanism. 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.

Structure and basic properties

According to single crystal X-ray diffraction the compound adopts a slightly distorted square planar structure. In analyzing the bonding, it is a complex of Rh(I), a ''d''⁸

transition metal In chemistry, a transition metal (or transition element) is a chemical element in the d-block of the periodic table (groups 3 to 12), though the elements of group 12 (and less often group 3) are sometimes excluded. The lanthanide and actinid ...

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.

Synthesis

Wilkinson's catalyst is usually obtained by treating rhodium(III) chloride hydrate with an excess of triphenylphosphine in refluxing ethanol. Triphenylphosphine serves as both a

ligand In coordination chemistry, a ligand is an ion or molecule with a functional group that binds to a central metal atom to form a coordination complex. The bonding with the metal generally involves formal donation of one or more of the ligand's el ...

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). :RhCl₃(H₂O)₃ + 4 PPh₃ → RhCl(PPh₃)₃ + OPPh₃ + 2 HCl + 2 H₂O

Catalytic applications

Wilkinson's catalyst is best known for catalyzing the

hydrogenation Hydrogenation is a chemical reaction between molecular hydrogen (H2) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to redox, reduce or Saturated ...

of olefins with molecular hydrogen. 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 In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in whi ...

product, e.g.: WilkinsonCycleJMBrown

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.Kevin Burgess, Wilfred van der Donk, Chul-Ho Jun, Young Jun Park, "Chlorotris(triphenylphosphine)-rhodium(I)" Encyclopedia of Reagents for Organic Synthesis 2005 John Wiley & Sons. Ethylene reacts with Wilkinson's catalyst to give RhCl(C₂H₄)(PPh₃)₂, but it is not a substrate for hydrogenation.

Related catalytic processes

Wilkinson's catalyst also catalyzes many other hydrofunctionalization reactions including hydroacylation, hydroboration, and hydrosilylation of alkenes. Hydroborations have been studied with catecholborane and pinacolborane. It is also active for the hydrosilylation of alkenes. 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.

Reactions

RhCl(PPh₃)₃ reacts with

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

to give bis(triphenylphosphine)rhodium carbonyl chloride, ''trans''-RhCl(CO)(PPh₃)₂. The same complex arises from the decarbonylation of aldehydes: :RhCl(PPh₃)₃ + RCHO → RhCl(CO)(PPh₃)₂ + RH + PPh₃ Upon stirring in benzene solution, RhCl(PPh₃)₃ converts to the poorly soluble red-colored dimer hCl(PPh₃)₂sub>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.Eduardo Peña-Cabrera "Hydridotetrakis(triphenylphosphine)rhodium" Encyclopedia of Reagents for Organic Synthesis, 2001, John Wiley & Sons.

References