Wilkinson's catalyst

Wilkinson's catalyst

Wilkinson's catalyst
Wilkinson's Catalyst
Names
IUPAC names (SP-4)chloridotris(triphenylphosphane) rhodium
Other names Rhodium(I) tris- (triphenylphosphine) chloride, Wilkinson's catalyst, Tris(triphenylphosphine)- rhodium chloride
Identifiers
CAS Number	14694-95-2
EC Number	238-744-5
Jmol 3D model	Interactive image
PubChem	84599
RTECS number	none
SMILES [Rh+](P(C1=CC=CC=C1)(C2=CC=CC=C2)C3=CC=CC=C3)(P(C4=CC=CC=C4)(C5=CC=CC=C5)C6=CC=CC=C6)P(C7=CC=CC=C7)(C8=CC=CC=C8)C9=CC=CC=C9.[Cl-]
Properties
Chemical formula	C54H45ClP3Rh
Molar mass	925.22 g/mol
Appearance	red solid
Melting point	245 to 250 °C (473 to 482 °F; 518 to 523 K)
Solubility in water	insoluble in water
Solubility in other solvents	benzene
Structure
Coordination geometry	square planar
Vapor pressure	{{{value}}}
Related compounds
Related compounds	triphenylphosphine Pd(PPh3)4 IrCl(CO)[P(C6H5)3]2
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Wilkinson's catalyst is the common name for chlorotris(triphenylphosphine)rhodium(I), a coordination compound with the formula RhCl(PPh₃)₃ (Ph = phenyl). It is named after the chemist and Nobel Laureate, Sir Geoffrey Wilkinson who popularized its use.

Structure and basic properties

The compound is a square planar, 16-electron complex. It is usually obtained in the form of a red-violet crystalline solid from the reaction of rhodium(III) chloride with excess triphenylphosphine. The synthesis is conducted in refluxing ethanol which helps with the reduction. Triphenylphosphine serves as the reducing agent yielding triphenylphosphine oxide.

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

Catalytic applications

Wilkinson's catalyst catalyzes the hydrogenation of alkenes. 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, intramolecular hydride transfer (olefin insertion), and reductive elimination results in extrusion of the alkane product, e.g.:

File:RevisedCatCycle.png

Other applications of Wilkinson's catalyst includes the catalytic hydroboration of alkenes with catecholborane and pinacolborane, and the selective 1,4-reduction of α, β-unsaturated carbonyl compounds in concert with triethylsilane. When the triphenylphosphine ligands are replaced by chiral phosphines (e.g., chiraphos, DIPAMP, DIOP), the catalyst becomes chiral and converts prochiral alkenes into enantiomerically enriched alkanes via the process called asymmetric hydrogenation.

Other reactions of RhCl(PPh₃)₃

RhCl(PPh₃)₃ reacts with CO to give trans-RhCl(CO)(PPh₃)₂, which is structurally analogous to Vaska's complex (but much less reactive). 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 species Rh₂Cl₂(PPh₃)₄. This conversion further demonstrates the lability of the triphenylphosphine ligands.

Catalytic improvements

The addition of a Barton’s base such as 2-tert-Butyl-1,1,3,3-tetramethylguanidine poses dramatic implications on the classic Rh-(III)-mediated pathway of hydrogenations with Wilkinson's catalyst and H₂. Following the customary initial oxidative addition, a barrierless reductive elimination of HCl from the traditional Rh-(III)-H₂ intermediates instantly produces Rh-(I)-H remarkably reactive species. This mechanistic event translates into a notable boost on the performance of Wilkinson's catalyst on the hydrogenation of several alkenes and internal alkynes.

See also

• Rhodium-catalyzed hydrogenation