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Concept
Wilkinson's catalyst
Summary
Wilkinson's catalyst is the common name for chloridotris(triphenylphosphine)rhodium(I), a coordination complex of rhodium with the formula [RhCl(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 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), 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.
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 d8 transition 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.
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 and a two-electron reducing agent that oxidizes itself from oxidation state (III) to (V).
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