Wilkinson's catalyst

Wilkinson's catalyst, chemically known as chloridotris(triphenylphosphine)rhodium(I), is a coordination compound with the formula RhCl(PPh₃)₃. It is a homogeneous catalyst used in the hydrogenation of alkenes. The catalyst was first synthesized by the chemist Geoffrey Wilkinson, a Nobel Laureate, and its development marked a significant advancement in organometallic chemistry and homogeneous catalysis.

Structure and Properties

Wilkinson's catalyst is a square planar complex consisting of a rhodium center coordinated to three triphenylphosphine (PPh₃) ligands and one chloride ion. This configuration provides the catalyst with its characteristic reactivity and selectivity in chemical reactions. The compound is soluble in organic solvents such as benzene, chloroform, and ethanol, which facilitates its use in homogeneous catalysis.

Synthesis

The synthesis of Wilkinson's catalyst involves the reaction of rhodium(III) chloride with an excess of triphenylphosphine in a boiling alcohol solution. The reaction proceeds via the reduction of rhodium(III) to rhodium(I) and the formation of the complex:

RhCl₃·3H₂O + 3 PPh₃ → RhCl(PPh₃)₃ + 3 H₂O + 2 HCl

Mechanism of Action

In the hydrogenation of alkenes, Wilkinson's catalyst facilitates the addition of hydrogen (H₂) across the carbon-carbon double bond. The mechanism involves oxidative addition of H₂ to the rhodium(I) center to form a rhodium(III) dihydride species. This is followed by coordination of the alkene to the metal center, insertion of the alkene into one of the Rh-H bonds (hydrometalation), and reductive elimination of the alkane from the metal. The cycle is completed when the catalyst is regenerated.

Applications

Wilkinson's catalyst is primarily used in the hydrogenation of alkenes to alkanes. It exhibits high selectivity, allowing for the hydrogenation of terminal alkenes in the presence of internal alkenes. Additionally, it has been employed in the hydrogenation of other unsaturated compounds, including aldehydes, ketones, and nitriles. Its ability to selectively hydrogenate less reactive bonds under mild conditions makes it a valuable tool in organic synthesis.

Safety and Environmental Considerations

As with many organometallic compounds, Wilkinson's catalyst should be handled with care. It is sensitive to air and moisture, requiring storage under an inert atmosphere. Proper safety equipment, such as gloves and eye protection, should be used when handling the compound. Disposal should follow regulations for hazardous waste, considering its potential environmental impact.

See Also

• Homogeneous catalysis
• Hydrogenation
• Organometallic chemistry
• Rhodium