Organometallic Compounds and Catalysis

Organometallic chemistry is the chemistry of compounds which contain a metal carbon 

bond. A catalyst is defined as a substance that 

accelerates the rate of achieving chemical equilibrium,and which can berecovered unchanged at

the end of a reaction. Catalytic processes can be broadly defined into two categories: 1) 

homogeneous catalysis, a process where the catalysts and reactants remain in the same phase; 

and 2) heterogeneous catalysis, where the reactants and catalysts are in different phases. In most 

heterogeneous catalytic systems the catalyst is in the solid phase and the reactants are liquids or

gases,the homogeneous organometallic catalyst , RhCl(PPh3)3, which catalyzes the hydrogenation of olefins.

Wilkinson fully explored the scope, selectivity, andmechanismby which the complex 

catalyzes the hydrogenation of olefins and the compound is now commonly referred to as 

“Wilkinson’s catalyst.” The major mechanistic features of the reaction sequence can be shown 

by using what is known as a catalytic cycle ora Tolman loop, Figure 1.Wilkinson’s catalyst is not a catalystbut, rather a catalyst precursor! The actual catalyst is 

believed to be the solvento complex, (S)RhCl(PPH3)2. The problemof identifying the true active 

catalyst in catalytic systems is exceedingly difficult. Only through detailed mechanistic studies 

can an experimentalist gain any certainty of the active catalyst. There exist many reports in the 

scientific literature of‘catalysts’ which in reality are not catalysts at all. Often impurities or 

decomposition products catalyze the reactions of interest. 

In the catalytic cycle there are several important steps central to many organometallic reaction mechanisms.  A key example is the reaction of dihydrogen with the solvento complex to form a cis-dihydride species:

RhCl(P(C₆H₅)₃)₂(S) + H₂  cis - RhCl(P(C₆H₅)₃)₂(S)(H)₂

                                           A                                                             B

This reaction is known as an oxidative-addition reaction.  Note that in this chemical transformation, A is bound to only four ligands while B is bound to six.  We call species A a four-coordinate “coordinately unsaturated” compound and B is “coordinately saturated”.  Note also that species A is a Rh(I) complex with 16 total valence electrons and species B is a Rh(III) complex with 18 valence electrons.  Thus in an oxidative-addition reaction the coordination number of the metal changes from four to six and the oxidation state of the metal increases by two.  The reverse of an oxidative-addition reaction is also common and is termed a reductiveelimination reaction.

 A.    Synthesis and Characterization of Wilkinson’s Catalyst.

RhCl₃ ^. 3 H₂O  +  P(C₆H₅)₃    RhCl(P(C₆H₅)₃)₃

Place 5 mL of absolute ethanol in a round-bottom flask equipped with a magnetic stirring bar. Attach a water condenser and place the apparatus in a sand bath on a stirrer hot plate.  Heat the ethanol to just below its boiling point (78 ^oC).  Remove the condenser momentarily, add 150 mg of triphenylphosphine to the hot ethanol and stir until the solid is dissolved.  A small amount of solid may remain at this point.  Remove the condenser once again, add 25 mg of hydrated rhodium(III) chloride to the solution and continue to stir.  Heat the solution to a gentle reflux for ~ 30 minutes.  Bright shiny burgundy-red crystals should form during this time.  Collect the product crystals by suction filtration on a Hirsch funnel while the solution is hot.  Wash the crystals with three 1-mL portions of ether.  Dry the crystals on the filter by continuous suction. Calculate the percentage yield and determine the melting point of the product.  Obtain the IR spectrum and the ¹H NMR spectrum of the compound.  Save the product in a labeled vial.

B.     Absorption of Hydrogen by Wilkinson’s Catalyst.

RhCl(P(C₆H₅)₃)₃  +  H₂    RhCl(P(C₆H₅)₃)₂H₂

Place 25 mg of RhCl(PPh₃)₃ and a stir bar in a small flask fitted with a septum and a needle outlet.  Purge the apparatus with N₂ (rubber tubing and a needle) for 5 minutes.  Add 3 mL of chloroform to a different flask and bubble with H₂ for 10 minutes.  Using a syringe, add the chloroform to the RhCl(PPh₃)₃ with stirring.  Allow the reaction to proceed for 5 minutes.  Concentrate the solution under the flow of H₂ gas.  When the solution is sufficiently concentrated (~0.2 mL) add deoxygenated ether dropwise until precipitation occurs.  Cool the flask in an ice water bath and collect the light yellow crystals by suction filtration using a Hirsch funnel.