Organometallic chemistry is the chemistry of compounds which contain a metal carbon bond. Research interest in this area is largely fueled by potential applications of organometallic compounds as catalysts in industrial chemistry. A catalyst is defined as a substance that accelerates the rate of achieving chemical equilibrium, and which can be recovered 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. In this experiment you will prepare the most successful homogeneous organometallic catalyst to date, RhCl(PPh3)3, which catalyzes the hydrogenation of olefins. Interestingly, the catalytic discovery was made independently and nearly simultaneously by Wilkinson and Coffey in 1964.2 Wilkinson fully explored the scope, selectivity, and mechanism by 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 or a Tolman loop, Figure 1. While a detailed discussion of the underlying mechanism is beyond the scope of this lab, a few general comments are worthwhile. Note how RhCl(PPh3)3 does not appear in the catalytic cycle of Figure 1. Wilkinson's catalyst is not a catalyst but, rather a catalyst precursor! The actual catalyst is believed to be the solvento complex, (S)RhCl(PPH3)2. The problem of 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.
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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
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 reductive-elimination reaction. In this experiment, you will observe the above reaction by a color change and will actually characterize a fluxional five-coordinate species which forms in a subsequent step.
Rhodium (III) chloride hydrate (CAS No. 20765-98-4) is harmful if swallowed, inhaled, or absorbed through the skin. ORL-RAT LD50: 1302 mg/kg.
Triphenylphosphine (CAS No. 603-35-0) is a mild lachrymator and can cause skin irritation. ORL-RAT LD50: 700 mg/kg.
Hydrogen (CAS No. 1333-74-0) is an explosive gas. There must be no open flames when hydrogen is in use. It is a non breathable gas, so care should be exercised.
A. Synthesis and Characterization of Wilkinson's Catalyst.
RhCl3 . 3 H2O + P(C6H5) 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.
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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.
Current Research Efforts
Organometallic chemistry is central to a significant fraction of academic inorganic research.3 Some important specific examples of organometallic catalysts include: 1) Hydroformylation, the reaction of an olefin with CO and H2 in the presence of a metal carbonyl to form aldehydes; 2) Wacker Process, an olefin is oxidized to an aldehyde or ketone in the presence of a soluble palladium salt; 3) Ziegler Natta Process, olefins are polymerized using an organoaluminum-titanium catalyst to form stereoregular polymers; and 4) Fischer-Tropsch Reactions, the reductive polymerization of CO to form straightchain hydrocarbons, olefins and alcohols. The role inorganic chemists often play in this field is to prepare new homogeneous catalysts and develop mechanistic models for their formation and reactivity. Homogeneous catalysts offer the possibility of understanding more complex heterogeneous catalytic systems on a molecular level. A growing area of research is the study of main group and lanthanide organometallic chemistry as opposed to the transition metal chemistry explored in this experiment.
Organometallics find practical uses in stoichiometric and catalytic processes, especially processes involving carbon monoxide and alkene-derived polymers. All the world's polyethylene and polypropylene are produced via organometallic catalysts, usually heterogeneously via Ziegler-Natta catalysis. Acetic acid is produced via metal carbonyl catalysts in the Monsanto process and Cativa process. Most synthetic aldehydes are produced via hydroformylation. The bulk of the synthetic alcohols, at least those larger than ethanol, are produced by hydrogenation of hydroformylation derived aldehydes. Similarly, the Wacker process is used in the oxidation of ethylene to acetaldehyde.
Organolithium, organomagnesium, and organoaluminium compounds are highly basic and highly reducing. They catalyze many polymerization reactions, but are also useful stoichiometrically.
III-V semiconductors are produced from trimethylgallium, trimethylindium, trimethylaluminum and related nitrogen / phosphorus / arsenic / antimony compounds. These volatile compounds are decomposed along with ammonia, arsine, phosphine and related hydrides on a heated substrate via metalorganic vapor phase epitaxy (MOVPE) process for applications such as light emitting diodes (LEDs).
Organometallic compounds may be found in the environment and some, such as organo-lead and organo mercury compounds are a toxic hazard.
Concepts and techniques
As in other areas of chemistry, electron counting is useful for organizing organometallic chemistry. The 18-electron rule is helpful in predicting the stabilities of metal carbonyls and related compounds. Chemical bonding and reactivity in organometallic compounds is often discussed from the perspective of the isolobal principle.
NMR and infrared spectroscopy are common techniques used to determine structure. The dynamic properties of organometallic compounds is often probed with variable-temperature NMR and chemical kinetics.
Organometallic compounds undergo several important reactions:
oxidative addition and reductive elimination
organometallic substitution reaction
carbon-hydrogen bond activation