Tributylphosphine

Tributylphosphine,^[1] formula P(C₄H₉)₃ or PBu₃, is a tertiary phosphine, most commonly encountered as a ligand in transition metal complexes. It its an oily liquid at room temperature, with a nauseating odour. It reacts slowly with atmospheric oxygen, and rapidly with other oxidizing agents, to give the corresponding phosphine oxide. It is usually handled using air-free techniques.

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Preparation

Tributylphosphine is prepared industrially by the addition of phosphine to butene: the addition proceeds by a free radical mechanism, and so the Markovnikov rule is not followed.

PH₃ + 3CH₂=CHCH₂CH₃ → P(CH₂CH₂CH₂CH₃)₃

Tributylphosphine can be prepared in the laboratory by reaction of the appropriate Grignard reagent with phosphorus trichloride although, as it is commercially available at reasonable prices, it is rare to have to perform the small-scale preparation.

3BuMgCl + PCl₃ → PBu₃ + 3MgCl₂

Use

Tributylphosphine finds some industrial use as a catalyst modifier in the cobalt-catalyzed hydroformylation of alkenes, where it greatly increases the ratio of straight-chain aldehydes to branched-chain aldehydes in the product mixture.^[2] However, tricyclohexylphosphine is even more effective for this purpose (although more expensive) and, in any case, rhodium catalysts are usually preferred to cobalt catalysts for the hydroformylation of alkenes.

Tributylphosphine is also a common ligand for the preparation of complexes of transition metals in relatively low oxidation states, as it is cheap and less air-sensitive. It has a conveniently low volatility, which makes it easier to handle than other trialkylphosphines. Although its complexes are generally highly soluble, they are often more difficult to crystallize compared to complexes of more rigid phosphines. Furthermore, the 1H NMR properties are less easily interpreted and can mask signals for other ligands. Compared to other tertiary phosphines, it is relatively compact (cone angle: 136°) and relatively basic (χ-parameter: 5.25 cm^–1)^[3]

Hazards

The main laboratory inconvenience of tributylphosphine is its extremely strong and unpleasant smell. All manipulations must be carried out in a efficient fume hood, and glassware which has come into contact with the compound must be decontaminated before leaving the hood. Because tributylphosphine must be protected from the atmosphere, it is usually handled using syringe techniques. The manipulation of large quantities requires specific precautions to prevent the release of the vapour into the environment. For transport purposes, it is classified as a "spontaneously flammable liquid" (group 4.2), although this is rarely a significant problem in the laboratory use of small quantities: it may not be transported by air.^[4]

References

1. ^ Tributylphosphine is the most widely used name for this compound. However Iupac nomenclature prefers the name tributylphosphane, which is also the U.S. proper shipping name (49 CFR 172.101).
2. ^ Bell, P.; Rupilus, W.; & Asinger, F. (1968). "Zur Frage der Isomerenbildung bei der Hydroformylierung Höhermolekularer Olefine mit Komplexen Kobalt- und Rhodiumkatalysatoren". Tetrahedron Lett.: 3262–66.
3. ^ Rahman, M. M.; Liu, H. Y.; Prock, A.; & Giering, W. P. (1987). "Steric and Electronic Factors influencing Transition-Metal–Phosphorus(III) Bonding". Organometallics 6: 650–58.
4. ^ 49 CFR 172.101.