Triphenylphosphine oxide

Triphenylphosphine oxide is the chemical compound with the formula OP(C₆H₅)₃. Often chemists abbreviate the formula by writing Ph₃PO or PPh₃O (Ph = C₆H₅). This white crystalline compound is a common side product in reactions involving triphenylphosphine. It is a popular reagent to induce the crystallizing of chemical compounds.

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Structure and properties

Ph₃PO is tetrahedral molecule related to POCl₃.^[1] The oxygen center is relatively basic. The rigidity of the backbone and the basicity of the oxygen center make this species a popular agent to crystallize otherwise difficult to crystallize molecules. This trick is applicable to molecules that have acidic hydrogen atoms, e.g. phenols.^[2]

As a by-product of organic synthesis

Ph₃PO is a by-product of many useful reactions in organic synthesis including the Wittig, Staudinger, and Mitsunobu reactions. It is also formed when PPh₃Cl₂ is employed to convert alcohols into alkyl chlorides.

Ph₃PCl₂ + ROH → Ph₃PO + HCl + RCl

Triphenylphosphine can be regenerated from the oxide by treatment with trichlorosilane.

Ph₃PO + SiHCl₃ → PPh₃ + 1/n (OSiCl₂)_n + HCl

Coordination chemistry

Ph₃PO is an excellent ligand for "hard" metal centers. A typical complex is the tetrahedral species NiCl₂(OPPh₃)₂.^[3]

Ph₃PO is a common impurity in PPh₃. The oxidation of PPh₃ by oxygen, including air, is catalyzed by many metal ions:

2 PPh₃ + O₂ → 2 Ph₃PO

References

1. ^ D. E. C. Corbridge "Phosphorus: An Outline of its Chemistry, Biochemistry, and Technology" 5th Edition Elsevier: Amsterdam. ISBN 0-444-89307-5.
2. ^ M. C. Etter and P. W. Baures (1988). "Triphenylphosphine oxide as a crystallization aid". J. Am. Chem. Soc. 110 (2): 639-640. doi:10.1021/ja00210a076.
3. ^ D. M. L. Goodgame and M. Goodgame (1965). "Near-Infrared Spectra of Some Pseudotetrahedral Complexes of Cobalt (II) and Nickel(II)". Inorg. Chem. 4 (2): 139-143. doi:10.1021/ic50024a002.

Other reading

• Triphenylphosphine oxide at Chemexper.com