Basicity of Phosphanes and Diphosphanes in Acetonitrile

Phosphanes are widely used in organic chemistry. Their basicity is important in determining their behavior in solutions of different acidity. We report a study of the basicities of various mono-and diphosphanes in acetonitrile, some of them of significant practical interest (BINAP, BIPHEP). The basicities of the studied phosphanes in acetonitrile are in the pK(a) range of 4-16. From the results, the relationship between the structure and basicity of the phosphanes has been analyzed and compared with that of amines of similar structure. A remarkably stronger change in basicity can be observed in the amines in comparison with the phosphanes when alkyl groups are replaced by aryl groups. This is mainly caused by resonance between the aryl group and the lone pair of the basicity center, which is strong in the case of the nitrogen atom and weak in the case of the phosphorus atom. The results of the basicity measurements of the diphosphanes in this work has led to two tentative conclusions: (1) there is no intramolecular hydrogen bond in protonated diphosphanes with an alkyl backbone, and (2) there may be a weak intramolecular hydrogen bond in protonated diphosphanes with an aromatic backbone.