The basicity of phosphines

Abstract

The basicities of the triarylphosphines P(4-XC6H4)3 (X = Cl, F, H, CH3, CH3O, (CH3)2N), P(3-CH3C6H4)3, and P(2-CH3C6H7)3 as well as the trialkylphosphines P(t-Bu)3 and PCy3 have been measured by the nitromethane titration method. The range of basicity available by aryl substitution is very large, being pKa = 8.65 for X = (CH3)2N to 1.03 for X = Cl. The most basic phosphine is P(t-Bu)3 whose pKa = 11.40. The measured basicities correlate well with σp, [Formula: see text], and ν as well as with the lone pair ionisation potentials of the triarylphosphines. Generally the 1H, 31P, and 13C nmr spectral parameters of the free and protonated phosphines do not correlate well with pKa.