Hydrogen bonding basicity of amidines

Abstract

The hydrogen bonding basicity of amidines (1)–(20) has been measured by means of the frequency shift Δν(OH) of methanol hydrogen bonded to amidines and of the formation constant K of their complexes with 4-fluorophenol. The electronic and steric effects of substituents on the imino nitrogen have been studied. Hydrogen bonding occurs at the imino nitrogen atom except for acetylformamidine (16) for which it occurs on the carbonyl group. For compounds 4-XC₆H₄NCHNMe₂, with X = NO₂, COMe, or CN, hydrogen bonding also takes place on the NO₂, COMe, or CN groups. From the logK/Δν(OH) correlation, two subsets, phenylformamidines and alkylformamidines, can be distinguished for the family of formamidines. For formamidines, Brönsted basicities correlate with Δν(OH) as for the pyridines.