Liaison hydrogène des arylamines : compétition des sites π et N

An IR study, in the region of OH stretching, of a reference hydrogen-bond donor, 4-fluorophenol, hydrogen bonded to primary, secondary, and tertiary arylamines differently substituted on the ring and on the nitrogen, shows the formation of two kinds of 1:1 complexes in CCl 4 solution: an OH···π and an OH···N hydrogen-bonded complex. The IR method gives only access to a global complexation constant K t . A method is proposed for separating K t into a K π component for hydrogen bonding to the π system and a K N component for hydrogen bonding to the nitrogen atom. This method is validated by comparing the estimated K π and K N values to theoretically calculated descriptors of basicity: the nitrogen lone pair orientation towards the aromatic ring, the molecular electrostatic potentials around the nitrogen and the π cloud, and the enthalpy of hydrogen bonding of hydrogen fluoride with the π system of selected arylamines. The main electronic and steric factors governing the competition between π and N sites are analysed. The strongest π and N bases among the arylamines are julolidine and Tröger's base, respectively. Triphenylamine and diphenylamine, which are nitrogen Brønsted bases, become π bases in hydrogen bonding. Moreover, there is no correlation between the pK HB and the pK BH+ scales of basicity of arylamines. The use of the pK BH+ scale is therefore not recommended in hydrogen-bonding studies.