Dipolar and calorimetric study of the complexes between N,N-dimethyl-N′-phenylformamidines and phenols

The dipole moments µb of N,N-dimethyl-N′-phenylformamidine and its 4-methylphenyl and 4-chorophenyl derivatives and the dipole moments µab of some 20 complexes of these bases with phenols were determined in cyclohexane at 298 K. Assuming the additivity of the bond moments, the angles αb between µb and the axis C1C4 phenyl group were computed for the three bases. Dipole increments [Formula: see text] were then computed assuming the hydrogen bond involved the electron pair of the sp2 hybridized imino nitrogen atom. The variation of the Δµ computed in this way with the complexation enthalpy ΔHab is described by the same curve as that for the systems phenols–anilines and phenols–pyridines. This is not the case when other structures are chosen for computing Δµ and this provides evidence for the 'imino' structure of the complexes. For a given amidine, the dipole increment increases when the proton donor becomes more acidic. The derivative d Δµ/dpKa here is of the order of −0.5 D per pKa unit whereas this value is only −0.2 for the pyridines. The absolute values of Δµ are also greater for the amidines. This can be ascribed in part to a higher proportion of H-bonds of the ion pair type [Formula: see text] when the amidine is complexed by the same phenol as the pyridines.