Push–pull substitution versus intrinsic or packing related N–N gauche preferences in azines. Synthesis, crystal structures and packing of asymmetrical acetophenone azines

The asymmetrical E,E-configured para-disubstituted 4-methoxyacetophenone azines with 4′-bromo-(1), 4′cyano-(2) and 4′-nitro-(3) substituents have been synthesized and their crystal structures have been determined. The synergetic reinforcement of ‘push’ and ‘pull’ effects does not suffice to overcome the intrinsic or packing induced N–N gauche preference, and azines 1–3 all assume distinctly gauche N–N conformations (dihedral angle of 115–140°). The crystal packing is characterized by offset T-shaped and parallel displaced face-to-face arene-arene contacts between pairs of azines with parallel or anti-parallel D→A orientations. This crystal architecture results in a net dipole of crystals of 1. The structural data are analysed in comparison with the symmetrical E,E-configured para-disubstituted acetophenone azines 4–7. With regard to this best possible reference data set, the structural parameters of 1–3 do not show any significant manifestation of special electronic interactions over the N-perturbed extended π-system to be associated with the asymmetry of the azines. While there exists no structural evidence of conjugation in these solid state structures, our results do not rule out asymmetry effects on the electronic structure of the ground or the excited states.