Conformational flexibility and role of aromatic-aromatic interactions in the crystal packing of the coordination compounds of some novel quadridentate Schiff bases

The synthesis and characterisation of two novel quadridentate Schiff base ligands N,N'- ethylenebis(1,5-diphenyl-4-iminopentan-2-one) (2a) and N,N'-ethylenebis[1,5-di(1-naphthyl)-4-iminopentan- 2-one] (2b) and five of their Ni(II)-, Co(II)- and Cu(II) complexes are described. The crystal structures of 2a and complexes 3-7 were established by single-crystal X-ray diffraction. Although the coordination geometries of 3-7 were found to be similar, the conformations of the aromatic side chains differed dramatically: very small changes in the coordination geometries, together with the high flexibility of the ligands, as confirmed by quantum mechanical ab initio calculations, seem to lead to remarkable conformational differences as well as to differences in the intermolecular aromatic-aromatic interactions and packing in the crystals.