Two- and three-dimensional packing diagrams of M(salophen) complexes

For better understanding of two- (2D) and three-dimensional (3D) crystal engineering of metal complexes, the 2D and 3D packing diagrams of metal salophen (=N,N'-o-phenylene-bis (salicylideneimine)) complexes are compared in detail. According to the 3D structures, determined by X-ray crystallography, N,N'-(o-phenylene)bis(4-hexyloxysalicylideneiminato)Cu(II) (1) and N,N'-(o-phenylene)bis(4-decyloxysalicylidene-iminato)Ni(II) (2) have a square-planar coordination geometry. 1 forms a host-guest complex with CH2Cl2 solvent molecule via intermolecular hydrogen bonds. In 2 the interdigitating alkyl chains allow the molecules to be packed in parallel sheets with a zigzag-type pattern while in 1 the molecules are packed in coplanar layers. In both structures the layers are connected by short {M}center dot center dot center dot H contacts (3.07 angstrom in 1, 3.15 angstrom in 2). The molecular packings in planes of the 3D structures are compared with previously determined 2D surface patterns of five equivalent Cu(II), Ni(II) and Co(II) complexes formed at the liquid-solid interface. The structures were studied at the interface by means of scanning tunneling microscopy. It is shown that in the case of 2 the 2D structures observed in the plane of a bulk crystal and at the liquid-solid interface are not comparable. As shown, even the metal ion and the alkyl chain length affect differently on the 2D and 3D packing. The role of molecular planarity on the packing diagrams is addressed by comparing the nearly planar single crystal structures of salophen complexes 1 and 2 to the structure of N,N'-ethylene-bis(4-octyloxysalicylideneiminato)Cu(II) which has a twisted salen moiety.