Coordination and Supramolecular Chemistry of New bis-Bidentate Shiff-Base Ligands

The coordination chemistry of a series of bis-bidentate Schiffbase ligands with NiII and CuII has been investigated. The ligands contain two N,O-bidentate chelating salicylaldiminato units attached to polymethylene or alpha,alpha'-ortho-xylidene central spacers, through ether linkages at the 3-position of the salicyl moieties. In this paper we present a series of seven new complexes displaying several novel structural types, three of which are crystallographically characterized. Among others, the double-stranded helicates with the metal ions in the trans-square-planar geometry are of particular interest. The ortho-xylidene ligand H2L, like the analogue H2L previously reported by us, forms dinuclear double-stranded helicates stable both in the solid state and in solution; in the case of the ligands H2L, the nuclearity and the structural motif of the related complexes depend on the number (n) of methylene units in the bridge. When n = 3, 4, thermodynamically stable products from their reaction with NiII and CuII are oligomeric or polymeric species, while under kinetic control novel supramolecular architectures become accessible. The NiII complexes of the ligands with n = 5, 6, 8 exist as dinuclear double-stranded helicates in the solid state, while in solution they exhibit a monomer-dimer interconversion process, which has been studied using Diffusion Ordered Spectroscopy (DOSY).