Metal-dependent assembly of a helical-[Co3L3] cluster versus a meso-[Cu2L2] cluster with O,N,N',O'-Schiff base ligand: structures and magnetic properties.

Self-assembly of a tetradentate ligand, N, N'-bi(salicylidene)-2,6-pyridinediamine (H 2L), with Cu(II) or Co(II), affords a dinuclear [Cu 2L 2] complex ( 1) or a trinuclear [Co 3L 3] complex ( 2), which were characterized by the single crystal X-ray diffraction study. The coordination geometry of the Cu (II) centers in 1 is between square planar and tetrahedral, with the ligand adopting a cis-cis conformation to give a centrally symmetric structure, which can be regarded as a mesocate. However, the coordination geometry of Co (II) centers in 2 is distortedly tetrahedral, and the ligand adopts a cis-trans conformation. The whole complex of 2 is of a pseudo- C 3 symmatrical, torus-like structure, which can be regarded as a circular helicate. Both the mesocate and the helicate exhibit expanded supramolecular structures due to elaborate intercomplex pi-stacking interactions. These two complexes were also characterized by element analysis, IR spectra, and TGA. To verify the stability of 2, ESI-MS was carried out on both the crystal and the powdered samples. Variable temperature magnetic susceptibility measurements reveal that both 1 and 2 display antiferromagnetic properties. DFT calculations were carried out on 1 to verify the antiferromagnetic coupling between intracluster metal centers.