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 (H₂L), with Cu(II) or Co(II), affords a dinuclear [Cu₂L₂] complex (1) or a trinuclear [Co₃L₃] 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-C₃ 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 π-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.