Unusually Distorted Pseudo-Octahedral Coordination Environment Around Co^II from Thioether Schiff Base Ligands in...

The synthesis, structural characterization, and magnetic behavior of a new family of binuclear Co^II-Ln^III complexes of formula [Ln^IIICo^IIL₂(NO₃)₃]·H₂O (Ln = La, 1; Gd, 2; Tb, 3; Dy, 4; Ho, 5; HL = 3-methoxy-N-(2-(methylsulfanyl)phenyl)salicylaldimine) have been reported. The chosen ligand system HL has adjacent soft ONS and hard OO binding pockets ideal for selective coordination of Co^II and 4f metal ions. All the complexes 1-5 exhibit a Co^II center in a highly distorted octahedral geometry with the Ln^III centers in bicapped square-antiprism geometry. The unique distortion about the Co^II center is introduced by the coordination of 4f metal ions in the hard OO coordination site. The distortion is further supported by the presence of -SMe groups giving an S donor atom which owing to its larger size can support large bond distances and angles. The geometry around the Co^II centers is intermediate between meridional and facial geometric isomers. The magnetic properties of these complexes have been investigated by a "full model" approach using CONDON with the experimental magnetochemical analysis revealing ferromagnetic Co-Ln coupling in compounds 2-5. Ab initio calculations on the X-ray crystal structures of 1-5 paint a semiquatitative picture about the contribution of the individual anisotropic centers toward the overall magnetic properties of the compounds. Theoretical analysis predicts 1 and 2 as weak single-ion magnet (SIM) and single-molecule magnet (SMM) respectively with Co^II being solely responsible for the complex anisotropy. In 2, J_CoGd plays a crucial role in preserving the anisotropy contribution of Co by channelizing relaxation via a higher excited exchange doublet. Because of the inefficiency of J_CoTb, J_CoDy, and J_CoHo in quenching single-ion Ln fragment transverse anisotropy and preserving Co^II high axial anisotropy (favoring rhombicity), 3-5 lack SMM behavior.