Symmetry control and magnetic exchange coupling in SMMs based on Co(ii) complexes

Abstract

This work reports on a mononuclear trigonal prismatic complex [Co^IIL](ClO₄)₂ (1) and a trinuclear linear phenoxido-bridged complex [(Co^IIL1)₂Co^II] (2), where L and H₃L1 are N₆ and N₃O₃ tripodal pro-ligands derived from the respective condensation of the cis,cis-1,3,5-tiaminocyclohexane with either 1-methylimidazol-2-carbaldehyde or salicylaldehyde. These compounds have been studied by X-ray single-crystal diffraction, dc and ac magnetism, High-Frequency and -Field Electron Paramagnetic Resonance spectroscopy (HFEPR), Far Infrared Magnetic Spectroscopy (FIRMS) and quantum chemical calculations. The results obtained for 1 show that N₆-tripodal Schiff-base ligands incorporating 1-methylimidazole donors yield trigonal-prismatic mononuclear complexes that typically display C₃ symmetry and exhibit very large, purely easy-axis magnetic anisotropy. In fact, complex 1 exhibits an energy gap between the ground and the first excited state (2D) of 228 cm⁻¹ (directly measured by FIRMS), that can be considered the maximum limit for the easy-axis magnetic anisotropy in this type of complexes. The symmetry-driven large 2D value, together with the parallel alignment of the anisotropy axes, reduces QTM (Quantum Tunnelling of Magnetization) and yields mononuclear single-molecule magnet (MSMM) behaviour, with the observation of magnetic relaxation through a Raman process and open hysteresis at zero field. In compound 2, the combination of the strong easy-axis anisotropy of the local Co^II ions, collinearity of the local anisotropy axes and sizable intramolecular magnetic exchange interactions between the Co^II ions in triple phenoxido-bridged linear trinuclear complexes causes the full quenching of the QTM and the observation of SMM behaviour with open hysteresis at zero field. Theoretical calculations point out that the magnetic interaction between the ground Kramers doublets (KDs) of the local Co^II ions generates four KDs and the magnetic relaxation occurs through the first excited KD via an Orbach process. Supporting this suggestion, the experimental value for the effective thermal energy barrier extracted from FIRMS of 76.8 cm⁻¹ is not too far from the theoretical calculated value of 59.54 cm⁻¹.