Zero-Field SMM Behavior Triggered by Magnetic Exchange Interactions and a Collinear Arrangement of Local Anisotropy Axes in a Linear Co ₃ ^II Complex

Abstract

A new linear trinuclear Co(II) ₃ complex with a formula of [{Co(μ-L)} ₂Co] has been prepared by self-assembly of Co(II) ions and the N ₃O ₃-tripodal Schiff base ligand H ₃L, which is obtained from the condensation of 1,1,1-tris(aminomethyl)ethane and salicylaldehyde. Single X-ray diffraction shows that this compound is centrosymmetric with triple-phenolate bridging groups connecting neighboring Co(II) ions, leading to a paddle-wheel-like structure with a pseudo- C ₃ axis lying in the Co–Co–Co direction. The Co(II) ions at both ends of the Co(II) ₃ molecule exhibit distorted trigonal prismatic CoN ₃O ₃ geometry, whereas the Co(II) at the middle presents an elongated trigonal antiprismatic CoO ₆ geometry. The combined analysis of the magnetic data and theoretical calculations reveal strong easy-axis magnetic anisotropy for both types of Co(II) ions (| D| values higher than 115 cm ^–1) with the local anisotropic axes lying on the pseudo- C ₃ axis of the molecule. The magnetic exchange interaction between the middle and ends Co(II) ions, extracted by using either a Hamiltonian accounting for the isotropic magnetic coupling and ZFS or the Lines’ model, was found to be medium to strong and antiferromagnetic in nature, whereas the interaction between the external Co(II) ions is weak antiferromagnetic. Interestingly, the compound exhibits slow relaxation of magnetization and open hysteresis at zero field and therefore SMM behavior. The significant magnetic exchange coupling found for [{Co(μ-L)} ₂Co] is mainly responsible for the quenching of QTM, which combined with the easy-axis local anisotropy of the Co ^II ions and the collinearity of their local anisotropy axes with the pseudo- C ₃ axis favors the observation of SMM behavior at zero field.

Abstract

The simultaneous occurrence of considerable magnetic exchange coupling, strong easy-axis local anisotropy, and collinear arrangement of anisotropic axes in a unique linear Co ₃ ^II complex with triple-phenolate bridging groups leads to the suppression of QTM and the observation of slow magnetic relaxation and open hysteresis under zero field.