Analysis of Magnetic Anisotropy and the Role of Magnetic Dilution in Triggering Single-Molecule Magnet (SMM) Behavior in a Family of Co^IIY^III Dinuclear Complexes with Easy-Plane Anisotropy

Three new closely related Co^IIY^III complexes of general formula [Co(μ-L)(μ-X)Y(NO₃)₂] (X⁻=NO₃ ⁻ 1, benzoate 2, or 9-anthracenecarboxylato 3) have been prepared with the compartmental ligand N,N′,N′′-trimethyl-N,N′′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H₂L). In these complexes, Co^II and Y^III are triply bridged by two phenoxide groups belonging to the di-deprotonated ligand (L²⁻) and one ancillary anion X⁻. The change of the ancillary bridging group connecting Co^II and Y^III ions induces small differences in the trigonally distorted CoN₃O₃ coordination sphere with a concomitant tuning of the magnetic anisotropy and intermolecular interactions. Direct current magnetic, high-frequency and -field EPR (HFEPR), frequency domain Fourier transform THz electron paramagnetic resonance (FD-FT THz-EPR) measurements, and ab initio theoretical calculations demonstrate that Co^II ions in compounds 1–3 have large and positive D values (≈50 cm⁻¹), which decrease with increasing the distortion of the pseudo-octahedral Co^II coordination sphere. Dynamic ac magnetic susceptibility measurements indicate that compound 1 exhibits field-induced single-molecule magnet (SMM) behavior, whereas compounds 2 and 3 only display this behavior when they are magnetically diluted with diamagnetic Zn^II (Zn/Co=10:1). In view of this, it is always advisable to use magnetically diluted complexes, in which intermolecular interactions and quantum tunneling of magnetism (QTM) would be at least partly suppressed, so that “hidden single-ion magnet (SIM)” behavior could emerge. Field- and temperature-dependence of the relaxation times indicate the prevalence of the Raman process in all these complexes above approximately 3 K.