Magnetic relaxation in cobalt(ii)-based single-ion magnets influenced by distortion of the pseudotetrahedral [N2O2] coordination environment.

The synthesis, structure, and magnetic properties of two mononuclear cobalt(ii) complexes [Co(LSal,2-Ph)2] (1) and [Co(LNph,2-Ph)2] (2) are reported. The utilized sterically demanding Schiff-base ligands HLSal,2-Ph (2-(([1,1'-biphenyl]-2-ylimino)methyl)phenol) and HLNph,2-Ph (1-(([1,1'-biphenyl]-2-ylimino)methyl)naphthalen-2-ol) lead to a strong distortion of the [N2O2] coordination environment in the complexes 1 and 2, which can be primarily attributed to the variation in the dihedral angle between the planes of the two chelate ligands. Magnetic susceptibility and FD-FT THz-EPR measurements as well as ab initio calculations reveal that both complexes exhibit an easy-axis type of anisotropy. For both compounds frequency-dependent ac susceptibility measurements show an out-of-phase susceptibility under applied static fields of 400 and 1000 Oe. A detailed analysis of the underlying relaxation process is given, revealing significant differences in the contributions of Orbach, Raman, and direct processes within the observed temperature range. Fitting of the magnetic data leads to a spin-reversal barrier of 49 cm-1 for 1 at an applied field of 1000 Oe. For 2 the barrier is not well defined by the analysis of the relaxation times and is, therefore, approximated by the experimental barrier derived from FD-FT THz-EPR measurements (62.8 cm-1). The results from ab initio calculations and FD-FT THz-EPR measurements show that the distortion of the coordination sphere in complexes 1 and 2 from the pseudotetrahedral towards a square-planar coordination geometry leads to an increase in both the axial (D) and the rhombic zero-field splitting (E).