Kink distortion of the pseudo-S₄ axis in pseudotetrahedral [N₂O₂] bis-chelate cobalt(ii) single-ion magnets leads to increased magnetic anisotropy

Two new mononuclear cobalt(II) complexes with the general formula [Co(L-1,L-2)(2)] (1 and 2) were synthesized using bidentate Schiff base ligands with NO donor set, namely, 2-(benzothiazole-2-ylimino) methyl-5-(diethylamino)phenol (HL1) and its methyl substituted derivative 2-(6-methylbenzothiazole-2-ylimino)methyl-5-(diethylamino)phenol (HL2). X-ray structure analysis reveals a distorted pseudotetrahedral coordination sphere at the cobalt(II) ion, that cannot be described by a simple twisting of the two ligand chelate planes with respect to each other, which would imply a rotation about the pseudo-S4 axis of the complex. Such a pseudo-rotation axis would approximately be colinear with the two vectors defined by the cobalt ion and the two centroids of the chelate ligands, where the angle. between the two vectors would be 180 degrees in an ideal pseudotetrahedral arrangement. For complexes 1 and 2, the observed distortion can be characterized by a significant bending at the cobalt ion with angles. of 163.2 degrees and 167.4 degrees, respectively. Magnetic susceptibility and FD-FT THz-EPR measurements together with ab initio calculations reveal an easy-axis type of anisotropy for both complexes 1 and 2, with a spin-reversal barrier of 58.9 and 60.5 cm(-1), respectively. For both compounds, frequency-dependent ac susceptibility measurements show an out-of-phase susceptibility under applied static fields of 40 and 100 mT, which can be analyzed in terms of Orbach and Raman processes within the observed temperature range.