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(L1,2)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 κ between the two vectors would be 180° 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 κ of 163.2° and 167.4°, 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.

Magnetic Anisotropy and Relaxation of Pseudotetrahedral [N2 O2 ] Bis-Chelate Cobalt(II) Single-Ion Magnets Controlled by Dihedral Twist Through Solvomorphism.

The methanol solvomorph 1 ⋅ 2MeOH of the cobalt(II) complex [Co(LSal,2-Ph )2 ] (1) with the sterically demanding Schiff-base ligand 2-(([1,1'-biphenyl]-2-ylimino)methyl)phenol (HLSal,2-Ph ) shows the thus far largest dihedral twist distortion between the two chelate planes compared to an ideal pseudotetrahedral arrangement. The cobalt(II) ion in 1 ⋅ 2MeOH exhibits an easy-axis anisotropy leading to a spin-reversal barrier of 55.3 cm-1 , which corresponds to an increase of about 17 % induced by the larger dihedral twist compared to the solvent-free complex 1. The magnetic relaxation for 1 ⋅ 2MeOH is significantly slower compared to 1. An in-depth frequency-domain Fourier-transform (FD-FT) THz-EPR study not only allowed the direct measurement of the magnetic transition between the two lowest Kramers doublets for the cobalt(II) complexes, but also revealed the presence of spin-phonon coupling. Interestingly, a similar dihedral twist correlation is also observed for a second pair of cobalt(II)-based solvomorphs, which could be benchmarked by FD-FT THz-EPR.