Magnetic Anisotropy and Relaxation of Pseudotetrahedral [N 2 O 2 ] Bis‐Chelate Cobalt(II) Single‐Ion Magnets Controlled by Dihedral Twist Through Solvomorphism

The methanol solvomorph 1  ⋅ 2MeOH of the cobalt(II) complex [Co(L Sal,2−Ph ) 2 ] ( 1 ) with the sterically demanding Schiff‐base ligand 2‐(([1,1′‐biphenyl]‐2‐ylimino)methyl)phenol (HL Sal,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.