Magnetic Anisotropy and Relaxation of Pseudotetrahedral [N2O2] Bis-Chelate Cobalt(II) Single-Ion Magnets Controlled by Dihedral Twist Through Solvomorphism

The methanol solvomorph 1 center dot 2MeOH of the cobalt(II) complex [Co(L-Sal,L-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 center dot 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 center dot 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.