Magnetic Anisotropy: Structural Correlation of a Series of Chromium(II)-Amidinate Complexes.

Systematic substituent variations on amidinate ligands bring delicate changes of CrN coordination in a family of chromium(II) complexes with the common formula of Cr(RNC(CH)NR), where R = Pr (), Cy (), Dipp (Dipp = 2, 6-diisopropylphenyl) (), and Bu (). With the largest substituent group, shows the largest distortion of the N coordination geometry from square-planar to seesaw shape, which leads to its field-induced single-molecule magnet (SMM) behavior. This is an indication that has the strongest axial magnetic anisotropy and/or optimized magnetic relaxation process. Combined with high-frequency/field electron paramagnetic resonance (HF-EPR) experiments and calculations, we deduce that the smallest energy gap between ground Ψ and the first excited Ψ orbitals in contributes the most to its strongest magnetic anisotropy. Moreover, the lower value of ensures its being a field-induced SMM. Specifically, the and values were found to be correlated to the dihedral angle between the ΔNCrN and ΔNCrN triangles, indicating that distortion from ideal square-planar geometry to the seesaw help increase axial magnetic anisotropy and suppress the transversal part. Thus, the study on this system not only expands the family of Cr(II)-based SMMs but also contributes to a deeper understanding of magneto-structural correlation in four-coordinate Cr(II) SMMs.