Systematic
substituent variations on amidinate ligands bring delicate
changes of CrN4 coordination in a family of chromium(II)
complexes with the common formula of Cr(RNC(CH3)NR)2, where R = iPr (1), Cy (2), Dipp (Dipp = 2, 6-diisopropylphenyl) (3),
and tBu (4). With the largest substituent
group, 4 shows the largest distortion of the N4 coordination geometry from square-planar to seesaw shape, which
leads to its field-induced single-molecule magnet (SMM) behavior.
This is an indication that 4 has the strongest axial
magnetic anisotropy and/or optimized magnetic relaxation process.
Combined with high-frequency/field electron paramagnetic resonance
(HF-EPR) experiments and ab initio calculations,
we deduce that the smallest energy gap between ground 4Ψ0 and the first excited 4Ψ1 orbitals in 4 contributes the most to its strongest
magnetic anisotropy. Moreover, the lower E value
of 4 ensures its being a field-induced SMM. Specifically,
the D and E values were found to
be correlated to the dihedral angle between the ΔN1CrN2 and ΔN3CrN4 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.