Magnetic Anisotropy in CoII X4 (X=O, S, Se) Single-Ion Magnets: Role of Structural Distortions versus Heavy Atom Effect.

Mononuclear four coordinate CoII complexes have drawn a great deal of attention as they often exhibit excellent single-ion magnet (SIM) properties. Among the reported complexes, the axial zero-field splitting parameter (D) was found to vary drastically both in terms of the sign as well as strength. There are various proposals in this respect such as structural distortions, heavier atom substitution, metal-ligand covalency, tuning secondary coordination sphere, etc. that are expected to control the D values. To assess the importance of structural distortions vs. heavier atom substitution effect, here we have undertaken detailed theoretical studies based on the ab initio CASSCF/NEVPT2 method to estimate zero-field splitting parameters for twelve complexes reported in the literature. Our test set includes the {CoII X4 } (where X=O, S, Se) core structure where the D value was found to vary from +19 to -118 cm-1 . Based on the structural variation, we have classified the complexes into three types (I-III) where type I complexes were found to exhibit the largest negative D value as desired for SIMs. The other two types (II and III) of complexes have been found to be inferior with respect to type I. The secondary coordination sphere was also found to influence D, as substitution on the secondary coordination sphere atom was found to significantly alter the magnitude of D values. Particularly, two structural parameters, namely, the dihedral angle between the two ligand planes and the ∠ X-Co-X polar angle were found to heavily influence the sign and strength of D values. Our analysis clearly reveals that these structural factors are much more important than the heavier atom substitution, or metal-ligand covalency. A large variation in the D and E/D values among these complexes despite possessing a very close structural similarity offers an exquisite playground for a chemist to design and develop new-generation CoII -based SIMs.