Magnetic Anisotropy in Co^IIX₄ (X=O, S, Se) Single‐Ion Magnets: Role of Structural Distortions versus Heavy Atom Effect

Abstract

Mononuclearf our coordinate Co II complexes have drawn ag reat deal of attention as they often exhibit excellent single-ion magnet (SIM) properties. Among the reported complexes, the axial zero-field splitting parameter (D)w as found to vary drastically both in terms of the sign as well as strength. There are variousp roposals in this respects uch as structural distortions,h eaviera tom substitution, metalligand covalency,t uning secondary coordination sphere, etc. that are expected to control the D values. To assess the importance of structurald istortions vs. heaviera tom substitution effect, herew eh ave undertaken detailed theoretical studies based on the ab initio CASSCF/NEVPT2 methodt o estimate zero-field splitting parameters for twelve complexes reported in the literature. Our test set includes the {Co II X 4 } (where X = O, S, Se) core structure where the D value was found to vary from + 19 to À118cm À1 .B ased on the structural variation, we have classified the complexes into three types (I-III)w here type I complexes were found to exhibit the largestn egative D value as desired for SIMs. The other two types (II and III)o fc omplexes have been found to be inferior with respect to type I. The secondary coordination spherew as also found to influence D,a ss ubstitution on the secondary coordination sphere atom was found to significantly alter the magnitude of D values. Particularly,t wo structuralp arameters, namely,t he dihedrala ngle between the two ligand planes and the ffX-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,o r metal-ligand covalency.Alarge variation in the D and E/D values among these complexesd espite possessing av ery close structuralsimilarity offers an exquisite playground for a chemistt odesign and develop new-generation Co II -based SIMs.

Role of Coordination Number and Geometry in Controlling the Magnetic Anisotropy in FeII, CoII, and NiII Single‐Ion Magnets

“…on this complex predicted a very large negative D value of −70 cm −1 [9a] . Magneto‐structural correlations performed on this type of {CoS 4 } system by us and others further revealed that the magnetic anisotropy of these complexes is controlled by the following factors: i) structural parameters such as bond angle and torsional angle around the first and second coordination sphere atoms, [12c, 39] ii) effect of donor atoms or ligand field strength, [39b, 40] iii) spin‐orbit coupling of the donor atoms [41] and also iv) counterions [39d, 42] . Soft donor groups such as S, Se/Br, I coordination bring down the 4 A 2 → 4 T 2 (ligand field states in T d symmetry) transition energy gap, and thus the D switches to negative from positive.…”

“…Particularly, complex [Co{N(SOCN i Pr2 ) 2 } 2 ] ( 32 ) was found to yield a larger D value of −118 cm −1 ; this value was predicted by theory and has not yet been verified by experiments (see Figure 6 d). [12c] The d orbital splitting obtained from AILFT analysis reveals that the d –d xy gap is close to 940 cm −1 and the first excited quartet state lies only 417 cm −1 higher in energy (see Figure 6 e). Other than S donor ligands, an N‐coordinated Co II complex (HNEt 3 ) 2 [Co(L) 2 ] ( 33 ) (where H 2 L=1,2‐bis(methanesulfonamido)benzene), possessing again D 2 d symmetry, was reported by van Slageren and co‐workers, where a huge negative D value (−115 cm −1 ) was confirmed from far‐infrared spectroscopy, CASSCF‐NEVPT2 calculations and other magnetic measurements (see Figure 6 f).…”

“… [44] Complex 33 showed an U eff of 118 cm −1 in a zero dc field, which is the best barrier height observed for any four‐coordinated Co II SIM. Ultimately, it was found that maintaining the D 2 d symmetry is key to achieving a large negative D value with four‐coordinated Co II complexes, and structural parameters always dominate over the soft or heavy atom substitutions [12c] …”

Exploiting host–guest chemistry to manipulate magnetic interactions in metallosupramolecular M4L6 tetrahedral cages

“…Deviation from ideal T d symmetry is well-known to result in a significant D value for tetrahedral Co II ions. 35–38 …”

“…Deviation from ideal Td symmetry is well-known to result in a significant D value for tetrahedral Co II ions. [35][36][37][38] For complex 7, the orbital degeneracy of the tetrahedral Ni II ion precluded convergence of the DFT calculations and thus no host-guest JNi-Ni exchange coupling could be estimated. NEVPT2 calculations yield a D value of +214 cm −1 for [NiCl4] 2− , a value much larger than that estimated from experimental susceptibility and magnetisation data.…”