Tuning Magnetic Anisotropy in a Class of Co(II) Bis(hexafluoroacetylacetonate) Complexes

Abstract

Tuning the magnetic anisotropy of metal ions remains highly interesting in the design of improved singlemolecule magnets (SMMs). We herein report synthetic, structural, magnetic, and computational studies of four mononuclear Co II complexes, namely [Co(hfac) 2 (MeCN) 2 ] (1), [Co(hfac) 2 (Spy) 2 ] (2), [Co(hfac) 2 (MBIm) 2 ] (3), and [Co (hfac) 2 (DMF) 2 ] (4) (MeCN = acetonitrile, hfac = hexafluoroacetylacetone, Spy = 4-styrylpyridine, MbIm = 5,6-dimethylbenzimidazole, DMF = N,N-dimethylformamide), with distorted octahedral geometry constructed from hexafluoroacetylacetone (hfac) and various axial ligands. By a building block approach, complexes 2-4 were synthesized by recrystallization of the starting material of 1 from various ligands containing solution. Magnetic and theoretical studies reveal that 1-4 possess large positive D values and relative small E parameters, indicating easy-plane magnetic anisotropy with significant rhombic anisotropy in 1-4. Dynamic alternative current (ac) magnetic susceptibility measurements indicate that these complexes exhibit slow magnetic relaxation under external fields, suggesting field-induced single-ion magnets (SIMs) of 1-4. These results provide a promising platform to achieve fine tuning of magnetic anisotropy through varying the axial ligands based on Co(II) bis (hexafluoroacetylacetonate) complexes.[a] Dr.

A porous cobalt(ii)–organic framework exhibiting high room temperature proton conductivity and field-induced slow magnetic relaxation

“…39 CASSCF/NEVPT2 methodology with the EHA approach has been very promising in computing the magnetic properties of Co( ii )-based complexes. 20 e ,21 c , 40 Our calculations yield a gap of 222.46 cm −1 between the ground and first excited K.D. The NEVPT2 computed effective g -values for the ground state KD of complex 1 are g min = 2.782, g mid = 4.008, and g max = 6.065, which are highly axial with modest transverse components.…”

Hydrogen-Bonded Framework of a Cobalt(II) Complex Showing Superior Stability and Field-Induced Slow Magnetic Relaxation

“…This large g value suggests a significant orbital contribution of the six-coordinate Co II ion to the magnetic susceptibility . Upon sample cooling, the χ M T values are almost unchanged before undergoing an obvious decline to a minimum value at the measured lowest temperature, reaching 1.624 cm 3 mol –1 K at 2 K. The shape of the χ M T vs T curve agrees well with the widely reported mononuclear Co II complexes . The decrease in the χ M T curves at low temperatures is mainly due to the zero-field splitting (ZFS) of the Co II ion and the superexchange interactions between neighboring Co II ions.…”

Tuning the Coordination Geometry and Magnetic Relaxation of Co(II) Single-Ion Magnets by Varying the Ligand Substitutions

“…It was established that the magnetic relaxation of SIMs can be influenced by many factors such as magnetic anisotropy, ligand field strength, symmetry of coordination geometry, intermolecular interaction, and molecular vibration. − ,,,− ,− The substituent of the Schiff-based ligand can regulate the coordination symmetry of Co­(II) ions and thus lead to a tetrahedral geometry for 1 and octahedral geometries for 2 and 3 . Different coordination geometries will produce a distinct electronic structure for the Co­(II) ion, which strongly influences the magnetic anisotropy and magnetic relaxation of the resulting products.…”

“…The magnetic relaxation behavior of Co­(II) SIMs is governed by magnetic anisotropy, which is strongly dependent on ligand field strength and coordination geometry. Several strategies such as decreasing coordination number, , changing coordination atom, − introduction of heavier coordination atom, − varying the counteranions, , and ligand substitution − have been successfully employed to tune the magnetic anisotropy and magnetic properties of Co­(II) SIMs. By ligand substitution, the symmetry of the coordination geometry can be changed while the coordination atoms stay the same.…”