Fine-Tuning of Uniaxial Anisotropy and Slow Relaxation of Magnetization in the Hexacoordinate Co(II) Complexes with Acidoligands

A new hexacoordinate Co(II) complex with the general formula [CoL(NCS)₂]·DMSO (1) (L = biscondensation product of diacetyl and 2-hydrazinyl-4,6-dimethylpyrimidine, DMSO = dimethylsulfoxide) exhibiting field-induced single-ion magnet (SIM) behavior has been obtained and characterized. To explain such field-induced SIM behavior, we have performed the analysis of the observed direct current (DC) magnetic properties based on the Griffiths Hamiltonian in combination with the high-level quantum-chemical calculations of the crystal field parameters. The complex has been shown to exhibit strong easy-axis-type magnetic anisotropy arising from a large negative value Δₐₓ = −1538.0 cm–¹ of the axial crystal parameter, which is accompanied by a weak rhombic contribution with |Δᵣₕ| = 219.3 cm–¹. Magnetic properties of 1 have been compared with the previously studied field-induced SIM compound [CoLCl₂]·H₂O (2). Introduction of a thiocyanide anion in 1 (instead of chloride in 2) in the Co(II) coordination sphere resulted in improvement of uniaxial magnetic anisotropy and switch of the relaxation process from the combination of direct and Raman mechanisms in 2 to Orbach-type one in complex 1. Effects of substitution of acidoligands in the Co(II) coordination sphere on the crystalline structure, DC magnetic properties, magnetic anisotropy, and magnetization relaxation processes have been rationalized.