Trigonally Distorted Hexacoordinate Co(II) Single-Ion Magnets

Abstract

By simple reactions involving various cobalt(II) carboxylates (acetate and in situ prepared pivalate and 4-hydroxybenzoate salts) and neocuproine (neo), we were able to prepare three different carboxylate complexes with the general formula [Co(neo)(RCOO)2] (R = –CH3 for 1, (CH3)3C– for 2, and 4OH-C4H6– for 3). The [Co(neo)(RCOO)2] molecules in the crystal structures of 1–3 adopt a rather distorted coordination environment, with the largest trigonal distortion observed for 1, whereas 2 and 3 are similarly distorted from ideal octahedral geometry. The combined theoretical and experimental investigations of magnetic properties revealed that the spin Hamiltonian formalism was not a valid approach and the L-S Hamiltonian had to be used to reveal very large magnetic anisotropies for 1–3. The measurements of AC susceptibility showed that all three compounds exhibited slow-relaxation of magnetization in a weak external static magnetic field, and thus can be classified as field-induced single-ion magnets. It is noteworthy that 1 also exhibits a weak AC signal in a zero-external magnetic field.

Magnetization Slow Dynamics in Mononuclear Co(II) Field-Induced Single-Molecule Magnet

“…Next, the energies of the six lowest Kramers doublets were used for the analysis of the parameters of the L-S Hamiltonian in eq . We already applied such a procedure for the investigation of several Co II compounds. − A good agreement with the CASSCF/DLPNO-NEVPT2 energy levels was found for Δ ax = −1508 cm –1 , Δ rh = −164 cm –1 , and α · λ = −223 cm –1 (Table ). Recently, this procedure was shifted further in order to also decipher values of α and λ separately .…”

“…Recently, this procedure was shifted further in order to also decipher values of α and λ separately. 50 Thus, temperature-and field-dependent magnetization data were calculated directly in an ORCA package resulting from CASSCF/DLPNO-NEVPT2 calculations. In the next step, these magnetic data were fitted to eq 1, but with fixed values of α•λ, Δ ax , and Δ rh , which enabled the determination of α = 1.86 and λ = −120 cm −1 , as given in Table 1.…”

Field-Induced Slow Magnetic Relaxation in CoII Cyclopropane-1,1-dicarboxylates

“…In both cases, the decrease in χ M T slowly accelerates with decreasing temperature, from 3.31 cm 3 K/mol and 3.26 cm 3 ·K·mol −1 (at 300 K) to 1.82 cm 3 ·K·mol −1 and 1.53 cm 3 ·K·mol −1 (at 2 K) for for 1 and 2 , respectively. In the case of Co 2+ complexes, this magnetic behavior is caused by the anisotropy of the Co 2+ ions and the Zeeman effect caused by the applied field [ 45 , 46 ].…”

Slow magnetic relaxation and spin crossover behavior in two mixed-valence Co(ii)/Co(iii) complexes

“…[31][32][33] Searching for welldefined correlations between different structural factors and the magnetic anisotropy in transition metal complexes remains a great challenge and much more work is required to extend our knowledge for the rational design of new magnetic materials. [33][34][35][36][37][38][39][40] In addition, spin crossover (SCO) can occur in transition metal complexes with a d 4 -d 7 electron configuration provided that a suitable ligand field is present. SCO complexes show reversible transformation between a high-spin (HS) and a lowspin (LS) state under relatively minor external stimuli such as temperature, pressure, or light, and have been extensively investigated in the past few decades as potential materials for sensors, memory, and other electronic devices.…”

“…31–33 Searching for well-defined correlations between different structural factors and the magnetic anisotropy in transition metal complexes remains a great challenge and much more work is required to extend our knowledge for the rational design of new magnetic materials. 33–40…”