Field-induced slow relaxation of magnetization in dinuclear and trinuclear CoIII⋯MnIII complexes

Abstract

Two new dinuclear, [Co(3EtO-L5)(μ-CN)Mn(L4a)Cl] (1) and [Co(3EtO-L5)(μ-CN)Mn(L4a)Br] (2), and two trinuclear [{Co(3EtO-L5)(μ-CN)}₂Mn(L4a)]I (3) and [{Co(3EtO-L5)(μ-CN)}₂Mn(L4a)](NO₃) (4) complexes were prepared and thoroughly characterized (H₂3EtO-L5 = N,N′-bis(3-ethoxy-2-hydroxybenzylidene)-1,6-diamino-3-azahexane, L4a²⁻ = N,N′-ethane-bis(salicylideneiminate)dianion). The crystal structures were determined for all four compounds, while the static and dynamic magnetic properties were studied only for compounds 1–3. It has been revealed by simultaneous fitting of temperature and field dependent magnetic data and by using the spin Hamiltonian formalism involving the axial anisotropy term that the manganese(III) atoms possess relatively large and negative axial magnetic anisotropy in 1–3, with D = −3.9(2) cm⁻¹ in 1, −4.9(2) cm⁻¹ in 2, and −4.1(1) cm⁻¹ in 3. These results were supported by ab initio CASSCF calculations which were in good agreement with the experimental ones, however, a small rhombicity was calculated contrary to the experimental evaluations: D_calc(E_calc/D_calc) = −3.2 cm⁻¹ (0.04) in 1, −3.0 cm⁻¹ (0.03) in 2, and −3.6 cm⁻¹ (0.04) in 3. The measurements of dynamic magnetic data confirmed that compounds 1–3 represent a new type of Mn(III) field-induced single-ion magnet. The peak maxima of the frequency dependent out-phase susceptibility were below the lowest accessible temperature in all the three cases (i.e. below 1.9 K) and this prevented us from the construction of the Argand diagram. Nevertheless, the approximate procedure for extracting the spin-reversal barrier (U_eff) and relaxation time (τ₀) was used, and the values of U_eff ranging from 11 to 20 K and t₀ from 0.1 to 19 × 10⁻⁷ s were obtained.