Slow magnetic relaxation in penta-coordinate cobalt(ii) field-induced single-ion magnets (SIMs) with easy-axis magnetic anisotropy

Abstract

Two penta-coordinate complexes of the general formula [Co(Lⁿ)(NCS)]ClO₄, where L¹ = {bis[(3,5-dimethyl-1H-pyrazol-1-yl)ethyl]-[(3,4-dimethoxypyridin-2-yl)methyl]}amine and L² = {bis[(3,5-dimethyl-1H-pyrazol-1-yl)ethyl]-[(4-methoxy-3,5-dimethylpyridin-2-yl)methyl]}amine, have been synthesized and thoroughly characterized. Each of the cobalt(II) atoms is penta-coordinated in the {CoN₅} donor set with a distorted square-pyramidal geometry in [Co(L¹)(NCS)]ClO₄·MeOH (1), while the vicinity of the central atom can be described as a distorted trigonal–bipyramidal geometry in [Co(L²)(NCS)]ClO₄ (2) as revealed using the SHAPE analysis. Differences in interatomic parameters among the cobalt(II) and donor atoms in 1 and 2 have definite impact on the magnetic features of both compounds. The complexes show an easy-axis magnetic anisotropy (D = −38.5 cm⁻¹ for 1 and D = −8.5 for 2), and both complexes reveal a large rhombicity with E/D = 0.21 for 1 and E/D = 0.29 for 2. The ZFS parameters (g, D and E) were also calculated using CASSCF/NEVPT2 methods and they are in good agreement with those determined from experimental data. A frequency dependent out-of-phase susceptibility has been observed in external magnetic field (B_dc = 0.1 T) revealing the following parameters of slow relaxation of magnetization for 1: energy of the spin reversal barrier, U_eff = 16.0 cm⁻¹ (U_eff/kB = 23.0 K) and the relaxation time, τ₀ = 1.28 × 10⁻⁶ s. In the case of complex 2, no maxima of frequency dependent out-of-phase susceptibility have been observed and thus, the value of U_eff = 17 cm⁻¹ has been estimated using the expression U_eff = |D| × (S² − 1/4). It has been demonstrated that the degree of substitution and the type of substituents on the pyridyl moieties of the tripodal ligands (L¹ and L²) used in these penta-coordinate cobalt(II) complexes have significant impact on structural and magnetic features.