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

• Published in: Dalton transactions : an international journal of inorganic chemistry 2020-08, Vol.49 (33), p.11715-11726
• Main Authors: Massoud, Salah S, Perez, Zoe E, Courson, Jessica R, Fischer, Roland C, Mautner, Franz A, Van o, Ján, ajan, Michal, Trávní ek, Zden k
• Format: Article
• Language: English
• Subjects: Cobalt compounds; Coordination compounds; Crystallography; Ligands; Magnetic anisotropy; Magnetic induction; Magnetic permeability; Magnetic properties; Magnetic relaxation; Magnets; Mathematical analysis; Parameters; Relaxation time
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d0dt02338h

Two penta-coordinate complexes of the general formula [Co(L n )(NCS)]ClO 4 , where L 1 = {bis[(3,5-dimethyl-1 H -pyrazol-1-yl)ethyl]-[(3,4-dimethoxypyridin-2-yl)methyl]}amine and L 2 = {bis[(3,5-dimethyl-1 H -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 {CoN 5 } donor set with a distorted square-pyramidal geometry in [Co(L 1 )(NCS)]ClO 4 ·MeOH ( 1 ), while the vicinity of the central atom can be described as a distorted trigonal-bipyramidal geometry in [Co(L 2 )(NCS)]ClO 4 ( 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, τ 0 = 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 | × ( S 2 − 1/4). It has been demonstrated that the degree of substitution and the type of substituents on the pyridyl moieties of the tripodal ligands (L 1 and L 2 ) used in these penta-coordinate cobalt( ii ) complexes have significant impact on structural and magnetic features. Two penta-coordinate [Co(L n )(NCS)]ClO 4 with substituted pyridyl based bispyrazolyl ligands have been structurally characterized. The complexes show an easy-axis magnetic anisotropy, large rhombicity and slow relaxation of magnetization.