A Study of the Lack of Slow Magnetic Relaxation in Mononuclear Trigonal Bipyramidal Cobalt(II) Complexes

Abstract

Two complexes of formula [Co(H 2 tba)(N 3)]⋅MeOH (1) and [Co-(H 2 tba)(NCS)] ⋅ 2DMF (2) [H 3 tba = tris(2-benzimidazolylmethyl) amine] were prepared and characterized aiming at investigating the influence of the structural and chemical effects on the Ising-type magnetic anisotropy in mononuclear trigonal bipyramidal cobalt(II) complexes. Each cobalt(II) ion in 1 is fivecoordinate by a monodentate azide ligand and a tetradentate monodeprotonated H 2 tbagroup building a slightly distorted trigonal bipyramidal surrounding. Compound 2 is also a mononuclear species whose structure was previously reported and only some of its structural data are considered in this work for comparative purposes. Detailed dc and ac cryomagnetic measurements of 1 and 2 show the lack of slow magnetic relaxation both in the presence and absence of applied dc fields. The profile of the χ M T against T plots together with the non-superimposable M against H/T curves for 1 and 2 support the occurrence of high-spin Co(II) ions with an appreciable magnetic anisotropy. Q-band EPR studies on polycrystalline samples of these compounds at low temperatures confirm the positive value of the zero-field splitting (D), a strong asymmetry in the g-tensors and hence an easy plane of magnetization indicating stabilization of M S = � 1/2 sublevels of the S = 3/2 spin state. A comparison of our results with previous magnetostructural reports on mononuclear trigonal bipyramidal cobalt (II) complexes that exhibit slow relaxation of the magnetization is carried out in order get deeper insights about the electronic and structural factors that would be at the origin of the lack of Single-Ion Magnet (SIM) behaviour for 1 and 2.

Field-induced mononuclear cobalt(ii) single-molecule magnet (SMM) based on a benzothiadiazole-ortho-vanillin ligand

“…These features are responsible for the extensive study of its complexes as candidates to mononuclear SMMs. [35][36][37][38][39][40][41][42] Indeed, six-coordinate Co(II) mononuclear complexes showing octahedral (O h ), [43][44][45] trigonal prismatic (TP) 46,47 or trigonal antiprismatic 48 coordination geometries are very good candidates for single-molecule magnets (SMMs). Usually, the octahedral complexes show a uniaxial anisotropy (D < 0) and only occasionally an easy-plane anisotropy (D > 0), but in all cases, the zfs is large.…”

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

“…24 Contrary to lanthanides, although the 3d orbitals of transition metal ions are valence orbitals and the orbital angular momentum can be readily quenched by ligand fields, transition metal coordination compounds that exhibit slow magnetic relaxation phenomena are at the forefront of the research on nanomagnetic materials. 25 Among them, special scientific attention has been paid to cobalt( ii ) complexes. In this respect, slow relaxation effects have been reported for two-, three-, four-, five-, six-, seven- and eight-coordinate mononuclear cobalt( ii ) complexes.…”