Synthesis and Molecular Magnetism in Cobalt(II) Complexes with Tridentate N-Donor Ligands


Jana Juráková,^1 Ivan Šalitroš,^1,2,3 Ján Moncoľ^2


1- Central European Institute of Technology, Brno University of Technology, Purkyňova 123, 61200
Brno, Czech Republic, e-mail: jana.jurakova@ceitec.vutbr.cz

2- Department of Inorganic Chemistry. Faculty of Chemical and Food Technology. Slovak University of
Technology in Bratislava. Bratislava SK-81237. Slovakia. e-mail: ivan.salitros@stuba.sk and
jan.moncol@stuba.sk

3- Department of Inorganic Chemistry. Faculty of Science. Palacký University. 17. listopadu 12. 771
46 Olomouc. Czech Republic


The single molecule magnets (SMMs) are defined as molecules that show the slow relaxation of
magnetisation based on purely molecular origin. This is the fundamental difference between the SMMs
and molecular ferro- or ferrimagnets. They illustrate the smallest possible magnetic storage
devices, which can possibly maintain information in a single molecule rather than in a magnetic
particle consisted of domains. Hysteresis loop in magnetization vs. field dependency below the
blocking temperature T[B] of SMMs is observed. The T[B] of the most molecule-based magnets is below
10 K, although the rare examples up to 80 K can be found [1], [2]. To be viewed as a good SMM, a
compound should exhibit a large effective anisotropy barrier against magnetization switch U[eff],
relatively high blocking temperature T[B] and high value of relaxation time t[0]. Over the time,
mononuclear and polynuclear complexes have been studied. Especially those, which contain Mn(III),
Fe(I), Fe(II), Co(II) and lanthanides are the subjects of particular interest [3].

The aim of this contribution is to discuss about synthesis, structural and magnetic properties of
new Co(II) coordination compounds with derivatives of 2,6-bis(1H-benzimidazolyl)pyridine (bzimpy).
First, ligands L1 (2,6-bis(1-octyl-1H-benzimidazol-2-yl)pyridine) and L2
(2,6-bis(1-dodecyl-1H-benzimidazol-2-yl)pyridine) were prepared and characterized by elemental
analysis (L1), IR and UV–VIS spectroscopy and NMR spectroscopy. The preparation of ligands L1 and
L2 was based on N-alkylation of imidazole rings of bzimpy. The main purpose of alkylation in this
case was to increase the solubility of ligands and corresponding complexes. Solubility of complexes
in solvents such as CH[2]Cl[2] or acetone, which can be considered as low boiling point solvents,
is important for further deposition using lithography techniques. In addition, the change of
magnetic properties with the alkyl chain lengths can be studied as well. Ligands were used for the
synthesis of mononuclear complexes. Four new pentacoordinated complexes expressed by general
formula [Co(L1)X[2]] (X = Cl^- for 1 and Br^- for 2) and [Co(L2)X[2]] (X = Cl^- for 3, Br^- 4) were
prepared. Newly prepared complexes were analysed by elemental analysis, IR, UV–VIS spectroscopy,
single crystal X-ray diffraction analysis and static magnetic measurements. The dynamic magnetic
investigations of complexes revealed slow magnetic relaxation under small applied static magnetic
field. Complexes 1 – 4 exhibit energy barrier U[eff] in the range of 23 to 37 K and relaxation time
t[0] of the order of 10^-7 s, which are values typical for Co(II) mononuclear complexes. The value
of relaxation time t of 1 at the lowest temperatures is 0.02 s, which is considered as relatively
high value among other similar compounds.

In addition, hexacoordinated Co(II) compounds 5 and 6 expressed by formula
[Co(L3)[2]Cl](C[7]H[7]O[3]S) and [Co(L3)[3]](CF[3]SO[3])[2], respectively, which employ ligand L3
(2-(6-(phenyldiazenyl)pyridin-2-yl)-1H-benzimidazole) were synthesised and structurally
characterised.



  [1]          F.S. GUO, et al. Angew. Chem. Int. Ed. 2017, 56, 11445

  [2]          F.S. GUO, et al. Science 2018, 362, 1400

  [3]          SLAGEREN, Introduction to Molecular Magnetism. Available from:
http://obelix.physik.uni-bielefeld.de/~schnack/molmag/material/123.pdf