Cis and Trans Isomers of Fe(II) and Co(II) Complexes with Oxadiazole Derivatives ‐ Structural and Magnetic Properties

Abstract

Four complexes with bidentate N,N-donors 2-(furan-2-yl)-5-(pyridin-2-yl)-1,3,4-oxadiazole (fpo) and 2-(pyridin-2-yl)-5-(thiophen-2-yl)-1,3,4-oxadiazole (pto) with general formula [M-(L) 2 (NCS) 2 ] (M = Co(II), L = fpo for (1); M = Co(II), L = pto for (2); M = Fe(II), L = fpo (3); M = Fe(II), L = pto (4)) are reported. Analysis and characterization of the samples was performed using standard physico-chemical techniques-elemental analysis, nuclear magnetic resonance, Fourier transform infrared spectroscopy, single-crystal X-ray diffraction. Magnetic properties for 1-4 revealed large magnetic anisotropy of Co(II) complexes, and AC susceptibility measurements confirmed their single-molecule magnetic behaviour. Furthermore, the theoretical calculations at DFT and CASSCF/NEVPT2 level of theory were exploited to better understand magnetism of these compounds.

Large easy-axis magnetic anisotropy in a series of trigonal prismatic mononuclear cobalt(ii) complexes with zero-field hidden single-molecule magnet behaviour: the important role of the distortion of the coordination sphere and intermolecular interactions in the slow relaxation

“…In good agreement with this, to the best of our knowledge and with one exception, all the compressed octahedral CoN 6 complexes containing two trans-thiocyanate groups exhibit, like 5, D > 0. 19,20,33 All the complexes with analogous structures to 5 exhibit a similar d ax /d eq ratio (d ax and d eq are the average axial and equatorial bond distances, respectively) within the 0.94-0.96 range, and it appears that to observe negative D values for this type of complexes, a comparative rather smaller d ax /d eq ratio is required. 18…”

“…The best fit led to the parameters indicated in Table 3, which are similar to those found for distorted octahedral Co II complexes with large easyplane anisotropy. 19,20,33,34,48 It should be noted that for complexes 1-5 under the optimal field, the Raman process predominates above approximately 4 K, whereas below this temperature QTM and/or direct processes dominate. In connection with this, the α values extracted from the Cole-Cole plot for compounds 2-4 (Fig.…”

“…Thus, Co II complexes combining strong axial and weak equatorial bonds (axially compressed octahedral geometry), as it occurs in [Co(tu) 4 Cl 2 ], stabilize a large axial anisotropy, whereas the combination of large axial bonds and short equatorial bonds, like in [Co(py) 4 Cl 2 ], favors large positive D values. 18,19 This qualitative magneto-structural correlation seems not to be of general applicability, because compressed octahedral CoN 6 complexes, such as [Co(py) 4 (NCS) 2 ] 19 and [Co(pto) 4 (NCS) 2 ] (pto = 2-(pyridine-2-yl)-5-thiophen-2-yl)-1,3,4-oxadiazole, 20 have a similar ratio between the axial and equatorial bond distances, however, they exhibit D values of opposite sign. Therefore, in order to clarify this appar-ent contradiction more examples of this type of complexes, like 5, where the sign of D is unambiguously supported by a combination of theoretical and experimental methods, such as HFEPR, PND and single-crystal torque anisotropy, are needed.…”

Trigonally Distorted Hexacoordinate Co(II) Single-Ion Magnets

“…Secondly, the active space of theoretical calculations was limited to only five d-orbitals, thus the ligand-based orbitals are The energies of the six lowest Kramers doublets were used for the analysis of the parameters of the Hamiltonian in Equation (1). Such a procedure we applied for the first time in the investigation of the above mentioned [Co(neo)(PhCOO) 2 ] polymorphs [34] and then also for other Co II complexes [60][61][62]. This procedure resulted in the values of α•λ, ∆ ax , ∆ rh , which are listed in Table 2 and graphically presented in Figure S9.…”

Effect of Ligand Chain Length for Tuning of Molecular Dimensionality and Magnetic Relaxation in Redox Active Cobalt(II) EDOT Complexes (EDOT=3,4‐Ethylenedioxythiophene)

“…Such low value of n suggests either the involvement of both acoustic and optical phonons in Raman relaxation process, [31] or the relaxation of the magnetism is governed by the phonon bottleneck effect. [32] EPR Studies EPR (X-band and Q-band) experiments were performed on powder samples of 1-4 at 10 K (Figures 8 and S52-S54), while the X-band EPR spectra were simulated with the EasySpin toolbox. [33] Spectra simulation parameters were listed in Table S8.…”