Suppressing of slow magnetic relaxation in tetracoordinate Co(II) field-induced single-molecule magnet in hybrid material with ferromagnetic barium ferrite

The novel field-induced single-molecule magnet based on a tetracoordinate mononuclear heteroleptic Co(II) complex involving two heterocyclic benzimidazole (bzi) and two thiocyanido ligands, [Co(bzi)2(NSC)2], (CoL4), was prepared and thoroughly characterized. The analysis of AC susceptibility data resulted in the spin reversal energy barrier U = 14.7 cm−1, which is in good agreement with theoretical prediction, Utheor. = 20.2 cm−1, based on axial zero-field splitting parameter D = −10.1 cm−1 fitted from DC magnetic data. Furthermore, mutual interactions between CoL4 and ferromagnetic barium ferrite BaFe12O19 (BaFeO) in hybrid materials resulted in suppressing of slow relaxation of magnetization in CoL4 for 1:2, 1:1 and 2:1 mass ratios of CoL4 and BaFeO despite the lack of strong magnetic interactions between two magnetic phases.

The discovery of the first Co(II) SIM followed next year, in 2011, T. Jurca et al. reported on SRM in pentacoordinate mononuclear Co(II) compounds with isothiocyanido and bis(imino)pyridine pincer ligands 6 . Further reports on Co(II) SIMs aim dominantly on low-coordinate species, such as tridentate 7 , tetracoordinate 8 , pentacoordinate 9 , but also hexacoordinate SIM complexes were reported 10 . The relatively frequent occurrence of the SIM phenomenon in Co(II) compounds motivated us to study magnetic anisotropy of tetracoordinate Co(II) compounds. These are synthetically easily available and usually sufficiently air-stable and therefore, such compounds are appropriate candidates for extensive and advanced studies.
In this work, we report on synthesis, crystal structure and magnetic properties of mononuclear tetracoordinate complex [Co II (bzi) 2 (NCS) 2 ] (CoL4), where bzi = benzimidazole. In order to prove field-induced SRM in CoL4 the thorough study of magnetic properties was done by analysis of experimental data acquired by DC and AC magnetometry. Additionally, the experimentally obtained results were compared with those acquired using ab initio (DFT and CASSCF) calculations. Moreover, inspired by our recent research involving a study of interactions between molecule-based metamagnet {[Ni(en) 2 ] 3 [Fe(CN) 6 ] 2 ⋅ 3H 2 O} n 11 and nanocrystalline magnetite Fe 3 O 4 resulting in magnetic superstructure 12 , we decided to investigate the impact of ferromagnetic barium ferrite BaFe 12 O 19 (BaFeO) upon herein reported static and dynamic magnetic properties of a single-ion molecule magnet CoL4. Therefore, the heterogeneous solid state mixtures of CoL4 and BaFeO in mass ratios 1:2, 1:1 and 2:1, respectively, were prepared by ball milling and characterized by DC magnetization and AC susceptibility measurements with the aim to answer remarkable questions regarding the possibility of dipolar/exchange magnetic interactions between two magnetic components and possible influence of ferromagnetic component on SRM of CoL4.

Results and Discussion
Synthesis and X-ray structure analysis. The synthesis of CoL4 is very straightforward and facile: CoCl 2 was mixed with KNCS in 1:2 molar ratio in methanol, producing characteristic cobalt-blue colored solution. Then, the stoichiometric amount of bzi was added during stirring. The solution was further stirred under heating for 15 minutes and then was filtered off through the paper filter. The dark blue single crystals appeared after 3 days of slow evaporation of mother liquor.
The crystal structure of CoL4 was determined by a single crystal X-ray diffraction analysis. The compound crystallizes in triclinic space group P-1. The molecular structure of CoL4 consists of two bzi and NCS − ligands coordinated to Co(II) center through nitrogen atoms, thus forming a pseudotetrahedral coordination polyhedron, with the {CoN 2 N' 2 } chromophore. The Co-N bond lengths are a bit shorter in the case of NCS − ligands as compared to bzi ones (in Å): d(Co-N NCS ) = 1.933(5), 1.948(4), d(Co-N bzi ) = 1.988(3), 1.993 (4). The angular distortion of the coordination polyhedron is more obvious while the chromophore angles differ from the ideal tetrahedral angle (α Td = 109.5°). Such distortion can be described by previously defined parameter δ 13 which includes the deviation of a sum of the angles α = N NCS -Co-N NCS and β = N bzi -Co-N bzi from the pair of the ideal tetrahedral angles: δ = 2·α Td -(α + β). The δ parameter adopts relatively large negative value of − 6.3°. However, there is another way how to define total angular distortion by sum of the deviations from ideal α Td calculated for all chromophore angles γ i : ∆ = Σ i |γ i − α Td |. and ∆(CoL4) = 22.3°. Summary of the selected structural parameters for CoL4 and other previously published tetracoordinate [Co(L) 2 (A) 2 ] compounds 14-17 are given in Table 1 (L1 = a monodentate ligand, A = an anionic ligand).
The crystal structure of CoL4 is stabilized by several weak non-covalent contacts. Firstly, the 2D sheet of the [Co(bzi) 2 (NCS) 2 ] molecules is formed due to the weak N-H···S contacts (Fig. 1). Each complex molecule acts simultaneously as a donor and acceptor of two symmetrically independent contacts with d(N···S) = 3.369(3) and 3.353(3) Å. Secondly, the supramolecular [Co(bzi) 2 (NCS) 2 ] ···[Co(bzi) 2 (NCS) 2 ] dimers are formed in the crystal structure due to the interactions of the aromatic rings (Fig. 1). The hydrogen bonding and π-π interactions are well-known mediators of the magnetic exchange interactions, therefore, in the case of CoL4 it can be doubt if the system is magnetically isolated. However, the lengths of the possible super-exchange pathways through the N-H···S contacts are too long (based on compound chromophore Co-L/Å Co-A/Å δ/°Δ/°D/cm −1 ref. [ the lengths of non-covalent contacts, vide infra, and Co···Co distances of involved complex molecules, 9.3540(9) and 9.5037(9) Å) and no directly coordinated atoms are involved in this kind of very weak hydrogen bonding 18 . The present π-π stacking is also quite distant with the shortest C···C distances equaled to 3.354 (5) Å (Fig. 1). On the other hand, it must be noted that such C···C distance might be indication of the non-covalent magnetic exchange pathway mediated by π-π stacking interaction 19 .

Ab initio calculations.
where the following spin Hamiltonian for a dimer was used Then, the isotropic exchange J-values were calculated by Ruiz's approach 21,22 as which resulted in a negligible value of J equaled to -0.01 cm −1 . The spin densities of the BS spin calculation are depicted in Fig. 1. Additionally, we performed ab initio calculations of ZFS parameters based on state average complete active space self-consistent field (SA-CASSCF) wave functions complemented by N-electron valence second order perturbation theory (NEVPT2) with the active space defined as CAS (7,5). The resulting values of ZFS parameters were as follows: D = − 11.1 cm −1 and E/D = 0.050. Negative D-parameter is in agreement with the magneto-structural correlation presented in Table 1. In addition, g-tensor values were found as: g 1 = 2.173, g 2 = 2.182, g 3 = 2.311 resulting in g iso = 2.222. Both g-tensor and D-tensor almost coincide as can be seen in Fig. 1, so we can conclude from their mutual relationship that g x = g 2 , g y = g 1 , and g z = g 3 . In next step, we utilized the respective ab initio CASSCF/NEVPT2 spin-orbit coupling, orbital and spin angular momentum matrices to calculate all 120 energy levels for any orientation of magnetic field B a , followed by integral calculation of both temperature and field dependent magnetization data, which are in good agreement with the experimental ones (Fig. 2). The same quality of agreement between theory and experiment we achieved for a hexacoordinate Co(II) field-induce single-ion magnet 23 , thus showing that this theoretical protocol is suitable for study of Co(II) complexes in relationship to their ZFS tensor and magnetic properties.
Static magnetic properties of CoL4. Temperature and field dependent magnetic data of CoL4 are depicted in Fig. 2. The effective magnetic moment μ eff starts at the value of 4.50 μ B at room temperature, which is considerably higher than the spin-only value for the S = 3/2 and g = 2.0 (μ eff /μ B = 3.87) as a result of significant contribution of the orbital angular momentum to the ground state. The μ eff /μ B starts to decrease on cooling below 30 K reaching the value of 3.62 at T = 1.9 K, which is caused by the zero-field splitting (ZFS). The same phenomenon is also responsible for the large deviation of the isothermal magnetization curves measured at T = 2 and 5 K up to 5 T from the theoretical Brillouin's function (Fig. 2). As a result of DFT calculations, we can consider Co(II) atoms magnetically well separated from each other and we can safely use the spin Hamiltonian formalism for a monomeric complex to interpret experimental data of CoL4 as follows 24 here D and E are the single-ion axial, and rhombic ZFS parameters, respectively, and a defines orientation of the magnetic field vector, B a = B(sinθcosφ, sinθsinφ, cosθ). The final calculated molar magnetization was calculated as an integral average in order to properly simulate powder sample signal, using FORTRAN subroutine QROMB 25 .
Both temperature and field dependent magnetization data were fitted simultaneously with the aim to obtain plausible parameters and it was found that satisfying fit was already obtained when rhombic anisotropy was neglected. As a result, the best-fitted parameters were found as g = 2.27, D = -10.1 cm −1 and χ TIP = 5.3 ⋅ 10 −9 m 3 mol −1 (Fig. 2), where χ TIP stands for the contribution of temperature-independent paramagnetism. 24 Dynamic magnetic properties of CoL4. The negative value of D-parameter of CoL4 encouraged us to measure also AC susceptibility data. In zero static magnetic field, there was no out-of-phase susceptibility signal ( Figure S1, Supplementary Information), but the field dependent measurement performed at T = 1.9 K revealed a slow relaxation of magnetization ( Figure S2). Therefore, AC susceptibility measurements were done in non-zero static field, B dc = 0.2 T at low temperatures, showing characteristic pattern for slow relaxation of magnetization typically observed for SMM species (Fig. 3). The analysis of susceptibility data for each temperature using the one-component Debye model resulted in isothermal (χ T ) and adiabatic (χ S ) susceptibilities, relaxation times (τ ) and distribution parameters (α) ( Table S2, Supplementary Information). This enabled us to construct the Argand (Cole-Cole) plot (Fig. 3) and using the Arrhenius expression for the temperature dependence of relaxation time resulted in τ 0 = 1.86⋅ 10 −8 s −1 and the spin reversal barrier U = 21.4 K / 14.7 cm −1 (Fig. 3), where only data having maxima in Argand diagram were used. The effective value of U is in good agreement with theoretical prediction, U theor . = |2D| = 20.1 cm −1 , based on the single-ion axial zero-field splitting parameter D derived from magnetic analysis.

Preparation and physical characterization of CoL4 : BaFeO mixtures. The solid mixtures of
CoL4 and BaFeO were prepared in mass ratios 1:2, 1:1 and 2:1 by ball milling technique. The resulting solids were characterized by a powder X-ray diffraction method in order to prove that the no solid state reaction occurred and that the chemical character of both compounds was preserved ( Figure S3, Supplementary Information). Furthermore, also FTIR spectroscopy confirmed that spectra of mixtures are simple sums of individual components spectra ( Figure S4). The composition of hybrid material was studied also by UV-VIS spectroscopy. 12 mg of 1:2, 1:1 and 2:1 mixtures were extracted in 40 ml of CHCl 3 at 40 °C for 20 min in an ultrasonic bath. Extracted solutions were then poured into 100 ml volumetric flasks and additional CHCl 3 was added to fill the volume up to 100 ml. Appropriate amounts of CoL4 (4 mg for 1:2, 6 mg for 1:1 and 8 mg for 2:1) were dissolved in CHCl 3 , poured into 100 ml volumetric flasks and the volume was filled up to 100 ml. The spectra were measured in the range of 200-1000 nm and compared with the pure phase and mixtures extracts. Using the Beer-Lambert law it was calculated that at least 85% of CoL4 can be extracted back to CHCl 3 solution from hybrid materials using the above mentioned method ( Figure S6) Thus, from the chemical point of view, the resulting mixtures can be considered as two phase systems.

Static magnetic properties of CoL4 : BaFeO mixtures.
In order to eventually identify any magnetic interaction between CoL4 and BaFeO in the studied mixtures, the magnetic hystereses were measured at 2 K in magnetic field range from − 5 to + 5 T. The commercially available BaFeO behaves as a ferromagnet with saturation magnetization equaled to 90.9 emu/g and coercive field equaled to 0.13 T. On the contrary, CoL4 compound shows no hysteresis at this temperature and magnetization reaches the value of 27.4 emu/g at 5 T. The magnetic properties of 1:2 mixture are shown in Fig. 4, and other mixtures behave in analogous manner. The magnetization of the particular mixture saturates at 70.3 emu/g, which is in good agreement with the calculated arithmetic average equaled to 69.7. Indeed, the calculated curve of arithmetic average coincides with the experimental data very well, there is a slight deviation only at low fields as evident in inset of Fig. 4. This comparison suggests that magnetic interactions between two phases, CoL4 and BaFeO, are either negligible or imperceptible.
Dynamic magnetic properties of CoL4 : BaFeO mixtures. The temperature dependent AC susceptibility data were firstly measured at non-zero (B DC = 0.2 T) DC field for all three mixtures. The results for mixtures 1:2 and 2:1 are plotted in Fig. 5, and for mixture 1:1 in Figure S7   Information). Evidently, the characteristic pattern of slow relaxation of magnetization visible both in real and imaginary components of AC susceptibility data of CoL4 is lost in all studied mixtures with BaFeO. The same effect is also manifested in the field depended AC susceptibility data measured at 1.9 K for varying B DC from 1.0 to 0.0 T. To ensure that the loss of SRM is not artefact of ball milling, we prepared also the mixture with diamagnetic matrix, Y 2 O 3 , in the mass ratio of 1:2 ( Figure S7). In his case, the SRM was clearly observed both in temperature and field dependent AC susceptibility data.

Discussion
The novel tetracoordinate field-induced single-ion Co(II) molecular magnet was prepared and characterized by various physical techniques resulting in identification of the height of its spin reversal barrier U = 21.4 K. The observation of field-induced SRM is inevitably connected to quenching of magnetic tunneling by shifting the energy levels by the Zeeman effect. In CoL4 compound, we observed that the field necessary to do so is relatively small (even below 0.1 T). This property of CoL4 offers an opportunity to test possibility of switching the CoL4 molecules to "field-induced" state simply by using a suitable ferromagnetic substrate. Therefore, the detailed study was performed in order to identify possible magnetic interactions with ferromagnetic barium ferrite (BaFeO). The experimental data proposed that albeit DC magnetic data show imperceptible interference of two mixed solid state phases, the AC susceptibility data surprisingly, but clearly, confirmed suppressing of SRM in CoL4 induced by the ferromagnetic component. It must be noted that the temperature dependent AC study was done in two distinct ways. Firstly, the hybrid samples were magnetized by the static magnetic field (B = 2 T) followed by setting zero static magnetic field in no overshoot regime in order to preserve remanent magnetization of BaFeO, and then the dynamic magnetic properties were measured resulting in no out-of-phase signal of CoL4. This motivated us to measure dynamic properties in non-zero static external field in which the pure CoL4 phase possesses maximal out-of-phase signal (B = 0.2 T). Again, no out-of-phase signal was observed (Fig. 5). The detailed field dependence AC susceptibility study confirmed that there is no out-of-phase signal of CoL4 in hybrid samples in any static magnetic field up to 1 T (Fig. 5).
The outcome of this work is hard to compare with other systematic works oriented at mutual interaction of SMM and ferro/antiferromagnetic phases, because most of them were done by depositing SMM molecules on various ferro/antiferromagnetic surfaces [26][27][28] or diamagnetic substrates 29,30 . In such cases, the interactions were dominantly investigated by acquiring hysteresis loops using element-resolved X-ray magnetic circular dichroism (XMCD), while dynamic magnetic information obtainable by AC susceptibility was unreachable. On the contrary, AC susceptibility measurements were successfully employed in SMM molecules attached on gold nanoparticles 31,32 . However, all the above mentioned methods did not preserve original crystal structure of SMM and therefore the molecular geometry is usually affected by processing the hybrid structures. In order to avoid this drawback, as the magnetic anisotropy of 3d metal complexes is significantly affected by change in their molecular geometry, we directly used the crystalline materials of CoL4 and BaFeO in the preparation of the studied mixtures. To summarize, this article reports on magnetic properties of a novel tetracoordinate field-induced single-molecule magnet CoL4. We studied its magnetic properties and we used it as a constituent for the preparation of mixtures with ferromagnetic barium ferrite (BaFeO). It was observed that suppressing of slow-relaxation of magnetization (SRM) on the complex molecule occurred in these materials by the interaction with barium ferrite. On the contrary, CoL4 in the mixture prepared from CoL4 and diamagnetic Y 2 O 3 preserves slow relaxation of magnetization as was observed in pure CoL4, which indirectly shows that the suppression of SRM in CoL4-BaFeO hybrid materials has origin in BaFeO ferromagnetic component. Crystallography. Single crystal X-ray diffraction data were collected using Oxford diffraction Xcalibur2 CCD diffractometer with a Sapphire CCD detector (Mo-Kα radiation, λ = 0.71073 Å). The structure was solved by direct methods using SHELXS97 33 incorporated into the WinGX program package 34 . The structure was refined using full-matrix least-squares on F o 2 − F c 2 with SHELXL-97 33 with anisotropic displacement parameters for non-hydrogen atoms. All the hydrogen atoms were found in differential Fourier maps and their parameters were refined using a riding model with U iso (H) = 1.2 U eq (atom of attachment). The crystal structure was visualized using the Mercury software 35  Physical methods. Elemental analysis (C, H, N) was performed on a Flash 2000 CHNO-S Analyzer (Thermo Scientific). Infrared (IR) spectra of the complexes were recorded on a Thermo Nicolet NEXUS 670 FT-IR spectrometer (Thermo Nicolet) employing the ATR technique on a diamond plate in the range of 400-4000 cm -1 . Temperature dependence of the magnetization at B = 0.1 T from 1.9 to 300 K and the isothermal magnetizations at T = 2.0 and 5.0 K up to B = 5 T were measured using MPMS XL-7 SQUID magnetometer (Quantum Design). The experimental data were corrected for diamagnetism. Measurements of AC susceptibility were carried out in a 3.8 Oe ac field oscillating at various frequencies from 1 to 1000 Hz and with various dc fields. SEM images and energy-dispersive X-ray (EDX) spectroscopy data were recorded on a Hitachi 6600 FEG microscope. Powder samples were placed on an aluminum holder with double-sided adhesive carbon tape. The accelerating voltages used were in the range of 5− 15 keV. The X-ray powder diffraction patterns of all solid samples were recorded on an MiniFlex600 (Rigaku) instrument equipped with the Bragg− Brentano geometry, and with iron-filtered Cu Kα 1,2 radiation.

Ab initio calculations.
All ab initio calculations were performed with ORCA 3.0.1 computational package 36 on the experimental X-ray structure of CoL4, without employing optimization of the molecular structure by computational methods. The relativistic effects were also included in the calculation with zero order regular approximation (ZORA) 37,38 together with the scalar relativistic contracted version of def2-TZVP(-f) basis functions. 39 The DFT calculations were based on B3LYP functional [40][41][42][43] and utilized the RI approximation with the decontracted auxiliary def2-TZV/J Coulomb fitting basis set and the chain-of-spheres (RIJCOSX) approximation to exact exchange 44 . Increased integration grids (Grid5 and GridX5in ORCA convention) and tight SCF convergence criteria were used also.
The calculations of ZFS parameters were based on state average complete active space self-consistent field (SA-CASSCF) 45  metal-based d-orbitals (CAS (7,5)). The state averaged approach was used, in which all ten quartet states and forty doublets states were equally weighted. The calculations utilized the RI approximation with the decontracted auxiliary def2-TZV/C and def2-SVP/C Coulomb fitting basis sets and the chain-of-spheres (RIJCOSX) approximation to exact exchange. Increased integration grids (Grid4 in ORCA convention) and tight SCF convergence criteria were used. The ZFS parameters, based on dominant spin− orbit coupling contributions from excited states, were calculated through quasi-degenerate perturbation theory (QDPT), 51 in which an approximations to the Breit-Pauli form of the spin-orbit coupling operator (SOMF approximation) 52 and the effective Hamiltonian theory 53 were utilized.