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18 August 2026

15 Pages

Diastereoisomerism and SIM Behavior in Mononuclear Co(II) Systems Based on Mepirizole

and
Departament de Química Inorgànica, Facultat de Química, Universitat de València, Av. Vicent Andrés Estellés, 19, Burjassot, 46100 Valencia, Spain
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Author to whom correspondence should be addressed.
Magnetochemistry2026, 12(8), 90;https://doi.org/10.3390/magnetochemistry12080090
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Abstract

Two diastereoisomers of bis(mepirizole)bis(isothiocyanato-κ,N)cobalt(II) (1 and 2) are isolated from the reaction of cobalt(II) thiocyanate and mepirizole, depending on the crystallization solvent (ethanol, 1; acetonitrile, 2). In both crystal structures, the Co(II) ions exhibit distorted octahedral [CoN4N′2] environment. The analysis of the packing frameworks shows a cooperative relationship between non-classical H-bonds C(sp3)-H···X (X = N, O, S, π) and π-hole bonds, which control the arrangement of the supramolecular 3D networks. The values of the shortest intermolecular metal–metal separation are 8.584(2) Å in 1 and 8.249(1) Å in 2. Both diastereoisomers exhibit magnetic behavior typical of mononuclear Co(II) systems with significant zero-field splitting (ZFS) values, with D being 75.8(1) and 52.9(2) cm−1 for 1 and 2, respectively. Q-band EPR studies confirm the positive value for the D parameters for both compounds. Alternating current dynamic susceptibility measurements show that 1 and 2 exhibit field-induced slow relaxation of the magnetization, which is reminiscent of single-ion magnet (SIM) behavior.

1. Introduction

Mepirizole, or 4-methoxy-2-(5-methoxy-3-methyl-1H-pyrazol-1-yl)-6-methylpyrimidine, is a non-steroidal anti-inflammatory drug which has been investigated for decades [1,2,3]. In addition to its biological activity, this pyrimidine-pyrazole derivative molecule has been studied in coordination chemistry as a bidentate ligand, through two free nitrogen atoms, one from each ring, with significant steric hindrance in the formation of metal complexes (Scheme 1) [4,5,6,7,8,9,10,11].
Scheme 1. Molecular structure of mepirizole (mep).
The mepirizole ligand has allowed the construction of systems of variable dimensionality generated from [L-M-X] and/or [L-M-X-L] molecular building blocks (MBBs), where L is the mepirizole, M is a 3d metal ion and X is an extended-bridging ligand as oxalate, squarato or pyrazolato [4,5,6,7,8,9,10]. Thus, we have previously reported the structural, spectroscopic and magnetic characterization of some transition metal networks ranging from mononuclear entities [4,5,6], through dinuclear compounds [7,8,9], to polymeric systems [10]. Significantly, no single-ion magnet (SIM) based on the mepirizole molecule has been reported up to date.
One of the current challenges in the design of new molecular materials is the preparation and characterization of supramolecular metal–organic materials (MOMs), in which the assembly of MBBs through intermolecular interactions can afford non-covalent networks. The presence of non-conventional interactions (NCIs) in MOMs has been relatively little studied and their role in the modulation of magnetic properties is in many cases controversial. Thus, the study of NCIs can become a difficult task in terms of their definition and type and also of their influence on the magnetic properties of crystalline solids [12,13,14,15,16,17,18,19,20,21,22,23,24,25].
Structural studies conducted in numerous systems involving N-containing heterocyclic ligands frequently show the formation of intricate structural frameworks supported by weak/moderate NCIs, such as non-classical H-bonds [26,27,28] and π-hole bonds [29,30], such as π···π interactions [31,32,33,34,35,36,37,38] and/or lone-pair···π/anion···π contacts [39,40,41,42,43,44,45,46,47,48,49,50,51,52]. In this context, the chemistry of multifunctional N-containing heterocyclic derivatives (especially those that combine the presence of both diazoles and diazines rings) has been a focus of interest in recent decades, partially due to the role that these systems develop in many biological processes [53]. Their coordinative capacity in conjunction with their proven ability to take part simultaneously in several NCIs provide the directional motifs necessary to lead to the self-assembly of new metallo-organic networks with interesting properties [54,55,56,57,58,59].
In the last decades, the study on the synthesis and characterization of the so-called single-molecule magnets (SMMs) and single-chain magnets (SCMs) has received notable attention in various research areas, particularly in the coordination chemistry and materials science fields [60,61,62,63,64,65,66,67,68]. In recent years, great attention has been focused on the characterization of coordination compounds in which the MBBs are neutral or charged entities with a single spin carrier with high magnetic anisotropy. When SMM-based systems show characteristic behavior of a single ion, they are known as single-ion magnets (SIMs). These requirements mean that certain 3–5d and 4f ions can be considered as potential candidates for the design and characterization of this type of systems [69,70,71,72,73,74,75,76]. One of the candidates that has aroused the most interest is the octahedral high-spin Co(II) (S = 3/2). The relative plasticity of their coordination sphere, the variability of zero-field splitting (ZFS) values, and the versatility of coordination environments are factors that make cobalt(II) ion an excellent candidate for building SIM-based systems [77,78,79,80,81,82,83,84,85,86,87,88,89,90].
We herein report the preparation, crystal structure, and magnetic characterization of two diastereoisomers of Co(II) based on mepirizole (mep) and thiocyanate ligands, with formula [Co(mep)2(NCS)2] (1 and 2). The analysis of their crystal lattices shows that these are supported by weak C(sp3)-H···X (X = N, O, S, π) interactions as well as π-hole contacts, the latter being those that would determine the differences between both supramolecular structures. The ac magnetic susceptibility study performed on 1 and 2 reveals field-induced slow relaxation of magnetization, which is typical of SIM behavior in both systems. Hence, 1 and 2 are the first examples of SIM systems based on the mepirizole molecule that have been reported up to date.

2. Results and Discussion

2.1. Description of the Crystal Structures

Compounds 1 and 2 are mononuclear systems that crystallize as purple crystals in the triclinic system (space group Pī) and as pink crystals in the monoclinic system (space group P21/n), respectively. Compound 1 is isostructural with the previously reported manganese(II) derivative of formula [Mn(mep)2(NCS)2] [6]. The crystal structure of both compounds is built up of isolated [Co(mep)2(NCS)2] molecules (or molecular building blocks, MBBs) connected through an extended and intricate π···π and weak C-H···X (X = N, O, S, π) interactions network. Compounds 1 and 2 are achiral, as they are centrosymmetric structures.
Figure 1 shows a simplified view of the mononuclear [Co(mep)2(NCS)2] entities with detailed labels of the atoms (Figure S1 displays a perspective view of the complexes with more extended labeled atoms). Selected bond distances and angles are listed in Table S1. Each cobalt(II) ion in 1 and 2 is surrounded by six nitrogen atoms, belonging to two thiocyanato κN ligands groups in the cis position and two mepirizole bidentate (κ2N,N’) ligands. The Co-N(mep) bond lengths (with mean values of approximately 2.20(1) (1) and 2.17(1) Å (2)) are longer than those corresponding to the Co-NCS ones (with mean values of ca. 2.06(1) and ca. 2.05(1) Å for 1 and 2, respectively), which describe a distorted octahedral environment around the metal ion (with CoN6 chromophores in both complexes).
Figure 1. (a) Diastereoisomer of [Co(mep)2(NCS)2] (1) as cis-isothiocyanato-κN and cis-Npymd; (b) diastereoisomer of [Co(mep)2(NCS)2] (2) as cis-isothiocyanato-κN and cis-Npyz (see text). Aromatic ring substituents and H atoms have been omitted for clarity. Only the labels of the coordinated N atoms are displayed.
In 1, a pyrimidine nitrogen atom (Npymd) and a pyrazole nitrogen atom (Npyz) of two different ligand molecules, along with two nitrogen atoms from two thiocyanato-κN ligands, define the equatorial plane around the metal ion. However, in 2, the equatorial plane is made up of two pyrazole nitrogen (Npyz) atoms and the two nitrogen atoms from the coordinated thiocyanates. The axial positions are occupied by two nitrogen atoms, Npymd and Npyz in 1 and two Npymd in 2 (Figure 1).
Consequently, these mononuclear octahedral [Co(mep)2(NCS)2] (1 and 2) complexes are diastereoisomers systems, as they are non-superimposable and non-mirror images [91,92]. The thiocyanate anions are nearly linear, with a mean NCS angle value of 179.1(1)° in 1 and 179.4(1)° in 2. All the molecular parameters observed for the mepirizole molecules are similar to those previously found in other mepirizole-containing complexes [4,5,6,7,8,9,10,11]. The dihedral angles between the pyrazole and pyrimidine rings vary between 11.5(1)° and 12.4(1)° in 1 and between 13.0(1)° and 14.0(1)° in 2. The octahedral distortion can be described through the Δ parameter (Δ = 0 for a regular octahedron and Δ = 1 for a trigonal prism) defined by Muetterties and Guggenberger [93]. Compounds 1 and 2 show Δ values of approximately 0.07 and 0.03, respectively. This indicates a slightly distorted octahedral geometry for both polyhedral systems, being more strongly marked in 1, which would affect the magnetic properties. This distortion could be produced by the bite angles of the mepirizole ligand on the metal ion (these values are 74.0(1)° for 1 and 76.1(1)° for 2, which are far from the value for a regular octahedron (90°)).
The crystal packing of both crystal structures is mainly determined by the interplay of non-classical H-bonds and π-hole bonds (π···π or anion···π interactions), but with subtle differences. With regard to the non-classical H-bonds network, the only possible H-donors are the groups C(sp2)-H (from pyrimidine and pyrazole rings) and C(sp3)-H (from methyl groups). On the other hand, the possible H-acceptors are the oxygen atoms (from methoxide groups), the nitrogen atoms (from heterocyclic rings as well as the thiocyanate groups) and the sulfur atoms from the thiocyanate ligand. Additionally, weak C-H···π interactions may contribute to the stabilization of the supramolecular framework. In the characterized compounds, only evidence of C(sp3)-H···X contacts (X = N, O, S, π) has been found. It must be pointed out that the main differences observed in the three-dimensional frameworks of 1 and 2 are the presence of significant π···π interactions in 1, but not in 2, and the presence of NCS···π contacts in 2, but not in 1 (Figures S2 and S3).
Packing of the MMBs in the crystals is mainly controlled by offset face-to-face (OFF) π–π interactions involving pyrazole and pyrimidine aromatic rings from mepirizole, with significant differences between both compounds. Therefore, in the structure of 1, each [Co(mep)2(NCS)2] entity is joined to a centrosymmetrically related one by means of two OFF π(pyrazole)···π(pyrimidine) interactions (centroid-to-centroid distance = 3.42(1) Å and plane-to-plane distance = 3.66(1) Å) (Figure 2 and Figure S2). The interdinuclear Co···Co distance is ca. 8.58(2) Å. In addition, four weak C-H···X (X = N, O) interactions contribute to stabilize the dinuclear entities (Table S1). Then, these dimers are further connected to each other through a weaker OFF π(pyrimidine)···π(pyrimidine) interaction to afford infinite chains running parallel to the [101] direction, with the shortest intermetallic Co···Co separation being ca. 10.62(2) Å (Figure 2).
Figure 2. View of the π···π interactions (dotted green line) between adjacent [Co(mep)2(NCS)2] entities generating a 1D structure along the [101] direction in 1 (top). View of the weak C-H···S (dotted blue line) interactions between neighboring [Co(mep)2(NCS)2] units showing the formation of a 1D structure running parallel to the [202] direction in 2 (bottom).
With regard to the packing of 2, the main interactions connecting the neighboring [Co(mep)2(NCS)2] entities are based on the C16-H16A···S2 interaction (Figure S3 and Table S1). This interaction forms infinite chains (the shortest interdinuclear Co···Co distance being ca. 8.25(1) Å) and is roughly running parallel to the [202] direction (Figure 2).
In both 1 and 2, additional weak C-H···X (X = N, O, S) interactions generate 2D networks (Figure 3). In 1, the C17-H17B···O3 and C11-H11B···S2 interactions connect the chains together into sheets parallel to the crystallographic ac plane. On the other hand, the layered structure in 2 is formed by cross-linking the above-described chains by means of the intermolecular C17-H17B···O1 and C14-H14A···O1 interactions (Figure 3).
Figure 3. (a) View along the crystallographic b axis of the 2D network formed through intermolecular C-H···X [X = N, O and S] and π···π interactions in 1. (b) View along the crystallographic b axis of the 2D network formed mainly through intermolecular C-H···X [X = N, O and S] interactions in 2.
Finally, in both systems, the third dimension of the supramolecular assembly is generated by the connection of these layers via additional C-H···X (X = N, O, S) interactions, which contribute to the stabilization of the crystal structures of 1 and 2 (Table S1).

2.2. Magnetic Properties

2.2.1. Dc Magnetic Susceptibility

The dc magnetic susceptibility measurements were carried out on microcrystalline samples of 1 and 2 (Figure 4) in the 2–300 K temperature range and under an external magnetic field of 0.1 T. The thermal variation of the χMT product plots (χM being the molar magnetic susceptibility per mononuclear Co(II) unit) for 1 and 2 are shown in Figure 5a and Figure 5b, respectively. At 300 K, the χMT values are approximately 3.00 (1) and 3.17 cm3mol−1K (2), which are somewhat greater than that expected for a magnetically isolated center with S = 3/2 and g = 2.00, indicating a significant orbital contribution typical of mononuclear high-spin (HS) Co(II) complexes [85,86,87]. The χMT values in both compounds decrease with temperature slowly until they reach 100 K, then they decrease much faster, reaching a minimum value of 1.80 (1) and 1.90 cm3mol−1K (2) at ca. 2.0 K (Figure 5).
Figure 4. (a) Plot of the theoretical and experimental powder X-ray diffraction (PXRD) patterns (2θ/°) in the range 0–50° for 1; (b) plot of the theoretical and experimental powder X-ray diffraction (PXRD) patterns (2θ/°) in the range 0–50° for 2.
Figure 5. Thermal variation of the χMT product for 1 (a) and 2 (b), with the red solid line being the best theoretical curve. The insets in (a,b) display the field dependence of the magnetization at 2.0 K for both compounds, with the black solid line being a guide for the eye.
Considering the large metal···metal distance indicated in the crystal structure description of compounds 1 and 2 (ca. 8.2–8.5 Å), the decrease in their χMT values may be assigned to zero-field splitting (ZFS) effects rather than to the presence of strong intermolecular interactions. The field dependence of the molar magnetization (M) plot at 2.0 K for 1 and 2 is given in the respective insets of Figure 5. As observed for both compounds, this plot displays a continuous increase in the M values with the applied magnetic field, and no hysteresis loop is detected at the studied temperature, with the higher M values being approximately 2.2 and 2.4 µB for 1 and 2, respectively. These values agree with those observed for previously reported mononuclear Co(II) complexes exhibiting similar structural parameters [85,86,87,94].
H ^ =   D [ S ^ z 2   −   S   ( S   +   1 ) / 3 ]   +   E   ( S ^ x 2   −   S ^ y 2 )   +   g μ B H S ^
The experimental data of the molar magnetic susceptibility of 1 and 2 were analyzed by means of the Hamiltonian in Equation (1) [94] where S is the spin ground state (quartet) for Co(II) metal ion, D and E are the axial and rhombic zero-field splitting (ZFS) parameters, respectively, and g is the Landé factor, µB is the Bohr magneton, and H is the applied magnetic field. In the expected octahedral environment of both compounds, the D value would lead to the splitting of the four sublevels into the MS = ±1/2 and MS = ±3/2 doublets of the paramagnetic Co(II) metal ion (S = 3/2).
In order to avoid any overparameterization, it was considered that g = g ∥ = g⊥ for both compounds. As in previous works [25,95], the experimental data for 1 and 2 were fitted by using the PHI program in the 2–300 K temperature range [96], with the best fitting process generating the following magnetic parameters: D = 75.8(1) cm−1, E/D = 0.11(1) and g = 2.52(1) with R = 3.7 × 10−5 for 1 and D = 52.9(2) cm−1, E/D = 0.00(0) and g = 2.64(2) with R = 4.1 × 10−5 for 2, with R being the agreement factor which is equal to ∑i[(χMT)obs(i) − (χMT)calc(i)]2/∑i[(χMT)obs(i)]2.
As shown in Figure 5, the theoretical curve for 1 and 2 (red solid line) reproduces quite well the experimental magnetic data in the whole studied temperature range. The computed magnetic parameters, mainly D and g, are found to be consistent with those previously reported for systems based on mononuclear Co(II) complexes [85,86,87,94]. The distinguishable D values obtained for 1 and 2 would account for the different geometries that these diastereoisomers exhibit, with 1 displaying a higher D value and a more distorted octahedral geometry than 2.
Given that it is difficult to obtain a reliable sign of D just from magnetic susceptibility measurements performed on microcrystalline powder samples, EPR measurements were also carried out on our systems. Thus, additional powder Q-band EPR measurements support a positive sign of the D parameter for both 1 and 2 (Figure S4), as in these spectra it is observed that the signal intensities invariably decrease when the temperature is increased. These features are consistent with a positive value of D, as previously observed in systems exhibiting these transitions of the Kramers’ doublet of hexacoordinate Co(II) complexes, with the Kramers’ doublet Ms = ±3/2 not contributing to these signals [94,97]. Indeed, the magnitude and the sign of the D value obtained for compounds 1 and 2 would indicate that these mononuclear systems behave at low temperature as a doublet rather than as a quartet, with the MS = ±1/2 doublet lying below the MS = ±3/2 doublet in energy [94,97,98].

2.2.2. Ac Magnetic Susceptibility

In order to explore the possible single-ion magnet (SIM) phenomenon in 1 and 2, ac susceptibility measurements were performed on microcrystalline samples of both compounds in the temperature range of 2–12 K and under a 5.0 G ac field oscillating at several frequencies (300–10,000 Hz). No out-of-phase ac signals (χ″M) are observed at Hdc = 0 G for either 1 or 2, a fact which could suggest a fast quantum tunneling (QT) of the magnetization (QTM) or strong hyperfine interaction with the I = 7/2 nuclear spin of the Co(II) metal ion, as previously reported [98]. However, both compounds exhibit out-of-phase ac signals at very low temperatures when an external dc magnetic field is applied, which is indicative of a system showing slow relaxation of magnetization typical of SIM behavior. Several dc magnetic fields were studied in the 1000–5000 G range, with Hdc = 1000 G being the optimal one. Out-of-phase (χ″M) ac signals for 1 and 2 are shown in their respective temperature-dependent ac magnetic susceptibility plots at the 300–10,000 Hz range of frequencies in Figure 6.
Figure 6. Temperature dependence of out-of-phase ac magnetic susceptibility signals, under a dc field of 1000 G and frequencies of 300–10,000 Hz, for 1 (a) and 2 (b). Each inset in (a,b) shows the ln (τ) versus 1/T plot with the best fit (red solid line) considering the contribution of two mechanisms (direct + Raman).
In both cases, the χ″M versus T plot maxima are observable in the temperature ranges 3–6 K for 1 and 2–6 K for 2, and decrease their intensity with increasing temperature and frequency (Figure 6). The ln(τ) versus 1/T plot for 1 and 2 show experimental data drawing a straight line (high-temperature region) followed by a small curved line (low-temperature region), most likely indicating the occurrence of a sum of diverse relaxation processes, such as thermally activated, Orbach, QT, direct or Raman mechanisms, which could contribute to the relaxation of the magnetization (Figure 6). Consequently, the high-temperature data were fitted to the Arrhenius equation (τ = τoexp(Ea/kBT)), thus evaluating the anisotropy energy barrier to the magnetization reorientation (Ea) and the pre-exponential factor (τo) in this region for 1 and 2 (kB being the Boltzmann constant), just for comparison with previously reported values (Table 1).
Table 1. Parameters of the magnetic relaxation obtained through the dc applied magnetic field of 1000 G for compounds 1 and 2.
The values obtained for the parameters Ea and τo are gathered in Table 1. Compounds 1 and 2 show Ea values, which would be associated with a high-temperature Orbach relaxation process, which are close in order of magnitude to 20 cm−1, which has been previously reported for other similar hexacoordinated Co(II) complexes with SIM behavior and D > 0. However, the Ea values obtained for 1 and 2 should be carefully considered, as their orders of magnitude are smaller than the energy gap between the ground state and the next excited energy level, which would involve a real transition to an intermediate magnetic energy level that actually does not exist at such energies, as previously reported [94,98]. Nevertheless, the whole experimental ln(τ) versus 1/T curves of 1 and 2 were well reproduced when a direct and a Raman mechanism were considered simultaneously in the expression τ−1 = AT + CTn, where the first and second terms are assigned to one-phonon direct and two-phonon Raman processes, respectively (Figure 6). However, the existence of other involved processes of relaxation cannot be totally excluded.
The final A, C and n parameters obtained for 1 and 2 are listed in Table 1. These calculated values are in agreement with those previously reported for SIMs based on octahedral Co(II) complexes with similar structural parameters [94,97,98,99]. The n values obtained for 1 and 2 are less than 9 (a figure which is associated with the Raman relaxation of Kramer ions) [97,98,99,100]. Nevertheless, the n values tabulated in Table 1 fall into the range typical of SIMs with relaxation through optical and acoustic Raman-like processes, as studied previously on both 3–5d-based and 4f-based SIMs [97,98,99,100].
Cole–Cole curves for 1 and 2 obtained in the range of frequencies 100–10,000 Hz, in the range of temperatures 4.0–5.5 K and under a dc magnetic field of 1000 G are plotted in Figure 7. The experimental data drew semicircular curves, which were accordingly treated through the Cole–Cole expression χ = χS + [χT − χS/(1 + (iωτ)1−α)], which is written for magnetic susceptibility, where 0 ≤ α <1 and χS and χT are the adiabatic and isothermal magnetic susceptibilities, respectively [101,102,103,104,105,106]. The final α values that were obtained cover the ranges 0.03–0.15 for 1 and 0.05–0.17 for 2 in the explored temperatures selection (Figure 7). Given that these α values are close to zero (and far from α = 1), a very narrow distribution of slow relaxation times in the regions of relaxation of our mononuclear Co(II) complexes would be expected [94]. Hence, our results support a single relaxation process in the temperature range investigated for these Co(II)-based SIM systems, thus discarding any possible behavior of spin-glass material [106,107,108,109,110,111].
Figure 7. Cole–Cole plots for 1 (a) and 2 (b) obtained from ac magnetic susceptibility measurements at the indicated temperatures and under an applied dc magnetic field of 1000 G. The solid lines are the best-fit curves.

3. Materials and Methods

3.1. Reagents and Instruments

All the reagents used in this work were used as supplied by commercial sources, and the syntheses were carried out under aerobic conditions. Elemental analyses were performed in an analyzer in the Central Service for the Support to Experimental Research (SCSIE) at the University of Valencia (Hitachi High-Tech, GLOBAL, Minato-ku, Tokyo, Japan). Infrared spectra were recorded (as KBr pellets) with a PerkinElmer Spectrum 65 FT-IR spectrometer in the 4000–400 cm−1 region (PerkinElmer, Inc., Waltham, MA, USA; OMNIC 9.0 software). EPR spectra were obtained on a Bruker ELEXSYS E580 EPR spectrometer (Billerica, MA, USA), working on a solid sample at temperatures ranging from 4 to 20 K, with Q-band (~34 GHz) and applied magnetic fields up to 2 T (Figure S4). Variable-temperature, solid-state (dc and ac) magnetic susceptibility data down to 2.0 K were collected on Quantum Design MPMS-XL SQUID and Physical Property Measurement System (PPMS) magnetometers (Quantum Design, San Diego, CA, USA). The experimental magnetic data were corrected for the diamagnetic contributions of the constituent atoms as well as for the sample holder [112].

3.2. Preparation of the Compounds

3.2.1. Synthesis of Compound 1

An ethanolic solution of mepirizole (1.0 mmol, 20 mL) was added dropwise and with continuous stirring to Co(II) thiocyanate (0.5 mmol) dissolved in ethanol (20 mL). The resulting violet solution was filtered off and left to evaporate at 20–25° C. Purple crystals of 1 were obtained after 1 week. These crystals were filtered off and washed first with cold ethanol and then with diethyl ether. Yield: ca. 45%. Anal. Calcd. for C24H28N10O4S2Co (1): C, 44.8; H, 4.4; N, 21.8. Found: C, 45.0; H, 4.9; N, 21.7. IR peaks (KBr pellets, ν/cm−1): 3432(w), 3100(m), 2979(m), 2944(m), 2875(w), 2073(vs), 1611(vs), 1559(s), 1500(s), 1437(s), 1372(s), 1330(m), 1257(m), 1225(m), 1189(s), 1077(m), 1055(m), 1037(m), 1005(m), 932(w), 848(m), 794(m), 762(m), 682(w), 620(w), 582(m), 550(m), 477(m).

3.2.2. Synthesis of Compound 2

Compound 2 was prepared following the same synthetic procedure as that of 1, but by using acetonitrile instead of ethanol, thus obtaining pink crystals of 2. Yield: ca. 35%. Anal. Calcd. for C24H28N10O4S2Co (2): C, 44.8; H, 4.4; N, 21.8. Found: C, 44.9; H, 4.3; N, 21.5. IR peaks (KBr pellets, ν/cm−1): 3430(w), 3100(s), 2978(m), 2944(m), 2876(w), 2068(vs), 1610(vs), 1558(s), 1500(s), 1435(m), 1373(m), 1330(m), 1257(m), 1225(m), 1190(m), 1075(m), 1055(m), 1036(m), 1005(m), 905(w), 800(m), 778(w), 682(w), 620(w), 581(m), 551(m), 480(m), 421(w).

3.3. X-Ray Diffraction Data Collection and Structure Refinement

X-ray diffraction (XRD) data collection from single crystals of 1 and 2 was performed using a Bruker D8 Venture diffractometer (Bruker, Billerica, MA, USA) with graphite-monochromated Mo–Kα radiation (λ = 0.7107 Å). Both crystal structures were solved by standard direct methods and subsequently completed by Fourier recycling by using the SHELXTL (version 2017/1; Bruker Analytical, Madison, WI, USA) software packages. The obtained model was refined with the SHELXL program against F2 on all data by full-matrix least squares [113]. All non-hydrogen atoms were anisotropically refined, whereas the hydrogen atoms of the mepirizole molecule were set in calculated positions and refined isotropically by using the riding model. The graphical manipulations were performed with the Mercury 3.8 program [114]. Powder X-ray diffraction (PXRD) measurements were performed using a PANalytical Empyrean diffractometer (PANalytical B.V., Almelo, The Netherlands, with PIXcel detector and Cu–Kα radiation, λ = 1.5406 Å). The 2θ peaks of both systems compare well with the theoretical diffractograms obtained from Mercury program, thus indicating homogeneity of the bulk samples for 1 and 2 (Figure 4). The CCDC numbers for 1 and 2 are 2574445 and 2574446, respectively.

4. Conclusions

In this work, we showed that the use of solvents of different polarity allowed us to isolate two diastereoisomers of the complex [Co(mep)2(NCS)2] (mep = mepirizole). Both mononuclear cobalt(II) complexes had two thiocyanato-kN ligands (in cis positions) and four nitrogen atoms from two mepirizole molecules (1, cis-isothiocyanato-κ1N, cis-Npymd; 2, cis-isothiocyanato-κ1N, cis-Npyz), describing somewhat distorted octahedral surroundings. Both diastereoisomers differed in the spatial arrangement of the mepirizole molecules as well as in the role played by the π-hole interactions in the organization of the supramolecular networks. Alternating current magnetic susceptibility measurements show that both diastereoisomers exhibit field-induced slow relaxation of the magnetization with not-negligible differences in their relaxation dynamics, which can be associated with some variations in their respective geometries and spin lattices. The reported systems are the first examples of SIMs based on the mepirizole molecule and, no doubt, many other mepirizole-based SIMs will be investigated and reported in future research works.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/magnetochemistry12080090/s1, Figure S1: Perspective view and atomic numbering of [Co(mep)2(NCS)2] entities in the crystal structures of compounds 1 and 2; Figure S2: Perspective view of the [Co(mep)2(NCS)2] units showing the π···π and the C-H···X (X = N, O) interactions (dotted blue lines) of compound 1; Figure S3: Perspective view of NCS···π contacts in 2; Figure S4: Q-band polycrystalline powder EPR spectra of 1 and 2; Table S1: Selected non-conventional hydrogen bonds (D-H···A) and contacts (D···A) (Å) and angles (°).

Author Contributions

Conceptualization and methodology, E.E.; formal analysis and investigation, E.E. and J.M.-L.; writing—original draft preparation, E.E. and J.M.-L.; writing—review and editing, E.E. and J.M.-L.; funding acquisition, J.M.-L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was mainly funded by the Universitat de València (UV), Project REDAMET, with grant number UV-INV-AE25-4216008.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The reported data are available on request from the corresponding author.

Acknowledgments

The authors would like to thank José María Martínez and Nicolás Moliner for their advice and assistance in EPR and SQUID measurements, respectively.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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