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Spin Frustrated Pyrazolato Triangular CuII Complex. Structure and Magnetic Properties, an Overview.

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Abstract
The synthesis and structural characterization of a new triangular Cu3-3OH pyrazolato complex of formula, [Cu3(μ3-OH)(pz)3(Hpz)3][BF4]2 (1-Cu3), Hpz = pyrazole, is presented. The triangular unit forms a quasi-isosceles triangle with Cu-Cu distances of 3.3739(9), 3.3571(9), and 3.370(1) Å. This complex is isostructural to the hexanuclear complex [Cu3(μ3-OH)(pz)3(Hpz)3](ClO4)2]2 (QOPJIP). A comparative structural analysis with other reported triangular Cu3-3OH pyrazolato complexes has been done, showing that, depending on the pyrazolato derivative, auxiliary ligand or counter-anion can affect the nuclearity and/or the dimensionality of the system. The magnetic properties of 1-Cu3 are analyzed by experimental data and DFT calculation. A detailed analysis is done on the magnetic properties comparing experimental and theoretical data of other molecular triangular Cu3-3OH complexes, showing that the displacement of the μ3-OH- from the Cu3 plane, together with the type of organic ligands, influences the nature of the magnetic exchange interaction between the spin-carrier centers, since it affects the overlap of the magnetic orbitals involved in the exchange pathways. Finally, a detailed comparison of the magnetic properties of 1-Cu3 and QOPJIP was done, which allowed us to understand the differences in their magnetic properties.
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Subject: Chemistry and Materials Science  -   Electronic, Optical and Magnetic Materials

1. Introduction

Triangular CuII complexes have been largely studied in the literature, and among them, several systems present a μ3-X- (X = Cl-, Br-, OH-, O2-) bridging unit that, together with other organic auxiliary ligands enables to obtain very stable systems [1,2,3]. Due to their high stability, these triangular fragments can be used as secondary building units (SBU) in constructing several coordination polymers or MOFs systems [4,5,6].
Moreover, triangular complexes are an interesting class of materials since they have been suggested as possible qubits, as they can present spin-electron coupling due to the interplay between three main factors (spin exchange, spin-orbit interaction, and chirality) [7,8,9,10,11,12]. Spin Frustration (SF) has been suggested as the origin of the abovementioned features. This phenomenon originates when an odd number of non-integer spin carriers that are antiferromagnetically coupled cannot be satisfied simultaneously, like in a triangular system [13]. Thus, the energy of the ground state is doubly degenerate, but distortions of the C3 symmetry of the triangle or by the antisymmetric exchange, which is related to spin-orbit interactions, can break this degeneracy by lowering the symmetry of the system [14,15].
These triangular CuII systems have been largely studied since they formed the simplest spin-triangle. This has allowed the possibility of studying in detail the magnetic properties of geometrically spin-frustrated systems [16]. Among these systems, the ones with a hydroxy bridge (μ3-OH-) are among the most reported in the literature [17,18]. Systems presenting pyrazolato, triazolato, and other types of auxiliary organic ligands have been magnetically studied in the literature [19,20]. In general, the displacement of the μ3-OH- from the Cu3 plane, together with the type of organic ligands, have been related to the nature of the magnetic exchange interaction between the spin-carrier centers since they affect the overlap of the magnetic orbitals involved in the exchange pathways [21].
Among all the mentioned systems, Cu3-μ3OH pyrazolato complexes are among the most reported systems, and they present strong antiferromagnetic properties [22,23]. However, they have not been extensively analyzed in search of magneto-structural features, as has been done for the triazolato complexes [24]. These compounds, depending on the pyrazolato derivative, auxiliary ligand, or counter-anion, may present different nuclearity and/or dimensionality [3,19].
In this work, we present the synthesis and structural characterization of a triangular Cu3-μ3OH pyrazolato complex of formula, [Cu3(μ3-OH)(pz)3(Hpz)3][BF4]2 (1-Cu3), Hpz = pyrazole. Interestingly, this trinuclear complex is isostructural to the hexanuclear QOPJIP structure [Cu3(μ3-OH)(pz)3(Hpz)3(ClO4)2]2, since the perchlorate anions connect the two triangular units [25]. An extensive structural analysis with other reported triangular Cu3-μ3OH pyrazolato complexes has been done. The magnetic properties of 1-Cu3 are analyzed by experimental data together with DFT calculation, showing that strong antiferromagnetic interactions exist between the CuII centers. We present a detailed analysis of the magnetic properties of 1-Cu3 and compare them with the experimental data of other molecular triangular Cu3-μ3OH pyrazolato complexes and with the theoretical magnetic properties of a previously reported Cu3-μ3OH complex [21]. Finally, we perform a detailed study of the magnetic properties of 1-Cu3 and QOPJIP to understand the differences in their magnetic properties.

2. Results and Discussion

ESI-Mass and FTIR Spectra

ES-MS in positive mode (acetonitrile) shows the existence of different fragments of the [Cu3-μ3OH]n+ unit, such as: {[Cu3(μ3-OH)(pz)3(Hpz)3][BF4]}+ (m/Z = 700); {[Cu3(μ3-OH)(pz)3(Hpz)3]+1e-}+ (m/Z = 613); {[Cu3(μ3-OH)(pz)3(Hpz)2]+1e-}+ (m/Z = 544); {[Cu3(μ3-OH)(pz)3(Hpz)1] ]+1e-}+ (m/Z = 476) and {[Cu3(μ3-OH)(pz)3}+ (m/Z = 408). See Figure S1. Complementary analyses (EA and FTIR spectroscopy) confirm the purity of the crystalline material (see supporting information, section S2, FTIR).

Structure Analysis

The triangular complex (1-Cu3) crystallizes in the centrosymmetric monoclinic space group P21/c (For more information, see CIF file and section S3). The molecular structure consists of a triangular [Cu3-μ3OH]n+ core surrounded by three protonated Hpz and three deprotonated pz- ligands, forming the cationic complex [Cu3(μ3-OH)(pz)3(Hpz)3]2+, which is counterbalanced by two tetrafluoroborate anions. The trinuclear unit is formed by two CuII (Cu1 and Cu3) centers with a square pyramid (SqP) geometry and an octahedral Cu2 center (Oh) with a Jahn-Teller distortion. This triangular unit presents pseudo-three-fold symmetry forming an isosceles triangle, with copper-copper distances of 3.3740(8), 3.3574(8), and 3.3702(8) Å for Cu1-Cu2, Cu2-Cu3, and Cu1-Cu3, respectively. As observed for similar systems, the μ3-OH- group is not coplanar with the plane formed by the three copper centers, displaced by 0.439 Å. Other displacements reported in the literature for the [Cu3-μ3OH]n+ are in the range of 0.363 and 0.759 Å [23,26]. The metal centers present three different types of Cu-O, Cu-N, and Cu-F bonds. The first is around 2.00 Å, the second is between 1.98 and 2.02 Å, and the third is between 2.48 and 2.58 Å (Figure 1).
An original aspect of 1-Cu3 is the presence of two [BF4]- counter-anions (B1 and B2) coordinated to the copper centers of the trinuclear unit. In fact, 1-Cu3 is the first example of a triangular pyrazolato complex with this type of counter-anion. One of the [BF4]- anions (B1) presents a μ3 coordination mode with three F atoms coordinated to the three CuII centers (with F-Cu distances of 2.483(3), 2.530(4), and 2.581(4) Å). The other [BF4]- anion (B2) is only coordinated by one F atom to a single CuII center (Cu2-F5 = 2.557(4) Å). The structure presents an inversion center (outside the complex) that generates a second triangular [Cu3(μ3-OH)(pz)3(Hpz)3][BF4]2 unit, where the fluorine atom (F7) of the [BF4]- anion (B2) is semi-coordinated to Cu1 with a long distance of 2.812(3) Å. Finally, it is worth mentioning that between the triangular units, there are some hydrogen bonds that stabilize the crystal lattice of the complex, with inter-cluster Cu···Cu distances ranging between 7.309(1) and 13.4522(9) Å (Figure 2).
According to the CCDC database, there are at least 96 structures based on pyrazolato (R-pz-) ligands, forming complexes with the general formula [Cu3(μ3-OH)(R-pz)3(L)3]n+/-, where R = -H, -CH3, -NO2, among others, and L = pz0/-, Cl-, H2O, NO3-,… [2,6,22,23,25,26,27,28,29,30]. The nature of the axial ligand and the type of substitution of the pz- ligand leads to the formation of either high dimensional systems (usually for R = -COO-) or discrete complexes. If the axial ligand is monodentate or acts as a chelate or if the pz- ligand's substituent group cannot coordinate with other metal centers, discrete (0D) systems are formed, see Table 1. As a general trend, we observe that when larger ligands are present either as axial or auxiliar ligands, the distance between the Cu3 plane and the μ3-OH- group increases. We also observe that when auxiliary ligands are present, the triangular units can form hexanuclear units by coordinating these auxiliar ligands to the metal centers of the closest triangular units (RUYGEX, RUYGIB, RUYHEY, RETQUD, QOPJIP, DIBXOC, EGIXUQ, EHOLIZ).
Among the compounds of Table 1, QOPJIP: [Cu3(μ3-OH)(pz)3(Hpz)3][ClO4]2) [25] is isostructural to 1-Cu3 ([Cu3(μ3-OH)(pz)3(Hpz)3][BF4]2), although there are some differences, mainly related to the nature of the counter-anion. The smaller size of the [BF4]- unit located between the two triangular units (compared to ClO4-) leads to an important shortening of the distances between the Cu3-planes for 1-Cu3 (6.789 Å) as compared to QOPJIP (7.044 Å). This shortening allows the formation of a hydrogen bond between F8 and the hydrogen atom of the μ3-OH group (not observed in QOPJIP), enlarging the O-H bond in 1-Cu3 (0.991 Å), compared to QOPJIP (0.979 Å). The lower coordination capacity of BF4- compared to ClO4- is clearly observed in 1-Cu3, where the Cu3(μ3-OH) units are isolated (except for a very long semi-coordinated Cu1-F7 bond of 2.811(3) Å). In contrast, in QOPJIP, the ClO4- anion connects two triangular Cu3 units through four short Cu-O bonds (in the range 2.44-2.66 Å) to form a hexanuclear complex.

Magnetic Properties. dc Magnetic Measurements

The thermal variation of the product of the molar magnetic susceptibility per Cu3 unit, times the temperature for 1-Cu3, measured with a DC field of 100 mT, shows a value of around 0.5 cm3 K mol-1 at 300 K (Figure 3). This value is below the expected one for three uncoupled paramagnetic Cu(II) ions (1.125 cm3 K mol-1 with g = 2.0), indicating the existence of bulk antiferromagnetic interactions between the CuII atoms of the Cu3(μ3-OH) core. When the temperature is lowered, χmT steadily decreases, reaching a plateau between 130 and 100 K. Below 100 K, χmT further decreases and reaches a value of 0.26 cm3 K mol-1 at 2 K. The χmT value in the plateau is 0.38-0.40 cm3 K mol-1, which is the expected value for a trinuclear unit with an S = 1/2 ground state [24,25]. The field dependence of the magnetization at 2 K for 1-Cu3 shows at 5 T a value of around 0.7 μB, corresponding to ca. 0.7 electrons, although saturation is not fully reached at 5 T (Figure 3). This behavior is typical of systems with a μ3-hydroxido moiety with a ground state of S = 1/2 (M = 1 μB) that present magnetization values below the expected ones and do not reach saturation, even at high fields [17,35]. Comparing the experimental data with those calculated by the PHI program (see below) for 1-Cu3 shows a good agreement between them. The lower values of the experimental data confirm the presence of antisymmetric exchange.
The magnetic behavior observed at low temperatures for the χT data can be associated with the spin frustration phenomena, which allows the existence of an antisymmetric exchange, as described by Ferrer et al.[24]. This work described in detail the antisymmetric exchange interaction in a triangular Cu3 system based on triazolato derivatives. Additionally, we cannot discard that geometry distortions on the local coordination environment may influence the overall magnetic properties. In this sense, several discussions have arisen from this point, and according to Niedner-Schatteburg et al., spin frustration leads to geometric distortion [38,39,40].
The fit of the dc experimental data was done using the PHI program [41]. At first, only the isotropic interactions for the triangular arrangement were considered, giving a good fit in the 50-300 K range. The best fit in the whole temperature range was obtained by adding to the model the antisymmetric exchange (ASE; Gij), which is a non-isotropic interaction [15,24]. Thus, based on the structural arrangement of the CuII triangles, we have used a model with two isotropic exchange interactions (J1 and J2) for an isosceles triangle and an antisymmetric exchange, using the Hamiltonian shown below:
H ^ = − J 1 S 1 S 2 + S 2 S 3 − J 2 S 1 S 3 − G S ^ 1 × S ^ 2 + S ^ 2 × S ^ 3 + S ^ 1 × S ^ 3 + μ B g H ∑ i = 1 3 S ^ i
The best-fit parameters obtained for the isotropic exchange interactions are J1 = -193.5(6) cm-1 and J2 = -205.5(3) cm-1 with an antisymmetric exchange parameter, |GZ| = 28 cm-1 (solid line in Figure 3). These values are listed in Table 2, together with the magnetic parameters of selected molecular [Cu3-μ3OH]n+/- pyrazolato complexes. The isotropic interaction values are strongly antiferromagnetic, being similar to those reported for other pyrazolato and triazolato triangular Cu3OH complexes. The antisymmetric exchange interactions for triangular CuII hydroxido pyrazolato complexes have only been reported for two systems, VAZCOR (|GZ| = -18.2 cm-1) and YIFGIG (|GZ| = -31.2 cm-1) [17,35]. However, for complexes based on the triazolato ligand, there are more examples in the literature, with |GZ| values between 17.5 and 44 cm-1 [24]. Thus, the isotropic and antisymmetric exchange interactions obtained for 1-Cu3 are within the range observed for other triangular CuII hydroxy compounds (see Table 2).
Magneto-structural analysis on triangular systems was done using the experimental data of Table 2. According to the literature, two structural parameters have been selected to study their influence on the magnetic properties of these triangular systems. The first is the displacement of the μ3-OH- from the Cu3 plane, where the magnetic interaction becomes more antiferromagnetic when the displacement is smaller [42]. The second corresponds to the Cu-(μ3-X)-Cu angle, which seems to be sensitive to the magnetic coupling interaction. The magnetic coupling interaction is switched from ferromagnetic to antiferromagnetic when the angle varies from 76° to 120° [43]. The analysis of these structural parameters with the average magnetic exchange interactions shows that a general tendency is observed only with the displacement of the μ3-OH- from the Cu3 plane (Figure 4).
As mentioned in the Structural Analysis section, 1-Cu3, and QOPJIP are isostructural crystalline systems. According to the literature, the displacement of the μ3-OH- group from the Cu3 plane influences the magnetic properties. This effect shows that a larger displacement causes a weaker antiferromagnetic interaction, which can be related to a weaker overlap of the magnetic orbitals of the CuII centers in the triangular system [44]. However, the smaller displacement observed for 1-Cu3 (0.439 Å) than for QOPJIP (0.466 Å) suggests that 1-Cu3 should present a stronger antiferromagnetic interaction between the copper centers than QOPJIP. However, the opposite phenomenon is observed (Figure 5).
We have performed DFT calculations to rationalize the magnetic properties observed for 1-Cu3 (see Materials and Methods section) [18]. The results were compared to a previously reported theoretical study of the magnetic properties of several μ3-OH- bridged trinuclear CuII complexes [21]. The theoretical calculations for 1-Cu3 were done under the same level of theory as for the study mentioned above. The geometrical array of the triangular unit for 1-Cu3 permits to define three exchange pathways, with magnetic exchange interactions of J1 = -94,9 cm-1, J2 = -87,7 cm-1, and J3 = -98.6 cm-1. For QOPJIP, previously reported DFT calculations also describe three exchange constants: J1 = -118.3 cm-1, J2 = -106.0 cm-1, and J3 = -120.6 cm-1. The difference observed in the magnitude of the magnetic exchange interaction between the calculated and the one obtained by the fitting experimental data for both systems may be related to the so-called strong interaction limit, in which the weak interaction limit treatment of Noodleman would result in J-values generally twice as larger [18]. This difference could also be due because the experimental J values are obtained from bulk magnetic data that include other magnetic phenomena in the crystalline lattice. On the other hand, DFT calculations can isolate the magnetic phenomena for the molecular structure.
The DFT calculation of 1-Cu3 was completely validated since the overlap parameters, together with their calculated magnetic exchange interactions, fit well on the plot of the J values of the seven studied complexes as a function of the square of the overlap depicted in the previous work of reference [21]. A linear relationship can be observed, as expected from the Kahn-Briat overlap model (Figure 6). These results permit us to infer that the μ3-OH- bridged contributes to the exchange phenomenon, together with the other bridges. Finally, Mulliken spin density values were determined for four spin configurations. The obtained values for the CuII atoms are in 0.60-0.68 e- range, similar to those obtained for other similar CuII systems [18,21]. These results reflect that most of the electron spin density is located on the metal centers, and the rest of the spin density appears over the atoms of the first coordination sphere through a delocalization mechanism of the spin density. Figure S3 presents the spin density surfaces for the ferromagnetic solution ST = 3/2 and three broken-symmetry solutions ST = 1/2 for 1-Cu3. It is possible to observe that no polarization mechanism of the spin density is observed for the corresponding second coordination spheres.
Finally, from all the results discussed above, it is possible to conclude that both compounds, 1-Cu3 and QOPJIP, have a similar trinuclear structure with a μ3-OH- and μ2-pz- bridges and both systems show a tetrahedral anion with a μ3 coordination mode ([BF4]- and [ClO4]-). The average DFT calculated J values for 1-Cu3, and QOPJIP are -93.7 and -114.9 cm-1, respectively. The displacement of the μ3-OH- group from the plane of the three copper atoms is smaller for 1-Cu3 (0.439 Å) than for QOPJIP (0.466 Å); thus, the first system should have stronger antiferromagnetic interactions, in contrast with the experimental values. These results suggest that the μ3-ClO4- anion is not an innocent ligand and favors an antiferromagnetic exchange between the CuII centers, resulting in a stronger antiferromagnetic coupling in QOPJIP.

3. Materials and Methods

Synthesis of [Cu3(μ3-OH)(pz)3(Hpz)3][BF4]2 (1-Cu3)

Cu(BF4)2∙H2O (765.5 mg, 3 mmol) was dissolved in 20 mL of methanol. Then, a solution of pyrazole (204.2 mg, 3 mmol) and dimethylamine (135.2 mg, 3 mmol) in 15 mL of methanol was added to the first solution. After adding the second solution, the color changes from light blue to greenish blue in the final solution. Greenish blue crystals of 1-Cu3, suitable for X-ray diffraction, were obtained within three days by slow evaporation of the filtered solution at room temperature. Elemental analysis (Found: C, 27.9 %; N, 19.5 %; H, 3.2 %. Calc. for Cu3C18H22N12OB2F8: C, 27.5 %; N, 21.4 %; H, 2.8 %). Elemental ratio estimated by electron probe microanalysis (EPMA): (Exp.) Theo. Cu : F = (2.89)3 : (8.03)8. ES-MS in positive mode (acetonitrile) shows the existence of only [Cu3(μ3-OH)]2+ unit, confirmed by mass-spectrometry. The experiments show the existence of the {[Cu3(μ3-OH)(pz)3(Hpz)3][BF4]}+ (m/Z = 700); {[Cu3(μ3-OH)(pz)3(Hpz)3]+1e-}+ (m/Z = 613); {[Cu3(μ3-OH)(pz)3(Hpz)2]+1e-}+ (m/Z = 544); {[Cu3(μ3-OH)(pz)3(Hpz)1] ]+1e-}+ (m/Z = 476); {[Cu3(μ3-OH)(pz)3}+ (m/Z = 408). See Figure S1. IR data (KBr, νmax/cm-1) 3400m [ν(NH)], 3137w [ν(μ3-OH-)], 1650w, and 1200w [νas(CN aromatic)]. See Figure S2.

Physical Characterization

Fourier transform infrared spectroscopy (FTIR) was performed using a NICOLET 5700 (Thermofisher Scientific, Waltham, MA, USA) in the range 4000-650 cm-1. Elemental analysis (C, N, H) was performed by microanalytical procedures using an EA 1108 elemental analyzer (CE Instruments, Wigan, UK). Electrospray ionization mass spectrometry (ESI-MS) studies of 1-Cu3 were performed with a QTOF Premier instrument with an orthogonal Z-spray-electrospray interface (Waters, Manchester, UK). A capillary voltage of 3.5 kV was used in the positive scan mode, and the cone voltage was set to 10 V to control the extent of fragmentation.

X-ray Diffraction

A single crystal of compound 1-Cu3 was mounted on a glass fiber, using a hydrocarbon oil to coat the crystal, and then transferred directly to the cold nitrogen stream for data collection. X-ray data were collected at 120 K on a Supernova diffractometer (Rigaku, Austin, TX, USA) equipped with a graphite-monochromated Enhance (Mo) X-ray Source (λ = 0.71073 Å). The program CrysAlisPro, Oxford Diffraction Ltd., was used for unit cell determination and data reduction. Empirical absorption correction was performed using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. The structure was solved with the ShelXT structure solution program [45] and refined with the SHELXL-2018 program [46] using Olex 2 [47]. Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated positions that were refined using idealized geometries (riding model). A summary of the data collection and structure refinements is provided in Table S1. CCDC-2174487 (1-Cu3) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.

Magnetic Susceptibility Measurements

Variable temperature susceptibility measurements were carried out for 1-Cu3 in the temperature range of 2-300 K with an applied magnetic field of 100 mT on a ground polycrystalline sample (with a mass of 37.64 mg) with a Quantum Design (San Diego, CA, USA) MPMS XL-5 SQUID magnetometer. The susceptibility data were corrected for the diamagnetic contributions of the sample using Pascal´s constants [48]. Isothermal magnetization measurements were made between 0 and 5 T at 2 K.

DFT Calculations of the Magnetic Properties

Spin-unrestricted calculations under the Density Functional Theory approach were done using the hybrid B3LYP functional [49,50] and a triple-ζ all-electron basis set for all atoms in all the calculations [51]. A guess function was generated using the Jaguar 5.5 code [52]. Total energy calculations were performed with the Gaussian09 code [53], using the quadratic convergence approach with a convergence criterion of 10−7 a.u. Mulliken spin densities were obtained from the Single Point calculations using Gaussian09.
The Heisenberg-Dirac-van Vleck spin Hamiltonian was used to describe the exchange coupling in the trinuclear complex is   H ^ = − ∑ i > j J i j S i S j ; where Si and Sj are the spin operators of the paramagnetic centers of the compound. The Ji parameters are the magnetic coupling constants between neighboring centers with unpaired electrons. Four different spin distributions (three antiferromagnetic and one ferromagnetic) for the system were calculated, and the obtained energies permit to evaluate the magnetic exchange constants of the system.
Utilizing the non-projected energy of the broken symmetry solution as the energy of the low spin state within the DFT methodology gives good results because it avoids the cancellation of the non-dynamic correlation effects, as has been stated in studies carried out by Ruiz et al. Thus, the J value is obtained using the non-projected method [54,55].

4. Conclusions

A new trinuclear cationic [Cu3-μ3OH]n+ complex based on the pyrazolato ligand has been obtained, [Cu3(μ3-OH)(pz)3(Hpz)3][BF4]2 (1-Cu3). The triangular complex presents the [BF4]- as counter-anion and is isostructural with the QOPJIP system. Nevertheless, the smaller size of the BF4- anion in 1-Cu3, compared to the ClO4- anion in QOPJIP, prevents the connection of the triangular units in 1-Cu3, in contrast to what is observed for the isostructural complex QOPJIP.
The magnetic data shows that strong antiferromagnetic interactions, together with antisymmetric interactions, exist in the triangular unit. The analysis of the experimental data and theoretical DFT results lead to the conclusion that there is a correlation between the displacement of the μ3-OH- from the Cu3 plane and the magnetic exchange interactions of the triangular Cu3 pyrazolato systems. However, the presence of other bridging organic ligands also plays a role in the magnetic exchange. These features affect the overlap of the magnetic orbitals according to the Khan-Briat model, suggesting that a strong overlap of magnetic orbitals exists in these systems.
The differences in the magnetic properties between 1-Cu3 and QOPJIP were analyzed and rationalized, showing that the different structural parameters, such as the displacement of the μ3-OH- from the Cu3 plane, the nature of the bridging organic ligands and also the size of the counter-anion affect the overall magnetic properties of these systems.

Supplementary Materials

The following supporting information can be downloaded at the website of this paper posted on Preprints.org, Figure S1. Electrospray-Mass Spectrometry 1-Cu3 measurements in the positive mode with the different simulated fragments patterns Figure S2. FTIR Spectra of 1-Cu3. Figure S3. Spin density surfaces for 1-Cu3 of the antiferromagnetic configurations and the ferromagnetic one. Table S1. Crystal data and structure refinement for 1-Cu3. Table S2. Fractional Atomic Coordinates and Equivalent Isotropic Displacement Parameters for 1-Cu3. Table S3. Anisotropic Displacement Parameters for 1-Cu3. Table S4. Bond Lengths for 1-Cu3. Table S5. Bond Angles for 1-Cu3. Table S6. Hydrogen Atom Coordinates and Isotropic Displacement Parameters for 1-Cu3.

Author Contributions

Conceptualization, W.C-M., D.V-Y, E.S. and C.J.G-G.; formal analysis, W.C-M. and P.H-I..; investigation, W.C-M., P.H-I. and C.J.G-G.; writing-original draft preparation, W.C-M., P.H-I. and V.P.-G.; writing-review and editing, W.C-M., D.V-Y, E.S., V.P.-G. and C.J.G-G.. All authors have read and agreed to the published this version of the manuscript.

Funding

Authors acknowledge Financiamiento Basal Program AFB220001 for partial financial support. Powered@NLHPC: This research was partially supported by the supercomputing infrastructure of the NLHPC (ECM-02), Center for Mathematical Modelling CMM, Universidad de Chile. The authors acknowledge CONICYT-FONDEQUIP/EQM130086-EQM140060. This work was done under partial support of the Chilean-French International Research Program “IRP-CoopIC”. This study forms part of the Advanced Materials program and was supported by MCIN with funding from European Union Next Generation EU (PRTR-C17.I1) and the Generalitat Valenciana (project MFA-2022-057). We also thank the Generalidad Valenciana (Prometeo/2019/076) and the project PID2021-125907NB-I00, financed by MCIN/AEI/10.13039/501100011033/FEDER, UE, for financial support.

Acknowledgments

The authors thank Dr. Guillermo Mínguez Espallargas from Instituto de Ciencia Molecular (ICMol), Universitat de Valencia, for performing the single crystal XRD and for fruitful discussion on the crystallographic and structural data.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Crystal structure of the triangular complex [Cu3(μ3-OH)(pz)3(Hpz)3][BF4]2 (1-Cu3), Color code: Cu = green, O = red, N = blue, C = grey, H = white, B = light pink and F = light yellow.
Figure 1. Crystal structure of the triangular complex [Cu3(μ3-OH)(pz)3(Hpz)3][BF4]2 (1-Cu3), Color code: Cu = green, O = red, N = blue, C = grey, H = white, B = light pink and F = light yellow.
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Figure 2. Crystal packing of the triangular complex [Cu3(μ3-OH)(pz)3(Hpz)3][BF4]2 (1-Cu3), Color code: Cu = green, O = red, N = blue, C = grey, H = white, B = light pink and F = light yellow.
Figure 2. Crystal packing of the triangular complex [Cu3(μ3-OH)(pz)3(Hpz)3][BF4]2 (1-Cu3), Color code: Cu = green, O = red, N = blue, C = grey, H = white, B = light pink and F = light yellow.
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Figure 3. A. Thermal dependence of χmT for 1-Cu3 at 100 mT. The solid line is the best fit obtained considering an isosceles triangle and the antisymmetric exchange; see equation 1 (PHI code). B. Experimental M(H) plot at 2 K for 1-Cu3, purple line - calculated magnetization curve using the PHI code.
Figure 3. A. Thermal dependence of χmT for 1-Cu3 at 100 mT. The solid line is the best fit obtained considering an isosceles triangle and the antisymmetric exchange; see equation 1 (PHI code). B. Experimental M(H) plot at 2 K for 1-Cu3, purple line - calculated magnetization curve using the PHI code.
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Figure 4. Relation between the displacement of the μ3-OH- from the Cu3 plane and the average magnetic exchange interactions of triangular Cu3 pyrazolato systems.
Figure 4. Relation between the displacement of the μ3-OH- from the Cu3 plane and the average magnetic exchange interactions of triangular Cu3 pyrazolato systems.
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Figure 5. A. Molecular structure of 1-Cu3 and the magnetic exchange interaction obtained from the experimental data. B. Molecular structure of QOPJIP and the magnetic exchange interaction obtained from reference 25.
Figure 5. A. Molecular structure of 1-Cu3 and the magnetic exchange interaction obtained from the experimental data. B. Molecular structure of QOPJIP and the magnetic exchange interaction obtained from reference 25.
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Figure 6. Dependence of the calculated J values with the square of the overlap integral of the magnetic orbitals. Adapted from Ref. [28], Copyright (2013), with permission from Springer.
Figure 6. Dependence of the calculated J values with the square of the overlap integral of the magnetic orbitals. Adapted from Ref. [28], Copyright (2013), with permission from Springer.
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Table 1. Structural parameters of triangular Cu3 systems of the type [Cu3(μ3-OH)(R-pz)3(L)3]n+/-.
Table 1. Structural parameters of triangular Cu3 systems of the type [Cu3(μ3-OH)(R-pz)3(L)3]n+/-.
CCDC
Code		 CuII-CuII (Å) Cu3(plane)-OH (Å) Cun-OH (Å) Cun-N(pz) (Å) Cun-1-Cun-Cun+1 (°) Cun-OH-Cun+1 (°) Ref.
1-Cu3 3.3740(8)
3.3574(8)
3.3702(9)		 0.439 2.005(3)
1.978(3)
1.995(3)		 1.942(4) to 1.965(3) 59.71(2)
60.09(2)
60.20(2)		 115.8(2)
115.3(2)
114.8(2)		 This
work
AMACIC 3.3020(6)
3.2561(5)
3.3927(6)		 0.553 1.977(2)
2.001(2)
2.005(2)		 1.932(3) to 1.947(2) 58.19(1)
62.30(1)
59.51(1)		 112.20(9)
116.9(1)
108.73(9)		 [27]
ASUNIN 3.3456(1)
3.3266(6)
3.3456(1)		 0.510 2.011(1)
1.932(5)
2.042(5)		 1.918(3) to 1.952(1) 59.62(1)
60.19(1)
60.19(1)		 116.1(1)
113.6(2)
111.3(1)		 [28]
BOFLEP 3.349(2)
3.239(2)
3.355(2)		 0.580 2.005(4)
2.001(4)
1.995(3)		 1.924(5) to 1.958(4) 57.78(2)
61.20(2)
61.02(2)		 113.5(2)
108.3(2)
114.1(2)		 [29]
DEFSEN 3.384(1)
3.2503(9)
3.2950(9)		 0.567 1.975(3)
2.008(3)
2.000(2)		 1.928(4) to 1.948(4) 58.22(2)
59.52(2)
62.26(2)		 116.3(1)
108.4(1)
112.0(1)		 [22]
DIBXOC 3.2972(5)
3.2972(5)
3.3843(4)		 0.609 2.008(2)
2.030(2)
2.008(2)		 1.946(2) to 1.1.957 59.12(1)
61.76(1)
59.12(1)		 109.5(1)
109.5(1)
114.9(1)		 [31]
EGIXOK 3.3540(5)
3.3874(6)
3.4036(6)		 0.363 1.979(2)
1.993(2)
1.985(3)		 1.921(3) to 1.941(2) 60.16(1)
60.64(1)
59.19(1)		 115.2(2)
116.7(2)
118.3(2)		 [26]
EGIXUQ 3.268(1)
3.379(1)
3.350(1)		 0.148 1.936(5)
1.943(4)
1.913(5)		 1.914(5) to 1.942(5) 61.39(2)
60.50(2)
58.11(2)		 114.8(2)
121.0(2)
122.4(2)		 [26]
EHOLIZ 3.389(5)
3.389(5)
3.389(5)		 0.274 2.046(10)
1.941(10)
1.941(10)		 1.92(1) to 1.97(2) 60.0(1)
60.0(1)
60.0(1)		 116(1)
122(1)
116(1)		 [32]
JEWWEO 3.3416(8)
3.3825(8)
3.3502(7)		 0.461 1.988(3)
2.010(3)
1.982(3)		 1.923(4) to 1.943(4) 60.73(2)
59.76(2)
59.51(2)		 113.4(1)
115.9(2)
115.1(2)		 [2]
JEWWIS 3.387(1)
3.309(1)
3.350(1)		 0.486 1.976(6)
2.021(5)
1.985(6)		 1.919(7) to 1.952(8) 58.84(3)
60.03(3)
61.13(3)		 115.9(3)
111.4(3)
115.5(3)		 [2]
MUZQUU 3.3696(5)
3.3461(5)
3.3788(5)		 0.455 1.982(2)
2.003(2)
2.001(2)		 1.947(3) to 1.960(2) 59.45(1)
60.41(1)
60.14(1)		 115.45(9)
113.39(9)
116.05(9)		 [6]
*QIMSIQ-a 3.2977(4)
3.1704(4)
3.3126(4)		 0.688 2.016(2)
2.012(2)
1.987(2)		 1.938(2) to 1.959(2) 57.32(1)
61.58(1)
61.10(1)		 109.91(7)
104.90(7)
111.68(8)		 [23]
*QIMSIQ-b 3.3911(4)
3.3023(4)
3.3214(4)		 0.512 2.000(1)
1.994(2)
1.989(2)		 1.944(2) to 1.959(2) 58.93(1)
59.48(1)
61.59(1)		 116.20(8)
111.99(7)
112.72(7)		 [23]
*QIMSOW-a 3.2559(7)
3.342(1)
3.2345(9)		 0.713 1.992(3)
2.032(3)
2.044(3)		 1.941(4) to 1.958(4) 61.98(2)
58.69(2)
59.32(2)		 108.0(1)
110.2(1)
106.6)1)		 [23]
*QIMSOW-b 3.2045(6)
3.1837(8)
3.2007(9)		 0.759 1.985(3)
2.011(2)
1.990(3)		 1.948(3) to 1.960(4) 59.61(2)
60.13(2)
60.25(2)		 106.6(1)
105.4(1)
107.3(1)		 [23]
QOPJIP 3.355(1)
3.386(1)
3.368(1)		 0.466 1.994(5)
2.000(4)
2.007(5)		 1.929(6) to 1.958(6) 59.94(3)
60.49(3)
59.57(3)		 114.3(2)
114.4(2)
115.6(2)		 [25]
QUSMEX 3.344(2)
3.286(2)
3.392(2)		 0.475 1.955(8)
2.017(6)
1.992(9)		 1.933(9) to 1.978(9) 58.39(4)
61.53(4)
60.07(4)		 114.7(4)
110.1(4)
118.5(4)		 [30]
QUSMIB 3.289(2)
3.289(2)
3.289(2)		 0.489 1.961(1)
1.962(1)
1.960(1)		 1.89(1) to 1.930(8) 60.00(4)
60.00(4)
60.00(4)		 114.0(1)
114.0(1)
114.0(1)		 [30]
QUSMUN 3.3550(5)
3.3615(5)
3.3439(6)		 0.471 1.985(2)
2.005(2)
1.987(2)		 1.937(2) to 1.951(2) 60.24(1)
59.72(1)
60.04(1)		 114.42(9)
114.68(9)
114.65(9)		 [30]
RETQUD 3.3833(6)
3.3629(6)
3.3769(5)		 0.542 2.026(2)
2.028(3)
2.013(2)		 1.942(3) to 1.961(2) 59.66(1)
60.07(1)
60.26(1)		 113.1(1)
112.7(1)
113.5(1)		 [19]
RETRAK 3.365(1)
3.3650(9)
3.3886(8)		 0.565 2.023(3)
2.041(3)
2.019(2)		 1.933(3) to 1.962(5) 59.77(2)
60.46(2)
59.77(2)		 111.8(1)
111.9(1)
113.9(1)		 [19]
RETREO 3.3442(6)
3.3975(6)
3.3022(7)		 0.625 2.024(2)
2.033(2)
2.038(2)		 1.936(3) to 1.957(3) 61.48(1)
58.65(1)
59.87(1)		 111.0(1)
113.1(1)
108.7(1)		 [19]
RUYGEX 3.4471(9)
3.206(1)
3.4227(9)		 0.524 1.987(3)
2.024(3)
2.035(3)		 1.940(4) to 1.953(4) 55.55(2)
62.01(2)
62.44(2)		 118.5(1)
104.4(1)
117.3(1)		 [3]
RUYGIB 3.2473(8)
3.4007(6)
3.4305(8)		 0.507 2.014(3)
2.017(2)
1.989(2)		 1.933(4) to 1.952(4) 61.16(1)
62.08(1)
56.76(1)		 107.3(1)
116.2(1)
118.0(1)		 [3]
RUYHEY 3.414(1)
3.253(1)
3.277(1)		 0.613 2.012(5)
2.006(4)
2.016(3)		 1.929(7) to 1.950(5) 58.15(3)
58.82(3)
63.03(3)		 116.4(2)
108.0(2)
108.9(2)		 [3]
SIJKOL 3.112(1)
3.321(1)
3.321(1)		 0.658 2.000(1)
2.000(1)
1.977(1)		 1.942(1) to 1.967(4) 62.06(1)
62.06(1)
55.88(1)		 102.2(1)
113.3(1)
113.3(1)		 [33]
UZIWEI 3.3695(6)
3.2840(5)
3.2953(5)		 0.595 1.998(2)
2.004(2)
2.104(2)		 1.937(2) to 1.952(2) 59.03(1)
59.36(1)
61.61(1)		 114.68(8)
109.64(8)
110.42(8)		 [34]
VAZCOR 3.1913(9)
3.391(1)
3.353(1)		 0.599 2.032(4)
2.030(4)
1.959(3)		 1.933(6) to 1.960(5) 62.36(2)
61.16(2)
56.49(2)		 103.6(2)
116.4(2)
114.3(2)		 [35]
VIMYEX 3.2639(7)
3.1851(8)
3.299(1)		 0.712 2.027(2)
1.991(2)
2.003(2)		 1.935(2) to 1.950(2) 58.06(1)
61.52(1)
60.41(1)		 108.67(7)
105.79(7)
109.9387)		 [36]
XOKXAX 3.347(1)
3.403(1)
3.320(1)		 0.491 1.998(4)
2.000(4)
2.000(4)		 1.939(5) to 1.963(6) 61.38(2)
58.92(2)
59.70(2)		 113.6(2)
116.6(2)
112.3(2)		 [37]
YIFGIG 3.3500(8)
3.2440(7)
3.3519(6)		 0.521 1.978(2)
1.968(2)
2.008(2)		 1.928(2) to 1.953(2) 57.90(1)
61.08(1)
61.02(1)		 116.20(9)
109.37(9)
114.48(9)		 [17]
*In QIMSIQ and QIMSOW, the letters a and b denote the structure that presents two different triangular Cu3 units.
Table 2. Selected examples of the magnetic and structural parameters of triangular Cu3 pyrazolato systems of general formula, [Cu3(μ3-OH)(R-pz)3(L)3]n+/-.
Table 2. Selected examples of the magnetic and structural parameters of triangular Cu3 pyrazolato systems of general formula, [Cu3(μ3-OH)(R-pz)3(L)3]n+/-.
CCDC
Code		 d(CuII-CuII) (Å) Cu3(plane)-OH (Å) J(CuII···CuII)
(cm-1)		 g zJ´
(cm-1)		 |GZ|
(cm-1)		 Ref
1-Cu3 3.3740(8)
3.3574(8)
3.3702(9)		 0.439 -193.5(6)
-205.5(6)		 2.09 - 28 This work
BOFLEP# 3.349(2)
3.239(2)
3.355(2)		 0.580 - - - - [29]
DEFSEN 3.384(1)
3.2503(9)
3.2950(9)		 0.567 -117.7
-90.3		 2.047 -3.0 - [22]
QISOW-a* 3.2559(7)
3.342(1)
3.2345(9)		 0.713 -140 2.07 - - [23]
QISOW-b* 3.2045(6)
3.1837(8)
3.2007(9)		 0.759 -109 2.07 - - [23]
QOPJIP 3.355(1)
3.386(1)
3.368(1)		 0.466 -241.9 2.07 -23.0 - [25]
SIJKOL 3.112(1)
3.321(1)
3.321(1)		 0.658 -148
-23		 2.17 - - [33]
VAZCOR 3.1913(9)
3.391(1)
3.353(1)		 0.599 -298
-257		 2.12 -0.37 18.2 [35]
YIFGIG 3.3500(8)
3.2440(7)
3.3519(6)		 0.521 -392
-278		 2.09 - 31.2 [17]
# In this structure, the magnetic properties are qualitatively described, and no analytical interpretation was done.
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