Ferro- and Antiferromagnetic Interactions in Oxalato-Centered Inverse Hexanuclear and Chain Copper(II) Complexes with Pyrazole Derivatives

Two novel copper(II) complexes of formulas {[Cu(4-Hmpz)4][Cu(4-Hmpz)2(µ3-ox-κ2O1,O2:κO2′:κO1′)(ClO4)2]}n (1) and {[Cu(3,4,5-Htmpz)4]2[Cu(3,4,5-Htmpz)2(µ3-ox-κ2O1,O2:κO2′:κO1′)(H2O)(ClO4)]2[Cu2(3,4,5-Htmpz)4(µ-ox-κ2O1,O2:κ2O2′,O1′)]}(ClO4)4·6H2O (2) have been obtained by using 4-methyl-1H-pyrazole (4-Hmpz) and 3,4,5-trimethyl-1H-pyrazole (3,4,5-Htmpz) as terminal ligands and oxalate (ox) as the polyatomic inverse coordination center. The crystal structure of 1 consists of perchlorate counteranions and cationic copper(II) chains with alternating bis(pyrazole)(µ3-κ2O1,O2:κO2′:κO1′-oxalato)copper(II) and tetrakis(pyrazole)copper(II) fragments. The crystal structure of 2 is made up of perchlorate counteranions and cationic centrosymmetric hexanuclear complexes where an inner tetrakis(pyrazole)(µ-κ2O1,O2:κ2O2′,O1′-oxalato)dicopper(II) entity and two outer mononuclear tetrakis(pyrazole)copper(II) units are linked through two mononuclear aquabis(pyrazole)(µ3-κ2O1,O2:κO2′:κO1′-oxalato)copper(II) units. The magnetic properties of 1 and 2 were investigated in the temperature range 2.0–300 K. Very weak intrachain antiferromagnetic interactions between the copper(II) ions through the µ3-ox-κ2O1,O2:κO2′:κO1′ center occur in 1 [J = −0.42(1) cm−1, the spin Hamiltonian being defined as H = −J∑S1,i · S2,i+1], whereas very weak intramolecular ferromagnetic [J = +0.28(2) cm−1] and strong antiferromagnetic [J’ = −348(2) cm−1] couplings coexist in 2 which are mediated by the µ3-ox-κ2O1,O2:κO2′:κO1′ and µ-ox-κ2O1,O2:κ2O2′,O1′ centers, respectively. The variation in the nature and magnitude of the magnetic coupling for this pair of oxalato-centered inverse copper(II) complexes is discussed in the light of their different structural features, and a comparison with related oxalato-centered inverse copper(II)-pyrazole systems from the literature is carried out.


Materials
Oxalic acid, 4-Hmpz, 3,4,5-Htmpz, copper(II) perchlorate hexahydrate, and triethylamine were purchased from commercial sources and they were used as received without any further purification.   4 ][Cu(4-Hmpz) 2 (µ-ox-κ 2 O 1 ,O 2 :κO 2 :κO 1 )(ClO 4 ) 2 ]} n (1) A water/methanol (1:1 v/v) solution (30 mL) of 4-Hmpz (0.245 g, 3.0 mmol) was poured into an aqueous solution (20 mL) of copper(II) perchlorate hexahydrate (0.370 g, 1.0 mmol). Then, an aqueous solution (10 mL) containing oxalic acid (0.045 g, 0.5 mmol) and triethylamine (0.2 mL, 1.0 mmol) was then added to the previous mixture and the whole was kept under continuous stirring for 20 min at room temperature and filtered to remove any remaining small particle. Single crystals of 1 as deep blue prisms were grown from the resulting blue solution after some days by slow evaporation at room temperature. They were collected and dried on filter paper. Yield: 0.28 g, 55%. Anal. Calcd. for C 26  3,4,5-Htmpz (0.275 g, 2.5 mmol) dissolved in a water/methanol (1:1 v/v) solvent mixture (30 mL) was added to an aqueous solution (20 mL) of copper(II) perchlorate hexahydrate (0.370 g, 1.0 mmol) under continuous stirring. An aqueous solution (10 mL) of oxalic acid (0.045 g, 0.5 mmol) and triethylamine (0.2 mL, 1.0 mmol) was then poured into the above solution and the whole was stirred for 20 min at room temperature. The resulting blue solution was filtered to remove any remaining small particle and allowed to evaporate in a hood under ambient conditions. X-ray suitable deep blue-greenish prisms of 2 were formed after a few days. They were collected by filtration and dried on filter paper. Yield: 0. 29

Physical Techniques
Elemental analyses (C, H, N) were performed by the Servei Central de Suport a la Investigació Experimental de la Universitat de València. FT-IR spectra were recorded on a Nicolet-5700 spectrophotometer as KBr pellets. Powder X-ray diffraction (XPRD) patterns of powdered crystalline samples were collected at room temperature on a D8 Avance A25 Bruker diffractometer by using graphite-monochromated Cu-Kα radiation (λ = 1.54056 Å). Variable-temperature (2.0-300 K) direct current (dc) magnetic susceptibility measurements were carried out with a SQUID magnetometer under applied fields of 5.0 kOe (T > 20 K) and 250 Oe (T < 20 K) to prevent for saturation effects at low temperature. The experimental magnetic susceptibility data were corrected for the diamagnetic contributions of the constituent atoms and the sample holder (a plastic bag), as well as for the temperature-independent paramagnetism (tip) of the copper(II) ion (60 × 10 −6 cm 3 ·mol −1 ).

Crystallographic Data Collection and Refinement
X-ray diffraction data on single crystals of 1 and 2 were collected at room temperature with Bruker D8 Venture with PHOTON II detector (1) and Bruker-Nonius X8-APEXII CCD area detector (2) diffractometers by using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The data were processed through the SAINT [51] reduction and SADABS [52] absorption software. The structures were solved using direct methods and subsequently completed by Fourier recycling using the SHELX-2018 software package [53,54], then refined by the full-matrix least-squares refinements based on F 2 with all observed reflections. All non-hydrogen atoms of 1 and 2 were refined anisotropically. The hydrogen atoms of the polymethyl-substituted pyrazole ligands in 1 and 2 were placed on calculated positions and refined using a riding model. The hydrogen atoms of the water molecules in 2 were initially located from the Fourier difference map and a few suitable hydrogen bonds were found; eventually, they were excluded from the refinement as their location was losing reliability after some refinement cycles, coming too close to other atoms in the structures, probably due to the confined space occupied by the water molecules in the structure. The perchlorate counterions in 2 were found to be involved in somewhat standard disorders, and almost all of the oxygen atoms were modeled over two sites. These disorders were refined freely within SHELXL, using similarity restraints on 1,2-and 1,3-distances as well as rigid-bond restraints, while constraining the sum of the occupancies to unit [54]. The final geometrical calculations and graphical manipulations for 1 and 2 were performed using the XP utility within SHELX [53] and the CrystalMaker program [55]. Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre with CCDC reference numbers 2076622 (1) and 2076621 (2). A summary of the crystallographic data and structure refinement for 1 and 2 is given in Table 1. Selected bond distances and angles and hydrogen bonds for 1 and 2 are listed in Tables S1-S4 (see Supplementary Materials).
In the following, a comprehensive description of the crystal structures of 1 and 2 is given in the framework of the ICC approach, a particular attention being paid to the elucidation of the possible magneto-structural correlations along this oxalate-centered copper(II)-pyrazole family.  The crystal structure of 1 consists of oxalato-centered cationic copper(II) chains showing a regular alternation of mononuclear bis(pyrazole)oxalatocopper(II) and tetrakis(pyrazole)copper(II) units, [Cu(4-Hmpz)2(ox)] and [Cu(4-Hmpz)4] 2+ respectively, together with weakly coordinated perchlorate anions (Figures 1 and 2).     Table S2). The C-C and C-O bonds from oxalate are depicted by black sticks to follow the chain development. The intrachain N-H···O hydrogen bonds between the pyrazole and the oxalate ligands or the perchlorate anions are depicted by black dotted lines, while the intra-and interchain C-H···π type interactions between the neighboring pyrazole ligands are depicted by thick black and gray dashed lines, respectively; (b) projection view of the stacking of the cationic copper(II) chains along the crystallographic a axis [symmetry code:    Table S2). The C-C and C-O bonds from oxalate are depicted by black sticks to follow the chain development. The intrachain N-H···O hydrogen bonds between the pyrazole and the oxalate ligands or the perchlorate anions are depicted by black dotted lines, while the intra-and interchain C-H···π type interactions between the neighboring pyrazole ligands are depicted by thick black and gray dashed lines, respectively; (b) projection view of the stacking of the cationic copper(II) chains along the crystallographic a axis [symmetry code:  [59]. Overall, this asymmetric bidentate/bis(monodentate) coordination mode of oxalato for 1 gives rise to a unique inverse triangular-based oxalato-centered copper(II) chain of branch-type topology featuring vertex-shared triangular copper(II) entities (Figure 1b,c). The two-fold symmetryrelated Cu(1) and Cu(1b) atoms within each alternating chain of 1, stand on the central chain axis while the centrosymmetrically-related Cu (2) and Cu(2a) atoms are alternatively located in opposite peripheral branches along the chain (Figure 1b,c). The mean equatorial planes of the two-fold symmetry-related propeller-like [Cu(4-Hmpz) 4 (Figure 1c). The regular alternation of left and right rotations along the chain leads to an overall achiral (meso-helical) chain. Hence, 1 constitutes a rare example of an oxalato-centered inverse copper(II) meso-helix [60].

Description of the Structures
In the crystal lattice of 1, the meso-helical copper(II) chains running in the [001] direction are parallel stacked leading to segregated layer arrays of cationic chains within the crystallographic ac plane (Figure 2). Within each meso-helical chain, the pyrazole ligands establish moderate to weak hydrogen bonds with the carboxylate oxygen atoms from the oxalate centroligands and the weakly coordinated perchlorate anions (Figure 2a). The adjacent chains interact through additional very weak contacts involving the methyne and methyl groups from pyrazole ligands and the weakly coordinated perchlorate anions, leading to a supramolecular 3D network of meso-helical chains (Figure 2c). There are also moderate to weak, intra-and interchain C-H···π type interactions between the methylpyrazole groups from the [Cu(4-Hmpz) 4 [50]. However, 5a and 5b show a significant asymmetry of the metal-to-oxalato bonds [R = 1.98 (5a)/1.97 Å (5b) and R' = 2.12 (5a)/2.11 Å (5b), with ∆R = R ' − R = 0.15 (5a)/0.14 Å (5b)] resulting from the five-coordinate trigonal bipyramidal geometry [50]. For symmetry reasons, the mean equatorial planes at each copper atom within the central dinuclear motif of 2 are parallel (ψ = 0 • ). The asymmetric bidentate/bis(monodentate) coordination mode of the oxalate center (III-3 Overall, these structural features give rise to a unique inverse triangular-based oxalatocentered hexacopper(II) skeleton of branch-type topology featuring two parallel planar triangular copper(II) entities ( Figure 3b). Interestingly, the hexacopper(II) entity possesses a S-shaped conformation with two void spaces that are filled by the two weakly coordinated perchlorate anions, giving rise to an oblate spheroid-like shape capsule (Figure 3c,d). Hence, 2 constitutes a rare example of anionic guest encapsulation by an oxalato-centered inverse polynuclear copper(II) complex in the solid-state [61,62].
In the crystal lattice of 2, the cationic hexacopper(II) units are well-separated from each other by the remaining non-coordinated perchlorate anions (Figure 4d). The hexanuclear entities are involved in a variety of moderately to relatively weak hydrogen bonds with both the weakly coordinated and free perchlorate anions and the water molecules of crystallization through their N-H pyrazole groups, the carbonyl oxygen atoms from oxalate, and the coordinated water molecules (Figure 4a). Moreover, there are moderately to relatively weak, intramolecular hydrogen bonds between the N-H pyrazole groups and the carbonyl-oxygen atoms from oxalato and intermolecular hydrogen bonds between the perchlorate anions and the water molecules of crystallization. Very weak intramolecular contacts between the methyl groups from pyrazole ligands and the carbonyl-oxygen atoms from oxalato and intermolecular contacts between the hydrogen-bonded perchlorate anions and the methyl groups from pyrazoles of adjacent {Cu II 6 } entities also occur (Figure 4a,b). They give rise to a dense-packed multilayer supramolecular structure made up of corrugated layers growing in the crystallographic bc plane that are parallel stacked along the crystallographic a axis (Figure 4b,c).  Figures 5 and 6 show the magnetic properties of 1 and 2 in the form of the χMT versus T plots (χM being the dc molar magnetic susceptibility per formula unit and T the absolute temperature).  1 exhibits a magnetic behavior which is typical of very weak antiferromagnetically coupled copper(II) ions ( Figure 5). At room temperature, χMT for 1 is equal to 0.83 cm 3 mol −1 K, a value which is as expected for two magnetically isolated copper(II) ions [χMT = 2 × (Nβ 2 gCu 2 /3kB)SCu(SCu + 1) = 0.83 cm 3 mol −1 K with gCu = 2.1 and SCu = 1/2]. Upon cooling, χMT remains constant down to 20 K, and it further decreases to 0.70 cm 3 mol −1 K at 2.0 K (inset of Figure 5). This small negative deviation from the Curie law in the low-temperature region is most likely due to very weak intrachain antiferromagnetic interactions between the copper(II) ions across the µ3-ox-κ 2 O 1 ,O 2 :κO 2' :κO 1' centroligand (see Figure 1b).  Figures 5 and 6 show the magnetic properties of 1 and 2 in the form of the χMT versus T plots (χM being the dc molar magnetic susceptibility per formula unit and T the absolute temperature).  1 exhibits a magnetic behavior which is typical of very weak antiferromagnetically coupled copper(II) ions ( Figure 5). At room temperature, χMT for 1 is equal to 0.83 cm 3 mol −1 K, a value which is as expected for two magnetically isolated copper(II) ions [χMT = 2 × (Nβ 2 gCu 2 /3kB)SCu(SCu + 1) = 0.83 cm 3 mol −1 K with gCu = 2.1 and SCu = 1/2]. Upon cooling, χMT remains constant down to 20 K, and it further decreases to 0.70 cm 3 mol −1 K at 2.0 K (inset of Figure 5). This small negative deviation from the Curie law in the low-temperature region is most likely due to very weak intrachain antiferromagnetic interactions between the copper(II) ions across the µ3-ox-κ 2 O 1 ,O 2 :κO 2' :κO 1' centroligand (see Figure 1b). 1 exhibits a magnetic behavior which is typical of very weak antiferromagnetically coupled copper(II) ions ( Figure 5). At room temperature, χ M T for 1 is equal to 0.83 cm 3 mol −1 K, a value which is as expected for two magnetically isolated copper(II) ions [χ M T = 2 × (Nβ 2 g Cu 2 /3k B )S Cu (S Cu + 1) = 0.83 cm 3 mol −1 K with g Cu = 2.1 and S Cu = 1/2]. Upon cooling, χ M T remains constant down to 20 K, and it further decreases to 0.70 cm 3 mol −1 K at 2.0 K (inset of Figure 5). This small negative deviation from the Curie law in the low-temperature region is most likely due to very weak intrachain antiferromagnetic interactions between the copper(II) ions across the µ 3 -ox-κ 2 O 1 ,O 2 :κO 2 :κO 1 centroligand (see Figure 1b).
In a first attempt, the magnetic susceptibility data of 2 were accordingly analyzed by the appropriate expression for the sum of magnetically non-interacting inner The magnetic behavior of 2 is characteristic of the coexistence of strong antiferro-(high temperature region) and very weak ferromagnetic (low temperature region) interactions within the hexacopper(II) unit ( Figure 6). The value of χ M T of 2.07 cm 3 mol −1 K at room temperature is rather lower than that expected for six magnetically non-interacting copper(II) ions [χ M T = 6 × (Nβ 2 g Cu 2 /3k B )S Cu (S Cu + 1) = 2.49 cm 3 mol −1 K with g Cu = 2.1 and S Cu = 1/2]. χ M T continuously decreases upon cooling reaching a sort of plateau in the temperature range 100 to ca. 20 K with a value of χ M T equal to 1.68 cm 3 mol −1 K. This value is very close to the expected one for four magnetically isolated copper(II) ions [χ M T = 6 × (Nβ 2 g Cu 2 /3k B )S Cu (S Cu + 1) = 1.66 cm 3 mol −1 K with g Cu = 2.1 and S Cu = 1/2]. Both features unambiguously support the occurrence of a diamagnetic singlet (S = 0) spin ground state for the inner dicopper(II) unit due to the expected strong intramolecular antiferromagnetic interaction between the copper(II) ions through the µ-ox-κ 2 O 1 ,O 2 :κ 2 O 2 ,O 1 centroligand (see Figure 3b). Below 20 K, χ M T slightly increases to attain a value of 1.75 cm 3 mol −1 K at 2.0 K (inset of Figure 6). This small increase of χ M T at very low temperatures is most likely due to a very weak intramolecular ferromagnetic interaction between the copper(II) ions through the µ 3 -ox-κ 2 O 1 ,O 2 :κO 2 :κO 1 centroligand (see Figure 3b).
In a first attempt, the magnetic susceptibility data of 2 were accordingly analyzed by the appropriate expression for the sum of magnetically non-interacting inner [Cu (1) In this expression, J and J' are the magnetic coupling parameters between the inner Cu(1) and Cu(1a) atoms and between the intermediate Cu(2)/Cu(2a) and the outer Cu(3)/Cu(3a) atoms, respectively (Scheme 2b), the corresponding spin Hamiltonian being defined as H = −JS 1 · S 1 − J'(S 2 · S 3 + S 2 · S 3 ) (see Figure 3b), and g is the average Landé factor of the six copper(II) ions (g = g 1 = g 2 = g 3 = g Cu ). The least-squares fit of the experimental data led to the following set of parameters: J = −348(2) cm −1 , J' = +0.28(2) cm −1 , and g = 2.110(1) with R = 3.0 × 10 −5 . The calculated curve (solid line in Figure 6) reproduces quite well the experimental data in the whole temperature range explored.

Magneto-Structural Correlations
In order to account for the unique structural features and magnetic properties of 1 and 2, we will refer to related examples of inverse polynuclear copper(II) complexes with oxalato as inverse coordination center and pyrazole or its polymethyl-substituted derivatives as terminal ligands (Scheme 3). Selected magneto-structural data for the broad oxalato-centered inverse copper(II)-pyrazole family are shown in Table 2.    2), asymmetric zig-zag chains (4), symmetric dinuclear complexes (5a/5b), and rectangular-based alternating sheet-like compounds (6). The Werner-type mononuclear complex (3) has been included for completeness. Table 2. Selected magneto-structural data for oxalato-centered inverse copper(II) complexes with pyrazole and polymethylsubstituted derivatives as peripheral ligands.  (6) [59]. In both cases, the unpaired electron at each copper(II) ion in a square pyramidal surrounding is delocalized in the d(x 2 -y 2 )-type magnetic orbital (the x and y axes being roughly defined by the short basal Cu-O bonds). A large spin delocalization is predicted on the µ-ox-κ 2 O 1 ,O 2 :κ 2 O 2 ,O 1 centroligand, due to the almost coplanar disposition of the metal basal planes and the mean oxalato plane [φ = 5.4 (2) and 2.8 • (6) with ψ = 0 (2) and 0 • (6); Table 2] [59]. This situation contrasts with the intermediate value of the antiferromagnetic coupling found for the related pair of cationic dicopper(II) complexes 5a and 5b (J = −129 and −161 cm −1 , respectively) (Scheme 3) [50]. This change in the size of the magnetic coupling is explained by the decrease of the spin delocalization of the unpaired electron from the trigonal bipyramidal copper(II) ions of 5a and 5b on the µ-ox-κ 2 O 1 ,O 2 :κ 2 O 2 ,O 1 centroligand, the magnetic orbital at each copper(II) ion being of the d(z 2 )-type (the z axis pointing toward one of the two Cu-O ox bonds). Then, a poor spin density is expected on the equatorial positions [50].

Compound a Coordination Mode
As far as the very weak ferro-or antiferromagnetic interactions found in 2 and 3 are concerned (J = +0.28 and −0.42 cm −1 , respectively; Table 2), they are as expected for an in-plane perpendicular planar conformation of the equatorial planes at the metal atoms [β = 17.7 (2) and 21.9 • (3) with ψ = 87.8 (2) and 85.7 • (3); Table 2]. In this case, the d(x 2 −y 2 )-type magnetic orbitals of the axially elongated octahedral copper(II) ions are perpendicular to each other in two different planes leading to a poor δ-type orbital overlap along the long apical position through the µ 3 -ox-κ 2 O 1 ,O 2 :κO 2 :κO 1 pathway (Scheme 4). A similar situation applies for the related weak antiferromagnetically coupled copper(II) chain 4 (J = −0.74 cm −1 ), where a quasi perpendicular conformation of the metal basal planes of the apically elongated square pyramidal copper(II) ions is achieved through the µ-ox-κ 2 O 1 ,O 2 :κO 1 pathway (β = 20.2 • and ψ = 81.5 • ; Table 2) (Scheme 3) [59]. In all these cases, the δ-type orbital overlap between the d(x 2 −y 2 )-type magnetic orbitals along the long apical position would mainly depend on the Cu-O-C bond angle (α). It seems that the orbital overlap would be strictly zero for α values close to that occurring in 2 (α = 138.1 • ; Table 2), so that the antiferromagnetic contribution, which is proportional to the square of the orbital overlap, becomes null and the ferromagnetic one dominates (example of accidental orthogonality) [40]. The orbital overlap would increase as the α value increases leading to a dominant antiferromagnetic contribution to the overall magnetic coupling for 1 and 4 [J = −0.42 (1) and −0.74 cm −1 (4) with α = 147.7 (1) and 177.0 • (4); Table 2] (Figure 7). That being so, the ferromagnetic and antiferromagnetic contributions would cancel each other for a critical value of α close to 141.5 • for which J = 0 (Figure 7). Interestingly, the values of R' increase continuously with the α values for this series of copper(II) complexes with an in-plane perpendicular planar conformation (inset of Figure 7). Indeed, the orbital overlap would also decrease with increasing the axial Cu-O distance and, consequently, the antiferromagnetic contribution to the overall magnetic coupling. However, this is the opposite trend to that found for 1, 2, and 4 (J = +0.28 (2), −0.42 (1) and −0.74 cm −1 (4) with R' = 2.341 (2), 2.377 (1) and 2.416 • (4); Table 2). This expected minor but non-negligible distance dependence of the antiferromagnetic contribution likely explains the observed non-linear angular dependence of the magnetic coupling along this series. In all these cases, the δ-type orbital overlap between the d(x 2 −y 2 )-type magnetic orbitals along the long apical position would mainly depend on the Cu-O-C bond angle (α). It seems that the orbital overlap would be strictly zero for α values close to that occurring in 2 (α = 138.1°; Table 2), so that the antiferromagnetic contribution, which is proportional to the square of the orbital overlap, becomes null and the ferromagnetic one dominates (example of accidental orthogonality) [40]. The orbital overlap would increase as the α value increases leading to a dominant antiferromagnetic contribution to the overall magnetic coupling for 1 and 4 [J = −0.42 (1) and −0.74 cm −1 (4) with α = 147.7 (1) and 177.0° (4); Table 2] (Figure 7). That being so, the ferromagnetic and antiferromagnetic contributions would cancel each other for a critical value of α close to 141.5° for which J = 0 ( Figure  7). Interestingly, the values of R' increase continuously with the α values for this series of copper(II) complexes with an in-plane perpendicular planar conformation (inset of Figure 7). Indeed, the orbital overlap would also decrease with increasing the axial Cu-O distance and, consequently, the antiferromagnetic contribution to the overall magnetic coupling. However, this is the opposite trend to that found for 1, 2, and 4 (J = +0.28 (2), −0.42 (1) and −0.74 cm −1 (4) with R' = 2.341 (2), 2.377 (1) and 2.416° (4); Table 2). This expected minor but non-negligible distance dependence of the antiferromagnetic contribution likely explains the observed non-linear angular dependence of the magnetic coupling along this series.   Table 2). The solid lines are only eye-guides. In all these cases, the δ-type orbital overlap between the d(x 2 −y 2 )-type magnetic orbitals along the long apical position would mainly depend on the Cu-O-C bond angle (α). It seems that the orbital overlap would be strictly zero for α values close to that occurring in 2 (α = 138.1°; Table 2), so that the antiferromagnetic contribution, which is proportional to the square of the orbital overlap, becomes null and the ferromagnetic one dominates (example of accidental orthogonality) [40]. The orbital overlap would increase as the α value increases leading to a dominant antiferromagnetic contribution to the overall magnetic coupling for 1 and 4 [J = −0.42 (1) and −0.74 cm −1 (4) with α = 147.7 (1) and 177.0° (4); Table 2] (Figure 7). That being so, the ferromagnetic and antiferromagnetic contributions would cancel each other for a critical value of α close to 141.5° for which J = 0 ( Figure  7). Interestingly, the values of R' increase continuously with the α values for this series of copper(II) complexes with an in-plane perpendicular planar conformation (inset of Figure 7). Indeed, the orbital overlap would also decrease with increasing the axial Cu-O distance and, consequently, the antiferromagnetic contribution to the overall magnetic coupling. However, this is the opposite trend to that found for 1, 2, and 4 (J = +0.28 (2), −0.42 (1) and −0.74 cm −1 (4) with R' = 2.341 (2), 2.377 (1) and 2.416° (4); Table 2). This expected minor but non-negligible distance dependence of the antiferromagnetic contribution likely explains the observed non-linear angular dependence of the magnetic coupling along this series.   Table 2). The solid lines are only eye-guides.  Table 2). The solid lines are only eye-guides.
This study provides further insights into the ligand design, structural chemistry, and magnetochemistry of the well-known class of oxalato-centered inverse copper(II) IPCs and ICPs with pyrazole and their polymethyl-substituted derivatives possessing varying nuclearity and dimensionality. They range from traditional Werner-type mononuclear (3) to inverse dinuclear complexes (5a and 5b) and chains (4), or from triangular-based hexanuclear complexes (2) and chains (1) to rectangular-based sheet-like polymers (6) with very weak to strong ferro-and/or antiferromagnetic couplings depending on the coordination mode of the oxalato centroligand. In this respect, new magneto-structural correlations have emerged for the series of oxalato-centered dicopper(II) complexes with an in-plane perpendicular planar conformation. This study completes and further extends the previous ones on the related series of symmetric and asymmetric oxalato-centered dicopper(II) complexes with a coplanar conformation, thus providing a unified vision on the magneto-structural correlations for this large class of oxalato-centered inverse copper(II)-pyrazole complexes.