Ligand Structure Effects on Molecular Assembly and Magnetic Properties of Copper(II) Complexes with 3-Pyridyl-Substituted Nitronyl Nitroxide Derivatives.

Reaction of Cu(hfac)2 with methyl- and bromo-3-pyridyl-substituted nitronyl nitroxides (L R ) leads to assemble a diverse set of coordination complexes: mononuclear [Cu(hfac)2L 2-Me ], binuclear [{Cu(hfac)2}2(H2O)L 2-Me ], trinuclear [{Cu(hfac)2}3(L 6-Br )2], pentanuclear [{Cu(hfac)2}5(L 2-Me )2], and [{Cu(hfac)2}5(L 2-Me )4], cocrystals [Cu(hfac)2(L 2-Br )2]·[Cu(hfac)2(H2O)2] and [Cu(hfac)2(L 2-Br )2]·2[Cu(hfac)2H2O], one-dimensional polymers [Cu(hfac)2L 2-Br ] n and [Cu(hfac)2L 6-Br ] n , and cyclic dimers [Cu(hfac)2L 5-Me ]2, [Cu(hfac)2L 5-Br ]2, and [Cu(hfac)2L 6-Me ]2. The molecular structures of the obtained complexes are strongly affected by the substituent type and its location in the pyridine heterocycle. Occupation of the second position of the pyridine ring increases the steric hindrance of both imine and nitroxide coordination sites of L 2-R , which is favorable for the formation of various conformers and precipitation of complexes with different molecular structures. The pentanuclear [{Cu(hfac)2}5(L 2-Me )2] and [{Cu(hfac)2}5(L 2-Me )4] complexes do not have prior analogues and are valuable model objects for investigation of the mechanism of formation of various coordination polymers. The arrangement of long Cu-ONO bonds in {CuO6} square bipyramids due to the weakening nitroxide donor site in complexes, based on L 2-Me , L 2-Br , and L 6-Br ligands, results in ferromagnetic exchange interactions between spins of Cu2+ ions and nitroxides. Complexes with substituents that do not considerably affect the coordination ability of ligands (L 5-Me , L 5-Br , and L 6-Me ) exhibit strong antiferromagnetic exchange interactions between spins of Cu2+ ions and nitroxides.

Introduction

The study of the effect of ligand structure modification on molecular and crystal structures of the resulting complexes is an important aspect of the rational design of materials with desirable structure and functional properties. Copper(II) complexes with nitroxide radicals are unique objects for studying the peculiarities of magnetic exchange pathways between different types of paramagnetic centers.[1,2] The magnetic behavior of Cu(II)–nitroxide heterospin systems strongly correlates with the geometry of the coordination sphere of Cu2+ Jahn–Teller ions, which is substantially affected by the molecular structure of the nitroxide ligand. The equatorial coordination of the nitroxide group (NO group) results in strong antiferromagnetic Cu2+-radical exchange coupling (J ≪ 0), whereas axial binding leads to ferromagnetic exchange interactions (J > 0).[1−3] There are several well-known factors that may favor one of the geometries.[2] Equatorial coordination is often preferred because of the additional stabilization in the case of spin pairing. Presence of other strong coordination sites in the side chain of ligand forces the axial binding of the NO group. Steric factors and intermolecular interactions between uncoordinated NO groups can favor both axial and equatorial coordination types, depending on the molecular structure and crystal packing of the resulting compounds. In some cases, the energy gap between equatorial and axial coordination is very weak, which allows the magnetic exchange interactions to be switched by changing the temperature, pressure, or light irradiation.[1−12]

However, only a few Cu(II) complexes with nitroxides exhibit structural-magnetic anomalies under external stimuli.[1−24] Even a minor modification of a ligand leads to significant changes in the molecular and crystal structures of the resulting Cu(II) complexes. This strongly affects the parameters of magnetic transitions and can result in the assembly of phases that cannot exhibit any magnetostructural anomalies.[5,7−13,23,24] Cu(hfac)2 complexes with various derivatives of 3-pyridyl-substituted nitronyl nitroxide (L) are of particular interest due to the diversity of molecular species and their magnetic properties (Scheme ).[4,7,12,16−18,25,26] Introduction of a Me group in the 4th position of the pyridine ring for L nitronyl nitroxide has led to the formation of Cu(II) complexes with structure and magnetic properties completely different from those of the [{Cu(hfac)2}4L2] compound based on the unsubstituted ligand.[4,12] Recently described copper(II) and mixed-metal copper(II)-lanthanide complexes with L and L nitronyl nitroxides have demonstrated various topological structures and different magnetic behaviors in the absence of structural and magnetic transitions.[16−18,25,26] However, the effect of substituents at different positions of the pyridine ring on the coordination ability of 3-pyridyl nitronyl nitroxide ligand, structure, and magnetic behaviors of the resulting Cu(II) complexes has not been studied systematically. Therefore, the Me group and Br atom were chosen as substituents for the 2nd, 5th, or 6th positions of the pyridine heterocycle of L nitroxide, because both of them have the same spatial size but different electronic properties (Scheme ).[27] This approach allowed us to trace the general relationship between the donor ability of nitroxide coordination sites and the molecular structure of resulting complexes, as well as a substitution effect on magnetostructural correlations inherent in their nature.

Scheme 1

Structures of Cu(hfac)2 and Methyl- and Bromo-3-pyridyl-substituted Nitronyl Nitroxides (L)

Results and Discussion

Nitronyl nitroxides L (R = Me, Br) were synthesized by condensation of the corresponding aldehydes with 2,3-bis(hydroxyamino)-2,3-dimethylbutane and subsequent oxidation of the resulting adduct with PbO2 according to a well-established approach derived from Ullman’s work.[28−30] The spin-labeled L (R = 2-Me, 2-Br, 5-Me, 5-Br, 6-Br) were successfully isolated as single crystals suitable for X-ray diffraction (XRD) analysis (Figures 1S–3S and Table 1S). It was shown that the N–•O bond lengths are typical for nitronyl nitroxides (1.27–1.28 Å).[30]

Reaction of synthesized Me- and Br-substituted nitronyl nitroxides with Cu(hfac)2 resulted in obtaining new heterospin coordination complexes: mononuclear [Cu(hfac)2L], binuclear [{Cu(hfac)2}2(H2O)L], trinuclear [{Cu(hfac)2}3(L)2], pentanuclear [{Cu(hfac)2}5(L)2] and [{Cu(hfac)2}5(L)4], cocrystals [Cu(hfac)2(L)2]·[Cu(hfac)2(H2O)2] and [Cu(hfac)2(L)2]·2[Cu(hfac)2H2O], one-dimensional polymers [Cu(hfac)2L] and [Cu(hfac)2L], and cyclic dimers [Cu(hfac)2L]2, [Cu(hfac)2L]2, and [Cu(hfac)2L]2. Interestingly, only one main product was obtained in the reactions of the 5th- and 6th-substituted nitronyl nitroxides, whereas L and L radicals generated several different complexes depending on the synthetic conditions and reagent ratio. Some of the resulting complexes are kinetic products and crystallized as admixtures. [{Cu(hfac)2}2(H2O)L], [{Cu(hfac)2}5(L)4], [Cu(hfac)2(L)2]·2[Cu(hfac)2H2O], and [{Cu(hfac)2}3(L)2] complexes were obtained in small amounts and could not be manually separated because of the small size of crystals and similar appearance to the main product. This allowed us to study only their crystal structures.

The main reaction product of Cu(hfac)2 and L in a 1:1 ratio is the mononuclear [Cu(hfac)2L] complex. The crystal structure comprises two crystallographically independent Cu(hfac)2L molecules (Figure a) with slightly different bond lengths and angles. The coordination environment of Cu atoms in both molecules is square pyramidal, where one of the Ohfac atoms is at the apex, and the base is formed by the other three Ohfac atoms and the NPy atom (Cu–NPy = 2.003(9) and 2.021(8) Å; Figure a and Table 2S). The nearest ONO···ONO distances between uncoordinated NO groups of neighboring [Cu(hfac)2L] molecules is 3.659(11) Å.

Figure 1

Structures of (a) [Cu(hfac)2L] (dashed line represents the shortest distances ONO···ONO between the molecules), (b) [{Cu(hfac)2}5(L)2], (c) [{Cu(hfac)2}5(L)4], and (d) [{Cu(hfac)2}2(H2O)L] (H atoms, geminal CH3 groups of L, and CF3 groups of hfac ligands are omitted for clarity).

The χMT in the temperature range 15–300 K for [Cu(hfac)2L] is 0.81 cm3·K·mol–1 (Figure a), in good agreement with the corresponding contribution of weakly correlated spins S = 1/2 of Cu2+ ion and nitroxide. The decrease of χMT below 15 K indicates weak antiferromagnetic exchange interactions between spin sites. According to the XRD data, JR–R in the pair of neighboring [Cu(hfac)2L] molecules could be a channel of exchange interactions. The experimental χMT(T) dependence is well approximated by an equation including the corresponding contribution from the {>N–•O···O•–N<} intermolecular exchange clusters of the pair of [Cu(hfac)2L] molecules (H = −2JR–RSRASRB) using the PHI program.[31] The weakly interacting spins of the Cu2+ ions that coordinate the donor NPy atoms were introduced in the analysis by using the Curie law. The obtained optimum parameters of magnetic exchange interactions gR = 2.00 (fixed), gCu = 2.25, JR–R = −2.49 cm–1, and zJ′ = −0.09 cm–1 are in good agreement with the hypothetical scheme of the intermolecular exchange interactions.

Figure 2

Experimental dependences of χMT(T) for (a) [Cu(hfac)2L] (●) and (b) [{Cu(hfac)2}5(L)2] (■) complexes; red solid thick lines represent the data calculated based on optimized parameters.

Dark-red crystals of pentanuclear [{Cu(hfac)2}5(L)2] complex are reproducibly obtained when the ratio of reagents Cu(hfac)2/L increases to 5:2. The molecular structure of the [{Cu(hfac)2}5(L)2] complex consists of five Cu(hfac)2 fragments connected by two L (Figure b). The vertices of the square bipyramidal coordination environment of the central Cucent atom are occupied by the ONO atoms (Cucent–ONO = 2.481(2) Å; Table 2S). The coordination environment of the four terminal Cuter atoms is a square pyramid, where the donor atom NPy of L occupies the axial position in the {CuO4N} coordination units (Cuter–NPy = 2.302(2) Å) and ONO atoms are in equatorial planes of the {CuO5} square pyramids (Cuter–ONO = 1.953(2) Å).

At 300 K, the χMT for [{Cu(hfac)2}5(L)2] is 1.22 cm3·K·mol–1 and does not change significantly when the temperature is lowered to 10 K (Figure b). The experimental dependence of χMT(T) was analyzed using the PHI program[31] with the contribution of {Cu2+A-JCuR-RA-J′CuR-Cu2+C-J′CuR-RB-JCuR-Cu2+B} five-spin fragments (H = −2JCuR(SCuASRA + SCuBSRB) – 2J′CuR(SRASCuC + SCuCSRB)). The weakly interacting spins of the Cu2+ ions that coordinate with the donor NPy atoms were introduced in the analysis by using the Curie law. The optimal values of the exchange parameters, gR = 2.00 (fixed), gCu = 2.06, JCuR = −842 cm–1, J′CuR = +57 cm–1, and zJ′ = +0.01 cm–1, indicate the strong antiferromagnetic exchange interactions between the spins of terminal Cu2+ ions and nitroxides in the {CuO5} coordination units due to direct overlapping of their magnetic orbitals. The almost-constant χMT value in the temperature range 10–300 K is in good agreement with the theoretical magnitude 1.30 cm3·K·mol–1 for uncompensated spins of three Cu2+ ions.

In addition, we determined the molecular and crystal structures of pentanuclear [{Cu(hfac)2}5(L)4] and binuclear [{Cu(hfac)2}2(H2O)L] complexes. The small amounts of these crystals were obtained as admixtures when synthesized with Cu(hfac)2/L ratios exceeding 1:1. A pentanuclear molecule of the [{Cu(hfac)2}5(L)4] complex is a linear oligomer, where four bidentate nitroxide ligands bind five Cu(hfac)2 units via the NPy and ONO atoms of the nitroxide ligands (Figure c). The vertices of the elongated octahedra of the {CuO6} coordination units are occupied by ONO atoms (Cucent–ONO = 2.522(4) Å; Table 2S). The axial positions of the {CuO4N2} square bipyramids are occupied by NPy donor atoms from two different ligands (Cu–NPy = 2.422(4) and 2.665(5) Å). The terminal {CuO5} square pyramids contain ONO atoms in the equatorial plane (Cuter–ONO = 1.947(2) Å). The molecular structure of the [{Cu(hfac)2}2(H2O)L] complex consists of two Cu(hfac)2 fragments connected by bridging L ligand (Figure d and Table 2S). The vertices of the {CuO4N} square pyramid are occupied by the NPy atoms (Cu–NPy = 2.298(3) Å). The {CuO6} square bipyramid contains ONO atoms and aqua ligand in the axial positions (Cu–ONO = 2.461(3) Å and Cu–OH = 2.344(3) Å). The shortest ONO···ONO contacts between uncoordinated NO groups of neighboring molecules for both [{Cu(hfac)2}5(L)4] and [{Cu(hfac)2}2(H2O)L] are quite long and are equal to 3.711(4) and 3.654(5) Å, respectively (Table 2S).

The main product in the {Cu(hfac)2 + L} synthetic system is a one-dimensional [Cu(hfac)2L] complex (Figure a). Nitroxide molecules carry out the bidentate bridging function via the ONO donor atoms of both NO groups in the coordination sphere of Cu atoms. The resulting polymer chains consist of two types of alternating [Cu(hfac)2L] monomers, which slightly differ in bond lengths and angles (Table 3S). The vertices of {CuO6} bipyramids are occupied by ONO atoms of NO groups (Cu–ONO = 2.554(3) and 2.619(2) Å), whereas the O atoms of the hfac anions are in the equatorial planes (Cu–Ohfac = 1.92–1.93 Å). The experimental χMT value for the [Cu(hfac)2L] polymer chain is 0.87 cm3·K·mol–1 at 300 K, which is close to the theoretical value of 0.81 cm3·K·mol–1 for two independent paramagnetic centers with S = 1/2. The χMT value gradually increases to 2.74 cm3·K·mol–1 upon lowering the temperature to 2 K, indicating weak ferromagnetic exchange interactions (Figure a) that are typical for complexes with axial coordination of NO groups.[1,2,10,13−15,18,24] The experimental dependence of χMT(T) was approximated using a high-temperature series expansion for the uniform chain model (H = −JCuR∑SASA)[32] on the assumption that the exchange interactions JA and JB are weakly ferromagnetic in nature and their magnitude should be approximately the same, i.e., JCuR ≈ JA ≈ JB. The estimated optimum parameters of magnetic exchange interactions, gR = 2.00 (fixed), gCu = 2.09, JCuR = +6.8 cm–1, are in good agreement with the proposed scheme of the intrachain exchange interactions.

Figure 3

Structures of (a) [Cu(hfac)2L], (b) [Cu(hfac)2(L)2]·[Cu(hfac)2(H2O)2], and (c) [Cu(hfac)2(L)2]·2[Cu(hfac)2H2O] (H atoms, geminal CH3 groups of L, and CF3 groups of hfac ligands are omitted for clarity).

Figure 4

Experimental dependence of χMT(T) for the [Cu(hfac)2L] complex (■); the red solid thick line represents the data calculated based on optimized parameters.

The reaction of Cu(hfac)2 with L can often give products containing coordinated water molecules. Violet needle-shaped crystals of the [Cu(hfac)2(L)2]·[Cu(hfac)2(H2O)2] complex were obtained from a 1:1 ratio of Cu(hfac)2 and L. XRD revealed that their crystal structure is a cocrystal of [Cu(hfac)2(L)2] and [Cu(hfac)2(H2O)2] complexes (Figure b and Table 3S). The vertices of the square bipyramid of the Cu atom in the [Cu(hfac)2(L)2] fragment are occupied by two ONO atoms (Cu–ONO = 2.582(3) Å), whereas Ohfac atoms are located in the equatorial plane (Cu–Ohfac = 1.92–1.93 Å). O atoms of the aqua ligands are located in the axial positions of a distorted octahedral [Cu(hfac)2(H2O)2] fragment (Cu–OH = 2.413(2) Å). The distances between OH atoms of the [Cu(hfac)2(H2O)2] molecules and uncoordinated NO groups and N atom of the pyridine ring from the [Cu(hfac)2(L)2] complex fragment are short (2.898(4) and 2.954(5) Å, respectively). This indicates the presence of weak O–H···ONO and O–H···NPy hydrogen bonds, which connect these [Cu(hfac)2(L)2] and [Cu(hfac)2(H2O)2] fragments into layers (Figure b).

After increasing the ratio of Cu(hfac)2 and L to 3:2, a small amount of the [Cu(hfac)2(L)2]·2[Cu(hfac)2H2O] compound was precipitated (Figure c). The molecular structure of [Cu(hfac)2(L)2] complex is similar to that described above. There are ONO atoms in the axial positions of the {CuO6} units (Cu–ONO = 2.651(9) Å). OH atoms occupy the vertices of {CuO5} square pyramids of the [Cu(hfac)2H2O] fragments (Cu–OH = 2.250(8) Å). Each [Cu(hfac)2(L)2] fragment is connected with two [Cu(hfac)2H2O] via hydrogen bonds (OH···ONO = 2.841(10) Å), whereas other hydrogen bonds (OH···NPy = 2.897(12) Å) are linked resulting in linear [Cu(hfac)2(L)2]·2[Cu(hfac)2H2O] chains forming ribbons (Figure c and Table 3S).

The reactions of Cu(hfac)2 with nitroxides bearing substituents at the 5th position of the pyridine ring produce the cyclic dimer complexes [Cu(hfac)2L]2 and [Cu(hfac)2L]2 with similar molecular structures and crystal packing. These dimer molecules consist of two Cu(hfac)2 fragments bridged by two nitroxide molecules via NPy and ONO donor atoms (Figures a and 4Sa). The coordination sphere of both crystallographically identical Cu atoms includes the ONO and NPy atoms from two paramagnetic ligands and two hfac anions in cis positions. The vertices of a square bipyramid are occupied by two Ohfac atoms (Cu–Ohfac = 2.358(3) and 2.368(3) Å). The remaining Ohfac atoms, the ONO atom, and NPy atom are located in the equatorial plane (Table 4S).

Figure 5

Structures of (a) [Cu(hfac)2L]2, (b) [Cu(hfac)2L], and (c) [{Cu(hfac)2}3(L)2] (H atoms, geminal CH3 groups of L, and CF3 groups of hfac ligands are omitted for clarity).

The temperature dependence of the molar magnetic susceptibility shows a similar magnetic behavior for both [Cu(hfac)2L]2 (R = Me, Br) complexes (Figures a and 4Sc). At 300 K, χMT values are equal to 0.04 and 0.06 cm3·K·mol–1 for [Cu(hfac)2L]2 and [Cu(hfac)2L]2, respectively, much smaller than the theoretical spin value (1.62 cm3·K·mol–1) for four independent paramagnetic centers with spins S = 1/2. By lowering the temperature to 2 K, the χMT decreases to 0.01 cm3·K·mol–1 for both complexes. The strong suppression of paramagnetic properties correlates well with the short Cu–ONO distances in the molecular structure of [Cu(hfac)2L]2 (R = Me, Br) dimers. The equatorial coordination of the nitroxide group to the central Cu2+ ion should lead to direct overlapping of their magnetic orbitals and mutual compensation of the spins in the {Cu–O•–N<} exchange clusters.[1,2,14,15,21,24] The χMT(T) dependences are well described by the dimer model with the Hamiltonian H = −2JCuR(SCuSR) using the PHI program.[31] The optimum values of the exchange parameters are as follows: gR = 2.00 (fixed), gCu = 2.15, JCuR= −493 cm–1, zJ′ = −0.10 cm–1 for [Cu(hfac)2L]2, and gR = 2.00 (fixed), gCu = 2.00, JCuR= -544 cm–1, zJ′ = −0.11 cm–1 for [Cu(hfac)2L]2. The residual χMT value is 0.01 cm3·K·mol–1 at 2 K for both the dimers, attributed to the free spin defects being less than 0.6%.

Figure 6

Experimental dependences of χMT(T) for (a) [Cu(hfac)2L]2 and (b) [Cu(hfac)2L]; red solid thick lines represent the data calculated based on optimized parameters.

Dark-red crystals of the [Cu(hfac)2L]2 dimer complex were isolated in the synthesis containing equimolar amounts of Cu(hfac)2 and L. The [Cu(hfac)2L]2 complex shows the same molecular structure with short Cu–NPy and Cu–ONO bond lengths (Cu–ONO ∼1.97 Å and Cu–NPy ∼2.04 Å; Figure 4Sb and Table 4S) as the above-mentioned Cu(II) compounds with nitroxide derivatives containing Me and Br substituents in the 5th position of the pyridine ring. The mutual arrangement of [Cu(hfac)2L]2 dimer molecules is different from that in the crystal structure complexes with 5-Me- and 5-Br-substituted nitroxides. However, the magnetic behavior, mainly caused by intramolecular magnetic exchange interactions, is similar to the above-described one for [Cu(hfac)2L]2 (R = Me, Br). The optimum values of the fitted exchange parameters gR = 2.15 (fixed), gCu = 2.15, JCuR= −460 cm–1, zJ′ = −0.10 cm–1 (H = -2JCuR(SCuSR)) show totally suppressed paramagnetism due to strong antiferromagnetic exchange interactions between spins in the {Cu–O•–N<} exchange clusters (Figure 4Sd).

The molecular structures of the obtained [Cu(hfac)2L]2, [Cu(hfac)2L]2, [Cu(hfac)2L]2 cyclic dimers and the previously described [Cu(hfac)2L]212 complex are similar. [Cu(hfac)2L]2 undergoes ferromagnetic exchange interactions in the temperature range 2–300 K,[12] whereas [Cu(hfac)2L]2, [Cu(hfac)2L]2, and [Cu(hfac)2L]2 exhibit suppressed paramagnetism that strongly correlates with the axial and equatorial type of the NO group bonding to the Cu2+ central ion. As the increasing of intermolecular Cu–ONO distances, associating with the change-over of spin multiplicity, can be initiated by increasing the temperature,[12] we performed the differential thermal and thermogravimetric analysis of the obtained dimer complexes. However, the TG-DTA curves for [Cu(hfac)2L]2, [Cu(hfac)2L]2, and [Cu(hfac)2L]2 (Figure 5S) did not show any specific heat effects associated with phase transitions up to the samples’ decomposition temperature. Thus, the strongly coupled state is the most stable one for the [Cu(hfac)2L]2, [Cu(hfac)2L]2, and [Cu(hfac)2L]2 dimer complexes in the temperature range 2–380 K.

The reaction of Cu(hfac)2 with the 6-Br-substituted derivative of nitronyl nitroxide yielded the one-dimensional [Cu(hfac)2L] and trinuclear [{Cu(hfac)2}3(L)2] complexes (Figure b,c and Table 4S). The trinuclear molecule of the [{Cu(hfac)2}3(L)2] complex is a linear oligomer, where two nitroxide ligands bind with three Cu(hfac)2 via ONO atoms. The vertices of the square bipyramid of the central Cucent atom in the {CuO6} coordination unit are occupied by the ONO atoms (Cucent–ONO = 2.577(2) Å). The coordination sphere of the Cuter atom of the terminal {CuO5} square pyramids contains an ONO atom in the equatorial plane (Cuter–ONO = 1.953(2) Å).

The structure of the [Cu(hfac)2L] polymer chain complex results from the bridging bidentate coordination of L (Figure b). Each Cu atom is surrounded either by two ONO atoms or by two NPy atoms from two different L, leading to a head-to-head motif of infinite chains. Both donor atoms of the ligands occupy the axial positions in the alternating {CuO6} and {CuO4N2} square bipyramids (Cu–ONO = 2.385(3) Å and Cu–NPy = 2.955(3) Å), whereas the equatorial planes are formed by the Ohfac atoms of the hfac anions (Table 4S). The shortest distances between ONO atoms from uncoordinated nitroxide groups of the ligands from neighboring chains are more than 4 Å.

The χMT for [Cu(hfac)2L] is 0.80 cm3·K·mol–1 at 300 K, in good agreement with the theoretical value (0.81 cm3·K·mol–1) for two independent spins S = 1/2 of Cu2+ ion and nitroxide radical per formula fragment [Cu(hfac)2L] (Figure b). χMT gradually increases with decreasing temperature and reaches 1.68 cm3·K·mol–1 at 2 K, indicating the domination of ferromagnetic exchange interactions, which are typical for axial coordination of NO groups to central Cu2+ ions.[1,2,10,14,24] The experimental dependence of χMT(T) is well described by the model including the contribution of the three-spin {RA-JCuR-Cu2+A-JCuR-RB} exchange cluster (H = −2JCuR(SCuASRA + SRBSCuA)) and isolated Cu2+ ion spins from coordination units {CuO4N2} according to the Curie law, with the best fit parameters as follows: gR = 2.00 (fixed), gCu = 2.10, JCuR = +16 cm–1, and zJ′ = +0.07 cm–1.

General Comparisons

Comparing data of the Cu(hfac)2 complexes with substituted derivatives of 3-pyridyl nitronyl nitroxides obtained within the scope of the current study and previously described elsewhere[4,12,26] showed that the substituent type and its position in the pyridine heterocycle considerably affect the structure and magnetic behavior of the resulting compounds. Initially, we suspected that a substituent at the 2nd position of the pyridine ring should sterically hinder the coordination of both imine and nitroxide donor sites. Occupation of the 4th or 6th positions should mainly weaken the donor properties of only the nitroxide or the imine, respectively. The location of the substituent at the 5th position of the pyridine heterocycle should have a much smaller effect on donor abilities of a ligand. Additionally, we assumed that Br substituent should considerably weaken the nearest donor site due to a strong negative inductive effect. However, the ability of ligand donor sites to allow coordination is only one of the factors affecting the molecular structures of complexes. The spatial arrangements of hfac and nitroxide ligands in different types of coordination units and the number of coordinated donor sites of ligands are strongly influenced by the reagents ratio, their concentration, the solvent used, ambient temperature, and humidity.[1,2,7,11,23,24] The low solubility of the resulting solids and the strong antiferromagnetic exchange interactions between paramagnetic centers can result in some particular molecular and high-dimensional species.[1,2,23,24]

The angle between the planes of the nitronyl nitroxide O•–N–C=N→O fragment and the pyridine ring is one of the key indicators of steric hindrance for 3-pyridyl-substituted nitronyl nitroxides (Scheme and Table 5S). It is 36 and 53° for crystallographically independent molecules in the crystal structure of L,[4] showing a broad range of possible conformations of the substituted derivatives. The smallest values of the CN2–Py angle (10–19°) were found for nitroxides with the Me group or Br atom at the 5th position of the pyridine ring. This angle increases to ∼37° when the Br atom is shifted to the 6th position of the pyridine heterocycle. The largest values were found for L and L [12] derivatives (47–72°), which are in perfect agreement with the expected increased steric hindrance.

Scheme 2

Angle between the Planes of the Nitronyl Nitroxide O•–N–C=N→O Fragment and the Pyridine Ring

A similar correlation between the CN2–Py interplanar angle and the arrangement of substituents in L (R = Me, Br) is found for the molecular structures of the complexes (Table 6S). However, this relationship is much more complicated and required consideration of the steric properties of bulky Cu(hfac)2 fragments in the different types of coordination units and the variation of a number of coordinated donor sites of ligands. Notably, the most sterically hindered L and L [12] radicals produce the largest number of complexes, whereas the use of ligands bearing substituents at the 5th and 6th positions of the pyridine heterocycle produces only one main coordination complex for each paramagnetic ligand. Apparently, a high steric hindrance for complexes with L and L derivatives induces the formation of many conformers with various CN2–Py angles in a solution, leading to the precipitation of compounds with different molecular structures.

The comparison of distances between Cu atoms and imine and nitroxide donor sites allowed us to clarify the key correlations between the type and position of the substituent in the pyridine heterocycle and the coordinating ability of nitroxide ligands (Table 6S). The short Cu–NPy (∼2.00 Å) and Cu–ONO (∼1.97 Å) distances have been found for dimer complexes containing ligands bearing Me and Br substituents at the 5th position of the pyridine ring. Similar cyclic dimers were obtained for Cu(hfac)2 complexes with L and L ligands. Shift of the Me group to the 6th position of the pyridine ring does not considerably change the donor ability of the imine site and Cu–NPy distance (∼2.04 Å). In contrast, the nitroxide donor site for L is more sterically hindered, which results in lengthening of the Cu–ONO bond (∼2.43 Å).[12]

The complexes with L ligand are the one-dimensional polymer [Cu(hfac)2L] and the trinuclear molecule [{Cu(hfac)2}3(L)2]. These are completely different from the [Cu(hfac)2L]2 cyclic dimer based on a Me-substituted analogue. The replacement of the Me group by a Br atom at the 6th position of the pyridine heterocycle substantially weakens the imine donor site due to a strong negative inductive effect, as indicated by the Cu–NPy bond lengths in the resulting complexes. Chains with a head-to-head motif of [Cu(hfac)2L] are similar to the previously described coordination polymers for complexes based on pyrazolyl-substituted derivatives of nitroxides.[9,10,23,24] Alternating octahedral {CuO6} and {CuO4N2} coordination units contain chelate hfac ligands in a coplanar arrangement. This arrangement should be less spatially crowded compared with the cis position spacing in {CuO5N} units for the [Cu(hfac)2L]2 dimer complex. Both NPy and ONO atoms are situated in axial positions of elongated octahedra (Cu–NPy = 2.954(3) Å and Cu–ONO = 2.385(2) Å). The structure of [{Cu(hfac)2}3(L)2] is similar to the previously described [{Cu(hfac)2}3(L)2].[26] The weak donor ability of the imine site leads to the absence of coordination to the central Cu2+ ion in both trinuclear complexes with L and L. The negative inductive effect inherent in L is comparable to the steric hindrance of L. The pentacoordinated terminal {CuO5} units have reduced spatial bulkiness and provide shorter Cu–ONO (1.953(2) Å) distances than those in the central {CuO6} coordination unit (2.577(2) Å).

The introduction of substituents at the 2nd position of the pyridine heterocycle sterically hinders both imine and nitroxide donor sites of the L ligand. Nevertheless, it turned out to be favorable for the formation of various conformers of L and complexes with different molecular structures. This is similar to some other stereochemically flexible Cu(II)–nitroxide systems, where a reaction of Cu(hfac)2 with polyfunctional nitroxides yields various solids with different structures and compositions.[1,2,7,9−14,23,24] At the same time, a variety of structural motifs generates the diversity of magnetic properties.[1,2,10,11,23,24] The short Cu–NPy bonds (2.003(9) and 2.021(8) Å) were observed only for the mononuclear [Cu(hfac)2L] complex due to the reduced spatial bulkiness of pentacoordinated {CuO4N} units. For the [{Cu(hfac)2}2(H2O)L] complex with two Cu(hfac)2 fragments, which coordinate the same L ligand, the Cu–NPy bond in the {CuO4N} units becomes longer (2.298(3) Å), whereas the coordination sphere of the other Cu atom tends to a distorted octahedral environment {CuO6} by including an ONO atom and H2O molecule in axial positions (Cu–ONO = 2.461(3) Å and Cu–OH = 2.344(3) Å).

The reproducibly obtained [{Cu(hfac)2}5(L)2] complex is supposedly the most thermodynamically stable product in the {Cu(hfac)2 + L} system. In spite of steric hindrance, both L ligands can perform tridentate coordination. As expected, Cu–ONO distances in central {CuO6} square bipyramids are long (2.481(2) Å). Long Cu–NPy (2.302(2) Å) and short Cu–ONO (1.953(2) Å) bonds in pentacoordinated {CuO4N} and {CuO5} units, respectively, indicate the higher steric effect on the imine donor site, comparable with nitroxide. Similar bond lengths for the L ligand donor sites are found for the linear pentanuclear [{Cu(hfac)2}5(L)4] complex (Cu–NPy = 2.422(4) and 2.665(5) Å, Cucent–ONO = 2.522(4) Å, and Cuter–ONO = 1.947(2) Å). Linear trinuclear complexes are often obtained using an excess of Cu(hfac)2 in the reaction mixture.[15,24] Recently, another linear pentanuclear complex with a different arrangement of coordination units was reported.[10] Reducing the spatial bulkiness of [{Cu(hfac)2}5(L)2] and [{Cu(hfac)2}5(L)4] complexes via formation of terminal pentacoordinated {CuO5} or {CuO4N} units prevents the assembly of high-dimensional species. However, the precipitation of intermediate oligomers in the form of new pentanuclear [{Cu(hfac)2}5(L)2] and [{Cu(hfac)2}5(L)4] complexes could clarify the mechanism for obtaining various polymers with chain, layered, or framework structures.

Location of the Br atom at the 2nd position of the pyridine heterocycle leads to considerable weakening of the imine site of L because of both the increasing steric hindrance and the negative inductive effect. Therefore, coordination of an imine atom to a Cu2+ ion is absent for the molecular structure in the [Cu(hfac)2L], [Cu(hfac)2(L)2]·[Cu(hfac)2(H2O)2], and [Cu(hfac)2(L)2]·2[Cu(hfac)2H2O] complexes. The chain polymer structure of [Cu(hfac)2L] consists of alternating nitroxide and Cu(hfac)2 fragments, with long Cu–ONO distances (2.554(2) and 2.619(2) Å) in distorted octahedral {CuO6} units. Despite the possibility of the existence of trinuclear oligomers like [{Cu(hfac)2}3(L)2], the molecular structures of the other two complexes includes mononuclear [Cu(hfac)2(L)2] molecules, which are bound by weak hydrogen bonds with [Cu(hfac)2(H2O)2] or [Cu(hfac)2H2O] fragments containing coordinated water molecules.

The magnetic properties of complexes correlate with their structural characteristics. The character of the magnetic exchange interactions for cyclic dimers with {CuO4N} coordination units mainly depends on the steric properties of the ligand. The strong antiferromagnetic coupling between spins of Cu2+ ions and nitronyl nitroxides is observed for the [Cu(hfac)2L]2, [Cu(hfac)2L]2, and [Cu(hfac)2L]2 dimers with short Cu–ONO bonds due to a weak steric effect. Ferromagnetic exchange interactions in the two-spin-coupled {Cu2+–O•–N<} exchange clusters for [Cu(hfac)2L]2 [12] results from lengthening of the Cu–ONO bonds due to steric hindrance of the nitroxide donor site of L. The appearance of {CuO5} coordination units with short Cu–ONO in complexes based on L and L ligands provides strong antiferromagnetic exchange interactions. This leads to full compensation of spins of Cu2+ ions and coordinated nitronyl nitroxides already at room temperature, as was found for [{Cu(hfac)2}5(L)2]. Coordination of a nitroxide group to the axial position of elongated {CuO6} octahedra is favorable for the formation of exchange channels with ferromagnetic interactions between spins of Cu2+ ions and nitroxides, as takes place for [Cu(hfac)2L] and [Cu(hfac)2L] species.

Conclusions

Tuning of the donor ability of nitroxide coordination sites is a powerful tool for the formation of Cu(II) complexes with diverse molecular and crystal structures and therefore various magnetic behaviors. Introduction of a methyl group or bromine atom at different positions of the pyridine heterocycle affects the donor abilities of the nitroxide ligand due to steric or/and electronic effects of substituents. Complexes in which substituents do not strongly reduce the coordination ability of ligands (L, L, L) exhibit strong antiferromagnetic exchange interactions in two-spin-coupled {Cu2+–O•–N<} clusters when NO groups are equatorially coordinated in {CuO4N} units. The arrangement of long Cu–ONO bonds in {CuO6} square bipyramids due to the weakening of the nitroxide donor site in complexes with L, L, and L leads to ferromagnetic exchange interactions between spins of Cu2+ ions and nitroxides. However, the design of complexes with predictable structures and magnetic behaviors should consider other steric factors, such as the bulkiness of hfac and nitroxide ligands in different types of coordination units and the number of coordinated donor sites of ligands. Spontaneously reducing steric hindrance due to formation of pentacoordinated {CuO5} units with short Cu–ONO distances allows the assembly of low-dimensional species with strong antiferromagnetic exchange interactions in two-spin-coupled {Cu2+–O•–N<} clusters.

The high steric hindrance for L ligands can lead to various conformers in solution, which provides additional diversity of molecular structures of the resulting complexes. This allows pentanuclear [{Cu(hfac)2}5(L)2] and [{Cu(hfac)2}5(L)4] complexes to be obtained with unique molecular structures. The [{Cu(hfac)2}5(L)2] and [{Cu(hfac)2}5(L)4] oligomers are suggested to be intermediate species in the formation of various chain polymers, as well as layered and framework complexes. We have plans for further investigations on the effect of bulky substituents in the pyridine heterocycle of nitroxide on the structure and magnetic properties of complexes. This will create additional opportunities in the design of complexes with novel molecular structures and desired magnetic properties.

Experimental Section

General Procedures

The reactions were monitored by TLC using Merck “Aluminum oxide 60 F254 neutral, aluminum sheets” and Merck “Silica gel 60 F254, aluminum sheets”. Column chromatography was carried out with the use of Al2O3 (Alumina activated 300, Nacalai Tesque). The reagents used without additional purification included 2-bromo-3-formylpyridine (96%, Sigma-Aldrich), 5-bromonicotinaldehyde (97%, Sigma-Aldrich), 6-bromo-3-formylpyridine (95%, Sigma-Aldrich), 3-bromo-2-methylpyridine (97%, Sigma-Aldrich), 3-bromo-5-methylpyridine (97%, Sigma-Aldrich), 5-bromo-2-methylpyridine (99%, Sigma-Aldrich), n-butyllithium (1.6 M in n-hexane, Kanto Chemical Co., INC), ethyl formate (FUJIFILM Wako Pure Chemical Corporation), and PbO2 (FUJIFILM Wako Pure Chemical Corporation). Cu(hfac)2,[33] 2,3-bis(hydroxyamino)-2,3-dimethylbutane,[34] and 2-methylnicotin aldehyde, 5-methylnicotin aldehyde, and 6-methylnicotin aldehyde[35] were synthesized by standard procedures. C, H, and N elemental analyses were carried out on a 2400 CHNS/O Series II System (100V) analyzer. Thermogravimetric and differential thermal analysis (TG-DTA) for polycrystalline samples [Cu(hfac)2L]2, [Cu(hfac)2L]2, and [Cu(hfac)2L]2 was performed using SII EXSTAR TGA operating under dry nitrogen with heating from 300 to 500 K at a rate of 5 K/min.

2-(2-Bromopyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L)

2-Bromo-3-formylpyridine (249.8 mg, 1.34 mmol) was added to a solution of 2,3-bis(hydroxyamino)-2,3-dimethylbutane (207.4 mg, 1.40 mmol) in MeOH (7.0 mL) at room temperature. The reaction mixture was stirred for 30 h; then, the precipitate was filtered off, washed with MeOH, and dried in air. This gave 3-(1,3-dihydroxy-4,4,5,5-tetramethyl-imidazolidin-2-yl)-2-bromopyridine (394.9 mg) as a white powder, which was subsequently used without additional purification. PbO2 (1.9 g) was added to a suspension of the adduct in MeOH (10.0 mL) and the mixture was stirred for 2 h. The resulting violet solution was filtered, the filtrate was evaporated, and the residue purified by column chromatography (Al2O3, EtOAc) followed by recrystallization from a CH2Cl2/n-hexane mixture. Yield: 271.0 mg (64%). Found (%): C 45.7; H 4.5; N 13.3. Calculated for C12H15N3O2Br (%): C 46.0; H 4.8; N 13.4. Other bromo- and methyl-substituted 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyls were obtained by an analogous procedure. 2-(2-Methylpyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L). Yield: 582.0 mg (53%). Found (%): C 62.8; H 7.5; N 16.9. Calculated for C13H18N3O2 (%): C 62.9; H 7.3; N 16.9. 2-(5-Bromopyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L). Yield: 161.0 mg (51%). Found (%): C 46.1; H 4.7; N 13.4. Calculated for C12H15N3O2Br (%): C 46.0; H 4.8; N 13.4. 2-(5-Methylpyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L). Yield: 210.0 mg (36%). Found (%): C 62.2; H 7.6; N 16.9. Calculated for C13H18N3O2 (%): C 62.9; H 7.3; N 16.9. 2-(6-Bromopyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L). Yield: 261.0 mg (41%). Found (%): C 46.0; H 4.5; N 13.2. Calculated for C12H15N3O2Br (%): C 46.0; H 4.8; N 13.4. 2-(6-Methylpyridin-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (L). Yield: 293.0 mg (61%). Found (%): C 62.4; H 7.3; N 16.9. Calculated for C13H18N3O2 (%): C 62.9; H 7.3; N 16.0.

[{Cu(hfac)2}5(L2-Me)2]

n-Hexane (3 mL) was added to a solution of Cu(hfac)2 (59.2 mg, 0.125 mmol) and L (12.5 mg, 0.050 mmol) in a mixture of Et2O (0.5 mL) and CH2Cl2 (1.0 mL). The volume of the dark-red solution was decreased to ∼2 mL with a slow flow of nitrogen. The reaction mixture was kept in a sealed flask at −5 °C for 12 h. The dark-red crystals suitable for XRD analysis were filtered off, washed with hexane, and dried in air. Yield: 63.7 mg (89%). Found (%): C 31.2; H 1.6; N 2.9. Calculated for Cu5C76H46N6O24F60 (%): C 31.6; H 1.6; N 2.9.

[{Cu(hfac)2}5(L)4]

[{Cu(hfac)2}5(L)4] crystals coprecipitated together with [{Cu(hfac)2}5(L)2] when the above-described reaction mixture was kept in an open flask at room temperature for 12–24 h. The crystals of [{Cu(hfac)2}5(L)2] and [{Cu(hfac)2}5(L)4] compounds are difficult to distinguish because they are similar in both color and habit. We could not collect enough [{Cu(hfac)2}5(L)4] manually with SC XRD identification for an extensive SQUID analysis and studied only the crystal structure.

[Cu(hfac)2L]

Cu(hfac)2 (59.9 mg, 0.125 mmol) and L (25.7 mg, 0.100 mmol) were dissolved in a mixture of Et2O (0.5 mL) and CH2Cl2 (1.0 mL). Then, n-hexane (3 mL) was added and the volume of the dark-red solution was reduced to ∼2 mL with a slow flow of nitrogen. The reaction mixture was kept in a sealed flask at −5 °C for 16 h. The green crystals suitable for XRD analysis were filtered off, washed with hexane, and dried in air. Yield: 65.1 mg (72%). Found (%): C 37.0; H 2.7; N 5.7. Calculated for CuC23H20N3O6F12 (%): C 38.1; H 2.8; N 5.8.

[{Cu(hfac)2}2(H2O)L]

A solution of Cu(hfac)2 (48.0 mg, 0.100 mmol) in n-heptane (20.0 mL) was placed in a 50 mL flask and was refluxed until the volume of the reaction mixture decreased to ∼8 mL. Then, a solution of L (12.5 mg, 0.050 mmol) in CH2Cl2 (2.0 mL) was added and the volume of the reaction mixture was reduced to ∼4 mL with a slow flow of nitrogen. The reaction mixture was kept in a sealed flask at −5 °C. After 12 h, a mixture of agglomerates of green crystals and violet powder was filtered off, washed with cold hexane, and dried in air. Several green crystals suitable for XRD were separated manually.

n-Hexane (3.0 mL) was added to a solution of Cu(hfac)2 (42.1 mg, 0.088 mmol) and L (15.6 mg, 0.050 mmol) in a mixture of Et2O (0.5 mL) and CH2Cl2 (1.0 mL). The volume of the resulting dark-red solution was reduced to ∼4 mL with a slow flow of nitrogen. The reaction mixture was kept at −5 °C in a sealed flask for 12–14 h. The resulting violet crystals were filtered off, washed with cold hexane, and dried in air. Yield: 34.7 mg (50%). Found (%): C 33.4; H 2.2; N 5.3. Calculated for CuC22H17N3O6F12Br (%): C 33.3; H 2.1; N 5.3.

[Cu(hfac)2(L)2]·[Cu(hfac)2(H2O)2]

n-Hexane (5.0 mL) was added to a solution of Cu(hfac)2 (48.0 mg, 0.100 mmol) and L (31.4 mg, 0.100 mmol) in CH2Cl2 (1.0 mL). Volume of the resulting mixture was reduced to ∼4 mL with a slow flow of nitrogen and kept in a sealed flask at −5 °C for 12–14 h. The violet crystals suitable for XRD analysis were filtered off, washed with hexane, and dried in air. Yield: 25.7 mg (32%). Found (%): C 32.0; H 2.3; N 5.0. Calculated for Cu2C44H38N6O14F24Br2 (%): C 32.7; H 2.4; N 5.2.

[Cu(hfac)2(L)2]·2[Cu(hfac)2H2O]

[Cu(hfac)2(L)2]·2[Cu(hfac)2H2O] was obtained as a coproduct of the [Cu(hfac)2(L)2]·[Cu(hfac)2(H2O)2] complex when the ratio of Cu(hfac)2/L was increased to 2:1. The crystals of [Cu(hfac)2(L)2]·2[Cu(hfac)2H2O] and [Cu(hfac)2(L)2]·[Cu(hfac)2(H2O)2] compounds are difficult to distinguish because they are similar in both color and habit. Several crystals suitable for SC XRD study were collected manually.

[Cu(hfac)2L]2

n-Hexane (5.0 mL) was added to a mixture of Cu(hfac)2 (47.8 mg, 0.100 mmol) and L (31.3 mg, 0.100 mmol) in CH2Cl2 (2.0 mL). The resulting dark-green solution was kept in an open flask at 18 °C for 12–14 h. Bulky red prisms were filtered off, washed with hexane, and dried in air. Yield: 73.0 mg (92%). Found (%): C 33.4; H 1.9; N 5.3. Calculated for Cu2C44H34N6O12F24Br2 (%): C 33.4; H 2.2; N 5.3. Other Cu(hfac)2 dimer complexes with L and L were obtained by following an analogous procedure. [Cu(hfac)2L]. Yield: 64.0 mg (88%). Found (%): C 37.9; H 2.4; N 5.7. Calculated for Cu2C46H40N6O12F24 (%): C 38.1; H 2.8; N 5.8. [Cu(hfac)2L]. Yield: 63.0 mg (86%). Found (%): C 37.7; H 2.4; N 5.7. Calculated for Cu2C46H40N6O12F24 (%): C 38.1; H 2.8; N 5.8.

[Cu(hfac)2L6-Br]

A solution of Cu(hfac)2 (71.9 mg, 0.150 mmol) in n-heptane (20.0 mL) was placed in a 50 mL flask and was refluxed until the volume of the reaction mixture reduced to ∼8.0 mL. Then, a solution of L (31.5 mg, 0.100 mmol) in CH2Cl2 (1.0 mL) was added and the volume of the reaction mixture was reduced to ∼3 mL with a slow flow of nitrogen. The reaction mixture was kept at −5 °C in a sealed flask for 10–16 h. Resulting dark-blue crystals were filtered off, washed with cold hexane, and dried in air. Yield: 46.1 mg (39%). Found (%): C 33.0; H 1.7; N 5.2. Calculated for Cu2C44H34N6O12F24Br2 (%): C 33.4; H 2.2; N 5.3.

The crystals of [{Cu(hfac)2}3(L)2] complex were obtained as an admixture by quickly decreasing the volume of the reaction mixture with a flow of nitrogen. Because of the small size of [{Cu(hfac)2}3(L)2] crystals, we collected only several of them manually for SC XRD study.

X-ray Crystallography (XRD)

X-ray diffraction intensities were collected for the selected single crystals individually mounted on glass fibers using Bruker diffractometers—SMART-APEX II with a CCD and D8-QUEST with a CMOS area detector (Mo Kα radiation). Data reduction was performed using SAINT, and intensities were corrected for absorption by using SADABS.[36,37] The structures were solved by direct methods and refined by the full-matrix least-squares procedure anisotropically for nonhydrogen atoms. The H atoms were calculated geometrically and included in the refinement as riding groups. All calculations were fulfilled with the program package SHELXT 2014/5 and SHELXT-2018/3.[38] The crystallographic data and details of experiments are presented in Tables 1S–4S in the Supporting Information.

Magnetic Measurements

The magnetic susceptibility measurements were performed using an MPMS-5 SQUID magnetometer (Quantum Design) in the temperature range 2–350 K in a 5 kOe magnetic field. The molar magnetic susceptibility was calculated using diamagnetic corrections for the complexes according to the Pascal scheme.[39] Analysis and fitting of magnetochemistry data for heterospin complexes was performed with the PHI program.[31]