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Article

A Unique Trinuclear, Triangular Ni(II) Complex Composed of Two tri-Anionic bis-Oxamates and Capping Nitroxyl Radicals

by
Vitaly A. Morozov
1,
Denis G. Samsonenko
2 and
Kira E. Vostrikova
2,*
1
International Tomography Center SB RAS, Institutskaya Str. 3a, 630090 Novosibirsk, Russia
2
Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Lavrentiev Avenue, 630090 Novosibirsk, Russia
*
Author to whom correspondence should be addressed.
Inorganics 2025, 13(7), 214; https://doi.org/10.3390/inorganics13070214
Submission received: 24 May 2025 / Revised: 22 June 2025 / Accepted: 23 June 2025 / Published: 25 June 2025
(This article belongs to the Section Coordination Chemistry)
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Abstract

Phenylene-based bis-oxamate polydentate ligands offer a unique opportunity for creating a large variety of coordination compounds, in which paramagnetic metal ions are strongly magnetically coupled. The employment of imino nitroxyl (IN) radicals as supplementary ligands confers numerous benefits, including the strong ferromagnetic interaction between Ni and IN. Furthermore, the chelating IN can act as a capping ligand, thereby impeding the formation of coordination polymers. In this study, we present the molecular and crystal structure and experimental and theoretical magnetic behavior of an exceptional neutral trinuclear complex [Ni(L3−)2(IN)3]∙5CH3OH (1) (L is N,N′-1,3-phenylenebis-oxamic acid; IN is [4,4,5,5-tetramethyl-2-(6-methylpyridin-2-yl)-4,5-dihydro-1H-imidazol-1-yl]oxidanyl radical) with a cyclic triangular arrangement. Moreover, in this compound three Ni2+ ions are linked by the two bis-oxamate ligands playing a rare tritopic function due to an unprecedented triple deprotonation of the related meta-phenylene-bis(oxamic acid). The main evidence of such a deprotonation of the ligand is the neutrality of the cluster, since there are no anions or cations compensating for its charge in the crystals of the compound. Despite the presence of six possible magnetic couplings in the trinuclear cluster 1, its behavior was reproduced with a high degree of accuracy using a three-J model and ZFS, under the assumption that the three different Ni-IN interactions are equal to each other, whereas only two equivalent-in-value Ni-Ni interactions were taken into account, with the third one being equated to zero. Our study indicates the presence of two opposite-in-nature types of magnetic interactions within the triangular core. DFT and CASSCF/NEVPT2 calculations were completed to support the experimental magnetic data simulation.

Graphical Abstract

1. Introduction

Polytopic organic ligands [1] of varying topologies are employed to create metallo-supramolecular assemblies that exhibit diverse structural and physical properties. Bis-oxamate phenylene-based polydentate ligands offer a unique potential for a greater variety of coordination compounds, both homo- and heterometallic, in which spin-bearing metal ions are strongly magnetically coupled [2,3,4,5,6]. Despite a thirty-year history of studying such systems, no polynuclear complexes have been identified in which N,N′-1,3-phenylenebis-oxamic acid (L) (Figure 1a) has been triple-deprotonated (L3−) (Figure 1b). In previous studies, the synthesis of the 3d metal complexes with a half-deprotonated ligand (L2−) (Figure 1c) [6,7,8,9,10] and a fully deprotonated ligand (L4−, see Figure 1d) [11,12,13,14,15,16,17,18,19,20] has been achieved.
Previous studies have demonstrated that nickel(II) complexes in distorted octahedral coordination environments can exhibit the combined optical and magnetic properties required for quantum bit (qubit) applications [21,22]. The employment of imino nitroxyl (IN) radicals as ancillary ligands offers several distinct advantages. These bidentate radicals are capable of inhibiting coordination polymer formation [23]. But more significantly, unlike nitronyl nitroxyl (NN) radicals, which display strong antiferromagnetic coupling in NiII−NN complexes [24,25,26,27], the NiII−IN interaction is ferromagnetic and strong, with exchange parameters reaching ~200 cm−1 [23,24,28,29,30,31,32,33,34,35,36,37]. This pronounced ferromagnetic behavior is particularly crucial for maximizing the total spin ground state of molecular clusters.
In this paper, we present both the molecular and crystal structure and experimental and theoretical magnetic behavior of [Ni(L3−)2(IN)3]∙5CH3OH (1), a unique neutral trinuclear complex with a cyclic triangular arrangement, where IN is a [4,4,5,5-tetramethyl-2-(6-methylpyridin-2-yl)-4,5-dihydro-1H-imidazol-1-yl]oxidanyl radical, Figure 1f. Furthermore, in this compound three Ni2+ ions are interconnected by the two bis-oxamate ligands playing a scarce tritopic function due to an unprecedented triple deprotonation of the related meta-phenylene-bis(oxamic acid) as a proligand.

2. Results and Discussion

2.1. Synthesis and Characterization

Two aqueous solutions, 1 and 2, were prepared simultaneously under agitation on magnetic stirrers at 80 °C. The utilized starting materials and their respective quantities are outlined in the Synthesis of [Ni3(L3−)2(IN)3]∙5CH3OH Section. Solution 1 contained a bis-oxamate ligand coordinated to the metal, while solution 2 contained the chelate complex [Ni(IN)(H2O)4]2+. Solution 2 was added dropwise to solution 1 without removing the heating. Following the heating and cooling stirring procedures, which were each 15 min in duration, the resultant red-orange precipitate was filtered through a porous glass filter. The precipitate was then washed with a small amount of water and dried on the filter until it became friable, while sucking in air. The solid was then dissolved in 5 mL of hot methanol. After filtering, the product solution was left undisturbed for several days. The dark red crystals that formed were used for structure determination. It is well established that crystals of the complex quickly lose solvate molecules when stored in air. For further characterization, we used either freshly prepared crystals (TG and IR) or powder brought to constant weight by heating at 80 °C (elemental analysis).
The results of the FTIR spectral and thermogravimetric investigations of [Ni3(L3−)2(IN)3]∙5CH3OH (1) are presented in the Supplementary Materials (SM). The thermic study of 1 is illustrated in Figure S1, SM. After loss of solvated methanol molecules in the range of 20–100 °C (theoretical mass loss is 10.46%), the solid phase of the trinuclear cluster is thermally stable up to 210 °C. After reaching a steady weight, the desolvated complex [Ni(L3−)2(IN)3] was further heated to 360 °C. At that temperature, the mass loss was 55.15% of the initial mass. This result supports the idea that the final product in this experiment was probably nickel(II) carbonate.
The FTIR spectrum (Figure S2, SM) of 1 confirms the absence of perchlorate anions in the starting nickel salt, indicating the neutrality of the cluster. The vibrations resulting from the participation of solvated alcohol molecules in the hydrogen bonding system generate an intense and broad band within the 3750–2700 cm−1 region (νmax = 3400 cm−1), encompassing the absorption bands from >N−H (shoulders at 3268 and 3102 cm−1) and C−H (2965, 2925, and 2857 cm−1) from the methyl groups of IN. An intense asymmetric absorption band is observed in the 1800–1500 cm−1 region with a maximum at 1590 cm−1, covering a few vibrations: the shoulder at 1717 cm−1 of (C=O), the band at ~1656 cm−1 of (O−C=O)asym, and the C=C and C=N stretching vibrations. The two peaks observed at 1448 cm−1 and 1335 cm−1 apparently correspond to the ν(C−NPy) and C=O vibrations, respectively. The remaining absorptions can be attributed to the following: a symmetric mode of O−C=O at 1472 cm−1, a shoulder at 1386 cm−1 to ν(N−O), and the vibrations at 1072 and 700 cm−1 to δC−H in-plane and δC−H out-of-plane, respectively. The other less intense absorption bands at 1200–400 cm−1 are mostly related to the IN ligand.

2.2. Crystal and Molecular Structure

The crystal structure of 1 contains trinuclear molecular clusters of [Ni3(L3−)2(IN)3] and solvate methanol molecules. It has been established that each Ni2+ cation coordinates an imino nitroxide (IN) paramagnetic ligand in a chelate manner. As illustrated in Figure 2, {Ni(IN)}2+ fragments are interconnected with two m-phenylene-bis(oxamate) anions, L3−, to form a trinuclear complex.
The metallic centers are positioned within a distorted octahedral coordination environment. Ni1 one is linked with two nitrogen atoms of the radical ligand (N11 and N12) and four oxygen atoms of two L3− anions (O41 and O43, and O53 and O53). Both Ni2 and Ni3 cations possess an identical coordination environment, comprising three nitrogen atoms: two of an IN ligand (N21 and N22 for Ni2, and N31 and N32 for Ni3), and one N atom from L3− anion (N51 for Ni2 and N41 for Ni3). In addition, the coordination environment incorporates three O atoms of two L3− anions (O44, O46, and O51 for Ni2, and O54, O56, and O42 for Ni3), which are linked in a fac manner. The lengths of Ni–N and Ni–O bonds, as well as the valence angles, are enumerated in Table S2. As demonstrated in Table S3, the coordination environment of each Ni ion is a distorted octahedron. The matching ratios are very close for Ni1 and Ni2, and the ratio for Ni3 slightly exceeds them.
The arrangement of the triangle molecules along the c axis gives rise to the formation of a supramolecular chain. In the latter, each complex is connected with adjacent molecules by means of pairwise hydrogen bonding between the amide groups >N–H (N42 and N52) and carbonyl species O=C− (O45 and O55) of two L3− ligands (Figure 3) with N⋯O distances of 2.795(5) and 2.829(5) Å. Interactions between neighboring chains are established via CH⋯O contacts between the pyridyl −CH group (C103) of the radical and the coordinated oxygen atom (O46) of the L3− anion. The aforementioned elements result in the formation of a wave-shaped supramolecular layer that is oriented parallel to the bc plane (Figure S3, SM) with a N⋯O distance of 3.182(8) Å. The layers exhibit an alternating pattern along the crystallographic axis a. The structure contains 28% solvate accessible void volume, which was estimated with PLATON [38]. The volume under consideration is occupied by guest methanol molecules. The latter forms a system of hydrogen bonds, involving CH3OH···HOCH3 and CH3OH···O(L3−) contacts, the O⋯O distances being in the range from 2.48(3) to 2.851(6) Å.
The triple deprotonation of the bis-oxamate ligand is confirmed primarily by the neutrality of cluster 1. Additional evidence come from FTIR spectroscopy (the presence of characteristic bands in the N–H stretching region) and single crystal X-ray diffraction (SCXRD) analysis: the Fourier difference map unambiguously determines the positions of the hydrogen atoms H42 and H52 bound to N42 and N52, respectively, which appear as residual electron density (Q peaks). In addition, these protons (H42 and H52) form a chain-like network of hydrogen bonds (Figure 3). The latter circumstance provides additional confirmation of the presence of the >NH fragment in the bridging ligand. Furthermore, elemental analysis data do not contradict the presence of an >NH group in the L3− complex.

2.3. Cryomagnetic Measurements

The temperature dependence of susceptibility for a polycrystalline sample of 1 is presented in Figure 4 as an χT vs. T graphic. The room temperature χT value (4.93 emu K mol−1) is close to the value corresponding to the sum of uncorrelated spins of constituent paramagnetic centers 4.8 emu K mol−1. As the temperature decreases to 2 K, the χT value steadily decreases, indicating the predominance of antiferromagnetic interactions in the solid phase of compound 1. This is also shown by the field dependence data of the magnetization (Figure 5), as the value of M(H) at H = 7 T is only 3.26 µB. Such a behavior is reminiscent of those of the carbonato-bridged neutral nickel(II) triangle cluster [39] heterometallic complex NiCu(obbz)∙6H2O, where obbz is an oxamidobis(benzoate) ligand.
In the context of AC magnetic measurements conducted at temperatures below 25 K, no significant correlation was identified between the magnetic susceptibility and the field oscillation frequency, irrespective of the presence or absence of an external magnetic field with a strength of B = 0.1 T.

2.4. Theoretical Model and Exhaustive Parameter Set Required for Magnetic Data Simulation

Theoretical modeling of the magnetic behavior of the investigated complex, considering two different types of magnetic couplings − between metal centers and between metal and radical − was performed using the spin-Hamiltonian (SH) expression outlined below
H ^ = − 2 J 1 S ^ M 1 · S ^ R 1 − 2 J 2 S ^ M 2 · S ^ R 2 − 2 J 3 S ^ M 3 · S ^ R 3 − 2 J 12 S ^ M 1 · S ^ M 2 − 2 J 13 S ^ M 1 · S ^ M 3 − 2 J 23 S ^ M 2 · S ^ M 3 + S ^ M 1 D 1 S ^ M 1 + S ^ M 2 D 2 S ^ M 2 + S ^ M 3 D 3 S ^ M 3       + g N i β ( S ^ M 1 + S ^ M 2 + S ^ M 3 ) · B 0 + g R β ( S ^ R 1 + S ^ R 2 + S ^ R 3 ) · B 0    
The initial line of the expression delineates the exchange interaction between nickel ions (Mn) and the IN radicals (Rn), while the subsequent line accounts for the M⋯M exchange interaction. The third and fourth lines represent the zero-field splitting (ZFS) for Ni2+-centers with S = 1 and the Zeeman interaction with the external magnetic field B0, respectively. To avoid the over-parameterization problem, the isotropic g-factor for nickel (gM) was utilized, and the value of gR was assumed to be 2. Scheme 1 presents a picture of the considered magnetic exchange couplings.
The results of DFT calculation of exchange integrals by the broken symmetry (BS) method using different functionals are summarized in Table 1. The latter shows that the nickel–radical magnetic couplings (J1, J2, and J3) are approximately equivalent within the method and ferromagnetic (FM) in nature, whereas the metal–metal interactions in the M1−M2 and M1−M3 pairs are antiferromagnetic. Hybrid functionals (B3LYP and TPSSh) give about one and a half times larger coupling values than the more recent range-separated DF variants. A separate calculation using the B3LYP method also showed that the exchange integral J23 between spins Ni2 and Ni3 is small and does not exceed 0.05 cm−1. Therefore, it was ignored and assumed equal to zero for SH.
To clarify and verify the results of DFT calculations, ab initio CASSCF/NEVPT2 computations with partial diamagnetic substitution of paramagnetic centers in 1 were also performed. For this purpose, the Ni2+-centers (M2 and M3) were replaced by Zn2+ ions in the trinuclear molecule. In addition, the radicals (1 and 2) were also replaced by their reduced analogs 4,4,5,5-tetramethyl-2-(6-methylpyridin-2-yl)-4,5-dihydro-1H-imidazol-1-ol with a hydroxyl amine moiety, >N–OH, instead of the nitroxyl group >N−O. Consequently, a cluster comprising solely an exchange-bonded pair of Ni2+ (S = 1) and an imino nitroxyl radical (S = 1/2) was obtained (see Figure 6). The ground state (GS) of such a spin system is a quartet, and the energy gap between GS and the nearest spin doublet can be calculated using the formula J1 = (E(D) − E(Q))/3. The results of the J1 calculation are summarized in Table 2.
A similar diamagnetic substitution approach (see Figure 7) was implemented for the CASSCF/NEVPT2 computation of the J12 exchange integral between Ni2+ ions (M1 and M2). The constant J12 in this case is found by the formula J12 = (E(S) − E(T))/2 (see Table 3).
A comparison of the data from Table 2 and Table 3 with those from Table 1 reveals a substantial discrepancy between the exchange integral values obtained from DFT calculations and CASSCF/NEVPT2 data, with a range from 4 to 7 times. It should be noted that the use of diamagnetically substituted structures in the calculation of magnetic coupling constants by the BS DFT method results in only a slight change in J values obtained from the calculations of full-spin complexes comprising three Ni and three IN radicals, as exampled in Table 1.
Usually such a large discrepancy between DFT and CASSCF calculations is observed only for strong antiferromagnetic exchanges between spins. The HS states with maximum total spin are well reproduced in DFT since they correspond to a single Slater determinant. This is not the case for BS spin states. Here a role is played not only by the impurity of the HS state, but also by impurities of excited spin states of the same multiplicity. In our case of the Ni(II)-IN spin pair, the DFT calculation error seems to be related to the overestimated value of the doublet BS state energy due to an excessive admixture of such excited spin doublets.
In order to estimate such SH parameters as the g-tensor and D-tensor for the paramagnetic center M1 (Ni2+, S = 1), in addition to the diamagnetic substitution shown in Figure 7, the substitution of the M2 center by a Zn2+ ion was also performed. The results of these calculations are summarized in Table 4. Given that the obtained anisotropy of the Ni g-tensor is less than 2%, only the isotropic part of the g-tensor is presented in Table 4. The table also provides the parameters corresponding to the standard form of the D tensor along the principal axis.
Finally, a fitting of the experimental χT(T) data using the spin Hamiltonian was performed. In accordance with DFT calculations and to avoid over-parameterization during the fitting process, it was assumed that the values of all magnetic exchange integrals in both Ni-IN and Ni-Ni pairs were equal to each other, i.e., J = J1 = J2 = J3, as well as J(Ni1−Ni2) = J(Ni1−Ni3). Furthermore, the g- and D-tensor parameters for all Ni centers were postulated to be the same and equal to each other. The results of the experimental data simulation performed in the PHI program are presented in Figure 4.
The best simulation parameters are as follows: gNi = 2.3; DNi = 2.0 cm−1; J = J1 = J2 = J3 = 36.2 cm−1; and J12 = J13 = −18.9 cm−1. These parameters were also used to fit the experimental magnetization data, M(H), measured at a temperature of 2K (see Figure 5).
The comparison of the optimal values for the exchange integrals, as well as for gNi and DNi, obtained from the χT(T) simulation with the results of quantum-chemical calculations (Table 2, Table 3 and Table 4), shows that the best value for gNi = 2.29 is provided by cas(8,5) calculation. DFT computation, particularly cam-B3LYP, most accurately reproduces DNi. The CAS(13,9)/NEVPT2 method gives a very close value for the Ni-IN exchange integral, J, but the ab initio approach cannot reproduce the J12 (Ni1-Ni2) exchange integral, while the CAM-B3LYP calculation is the most accurate in this case.
The BS DFT estimations of the magnetic exchange interactions between neighboring hydrogen-bonded triangular molecules of complex 1 (see Figure 3) yielded negative values of Jinter, with its absolute value ranging from 0.1 to 0.2 cm−1, indicating the weakness of intermolecular interactions in the solid phase of the compound. This finding provides a rationale for the omission of the Jinter parameter when modeling the magnetic behavior of complex 1 in the present study.

3. Materials and Methods

3.1. Instrumental and Physical Measurements

Elemental (C,H,N) analysis was performed on a Euro-Vector 3000 analyzer (Eurovector, Redavalle, Italy). FTIR spectra were registered with a NICOLET spectrophotometer (Thermo Electron Scientific Instruments LLC, Madison, WI, USA) in the 4000–400 cm−1 range. The thermal stability of 1 was studied by means of a Thermo Microbalance TG 209 F1 Iris (Netzsch, Selb, Germany). Powder XRD was carried out using a Shimadzu XRD-7000S diffractometer (Shimadzu, Kyoto, Japan) (CuKα radiation, Ni filter, 2θ angle range from 3° to 35°) using a Dectris MYTHEN2 R 1K detector (λ = 1.54178 Å, Kyoto, Japan). Simulated patterns from the X-ray crystal structure were obtained using the Mercury 2024.1.0 software from the CCDC. The cryomagnetic investigation of the compound was performed using a Quantum Design MPMS 5XL SQUID magnetometer (Quantum Design, Inc., San Diego, CA, USA) in the temperature range of 1.8–300 K and under a magnetic field of up to 7 T. The diamagnetic contribution for the magnetic susceptibility of complex 1 was corrected using Pascal’s constants for the constituent atoms and organic ligand bonds [40].
Single-crystal XRD experimental details are presented in Table S2 (SM). Crystallographic data were deposited with the Cambridge Crystallographic Data Centre (deposit number CCDC 2451012).

3.2. Theoretical Calculations

The quantum chemical study was fulfilled using crystallographic geometry. The calculations of magnetic exchange J integrals, D-tensors, and g-tensors were performed using both broken-symmetry DFT (BS-DFT) [41] and ab initio CAS/NEVPT2 methods with the def2-QZVPP basis set for Ni and def2-TZVP for other atoms using the Orca-6.0 software package [42,43]. Fitting of the experimental χT(T) and M(H) dependences to obtain optimal parameters of the employed spin Hamiltonian was carried out using the PHI 3.16 package [44].

3.3. Preparations

Solvents of reagent grade (EKOS-1, Moscow, Russia) were distilled prior to use. The complex was synthesized under ambient conditions. The radical 2-(6-methyl-2-pyridyl)-4,4,5,5,5-tetramethylimidazoline-1-oxyl (IN) was synthesized according to a literature procedure [24]. The tritopic derivative, Na3L, was prepared in situ by hydrolysis of ethyl-2-[3-[(2-ethoxy-2-oxo-acetyl)amino]anilino]-2-oxo-acetate [12].

Synthesis of [Ni3(L3−)2(IN)3]∙5CH3OH

Solution 1: Under stirring and heating (80 °C), a solution of IN (38 mg, 0.165 mmol) in 1.5 mL H2O was added dropwise to a solution of Ni(ClO4)2·6H2O (60 mg, 0.165 mmol) in 2.5 mL H2O.
Solution 2: The hydrolysis of ethyl-2-[3-[(2-ethoxy-2-oxo-acetyl)amino]anilino]-2-oxoacetate [45,46], the esterified form of 2-[3-(oxaloamino)anilino]-2-oxoacetic acid, was performed in 2 mL of distilled water, which was added to the mixture of ethyl-2-[3-[(2-ethoxy-2-oxo-acetyl)amino]anilino]-2-oxoacetate (33 mg, 0.11 mmol) and NaOH (18 mg, 0.45 mmol). The mixture was then stirred with a magnetic stirrer at a temperature of 80 °C for 15 min.
An elemental analysis of complex 1 was performed on a desolvated sample that was heated to 110 °C until constant weight was reached: Anal Calcd. for Ni3C59H64N13O15 (mass. %): C, 51.74; H, 4.71; N, 13.30; found: C, 51.68; H, 4.6; N, 13.25. IR spectrum (KBr, ν, cm−1): 3400, 3268, 3102, 2965, 2925, 2857, 1717, 1656, 1590, 1472, 1448, 1386, 1335, 1072, 700.

4. Conclusions

Among the complexes of d-metals with phenylene-based bis-oxamate ligands, the compound studied by us is distinguished by its atypical trinuclear triangular molecular structure, in which three metal centers are connected by only two anionic bridges, representing a triply deprotonated form of 2-[3-(oxaloamino)anilino]-2-oxoacetic acid, complexes with the form of the latter having not been previously known.
Despite the presence of six possible magnetic couplings in the trinuclear cluster 1, its magnetic behavior was well reproduced using the three-J model and magnetic anisotropy D under the assumption that three different Ni-IN interactions are equal to each other, and considering only two equivalent Ni-Ni interactions, with the third one equating to zero. This restriction was implemented to circumvent the issue of overparameterization. The magnetic plot simulation results indicate the presence of two opposite-in-nature types of magnetic interactions within the triangular core. From the model spin Hamiltonian with the obtained optimal parameters, it is found that, neglecting spin–orbital interaction, the ground state of the studied cluster is a quartet, the first excited state being a spin doublet lying 21 cm−1 above. Taking the spin–orbital interaction into account splits the ground spin quartet into two doublets with a gap of only 1.2 cm−1, which is significantly less than the splitting D = 2 cm−1 in the Ni(II) centers of the complex. Therefore, from the experimental data analysis, one can conclude that the anisotropy of the studied cluster will be less than the anisotropy of its constituent Ni(II) ions. DFT and detailed CASSCF/NEVPT2 computations were also performed, thereby providing support for the experimental modeling of the magnetic data.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/inorganics13070214/s1, Figure S1: TGA of [Ni3(L3−)2(IN)3]∙5CH3OH. Figure S2: FT-IR spectrum of [Ni3(L3−)2(IN)3]∙5CH3OH. Figure S3: Fragment of supramolecular layer in the structure of 1. H-bonds are shown with dashed lines. Figure S4: The diffractograms for the complex 1 registered at room temperature. Table S1: Continuous Shape Measures* calculation for 1. Table S2: Selected bond lengths and angles for 1. Table S3: Crystal data and structure refinement for complex 1. References [47,48,49] are cited in the supplementary materials.

Author Contributions

Conceptualization, K.E.V. and V.A.M.; methodology, K.E.V.; software, V.A.M.; validation, K.E.V., D.G.S. and V.A.M.; investigation, all; resources, K.E.V.; data curation, D.G.S.; writing—original draft preparation, K.E.V.; writing—review and editing, all; visualization, K.E.V. and D.G.S.; supervision, K.E.V. All authors have read and agreed to the published version of the manuscript.

Funding

The research was supported by the Ministry of Science and Higher Education of the Russian Federation, Projects 125020401317-8 and AAAA-A21-121012290043-3.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The CCDC2451012 entry contains the supplementary crystallographic data for this paper. This original data presented in the study are openly available in The Cambridge Crystallographic Data Center at https://www.ccdc.cam.ac.uk/structures/2451012. The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

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Inorganics 13 00214 g001
Figure 1. Organic entities pointed out in the text and their IUPAC names: (a) 2-[3-(oxaloamino)anilino]-2-oxo-acetic acid; (b) 2-[3-(carboxylatoformyl)azanidylanilino]-2-oxo-acetate; (c) 2-[3-[(carboxylatoformyl)amino]anilino]-2-oxo-acetate; (d) 2-[3-(carboxylatoformyl)azanidylphenyl]azanidyl-2-oxo-acetate; (e) ethyl-2-[3-[(2-ethoxy-2-oxo-acetyl)amino]anilino]-2-oxo-acetate; and (f) [4,4,5,5-tetramethyl-2-(6-methylpyridin-2-yl)-4,5-dihydro-1H-imidazol-1-yl]oxidanyl radical.
Figure 1. Organic entities pointed out in the text and their IUPAC names: (a) 2-[3-(oxaloamino)anilino]-2-oxo-acetic acid; (b) 2-[3-(carboxylatoformyl)azanidylanilino]-2-oxo-acetate; (c) 2-[3-[(carboxylatoformyl)amino]anilino]-2-oxo-acetate; (d) 2-[3-(carboxylatoformyl)azanidylphenyl]azanidyl-2-oxo-acetate; (e) ethyl-2-[3-[(2-ethoxy-2-oxo-acetyl)amino]anilino]-2-oxo-acetate; and (f) [4,4,5,5-tetramethyl-2-(6-methylpyridin-2-yl)-4,5-dihydro-1H-imidazol-1-yl]oxidanyl radical.
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Inorganics 13 00214 g002
Figure 2. Structure of [Ni3(L3−)2(IN)3]∙5MeOH (1). H atoms are omitted; ellipsoids of 50% probability.
Figure 2. Structure of [Ni3(L3−)2(IN)3]∙5MeOH (1). H atoms are omitted; ellipsoids of 50% probability.
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Figure 3. A fragment of supramolecular chain in the structure of 1 (H atoms are omitted; H-bonds are shown with dashed lines).
Figure 3. A fragment of supramolecular chain in the structure of 1 (H atoms are omitted; H-bonds are shown with dashed lines).
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Figure 4. Temperature dependence of the χT product at B = 0.25 T. The solid lines represent the theoretical simulation fitted to the experimental data (open circles).
Figure 4. Temperature dependence of the χT product at B = 0.25 T. The solid lines represent the theoretical simulation fitted to the experimental data (open circles).
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Figure 5. The magnetization curve measured for 1 at 2.8 K. The solid line represents the theoretical simulation fitted to the experimental data (open circles).
Figure 5. The magnetization curve measured for 1 at 2.8 K. The solid line represents the theoretical simulation fitted to the experimental data (open circles).
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Scheme 1. Topological structure of magnetic exchange interactions between spin carriers in complex 1, where Mn = Ni1, Ni2, and Ni3, and Rn = IN. The radicals Rn are directly coordinated to the Mn-centers at distances: M1–R1 = Ni1–N12 = 2.051, M2–R2 = Ni2–N22 = 2.040, and M3–R3 = Ni3–N32 = 2.022 Å. The distances between the Ni-centers are 5.347, 5.345, and 7.462 Å for the pairs M1⋯M2, M1⋯M3, and M2⋯M3, respectively.
Scheme 1. Topological structure of magnetic exchange interactions between spin carriers in complex 1, where Mn = Ni1, Ni2, and Ni3, and Rn = IN. The radicals Rn are directly coordinated to the Mn-centers at distances: M1–R1 = Ni1–N12 = 2.051, M2–R2 = Ni2–N22 = 2.040, and M3–R3 = Ni3–N32 = 2.022 Å. The distances between the Ni-centers are 5.347, 5.345, and 7.462 Å for the pairs M1⋯M2, M1⋯M3, and M2⋯M3, respectively.
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Figure 6. Partial diamagnetic substitution scheme employed to calculate the magnetic coupling constant J1 (see text).
Figure 6. Partial diamagnetic substitution scheme employed to calculate the magnetic coupling constant J1 (see text).
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Figure 7. Partial diamagnetic substitution scheme employed to calculate the magnetic coupling constant J12, see text.
Figure 7. Partial diamagnetic substitution scheme employed to calculate the magnetic coupling constant J12, see text.
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Table 1. Calculated values of coupling constants (in cm−1) using DFT methods.
Table 1. Calculated values of coupling constants (in cm−1) using DFT methods.
DFT LevelJ1J2J3J12J13
B3LYP157.4 1179.5146.1−26.6 1−26.3
TPSSh209.6231.4188.4−37.5−37.3
cam-B3LYP116.3137.9112.3−17.8−17.4
LC-BLYP131.4155.0125.6−25.7−25.6
wB97m-v98.2116.595.9−14.6−14.4
1 For diamagnetically substituted structures J1 = 145.0 and J12 = −25.8, see explanation in text.
Table 2. Calculated values 1 of coupling constant J1 (in cm−1) using ab initio methods.
Table 2. Calculated values 1 of coupling constant J1 (in cm−1) using ab initio methods.
CAS(n,m)
Roots(Q,D)		E(Q) − E(D)
CASSCF		J1
CASSCF		E(Q) − E(D) NEVPT2J1
NEVPT2
(13,9)/(2,2)−95.932.0−108.636.2
1 The values in bold are the closest to those simulated by means of the PHI program.
Table 3. Calculated values for energy levels and coupling constant J12 1.
Table 3. Calculated values for energy levels and coupling constant J12 1.
CAS(n,m)
Roots(Q,T,S)		E (Singlet)E (Triplet)E (Quintet)J12
CAS(10,9)/(2,2,2))03.510.6−1.75
NEVPT207.721.3−3.85
1 J12 = (E(S) − E(T))/2 (in cm−1).
Table 4. Calculated parameters of g- and D-tensors for the paramagnetic center M1 (Ni2+, S = 1).
Table 4. Calculated parameters of g- and D-tensors for the paramagnetic center M1 (Ni2+, S = 1).
SH Parameter 4Method
CAS(8,5) 1NEVPT2 2B3LYP 3TPSSh 3cam-B3LYP 3LC-BLYP 3wB97m-v 3
g2.292.222.172.122.182.152.14
D7.434.922.491.832.018.073.20
E/D0.0690.0870.3280.1830.2970.2430.162
1 roots(10,9); 2 CAS(8,5), roots(10,9); 3 SOMF(1X); 4 the values in bold are the closest to those simulated using the PHI program.
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Morozov, V.A.; Samsonenko, D.G.; Vostrikova, K.E. A Unique Trinuclear, Triangular Ni(II) Complex Composed of Two tri-Anionic bis-Oxamates and Capping Nitroxyl Radicals. Inorganics 2025, 13, 214. https://doi.org/10.3390/inorganics13070214

AMA Style

Morozov VA, Samsonenko DG, Vostrikova KE. A Unique Trinuclear, Triangular Ni(II) Complex Composed of Two tri-Anionic bis-Oxamates and Capping Nitroxyl Radicals. Inorganics. 2025; 13(7):214. https://doi.org/10.3390/inorganics13070214

Chicago/Turabian Style

Morozov, Vitaly A., Denis G. Samsonenko, and Kira E. Vostrikova. 2025. "A Unique Trinuclear, Triangular Ni(II) Complex Composed of Two tri-Anionic bis-Oxamates and Capping Nitroxyl Radicals" Inorganics 13, no. 7: 214. https://doi.org/10.3390/inorganics13070214

APA Style

Morozov, V. A., Samsonenko, D. G., & Vostrikova, K. E. (2025). A Unique Trinuclear, Triangular Ni(II) Complex Composed of Two tri-Anionic bis-Oxamates and Capping Nitroxyl Radicals. Inorganics, 13(7), 214. https://doi.org/10.3390/inorganics13070214

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