Novel Tetranuclear Heterometallic Mn₃Ni and Mononuclear Ni Complexes with an ONO Schiff Base Ligand: Synthesis, Crystal Structures, and Magnetic Properties

Abstract

A mononuclear Ni(II) complex, [Ni(HL1)₂], (1) and a novel tetranuclear heterometal Mn-Ni complex, [Mn₃Ni(L1)₄Cl₂(EtOH)₂], (2) [H₂L1 = N-(2-hydroxymethylphenyl)salicylideneimine], have been synthesized and characterized via X-ray crystal structure analyses, infrared spectra, and elemental analyses. The structure analyses revealed that the tridentate ligand, H₂L1, coordinates in a facial mode for Ni and a mer mode for Mn, respectively. Complex 2 includes Mn(II)Mn(III)₂Ni(II) tetranuclear metal core bridged by μ-phenoxo and μ-alkoxo oxygens. Magnetic measurements for 2 indicate that weak ferromagnetic interactions (J_{Mn(III)Ni(II)} = 2.23, J_{Mn(III)Mn(II)} = 0.46, J_Mn(II)Ni(II) = 1.78, and J_{Mn(III)Mn(III)} = 0.58 cm⁻¹) dominate in the tetranuclear core. Additionally, in alternating current (AC) magnetic measurements, frequency-dependent out-of-phase responses were observed.

1. Introduction

The field of molecular magnetism in discrete polynuclear complexes has seen significant research over the last few decades [1,2,3,4,5,6]. This research covers a wide range of objectives, from a fundamental understanding of the correlation between spin-exchange interactions and molecular structure [1,2,7,8,9,10] to the development of functional magnetic materials like single-molecule magnets (SMMs) [3,4,5,6,11,12]. While the study of homometallic complexes is well-established, the exploration of heterometallic complexes remains relatively limited. This is due to the complexities associated with synthesizing new compounds, even though such complexes can significantly impact the spin ground state, magnetic anisotropy, and magnetic exchange interactions, leading to the development of low-dimensional magnets, SMMs and SCMs (single-chain magnets) [5,13,14,15,16,17].

Three primary synthetic strategies have been employed for the synthesis of heterometallic complexes: (i) one-pot reactions, such as hydrothermal synthesis [18]; (ii) the design of ligands with multiple coordination sites that selectively bind specific metal ions [13,19]; and (iii) the reaction of complex ligands or bridging metal complexes with other metal ions [20,21]. While method (i) is suitable for synthesizing metal–organic frameworks (MOFs), it can be challenging to produce discrete complexes. Method (ii) offers opportunities for molecular design but can encounter obstacles in ligand synthesis. On the other hand, method (iii) holds an advantage as it allows for a relatively straightforward stepwise synthetic approach. In this method, tri- or tetradentate Schiff base ligands with bridgeable oxygen atoms at their terminals, similar to the Salen-type ligands in {[Cu(Mesalen)]₂VO)}(ClO₄)₂ (Mesalen = N,N′-bis(2-hydroxyacetophenone)ethylenediamine) [22], have often been used.

The tridentate ligand N-(2-hydroxymethylphenyl)salicylideneimine (H₂L1), which possesses bridgeable oxygen atoms (phenolic and alcoholic) at both terminals, has been employed to synthesize various polynuclear complexes with distinct bridging structures, such as tetranuclear Ni(II) complexes [23,24] and trinuclear Mn(III) complexes [25]. In our research, we have focused on leveraging the unique properties of the phenolic and alcoholic oxygen atoms at each end of this ligand. Through this feature, we have successfully synthesized heterometallic Mn(III)₂Ni(II)₂ complexes [24,26] and mixed-valence Co(II)₂Co(III)₂ complexes [27] and reported their crystal structures and unique magnetic properties. While these complexes were synthesized via one-pot reactions, we achieved a controlled structure due to the distinct properties of the bridging oxygen sites in the ligand. However, to attain even finer control over the structures, it is desirable to develop synthetic methods involving the isolation of mononuclear and/or binuclear complexes as complex ligands for reactions with other metal ions. In this study, we report the synthesis of a mononuclear Ni(II) complex using H₂L1, and the structure and magnetic properties of a novel heterometallic mixed-valence Mn(II)Mn(III)₂Ni(II) tetranuclear complex, which was obtained by utilizing the mononuclear Ni complex as a complex ligand.

2. Materials and Methods

2.1. Preparation

All chemicals were purchased and used as received, unless otherwise noted. Methanol and ethanol were purified by distillation over magnesium turnings. The ligand H₂L1 was obtained by the literature method [28]. The IR spectrum of H₂L1 is shown in Figure S1.

2.1.1. [Ni(HL1)₂] (1)

A methanol solution (10 mL) containing nickel(II) acetate tetrahydrate (0.124 g, 0.5 mmol) and H₂L1 (0.224 g, 1.0 mmol) was stirred with the addition of triethylamine (0.010 g, 0.10 mmol) for 90 min. The resulting pale yellowish-green solution was filtered and allowed to stand at room temperature. After 4 days, green single crystals suitable for X-ray analysis were obtained. The IR and PXRD spectra are shown in Figures S2 and S3, respectively. [Ni(HL1)₂]·0.5MeOH; yield: 88 mg, 34.0%. Anal. Calc. for C_28.5H₂₆N₂NiO_4.5: C, 64.93; H, 4.97; N, 5.31%. Found: C, 65.18; H, 4.81; N, 5.31%. IR data [$\tilde{\nu }$/cm⁻¹]: 3041 (w), 2912 (w), 1612 (s), 1595 (s), 1517 (m), 1448 (s), 1269 (m), 1178 (s), 1149 (s), 1001 (m), 916 (m), 849 (m), 748 (s), 725 (m), and 509 (m).

2.1.2. [Mn₃Ni(HL1)₄Cl₂(EtOH)₂] (2)

To an ethanol/dichloromethane mixed solution (1:3 in volume, 6 mL) of 1 (0.256 g, 0.5 mmol), an ethanol solution (1 mL) of manganese(II) chloride tetrahydrate (0.049 g, 0.25 mmol) was added. The resulting dark-brown solution was stirred for 90 min and then filtered. After allowing the filtrate to stand at room temperature for 2 days, dark brown single crystals suitable for X-ray analysis were obtained. The IR and PXRD spectra are shown in Figures S4 and S5, respectively. [Mn₃Ni(L1)₄Cl₂(EtOH)₂]; yield: 17 mg, 15.8%. Anal. Calc. for C₆₀H₅₆Cl₂Mn₃NiN₄O₁₀: C, 55.97; H, 4.38; N, 4.35%. Found: C, 55.86; H, 4.47; N, 4.36%. IR data [$\tilde{\nu }$/cm⁻¹]: 3198 (s), 2930 (w), 1605 (s), 1574 (m), 1533 (s), 1435 (s), 1315 (s), 1182 (s), 1149 (s), 1016 (s), 983 (m), 860 (m), 748 (s), 634 (s), and 550 (s).

2.2. Measurements

Elemental analyses for C, H, and N were obtained at the Elemental Analysis Service Center at Kyushu University. Fluorescent X-ray analysis was obtained on a Shimadzu energy dispersive X-ray spectrometer Rayny EDX-700HS (Shimadzu Co., Ltd., Kyoto, Japan). PXRD measurements were conducted on a Shimadzu X-ray diffractometer XRD-7000 (Shimadzu Co., Ltd., Kyoto, Japan). Infrared spectra were recorded on a Bruker VERTEX70-S FT-IR Spectrometer on the ATR (Attenuated Total Reflection) method. Reflection spectra were recorded on a PERKIN ELMER Lambda900Z UV/VIS/NIR Spectrometer (PerkinElmer, Inc., Waltham, MA, USA) and Ocean Optics USB2000+ Fiber Optic Spectrometer (Ocean Optics, Inc., Dunedin, FL, USA). The magnetic susceptibilities were measured on a Quantum Design MPMS-XL5R SQUID susceptometer (Quantum Design, Inc., San Diego, CA, USA) under an applied magnetic field of 0.1 T at room temperature for 1 and in the temperature range 2–300 K for 2 and 3. The susceptibilities were corrected for the diamagnetism of the constituent atoms using Pascal’s constant [29]. The DC-magnetic data were fitted using the PHI program 3.1.6 [30].

2.3. Single Crystal X-ray Diffraction

Single crystals suitable for X-ray analysis were obtained by slow evaporation of methanol for 1 and ethanol for 2, respectively. Diffraction data were measured on a Rigaku Vari-Max Saturn CCD 724 diffractometer (Rigaku Corp., Tokyo, Japan) with graphite monochromated Mo Ka radiation (λ = 0.71069 Å) at the Analytical Research Center for Experimental Sciences, Saga University. Data were collected and processed using CrystalClear-SM 2.0 r16 [31]. The crystal was kept at 113 K during data collection. Multi-scan correction for absorption was applied. The crystal data and experimental parameters are summarized in

Table 1. Crystallographic data and refinement parameters.

	1	2
Empirical formula	C30H32N2NiO6	C60H56Cl2Mn3N4NiO10
Formula weight	575.28	1287.51
Temperature/K	113.15	113.00
Crystal system	monoclinic	orthorhombic
Space group	P21/c	Aea2
a/Å	10.716(5)	12.180(6)
b/Å	20.271(8)	22.558(11)
c/Å	13.487(6)	19.913(11)
α/°	90	90
β/°	112.614(6)	90
γ/°	90	90
V/Å3	2704(2)	5471(5)
Z	4	4
Dcalc/g cm−3	1.413	1.563
μ(MoKα)/mm−1	0.765	1.178
F(0 0 0)	1208.0	2644.0
Crystal dimensions/mm3	0.29 × 0.19 × 0.18	0.20 × 0.20 × 0.20
Radiation	MoKα (λ = 0.71075)	MoKα (λ = 0.71075)
2θ range for data collection/°	6.25 to 54.974	6.402 to 55.02
No. of reflections collected	22,107	22,017
No. of independent reflections	6058	6184
Data/restraints/params	6058/6/363	6184/1/365
Goodness-of-fit indicator	1.071	1.077
R indices [I > 2.00σ(I)]	R1 = 0.0404	R1 = 0.0218
	wR2 = 0.1056	wR2 = 0.0546
R indices (all data)	R1 = 0.0476	R1 = 0.0220
	wR2 = 0.1123	wR2 = 0.0548
Largest diff. peak, hole/e Å−3	0.65, −0.55	0.30, −0.29
Flack parameter	----	0.010(4)
CCDC deposition number	2284205	2284228

. The structures were solved by direct methods (ShelXT-2016/6) and expanded using Fourier techniques [32]. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed geometrically in calculated positions and refined with a riding model. The final cycle of full-matrix least-squares refinement on F₂ using ShelXL-2016/6 [33] was based on observed reflections and variable parameters and converged with unweighted and weighted agreement factors of R and R_w. Olex2-1.5 [34] was used as an interface to the ShelX program package. The structure drawing of the molecules was performed using Mercury-2022.1.0 [35]. The Hirshfeld surface was generated using Crystal Explorer 3.1 [36].

3. Results and Discussion

3.1. Synthetic Outcomes and Characterization

The elemental analysis results for 1 were consistent with the calculated composition of [Ni(HL1)₂]·0.5MeOH. According to the molecular formula obtained from the single-crystal X-ray structural analysis (SCXRD), two methanols were initially included. However, crystalline decomposition was observed during the vacuum drying process, indicating that the loss and evaporation of methanol were responsible for this observation. The powder X-ray diffraction (PXRD) pattern of the dried sample does not match well with the simulated pattern obtained from SCXRD, further suggesting the loss of the crystal solvents (Figure S3). So far, crystal structures of Ni(II) complexes with H₂L1 have only been reported for tetranuclear complexes, [{Ni(L1)(EtOH)}₄] [23] and [{Ni₂(L1)(HL1)Cl}₂] [24], which have a cubane-like structure. This can be attributed to the strong bridging capability of deprotonated alkoxo-oxygen donors. In this study, our goal was to synthesize a mononuclear Ni(II) complex by carefully controlling the pH conditions, employing nickel(II) acetate as the metal source to prevent the dissociation of protons from benzyl alcohol. Remarkably, we successfully isolated complex 1 as a mononuclear Ni(II) complex (Scheme 1). The FT-IR spectrum revealed intense and characteristic bands corresponding to the azomethine $\tilde{\nu }$(C=N), phenolic $\tilde{\nu }$(C–O), and alcoholic $\tilde{\nu }$(C–O) stretching vibrations of free H₂L1, which were observed at 1615, 1279, and 1030 cm⁻¹, respectively. Upon complexation in 1, these bands were downshifted to 1612, 1178–1149, and 1001 cm⁻¹, respectively. The heterometal complex 2 was obtained by reacting the nickel complex 1 with manganese(II) chloride in ethanol. Initially, our aim was to produce a linear Ni(II)-Mn(II)-Ni(II) complex; therefore, 1 was reacted with Mn(II) in a 2:1 ratio. However, based on EDX measurements, the composition of 2 was determined to be in a 3:1:2 ratio of Mn, Ni, and Cl, respectively. Taking into account the EDX results and charge balance considerations, the oxidation states of the four metal ions in 2 are presumed to be Mn(II)Mn(III)₂Ni(II). Analytical data of 2 showed good agreement with its molecular formula obtained by single-crystal X-ray analysis. In previous studies, we have reported several heterometallic tetranuclear Mn(III)–Ni(II) complexes with H₂L1 and its homologs [24,26]. Complex 2 is a novel complex that exhibits characteristics of a mixed-valence electronic state in addition to being a heterometallic complex. In the FT-IR spectrum, a strong and sharp $\tilde{\nu }$(O–H) band was observed at 3196 cm⁻¹. This characteristic band likely originates from coordinated ethanol molecules, indicating the presence of strong intramolecular hydrogen bonds. The three IR bands stemming from the tridentate ligand were shifted to 1604 [$\tilde{\nu }$(C=N)], 1182—1149 [$\tilde{\nu }$(C–O)_phenolic], and 1016—983 [$\tilde{\nu }$(C–O)_alcoholic] cm⁻¹, respectively.

3.2. Structural Studies

3.2.1. Complex 1: [Ni(HL1)₂]·2MeOH

The molecular structure and packing diagram of 1 are shown in Figure 1a and Figure S6, respectively. The selected bond lengths and angles are listed in

Table 2. Selected bond distances and angles of 1.

Bond	Distance/Å	Angle	Angle/°
Ni1–O1	2.1064(15)	O1–Ni1–O2	95.38(6)
Ni1–O2	2.0113(15)	O1–Ni1–N1	87.89(6)
Ni1–O3	2.1103(16)	O1–Ni1–O4	89.93(6)
Ni1–O4	2.0319(15)	N1–Ni1–N2	97.12(7)
Ni1–N1	2.0531(17)	O2–Ni1–O4	89.90(7)
Ni1–N2	2.0486(17)	O2–Ni1–O3	173.67(6)
Ni1⋯Ni1 i	4.5896(17)

Symmetry code (ⁱ): 1 − x, 1 − y, 1 − z.

. The coordination environment surrounding the Ni(II) ion adopts an octahedral geometry, with a N₂O₄ donor set originating from the tridentate ligand (HL1)⁻. Previously reported mononuclear complexes with (HL1)⁻, such as [Co(HL1)₂]NO₃·2H₂O, has a mer-coordination mode [37]. However, due to the larger ionic radius of Ni(II) in comparison to trivalent transition metal ions, the tridentate ligands in this complex coordinate to the Ni atom in a fac mode. All three types of coordination atoms from the two ligands adopt a cis configuration. Consequently, complex 1 exhibits chirality around Ni.

The bond lengths of Ni–O_phenolic and Ni–N are closely comparable, falling within the range of 2.0113(15)–2.0531(17) Å. In contrast, the Ni1–O1 and Ni1–O3 distances, which involve protonated O atoms, are 2.1064(15) and 2.1103(16) Å, respectively, approximately 0.1 Å longer than the other coordination bonds (refer to Table 2). According to SHAPE analysis [38] score of 1, OC-6 = 0.501, indicating minimal distortion of the octahedral structure surrounding Ni. The benzyl alcohol moiety and the deprotonated phenolate group engage in intermolecular hydrogen bonds, specifically O1–H1D⋯O4ⁱ = 2.686(2) and O–H3D⋯O4ⁱ = 2.641(2) Å [symmetry code: (i) 1 − x, 1 − y, 1 − z], respectively. This interaction results in the formation of an enantiomeric pair, as illustrated in Figure 1b. The red-colored regions on the Hirshfeld surface clearly indicate the presence of these intermolecular hydrogen bonds.

3.2.2. Complex 2: [Mn₃Ni(L1)₄Cl₂(EtOH)₂]

Complex 2 is a heterometallic tetranuclear complex featuring a double cubane-like metal core structure comprising three Mn and one Ni ions. It crystallizes in the orthorhombic polar space group Aea2. The molecular structure and packing diagram of 2 are illustrated in Figure 2a and Figure S7, respectively. The selected bond lengths and angles are provided in

Table 3. Selected bond distances and angles of 2.

Bond	Bond Length/Å	Angle	Bond Angle/°
Ni1–O1	2.0937(18)	O1–Ni1–N1	85.24(8)
Ni1–O2	2.0454(18)	O2–Ni1–N1	85.91(7)
Ni1–N1	2.022(2)	O1–Ni1–N1 *	172.41(7)
Mn1–O1	1.9756(16)	O2–Ni1–O2 *	179.32(9)
Mn1–O2	2.1820(17)	O3–Mn1–N2	173.69(5)
Mn1–O3	1.8761(17)	O4–Mn1–N2	90.12(7)
Mn1–O4	1.8604(17)	O1–Mn1–N2	175.19(7)
Mn1–O5	2.2587(18)	O3–Mn1–O4	171.21(7)
Mn1–N2	2.0431(19)	O1–Mn2–O3	71.05(5)
Mn2–O1	2.3145(18)	O1–Mn2–O1 *	77.94(9)
Mn2–O3	2.1765(18)	Cl1–Mn2–Cl1 *	103.66(5)
Mn2–Cl1	2.4560(10)	O1–Mn2–Cl *	162.96(4)
Mn1⋯Mn2	3.223(1)	O3–Mn2–O3 *	153.32(9)
Mn1⋯Ni1	3.1004(10)	Mn1–O1–Mn2	97.09(6)
Mn1⋯Mn1 *	5.388(2)	Mn1–O1–Ni1	99.22(7)
Mn2⋯Ni1	3.3044(19)	Mn2–O1–Ni1	96.98(7)
		Mn1–O2–Ni1	94.29(6)
		Mn1–O3–Mn2	105.13(7)

Symmetry code (*): 1 − x, 1 − y, +z.

.

This molecule exhibits C₂ symmetry with the axis along the line connecting Ni1 and Mn2. Ni1 forms an octahedral [Ni(L1)₂]²⁻ unit, coordinated by two (L1)²⁻ ligands in a fac mode. However, unlike complex 1 where all three kinds of coordinating atoms were in cis positions, in this Ni unit, the phenolic oxygens adopt trans positions, introducing a rotational symmetry axis that was absent in complex 1. The coordination bond distances around Ni are Ni1–O1 = 2.0937(18), Ni1–O2 = 2.0454(18), and Ni1–N1 = 2.022(2) Å, respectively. The Ni1–O1 distance, corresponding to the benzyl alcohol oxygen, is approximately 0.15 Å shorter in complex 2 compared to 1 due to the dissociation of the alcohol proton. This benzyl alcohol oxygen, O1, acts as a μ₃-alkoxo bridge, connecting Ni1, Mn1, and Mn2. Mn1 is linked to Ni1 through this μ₃-alkoxo oxygen (O1) and a μ₂-phenoxo oxygen (O2). It forms an octahedral arrangement with another tridentate ligand’s donor set (O3, N2, O4) and the alcohol oxygen (O5) from the coordinating ethanol. The coordination bond distances within the square plane formed by the tridentate ligand and the μ₃-oxygen atom are 1.8761(17), 2.0431(19), 1.8607(17), and 1.9756(16) Å. In contrast, the Mn1–O2 distance for the μ₂-bridging oxygen is 2.1820(17) Å, and the Mn1–O5 distance for the coordinating ethanol moiety is 2.2587(18) Å, indicating elongation. This suggests that Mn1 is a Mn(III) ion with the O2–Mn1–O5 axis corresponding to the Jahn–Teller axis. On the other hand, Mn2 lacks chelating coordination from the tridentate ligand and exhibits a highly distorted six-coordinate structure formed by two μ₃-bridging alcoholic oxygens, two μ₂-bridging alcoholic oxygens, and two monodentate chloride ions. The bond distances around Mn2 are Mn2–O1 = 2.3145(18), Mn2–O3 = 2.1765(18), and Mn1–Cl1 = 2.4560(10) Å, which are longer than the bond distances around Mn1, indicating that Mn2 is in the Mn(II) oxidation state. To further confirm the oxidation state assignments of Mn1 and Mn2, bond valence sum (BVS) calculations were conducted for each manganese atom [39,40]. The calculated BVS values of 3.01 and 1.95 for Mn1 and Mn2, respectively, support the valence assignment based on the bond lengths [41]. To the best of our knowledge, only one Mn₃Ni complex has been reported [42], and in that complex, all Mn ions are in the trivalent state. Therefore, complex 2 represents the first report of a complex in which three Mn ions are in a mixed valence state.

The four metal ions are located on the same plane, forming a rhombus shape, as depicted in Figure 2b. The intramolecular metal-to-metal distances are as follows: Mn1⋯Ni1 = 3.1004(10), Mn1⋯Mn2 = 3.223(1), Ni1⋯Mn2 = 3.3044(19) Å, and Mn1⋯Mn1* = 5.388(2) Å (symmetry code: (*) 1 − x, 1 − y, +z), respectively. Due to the slight difference in length between the Mn1⋯Ni and Mn1⋯Mn2 sides, the rhombus shape is distorted towards Mn2. Comparing the bond angles around Ni1 and Mn2, the O2–N1–O2* angle corresponding to the longer side of the double-cubane framework is 179.32(9)°, while the O3–Mn2–O3* angle is 153.32(9)°, indicating significant distortion at Mn2. Consequently, the defective double-cubane metal core, comprising four metal ions and six bridging oxygen atoms, exhibits a distorted structure with Mn2 protruding from the double-cubane framework. This distortion can be attributed to the larger ionic radius of Mn2 and the steric repulsion of the coordinated two chloride ions. The SHAPE calculation results (OCT-6) further confirm the substantial distortion at Mn2, with values of 0.594 for Ni1, 0.964 for Mn1, and 2.459 for Mn2. Complex 2 exhibits hydrogen bonds between the O–H proton of the coordinated ethanol and the chloride ion, with a distance of O5–H5⋯Cl1 = 3.0745(19) Å. Additionally, weak π-π stacking interactions occur between the two phenyl rings defined by C9–C14 and C24–C28, with a centroid-to-centroid distance of 3.688(2) Å. These interactions are believed to stabilize the molecular structure.

3.3. Electronic Spectra

The electronic spectra of the present complexes were recorded using the diffuse reflectance technique. The obtained spectra are displayed in Figure 3, and the respective wavenumbers of the absorption bands are listed in

Table 4. Spectral data for d-d bands ($\tilde{\nu }$/10³ cm⁻¹) of 1 and 2.

Complex		Ni(II)			Mn(III)
	3A2 → 3T2	3A2 → 3T1	3A2 → 3T1(P)	5B1 → 5A1	5B1 → 5B2	5B1 → 5E
1	9.92	16.6	~22 a
2	~10 a	~16 a	~22 a	~13 a	~19 a	~20 a

^a Shoulder.

. In the reflection spectrum of 1, three distinct d-d transition bands were observed at approximately 9.92 × 10³, 16.6 × 10³, and ~22 × 10³ (shoulder) cm⁻¹. These transitions can be attributed to ³A_2g → ³T_2g, ³A_2g → ³T_1g, and ³A_2g → ³T_1g(P) transitions originating from the octahedral Ni(II) center [43]. Additionally, there’s an intense band at around 26.0 × 10³ cm⁻¹, which is attributed to ligand-to-metal charge transfer (LMCT) originating from either the phenolic or alcoholic oxygen in the ligand to the Ni(II) center. For complex 2, in addition to the d-d bands of Ni(II), three distinct d-d absorption bands are predicted. These bands arise from the octahedral Mn(III) ion with Jahn–Teller distortion and are assigned as ⁵B_1g → ⁵A_1g, ⁵B_1g → ⁵B_2g, and ⁵B_1g → ⁵E_g transitions [44]. In fact, more than six indistinct absorption bands were observed as shoulders in the reflectance spectrum of 2. Since the coordination environment of Ni(II) is not significantly different between 1 and 2, the three absorption bands at ~10 × 10³, ~16 × 10³, and ~22 × 10³ cm⁻¹ can be assigned to Ni(II) d-d transitions. The other three bands observed at around ~13 × 10³, ~19 × 10³, and ~20 × 10³ cm⁻¹ are likely associated with Mn(III) d-d transitions. The absorption peak at 26 × 10³ cm⁻¹ is assigned to LMCT from the ligands to the metal ions.

3.4. Magnetic Properties

Magnetic susceptibility measurements for 1 and 2 were performed with a SQUID (superconducting quantum interference device) magnetometer. The effective magnetic moment of 1 at 300 K is 1.26 cm³ mol⁻¹ K (3.17 μ_B), which falls within the expected range for mononuclear octahedral Ni(II) complexes.

For complex 2, the temperature dependence of magnetic susceptibility was measured in the temperature range from 2 to 300 K. Figure 4a displays the temperature dependence of direct-current (DC) molar magnetic susceptibilities (χ_M) and χ_MT vs. T plots of 2. At 300 K, the χ_MT value measures 11.18 cm³ mol⁻¹ K, slightly smaller than the expected spin-only value of 11.38 cm³ mol⁻¹ K for a magnetically uncoupled system comprising two high-spin Mn(III) ions (S = 2), one Mn(II) ion (S = 5/2), and one Ni(II) ion (S = 1). As temperature decreases, χ_MT values increase, reaching a maximum of 24.19 cm³ mol⁻¹ K at 8.0 K before decreasing rapidly to 15.27 cm³ mol⁻¹ K at 2.0 K.

This magnetic behavior suggests the presence of ferromagnetic coupling between the metal ions. The decline at low temperatures might arise from zero-field splitting (ZFS) or intermolecular antiferromagnetic interaction. The behavior was analyzed using the PHI program based on the magnetic coupling scheme shown in the inset of Figure 4a and spin Hamiltonian (Equation (1)).

H = −2J₁(S₁S₂ + S₁S₃) − 2J₂(S₂S₄ + S₃S₄) − 2J₃S₁S₄ − 2J₄S₂S₃

(1)

The obtained parameters are as follows: J₁ = 2.23 cm⁻¹, J₂ = 0.46 cm⁻¹, J₃ = 1.78 cm⁻¹, J₄ = 0.58 cm⁻¹, g_Mn(III) = 2.00, g_Mn(II) = 1.98, g_Ni = 2.00, zJ′ = −0.0158 cm⁻¹. Comparing these four J values reveals that the interactions between Ni and Mn and J₁ and J₃ are the main contributions to the magnetic behavior of 3. The average Mn–O–Ni angles, 96.76° for Mn(III)–O–Ni(II) and 97.09° for Mn(II)–O–Ni(II), fall within the 95–98° range known as the crossover angle, indicating ferromagnetic interaction [45,46].

Mn(III) and Ni(II) ions with distorted octahedral geometries often exhibit large ZFS and negative anisotropic D parameters. Although the D parameter may influence the decrease in the χT behavior, its value could not be satisfactorily analyzed with the χT vs. T plots. Magnetization (M) measurements were performed for a more accurate determination of the D parameter. Isothermal magnetization data were collected between 2 and 10 K at applied fields ranging from 0.001 to 5 T. The magnetization approaches saturation at 2 K, with a value of 14.8 Nμ_B, slightly below the expected value of 15 Nμ_B when g ~ 2.0 for S = 15/2 ground state. The M vs. H/T plots for 2, 3, and 4 K largely overlap on a single curve, while deviations are observed above 5 K, as shown in Figure 4b. These results suggest that the magnetic anisotropy D in 2 is notably small. Finally, the D value could not be precisely determined even through analysis of magnetization data (Figure S8).

Frequency-dependent alternating-current (AC) magnetization measurements were performed at frequencies of 1, 10, 100, and 1000 Hz (Figure 5a,b). The measurements without static DC field exhibited subtle out-of-phase (χ″_M) signals only. Using an applied magnetic field of 2000 Oe, no alteration was observed in χ’_M. However, a more prominent frequency dependence was observed in χ″_M below 10 K, indicating field-induced slow magnetic relaxation in magnetization. The energy barrier (E_a) and preexponential factor (τ₀) could be roughly estimated using the Debye model through Equation (2) [47].

$$\ln \frac{{\chi }^{″}}{{\chi }^{\prime }}=\ln \left(\omega {\tau }_0\right)+\frac{E_a}{k_{B}T}$$

(2)

The calculated results provide E_a values ranging from 6.13 to 9.45 cm⁻¹ and τ₀ values between 9.44 × 10⁻⁷ and 7.46 × 10⁻⁴ s for 2 under 2000 Oe DC field (Figure 6), corresponding to relatively slow magnetic relaxation, similar to metal complexes reported as weak SMMs [2,48].

4. Conclusions

The new mononuclear complex, [Ni(HL1)₂] (1), was synthesized via simple reactions between metal sources and the Schiff base ligand. The novel heteronuclear complex, [Mn₃Ni(L1)₄Cl₂(EtOH)₂] (2), was obtained through a stepwise synthesis using 1 as a complex ligand. The crystal structures of these complexes were elucidated through single-crystal X-ray analysis. Complex 1 represents the first example of a mononuclear complex with fac-coordinated H₂L1 ligands. The tetranuclear complex 2 features a unique Mn(II)Mn(III)₂Ni(II) metal core connected by two μ₃-alkoxo, two μ₂-alkoxo, and two μ₂-phenoxo bridges. Investigations into the magnetic properties of 2 through DC magnetic measurements revealed the presence of dominant ferromagnetic interactions among all metal ions, resulting in an S = 15/2 ground state. Additionally, 2 exhibits frequency-dependent out-of-phase signals below 10 K in AC studies, indicating relatively slow magnetic relaxation in magnetization.