Crystal Structures, Magnetic Properties, and Redox Behaviors of Carboxylato-Bridged Mn(II) Complexes with Ditopic Ligands Featuring N₃-Coordination Sites

Abstract

Carboxylato-bridged dinuclear and tetranuclear Mn(II) complexes 1–3 with ditopic ligands featuring two N₃-terminal coordination sites connected by hexyl (tphn), octyl (tpon), and p-xylyl (tpxn) linkers have been synthesized and characterized through X-ray single-crystal structure analyses, infrared spectroscopy, and elemental analyses. Complex 1 is a μ-fluorido-bis-μ-acetato dinuclear Mn(II) complex where the ligand tphn coordinates to both terminal sides of a dinuclear Mn unit. In contrast, complexes 2 and 3 are tetranuclear Mn(II) complexes with a macrocyclic structure, in which two dinuclear Mn units are linked by ligands tpon or tpxn. The redox behaviors of 1 and 2 were elucidated by cyclic voltammetry, revealing two metal-centered redox processes corresponding to Mn₂(II,II)/Mn₂(II,III) and Mn₂(II,III)/Mn₂(III,III).

1. Introduction

Manganese complexes with μ-carboxylato bridges have been extensively studied in a wide range of fields, including magnetic materials, catalysis, metal-organic frameworks (MOFs), and bioinorganic chemistry, due to various chemical and physical properties, which depend on changes in the oxidation states of Mn ions and the bridging ligands [1,2,3,4]. In these studies, carboxylates exhibit a variety of bridging modes. The bridges observed between two manganese ions include single [5,6], double [7,8], and triple bridges [9,10]. Not only does the number of bridges vary, but also the bridging modes of carboxylates themselves and the formation of mixed bridges combined with other bridging ligands provide a wide range of variations [11]. For example, the well-known Mn12 cluster, a single-molecule magnet, is a multinuclear manganese complex with multiple syn-syn-type carboxylates and oxo ions, and the metal core structure, including this bridging style, significantly contributes to its single-molecule magnet (SMM) properties [12]. Additionally, metalloenzymes such as arginase and manganese catalase have a dinuclear manganese active center bridged by carboxylates with both μ-O,O′ and μ-O bridging modes, and the flexibility of carboxylato bridging may contribute to enzyme function [13,14].

Dinuclear manganese complexes are often studied from a bioinorganic chemistry perspective, and many types of dinuclear manganese complexes have been reported as catalase model complexes [15]. However, unlike the Mn(III) complexes of the same type, which progress catalytic reactions via the Mn₂(II,II)/Mn₂(III,III) redox cycle similar to enzymes, μ-carboxylato dinuclear Mn(II) complexes have fewer reported examples due to their stability in air and labile activity of Mn(II) ions [16]. Most of these complexes have structures in which bidentate or tridentate terminal ligands are coordinated to fill the coordination sites on both sides of the carboxylate-bridged dinuclear manganese structure [17]. Due to the properties of labile Mn(II) ions, such complexes are expected to have limited catalytic activity in solution because the bridging structure cannot be maintained. To suppress the dissociation of the dinuclear structure, many complexes using dinucleating ligands that include phenoxo or alkoxo moieties in the ligand structure have been reported [18]. However, in these structures, the dissociation of phenoxo or alkoxo bridges is unlikely, so it cannot faithfully mimic the reaction mechanisms of arginase or Mn-catalase. Therefore, attempts have been made to stabilize the bridging structure and improve catalytic reaction efficiency by connecting terminal ligands on both sides of the dinuclear unit with a linker to form a cyclic structure [19].

The three ditopic ligands addressed in this study, as shown in Scheme 1, all have two N₃ coordination sites consisting of bis(pyridylmethyl)amine connected by a linker of six or more C atoms. The ligands tphn (N,N,N′,N′-tetrakis(2-pyridylmethyl)hexane-1,6-diamine) and tpon (N,N,N′,N′-tetrakis(2-pyridylmethyl)octane-1,8-diamine) are flexible ligands with C6 and C8 alkyl chains as linkers [20,21]. On the other hand, tpxn (N,N,N′,N′-tetrakis(2-pyridylmethyl)-p-xylene-α,α′-diamine) has a p-xylyl linker, which is considered to have a more rigid structure than the two former ligands [22]. In this study, we report the structures of newly obtained dinuclear and tetranuclear Mn(II) complexes with carboxylate bridging using these ligands, as well as their magnetic and electrochemical properties.

2. Materials and Methods

2.1. Materials

All common reagents and solvents were purchased and used as received, unless stated otherwise. Methanol was purified by distillation over magnesium turnings, while acetonitrile was dehydrated with P₂O₅ and distilled before use. Tetrabutylammonium perchlorate (TBAP), used as the supporting electrolyte, was recrystallized three times in a mixed solvent of dichloromethane and hexane. (Caution: TBAP is explosive and should be handled with great care!). The ligands tphn [20], tpon [21], and tpxn [22] were obtained by the literature method.

2.2. Preparation

2.2.1. [Mn₂F(OAc)₂(tphn)]BF₄ (1)

Manganese(II) tetrafluoroborate hexahydrate (0.336 g, 1.0 mmol) was dissolved in 6.0 mL of methanol, followed by the addition of a 4.0 mL aqueous solution of sodium acetate trihydrate (0.204 g, 1.5 mmol). After stirring for 15 min, 1.0 mL of a methanol solution of tphn (0.24 g, 0.50 mmol) was added, and the mixture was stirred overnight, yielding a white precipitate. Yield 0.130 g, 31.1%.

Single crystals suitable for X-ray crystallography were obtained as colorless transparent plate crystals by recrystallization from a mixed solvent of dichloromethane/toluene (v/v = 1/1). Complex 1: C₃₄H₄₁BF₅Mn₂N₆O₄ (814.41): calcd. C 50.14, H 5.20, N 10.32; found C 49.76, H 5.17, N 10.13. IR (ATR) [cm⁻¹]: 1597, 1417 (COO), 1049 (BF₄⁻), 766 (py).

2.2.2. [{Mn₂(OAc)₃(tpon)}₂](BPh₄)₂ (2)

A solution of manganese(II) acetate tetrahydrate (0.124 g, 0.50 mmol) in methanol (5 mL) was mixed with a solution of sodium acetate trihydrate (0.104 g, 0.76 mmol) in water (1 mL) and a solution of tpon (0.127 g, 0.25 mmol) in methanol (5 mL). The mixture was stirred for 30 min. To this solution, a solution of sodium tetraphenylborate (0.082 g, 0.24 mmol) in methanol (5 mL) was added and stirred for 30 min. The resulting white precipitate was collected by suction filtration. The crude product was recrystallized in a mixed solvent of THF/toluene, yielding colorless microcrystals. Yield: 0.185 g, 69.1%.

Single crystals suitable for X-ray crystallography were obtained as colorless transparent block crystals by recrystallization from a mixed solvent of THF/toluene (v/v = 1/2). Complex 2: C₁₂₄H₁₃₈B₂Mn₄N₁₂O₁₂ (2229.87): calcd. C 66.79, H 6.24, N 7.54; found C 67.05, H 6.22, N 7.52%. IR (ATR) [cm⁻¹]: 1620, 1602, 1422 (COO), 758 (py), 703 (BPh₄⁻).

2.2.3. [{Mn₂(OAc)₂(tpxn)(CH₃OH)(H₂O)₂}₂][{Mn₂(OAc)₃(tpxn)(CH₃OH)(H₂O)}₂](PF₆)₆ (3)

A solution of manganese(II) acetate tetrahydrate (0.125 g, 0.51 mmol) in methanol (5 mL) was mixed with a solution of sodium acetate trihydrate (0.104 g, 0.76 mmol) in water (1 mL) and stirred for 15 min. To this solution, 3 mL of a methanol solution of tpxn (0.122 g, 0.24 mmol) was added, and the resulting dark brown solution was further stirred for 15 min. Finally, a solution of potassium hexafluorophosphate (0.052 g, 0.28 mmol) in water (3 mL) was added and stirred for 30 min. The mixture was then filtered and allowed to stand for 24 h at room temperature. The dark brown precipitate was removed by filtration, and upon further incubation at 10 °C for another 24 h, colorless plate crystals suitable for X-ray crystallography were collected. Yield 0.008 g, 4%. Complex 3∙12H₂O: C₁₅₂H₁₈₆F₃₆Mn₈N₂₄O₃₀P₆ (4138.54): calcd. C 44.11, H 4.53, N 8.12; found C 43.88, H 4.28, N 8.32%. IR (ATR) [cm⁻¹]: 1603, 1549, 1445, 1421 (COO), 834 (PF₆⁻), 764 (py).

2.3. Measurements

Elemental analyses for C, H, and N were conducted at the Elemental Analysis Service Center, Kyushu University. Infrared spectra were recorded using a VERTEX70-S FT-IR Spectrometer (Bruker, Corp., Billerica, MA, USA) with the ATR (Attenuated Total Reflection) method. The magnetic susceptibilities were measured on a MPMS-XL5R SQUID (Superconducting Quantum Interference Device) susceptometer (Quantum Design, Inc., San Diego, CA, USA) under an applied magnetic field of 0.1 T in the temperature range 2–300 K for 1 and 2. The susceptibilities were corrected for the diamagnetism of the constituent atoms using Pascal’s constant [23]. The DC-magnetic data were fitted using the PHI program [24]. The cyclic voltammogram (CV) was obtained with an ALS Electrochemical Analyzer Model832A (BAS, Inc., Tokyo, Japan). CV measurements were carried out in dichloromethane containing 0.1 M TBAP as a supporting electrolyte. A three-electrode cell was used, equipped with a glassy carbon (ϕ = 1 mm) working electrode, a platinum wire counter electrode, and an Ag/Ag⁺ reference electrode.

2.4. Single Crystal X-ray Diffraction

Diffraction data were measured on a Vari-Max Saturn CCD 724 diffractometer (Rigaku, Corp, Tokyo, Japan) using graphite monochromated Mo Ka radiation (λ = 0.71069 Å) at the Analytical Research Center for Experimental Sciences, Saga University. Data were collected using CrystalClear [25]. Data processing was performed using CrysAlisPro [26] for 1 and CrystalClear for 2 and 3. A 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	3
Empirical formula	C34H42BF5Mn2N6O4	C136H162B2Mn4N12O15	C152H210F36Mn8N24O42P6
Formula weight	814.42	2446.15	4354.77
Temperature/K	301	113	113
Crystal system	triclinic	triclinic	monoclinic
Space group	$P\overline{1}$	$P\overline{1}$	P21/c
a/Å	10.3107(2)	9.257(2)	24.719(5)
b/Å	11.4913(2)	18.174(4)	20.878(5)
c/Å	15.7802(3)	19.196(4)	18.209(4)
α/°	84.2060(10)	99.904(3)	90
β/°	88.346(2)	101.628(4)	97.229(4)
γ/°	81.786(2)	96.449(4)	90
V/Å3	1840.86(6)	3079.7(12)	9323(4)
Z	2	1	2
Dcalc/g cm−3	1.469	1.319	1.551
μ(MoKα)/mm−1	0.757	0.470	0.691
F(0 0 0)	840.0	1292.0	4480.0
Crystal dimensions/mm3	0.61 × 0.56 × 0.55	0.20 × 0.20 × 0.20	0.61 × 0.20 × 0.20
Radiation	MoKα (λ = 0.71073)	MoKα (λ = 0.71075)	MoKα (λ = 0.71075)
2θ range for data collection/°	3.992 to 61.604	6.112 to 54.948	6.038 to 55.036
Reflections collected	16,511	25,676	74,634
Independent reflections	9644 (Rint = 0.0116)	13,566 (Rint = 0.0276)	21,041 (Rint = 0.0626)
Data/Restraints/Params	9644/170/518	13,566/75/788	21,041/8/1279
Goodness of fit indicator	1.053	1.044	1.074
R indices [I > 2.00σ(I)]	R1 = 0.0327	R1 = 0.0582	R1 = 0.0581
	wR2 = 0.0892	wR2 = 0.1493	wR2 = 0.1459
R indices (all data)	R1 = 0.0355	R1 = 0.0740	R1 = 0.0805
	wR2 = 0.0917	wR2 = 0.1673	wR2 = 0.1644
Largest diff. peak, hole/e Å−3	0.31, −0.24	1.52, −0.58	1.29, −0.98
CCDC deposition number	2361225	2361226	2361227

. Structures were solved by direct methods (ShelXT) and expanded using Fourier techniques [27]. Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed geometrically in calculated positions and refined with a riding model. Appropriate restrictions using fragment data were applied to the disordered components in 1 (BF₄⁻) and 2 (THF). The final cycle of full-matrix least-squares refinement on F² using ShelXL [28] was based on observed reflections and variable parameters and converged with unweighted and weighted agreement factors of R and R_w. Olex2 [29] was used as an interface to the ShelX program package. Molecular structure drawings were performed using Mercury [30]. The Hirshfeld surface was generated using Crystal Explorer 3.1 [31].

2.5. Study of Catalase Activity

A volumetric method was employed to determine the evolution of dioxygen. A 10 mL test tube, sealed with a silicone stopper, was connected to a 25 mL burette with a precision of 0.1 mL. A chosen concentration (1.2, 1.0, 0.85, and 0.67 mM) of aqueous H₂O₂ (1.0 mL) was injected through the sealed silicon stopper into the closed test tube containing 1.0 mL of an acetonitrile solution of the complex (1.0 mM), thermostatized at 25 °C. The volume of evolved oxygen was measured as a function of time.

3. Results and Discussion

3.1. Synthetic Aspects

The reaction of Mn(II) ions with three types of ditopic ligands in the presence of acetate ions yielded colorless Mn complexes 1–3. Elemental analysis suggests that these complexes have the following composition: [Mn₂F(OAc)₂(tphn)]BF₄, [Mn₂(OAc)₃(tpon)]BPh₄, and [Mn₄(OAc)₅(tpxn)₂(CH₃OH)₂(H₂O)₃](PF₆)₃∙6H₂O, respectively. Even with a composition in which the fluoride ion of 1 is replaced with a hydroxy ion, the elemental analysis values are sufficiently close. However, the characteristic sharp ν(OH) vibration of the μ-hydroxo bridge is not observed in the IR spectrum, indicating the presence of the fluoride ion generated by the hydrolysis of the tetrafluoroborate ion. Single crystals of 1 and 3 were obtained directly during synthesis, whereas single crystals of 2 were obtained by recrystallization from a mixed solvent (THF/toluene) different from that used during synthesis. Complex 3 has very high solubility in various solvents, making it difficult to crystallize. However, we confirmed that single crystals can be reproduced by following the synthetic procedure described in Section 2.2.3, although the yield is extremely low.

3.2. Crystal Structures

3.2.1. Complex 1

The cation structure of 1 is shown in Figure 1a, and the selected bond lengths and angles are listed in

Table 2. Selected bond distances and angles of 1.

Bond	Distance/Å	Angle	Angle/°
Mn1–F1	2.0637(7)	F1–Mn1–O1	100.11(4)
Mn1–O1	2.1199(10)	F1–Mn1–O3	89.92(4)
Mn1–O3	2.1536(11)	F1–Mn1–N1	168.45(4)
Mn1–N1	2.2883(12)	F1–Mn1–N3	98.49(3)
Mn1–N2	2.2753(11)	O1–Mn1–N3	154.74(4)
Mn1–N3	2.3910(11)	O3–Mn1–N2	167.61(4)
Mn2–F1	2.0429(8)	F1–Mn2–O2	94.70(4)
Mn2–O2	2.1334(10)	F1–Mn2–O4	93.04(4)
Mn2–O4	2.1312(10)	F1–Mn2–N4	105.64(4)
Mn2–N4	2.4314(12)	F1–Mn2–N5	91.58(4)
Mn2–N5	2.2593(11)	O2–Mn2–N4	157.97(4)
Mn2–N6	2.3271(13)	O4–Mn2–N5	160.67(4)
Mn1⋯Mn2	3.4309(3)	Mn1–F1–Mn2	113.33(3)

. Complex 1 was identified as a triply bridged dinuclear Mn(II) complex, [Mn₂(μ-F)(μ-acetato-O,O′)₂(tphn)]BF₄. The BF₄⁻ anion was treated as disordered, with occupancies of 0.51 and 0.49.

Two Mn atoms are bridged by two acetate ions and a fluoride ion. The bridging fluoride ion is believed to result from the partially hydrolyzed BF₄⁻ during synthesis. The tridentate N₃-coordination sites at both ends of the tphn ligand coordinate in a fac-manner at both sides of the dinuclear Mn core, forming a characteristic metallacyclic structure. The coordination environments of the two Mn atoms are similar, with average bond distances to each coordinating atom being Mn–F = 2.0533(8), Mn–O = 2.1346(10), and Mn–N = 2.3288(12) Å, respectively, suggesting an oxidation state of +2 for Mn. The difference of approximately 0.2 Å between the Mn–N distances and those of other bonds suggests a distorted octahedral geometry for the Mn ion.

The bond lengths, bond angles, and torsion angles of the two bridging acetate ions are summarized in

Table 3. Structural parameters and bridging modes of carboxylato bridges in Complex 1.

Bridging Acetate	Bond Length/Å Mn–O	Bond Angle/° Mn–O–C	Torsion Angle/° Mn–O–C–C	Torsion Angle/° Mn–O∙∙∙O–Mn	Bridging Mode
A (O1, O2)	2.1199(10)	130.52(9)	175.15(11)	6.67(6)	syn-syn
	2.1334(10)	136.17(10)	−165.71(12)		coplanar
B (O3, O4)	2.1536(11)	134.20(10)	174.47(15)	3.07(6)	syn-syn
	2.1312(10)	131.43(9)	−170.66(14)		coplanar

. The bridging modes of carboxylate groups are classified based on the differences between these parameters [32]. The torsion angles defined by Mn–O–C–C and Mn–O∙∙∙O–Mn range from 165.71(12) to 175.15(11)° and 3.07(6) to 6.67(6)°, respectively. These parameters fit well with the syn-syn coplanar bridging mode. The bridging structure of 1, as shown in Figure 1b, resembles those of μ-alkoxo/carboxylato-O-bis-μ-carboxylato-O,O′ bridged dinuclear Mn(II) complexes, except for the fluoride bridge [33]. The distance between the two Mn atoms is 3.4309(3) Å, which is shorter than reported tris-μ-carboxylato bridged Mn(II) complexes (~4 Å) [9,10].

The CPK model for 1 is presented in Figure 1c. Although the dinuclear Mn core appears to be surrounded and shielded by pyridyl groups and the alkyl linker, it is evident that the bridging fluoride ion remains unshielded. The Hirshfeld surface and 2D fingerprint plots for the complex cation of 1 are illustrated in Figure S4. The contact between the bridging F atom and the outer atom is about 0.6%, which is not large, but it indicates the possibility of external contact. Consequently, the dissociation of the fluoride ion may create a pathway for accessing the Mn ions, suggesting the potential for catalytic reactions.

Figure 1d illustrates the packing diagram of 1. Each dinuclear complex cation is stacked through weak π-π stacking interactions involving the pyridyl group, including N2, of tphn (centroid distance: 3.9103(12) Å).

3.2.2. Complex 2

The cation structure of 2 is depicted in Figure 2a, and selected bond lengths and angles are listed in

Table 4. Selected bond distances and angles of 2.

Bond	Distance/Å	Angle	Angle/°
Mn1–O1	2.095(2)	O1–Mn1–O3	99.68(9)
Mn1–O3	2.116(2)	O1–Mn1–O5	100.64(9)
Mn1–O5	2.1339(18)	O1–Mn1–N1	162.88(6)
Mn1–N1	2.253(2)	O3–Mn1–O5	104.96(8)
Mn1–N2	2.308(2)	O3–Mn1–N2	164.64(8)
Mn1–N3	2411(2)	O5–Mn1–N3	154.00(9)
Mn2–O2	2.137(2)	O2–Mn2–O4	94.02(9)
Mn2–O4	2.122(2)	O2–Mn2–O6	100.10(8)
Mn2–O6	2.1176(19)	O2–Mn2–N5i	168.08(8)
Mn2–N4i	2.319(2)	O4–Mn2–O6	109.64(8)
Mn2–N5i	2.268(2)	O4–Mn2–N4i	163.57(8)
Mn2–N6i	2.381(2)	O6–Mn2–N6i	151.47(7)
Mn1···Mn2	4.0293(9)
Mn1···Mn1i	13.368(3)
Mn1···Mn2i	13.348(2)
Mn2···Mn2i	14.496(3)

Symmetry Code i: 1 − x, −y, −z.

. Complex 2 was identified as a cyclic tetranuclear Mn complex, [{Mn₂(μ-acetato-O,O′)₃(tpon)}₂](BPh₄)₂·3THF, with an inversion center. Within the complex cation, two tris-μ-carboxylato triple-bridged dinuclear Mn units are present. One of the three THF molecules present as the crystal solvent was found to be disordered with an occupancy of 0.5 at a crystallographically special position.

The dinuclear Mn unit in 2 is illustrated in Figure 2b. Each Mn is hexacoordinated, with three oxygen atoms originating from acetate ions, three nitrogen atoms from two pyridyl groups, and one tertiary amine from the tpon ligand. The coordination geometry adopts a distorted octahedral structure. The Mn–N bond lengths range from 2.253(2) to 2.411(3) Å for Mn1 and from 2.268(2) to 2.381(2) Å for Mn2, while the Mn–O bond lengths range from 2.095(2) to 2.1339(19) Å and 2.1176(19) to 2.137(7) Å, respectively. These values closely resemble those reported for Mn(II) complexes, indicating that both Mn1 and Mn2 ions exist in the +2 oxidation state. The metal-to-metal distance Mn1···Mn2 is 4.0293(9) Å, similar to the distances observed in reported dinuclear Mn(II) complexes with tris-μ-carboxylato bridges (~4 Å) [9,10].

The intermetallic distances between two dinuclear units, Mn1···Mn1ⁱ, Mn1···Mn2ⁱ, and Mn2···Mn2ⁱ (symmetry code i: 1−x, −y, −z) are 13.368(3), 13.348(2), and 14.496(3) Å, respectively, which are more than three times longer than the distance between Mn1 and Mn2. As evident from the CPK model of the complex cation shown in Figure 2c, the alkyl linker moieties of the tpon ligands extend straight, indicating the presence of a “fastener effect” between the alkyl chains [34]. One reason for the structural differences between complexes 1 and 2 is believed to be the strong fastener effect due to the extension of the methylene chain of the linker moiety from C6 to C8.

The bridging acetate ions exhibit significant variations in bond angles and torsion angles with the Mn ions. The structural parameters related to the carboxylate-bridging mode are summarized in

Table 5. Structural parameters and bridging modes of carboxylato bridges in Complex 2.

Bridging Acetate	Bond Length/Å Mn–O	Bond Angle/° Mn–O–C	Torsion Angle/° M–O–C–C	Torsion Angle/° M–O∙∙∙O–M	Bridging Mode
A (O1, O2)	2.095(2)	162.1(2)	170.4(6)	3.01(17)	syn-syn
	2.137(2)	123.91(19)	−169.8(3)		coplanar
B (O3, O4)	2.116(2)	125.32(17)	147.2(2)	17.51(12)	syn-skew
	2.122(2)	156.3(2)	−165.2(4)		nonplanar
C (O5, O6)	2.1339(19)	132.27(17)	137.7(2)	29.71(11)	syn-skew
	2.1176(19)	142.41(18)	−176.3(2)		nonplanar

. This table shows significant differences in the Mn–O–C bond angle and in the torsion angles M–O–C–C and M–O∙∙∙O–M. The three acetato bridges in this complex are classified as a syn-syn coplanar type with a small torsion angle (bridge A) and as syn-skew nonplanar types with larger torsion angles (bridges B and C). Even in similar complexes with tris-μ-acetato bridges, two of the three bridging acetate ions are often twisted by about 15 to 35°, which is thought to be a common structure in tris-μ-acetato dinuclear structures [9,10].

The crystal packing diagram (Figure 2d) reveals the shortest intermolecular metal-to-metal distance between the complex cations to be 11.1480(19) Å, shorter than the intramolecular distance between the dinuclear units. No interactions such as hydrogen bonding or π–π stacking were observed in the crystal packing. The components in 2 are largely covered with hydrogen atoms, so the Hirshfeld surface analysis does not reveal any significant features (Figure S5).

3.2.3. Complex 3

Complex 3 is a double salt composed of two types of complex cations with different charges, [{Mn₂(μ-acetato-O,O′)(acetato-O,O′)(tpxn)(CH₃OH)(H₂O)₂}₂]⁴⁺ (Cation A) and [{Mn₂(μ-acetato-O,O′)(acetato-O,O′)(acetato-O)(tpxn)(CH₃OH)(H₂O)}₂]²⁺ (Cation B), along with six PF₆⁻ counter anions. Both complex cations form a cyclic tetranuclear structure in which two μ-acetato single bridged dinuclear Mn units are connected by two tpxn ligands. The composition and charge of the cations differ depending on the combination of non-bridging ligands (H₂O, CH₃OH, and CH₃COO⁻). The structures of cations A and B of Complex 3 are illustrated in Figure 3, and the selected bond lengths and angles are listed in

Table 6. Selected bond distances and angles of 3.

Bond	Distance/Å	Angle	Angle/°
Cation A
Mn1–O1	2.098(2)	O1–Mn1–N2	167.38(9)
Mn1–O3	2.182(2)	O3–Mn1–N3	162.33(9)
Mn1–O4	2.195(2)	O4–Mn1–N1	172.72(9)
Mn1–N1	2.270(2)	N1–Mn1–N2	96.10(9)
Mn1–N2	2.289(2)	N1–Mn1–N3	73.02(9)
Mn1–N3	2.348(2)	N2–Mn1–N3	74.59(9)
Mn2–O2	2.160(2)	O2–Mn2–O7	174.37(10)
Mn2–O5	2.238(3)	O5–Mn2–O6	54.74(10)
Mn2–O6	2.505(4)	O5–Mn2–N4i	81.63(10)
Mn2–O7	2.233(2)	O6–Mn2–N5i	83.60(10)
Mn2–N4i	2.309(3)	N4i–Mn2–N6i	71.33(9)
Mn2–N5i	2.331(3)	N5i–Mn2–N6i	70.68(9)
Mn2–N6i	2.400(3)
Mn1···Mn2	5.3487(13)
Mn1···Mn1i	15.039(2)
Mn1···Mn2i	11.981(2)
Mn2···Mn2i	10.868(2)
Cation B
Mn3–O8	2.114(2)	O8–Mn3–N8	164.21(9)
Mn3–O10	2.121(2)	O10–Mn3–N9	154.96(8)
Mn3–O12	2.197(2)	O12–Mn3–N7	170.74(9)
Mn3–N7	2.283(2)	N7–Mn3–N8	98.75(9)
Mn3–N8	2.328(2)	N7–Mn3–N9	72.58(9)
Mn3–N9	2.360(2)	N8–Mn3–N9	72.92(8)
Mn4–O9	2.163(2)	O9–Mn4–O15	175.04(9)
Mn4–O13	2.307(2)	O13–Mn4–O14	56.40(8)
Mn4–O14	2.333(2)	O13–Mn4–N10j	80.03(9)
Mn4–O15	2.209(2)	O14–Mn4–N11j	83.96(8)
Mn4–N10j	2.320(2)	N10j–Mn4–N12j	71.01(9)
Mn4–N11j	2.369(3)	N11j–Mn4–N12j	70.23(8)
Mn4–N12j	2.387(3)
Mn3···Mn4	5.3389(13)
Mn3···Mn3j	14.976(2)
Mn3···Mn4j	11.927(2)
Mn4···Mn4j	10.826(2)
Mn1···Mn3	8.6610(17)

Symmetry Code i: −x, 1 − y, 2 − z. j: −x, 1 − y, −z.

. Aside from differences in the ditopic ligands and acetate bridging moieties, the fundamental structures of the tetranuclear complex cations closely resemble those of Complex 2.

Each Mn has two coordination sites in addition to the coordination of the bridging acetate ion and tpxn. Mn1 in Cation A and Mn3 in Cation B exhibit six-coordinate octahedral structures with two monodentate ligands (H₂O or CH₃OH) binding. In contrast, Mn2 and Mn4 form seven-coordinate pentagonal bipyramidal structures with bidentate-chelating acetate ions and monodentate-coordinating methanol. This difference appears to correlate with the coordination modes of the tpxn ligands, where each N₃ site of tpxn coordinates in a fac-mode with the six-coordinate Mn1 and Mn3, whereas Mn2 and Mn4 are coordinated in a mer-mode within the equatorial plane of the pentagonal bipyramid. The average bond lengths around Mn are Mn–O = 2.218(2) and Mn–N = 2.327(2) Å, respectively, suggesting an oxidation state of +2 for all Mn ions. The metal-to-metal distances within the dinuclear unit were 5.3487(13) for Cation A and 5.3389(13) Å for Cation B, respectively. The distances are significantly longer than those observed in 1 and 2 due to the single acetato bridge in a syn-anti mode [5,6]. Therefore, although the phenyl rings of tpxn ligands are located in parallel, they are not close enough to allow π-stacking interactions between the phenyl rings [Cation A: 6.306(3) Å, Cation B: 6.316(3) Å]. When comparing the structures of 1–3 with previously reported dinuclear Mn complexes featuring shorter linkers such as tptn or tpbn [35], it can be concluded that when the linker length is shorter than C6, dinuclear Mn complexes are formed, whereas when it becomes longer or includes rigid structures like a phenyl ring, tetranuclear structures are formed.

The two cations are weakly connected by a hydrogen bond between the water molecule bound to Mn1 and the acetate ion bound to Mn3 (O3–H3A···O11 = 2.663 Å), forming a 1D chain structure. As a result, the intermetallic distance between dinuclear units is shorter between adjacent cations (Mn1···Mn3 = 8.6610(17) Å) than the distance within a cation (10.826(2)—15.039(2) Å). Additionally, numerous hydrogen bonds are formed between the crystal solvents, counter ions, and complex cations (Figure S6). The Hirshfeld surface analysis was performed on 3. The resulting Hirshfeld surface (d_norm) is shown in Figure 4a. Furthermore, 2D fingerprint plots of all atoms, H···O, and H···F are shown in Figure 4b–d. The Hirshfeld surface of the complex cation is mapped in red near the water molecules and PF6 ions, visualizing the presence of these strong interactions. The fingerprint plots show two characteristic sharp spikes, which are specific contacts between complex cations and water molecules, as seen in Figure 4a,c, indicating the network structure constructed by H···O contacts. The H···F contacts (Figure 4d) are distributed throughout (22.6%), likely because they are scattered throughout the 1D structure of the complex cation rather than being specific.

3.3. IR Spectroscopy

Infrared spectra of 1–3 are shown in Figures S1, S2, and S3, respectively. The γ(CH) vibrations of the pyridyl group are observed in the range of 764 to 759 cm⁻¹ in all complexes, indicating the presence of the pyridyl groups as terminal ligands. Intense peaks attributed to the symmetric (ν_s) and antisymmetric (ν_as) stretching vibrations of the carboxylate (COO) groups were observed in all complexes. As revealed by X-ray crystal structure analysis, these complexes exhibit significant differences in the coordination mode of acetate ions, which is reflected in the IR spectra. These differences are typically detected by the peak separation between ν_as and ν_s (Δν = ν_as − ν_s) [36]. In Complex 1, two intense peaks at 1597 and 1417 cm⁻¹ are assigned to ν_as and ν_s, respectively. The Δν value (180 cm⁻¹) indicates the presence of syn-syn type carboxylato bridges. For Complex 2, two ν_as vibrations and a slightly broad ν_s band are observed at 1620, 1602, and 1422 cm⁻¹, respectively. According to the X-ray results, separations of 198 cm^–1 and 180 cm^–1 suggest their assignment to syn-skew and syn-syn bridging coordination, respectively. In the case of Complex 3, two sets of ν_as and ν_s vibrations are observed at 1603, 1421 cm⁻¹ and 1549, 1445 cm⁻¹, respectively. Each Δν is 182 and 104 cm^–1, corresponding to syn-anti bridging and chelating bidentate coordination. Additionally, characteristic peaks originating from the counter anions, BF₄⁻, BPh₄⁻, and PF₆⁻, were observed around 1049, 703, and 834 cm⁻¹, respectively.

3.4. Magnetic Measurements

Magnetic susceptibility measurements for 1 and 2 were performed using a SQUID magnetometer. Unfortunately, due to the low yield of 3, magnetic and electrochemical measurements could not be conducted for it. The temperature dependence of magnetic susceptibility was measured in the temperature range of 2 to 300 K. Figure 5 shows χ_MT vs. T plots for 1 (red circles) and 2 (blue triangles).

At 300 K, Complex 1 exhibits a χ_MT value of 7.99 cm³mol⁻¹K, which is smaller than the expected spin-only value of 8.75 cm³mol⁻¹K for two magnetically uncoupled high-spin Mn(II) (S = 5/2). As the temperature decreases from room temperature, the χ_MT value gradually decreases, reaching 0.35 cm³mol⁻¹K at 2 K. This behavior indicates the presence of antiferromagnetic interaction between the Mn(II) ions. The behavior was analyzed by the PHI program [24] using the theoretical equation for the magnetic susceptibility of an S = 5/2 dinuclear model based on the isotropic Heisenberg spin-exchange Hamiltonian (H = −2JS₁S₂), where S₁ = S₂ = 5/2. The obtained magnetic parameters were J = −3.42 cm⁻¹ and g = 2.01, indicating weak antiferromagnetic interactions between the Mn(II) ions. The absolute value of J is smaller compared to the similar dinuclear Mn(II) complexes with μ-hydroxo-bis-μ-carboxylate bridges, e.g., [Mn₂(tpa)₂(OH)(OAc)₂] (J = −18 cm⁻¹) [17]. This suggests that the super-exchange spin coupling between the metal ions is inhibited due to the strong electronegativity of the fluorido bridge instead of the hydroxo group.

The χ_MT value of 2 at 300 K is 15.7 cm³mol⁻¹K, slightly smaller than the spin-only value of 17.5 cm³mol⁻¹K for four magnetically uncoupled high-spin Mn(II) ions at room temperature. The χ_MT value gradually decreased with decreasing temperature, dropping sharply around 70 K and reaching 0.235 cm³mol⁻¹K at 2 K. This behavior suggests that the antiferromagnetic interaction in 2 is much weaker than in 1. The data were analyzed using the PHI program based on the spin Hamiltonian in Equation (1):

H = −2J₁(S_Mn1S_Mn2 + S_Mn1ⁱS_Mn2ⁱ) − 2J₂(S_Mn1S_Mn2ⁱ + S_Mn1ⁱS_Mn2) − 2J₃S_Mn1S_Mn1ⁱ − 2J₄S_Mn2S_Mn2ⁱ

(1)

The obtained magnetic parameters are as follows: J₁ = −1.31 cm⁻¹, J₂ = −0.07 cm⁻¹, J₃ = −0.19 cm⁻¹, J₄ = −0.02 cm⁻¹, g = 1.95, indicating significantly weak antiferromagnetic interactions between the Mn(II) ions. The absolute value of the exchange parameter J₁ for the dinuclear Mn(II) unit is close to those of reported tris-μ-carboxylato-bridged dinuclear Mn(II) complexes [10,11]. This difference in J value between complexes 1 and 2 is thought to be caused by the presence of the twisted μ-acetato bridges in 2 instead of the fluoride bridge in 1, which reduces the overlap of magnetic orbits.

3.5. Electrochemistry

The electrochemical properties of 1 and 2 were investigated using cyclic voltammetry (CV) in dichloromethane containing tetrabutylammonium perchlorate (0.1 M) as the supporting electrolyte. Figure 6a,b depict the cyclic voltammograms for 1 and 2, respectively, measured in the range of −0.4 to +1.6 V (vs. Ag/Ag⁺) using a glassy carbon electrode. Differential pulse voltammograms under the same conditions are illustrated in Figure S7.

The CV of 1 displays two irreversible oxidation waves at +0.932 V and approximately +1.3 V. The free tphn ligand shows an irreversible oxidation peak at +0.734 V, which is attributed to the oxidation of the tertiary amine moiety (Figures S8 and S9). However, no corresponding oxidation wave is observed in the CV of 1. Therefore, these two oxidation processes of 1 are assigned to the oxidation of manganese centers, corresponding to Mn₂(II,II)/Mn₂(II,III) and Mn₂(II,III)/Mn₂(III,III). The observed potentials are slightly higher than those of the μ-hydroxo-bis-μ-carboxylato dinuclear Mn(II) complex [Mn₂(μ-OH)(μ-OAc)₂(Me₃-tacn)₂]ClO₄ [17], which features a different bridging ligand. It is suggested that the metal-centered oxidation processes are shifted to more positive potentials due to the high electronegativity of the bridging fluoride anion.

In contrast, the CV of 2 exhibits two distinct quasi-reversible redox couples at E_1/2 = +0.529 V and +1.20 V, respectively, corresponding to the Mn₂(II,II)/Mn₂(II,III) and Mn₂(II,III)/Mn₂(III,III) processes. These potentials are consistent with those reported for tris-μ-carboxylato dinuclear Mn(II) complexes [10]. In addition to these two redox processes, a small peak appears at +0.902 V, which is believed to be due to the tpon ligand. Since the oxidation peak of the free ligand is at +0.693 V, it is believed that some of the coordination sites are partially dissociated. In acetonitrile, the two quasi-reversible redox processes become irreversible, and this small peak disappears, suggesting that partial dissociation of the tpon ligand is unlikely to occur in acetonitrile (Figure S10). These results suggest that the small peak is not simply due to contamination and that the stability of 2 is affected by the solvent.

3.6. Catalase Activity

The catalytic activity of these complexes in the disproportionation of H₂O₂ into H₂O and O₂ was investigated using a volumetric method. When aqueous hydrogen peroxide is added to an acetonitrile solution of 1 and 2, bubbles due to oxygen evolution are observed only in 2, and the colorless solution gradually turns light brown. After around 60 s, the solution becomes very dark, and the evolution of oxygen intensifies. The time courses of dioxygen evolution catalyzed by 2 are shown in Figure 7. The amount of O₂ evolved after 900 s under each condition was 9.3–6.09 mL, corresponding to a turnover number (TON) of 414–272, close to the TON reported for dinuclear Mn(III) complexes [37]. By the time oxygen evolution subsides, the solution color approaches colorlessness but does not fully return to it, with a slight presence of brown precipitate observed. This suggests a small amount of decomposition of 2 during the reaction. Therefore, while the initial reaction might be catalyzed by the complex cation, it is anticipated that from three minutes onward, the catalytic reaction is mediated by manganese dioxide or other manganese species formed from the decomposition of the complex. As indicated by the results of the CV measurements, the terminal ligands and acetate bridging used in this study are considered insufficient to maintain the dinuclear Mn structure adequately in such a reaction condition. To improve the reactivity, adjustments to the complex structure, such as incorporating other auxiliary ligands into the bridging moiety, are necessary.

4. Conclusions

New carboxylate-bridged dinuclear and tetranuclear Mn(II) complexes with ditopic ligands containing two tridentate coordination sites connected by organic linkers were synthesized and characterized. Single-crystal X-ray analysis revealed that all of these complexes have macrocyclic structures. Complex 1 is a dinuclear complex where both ends of the dinuclear Mn unit are surrounded by the ligand tphn. On the other hand, complexes 2 and 3 are tetranuclear complexes where two dinuclear Mn units are dimerized by the ligands tpon and tpxn, respectively. Investigations into the magnetic properties of 1 and 2 through magnetic measurements revealed the presence of weak antiferromagnetic interactions among carboxylate-bridged dinuclear Mn(II) units. Electrochemical measurements showed that complexes 1 and 2 have redox potentials capable of carrying out catalase-like hydrogen peroxide disproportionation reactions. Indeed, when reacted with hydrogen peroxide, although signs of decomposition were observed, oxygen evolution due to the disproportionation reaction was confirmed for 2. The macrocyclic structures of these complexes can be utilized to develop substrate-selective new oxidation catalysts.