# Synthesis, Crystal Structure, Thermal Behavior, and Magnetic Properties of a Tetranuclear Mn(II) Complex Bridged by End-to-End Dicyanamide

Mariama Thiam, Amar Diop, Ibrahima Elhadji Thiam, Ousmane Diouf, Pascal Retailleau, Sylvain Bertaina, Mohamed Gaye

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## To cite this version:

Mariama Thiam, Amar Diop, Ibrahima Elhadji Thiam, Ousmane Diouf, Pascal Retailleau, et al. Synthesis, Crystal Structure, Thermal Behavior, and Magnetic Properties of a Tetranuclear Mn(II) Complex Bridged by End-to-End Dicyanamide. Asian Journal of Chemical Sciences, 2025, 15 (6), pp.114-127. 10.9734/ajocs/2025/v15i6410. hal-05416151

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# Synthesis, Crystal Structure, Thermal Behavior, and Magnetic Properties of a Tetranuclear Mn(II) Complex Bridged by End-to-End Dicyanamide

Mariama Thiam<sup>a</sup>, Amar Diop<sup>a</sup>, Ibrahima Elhadji Thiam<sup>a*</sup>, Ousmane Diouf<sup>a</sup>, Pascal Retailleau<sup>b</sup>, Sylvain Bertaina<sup>c</sup> and Mohamed Gaye<sup>a</sup>

## Authors' contributions

This work was carried out in collaboration among all authors. All authors read and approved the final manuscript.

## Article Information

DOI: https://doi.org/10.9734/ajocs/2025/v15i6410

**Open Peer Review History:**
This journal follows the Advanced Open Peer Review policy. Identity of the Reviewers, Editor(s) and additional Reviewers, peer review comments, different versions of the manuscript, comments of the editors, etc are available here: https://pr.sdiarticle5.com/review-history/148993

## Original Research Article

Received: 03/10/2025

Published: 13/12/2025

## ABSTRACT

A new tetranuclear compound [Mn₄(o-vanilin)₅(N(CN)₂)₂(MeOH)₂] (1) has been synthesized under reflux and structurally characterized by elemental analysis, IR, conductance measurement, variable temperature magnetic measurement and single crystal. The complex (1) crystallizes in the triclinic

<footnote><sup>a</sup> Department of Chemistry, University Cheikh Anta Diop, Dakar, 10700, Senegal.</footnote>

<footnote><sup>b</sup> Institut de Chimie des Substances Naturelles, CNRS UPR 2301, Université Paris-Sud, Université Paris-Saclay, 1, av. de la Terrasse, 91198 Gif-sur-Yvette, France.</footnote>

<footnote><sup>c</sup> CNRS, Aix Marseille Université, Université de Toulon, IM2PNInstitut des Matériaux, de Microélectronique et des Nanosciences de Provence, Marseille, France.</footnote>

<footnote>*Corresponding author. E-mail: i6thiam@yahoo.fr;</footnote>

<footnote>**Cite as:** Mariama Thiam, Amar Diop, Ibrahima Elhadji Thiam, Ousmane Diouf, Pascal Retailleau, Sylvain Bertaina, and Mohamed Gaye. 2025. "Synthesis, Crystal Structure, Thermal Behavior, and Magnetic Properties of a Tetranuclear Mn(II) Complex Bridged by End-to-End Dicyanamide". Asian Journal of Chemical Sciences 15 (6):114-127. https://doi.org/10.9734/ajocs/2025/v15i6410.</footnote>
space group P-1 with unit cell dimensions a = 9.1014 (4) Å, b = 11.1169 (4) Å, c = 14.8923 (4) Å, α = 93.698 (3)°, β = 96.960 (3)°, γ = 107.165 (3)°, V = 1421.07 (9) Å³, Z = 1, R₁ = 0.0345 and wR₂ = 0.0798. The compound consist of two units [Mn₂(o-Vanillin)₃(MeOH)] with double µ₁.₅-η¹'-η¹-dicyanamide (µ₁.₅-η¹'-η¹-DCA) bridges between neighboring manganese(II) atoms. Two Mn(II) are situated in an O₄N inner, while the two others Mn(II) are encapsulated by an O₅N inner. The two pentacoordinated Mn(II) ions are situated in square pyramidal environment, while the hexacoordinated Mn(II) ions environment is best described as octahedral geometry. Thermogravimetric analyze of the complex were carried out at 20–800 °C and showed that the complex were decomposed into three steps. Variable-temperature (5-300 K) magnetic susceptibility data reveal the existence of weak antiferromagnetic interaction in complex 1. The magnetic behavior is explained by considering the exchange coupled manganese-radical system, which is subjected to the Mn¹⁻O-Mn¹¹⁻ intradimer interactions. The least-squares fitting of the data results *J* = -0.8. Negligible interdimer exchange via Mn-DCA-Mn (Mn···Mn of 8.4530(5) Å) was observed.

Keywords: Crystal; manganese; square pyramidal; octahedral; magnetism; TGA.

# 1. INTRODUCTION

The design of polynuclear transition metal complexes has been the subject of considerable development because they have fascinating structures and functional applications as optical, electronic, catalytic, and magnetic material (Hasan et al., 2024; Kalita et al., 2015; Malinina et al., 2024; Nesterov & Nesterova, 2018; Noritake et al., 2013. Particular interest has been given to manganese complexes, widely used in the field of materials, notably for the synthesis of molecular magnets (Anwar et al., 2021; Karmakar et al., 2024; Reis Conceição et al., 2020; Wang & Qin, 2022). These single molecule magnets (SMMs) having the ability to maintain spin orientation at certain temperatures called blocking temperature, have the property of storing information at the nanometer scale. Several types of SMMs are developed with variable numbers of metal atoms (Bhaumik, 2024; Miyasaka et al., 2004; Tasiopoulos et al., 2004; Thompson & Dawe, 2015). The synthesis of these SMMs follows a strategy that consists of designing organic ligands with functional groups possessing particular donor sites and an adapted topology. The addition of co-ligands is an additional asset to increase the number of metal sites in the molecule (Han et al., 2022; Zhang et al., 2014). 2-hydroxy-3-methoxybenzaldehyde, which has three oxygen donor atoms, has been widely used as a ligand for the synthesis of polynuclear complexes due to the ability of the methoxy group to exhibit a variety of bonding geometries, such as chelating, mono- or bidentate bonding (Xu et al., 2017; Zhang et al., 2014). We are currently interested in the preparation of a series of o-vanillin-chelated model compounds, studying their molecular

structures and magnetic properties. The present study reports a novel tetranuclear manganese complex [Mn₄(o-vanillin)₆(N(CN)₂)₂(MeOH)₂] obtained using the o-vanillin ligand and sodium dicyanamide as co-ligand. The crystal structure and physicochemical properties are discussed.

# 2. EXPERIMENTAL MATERIAL

## 2.1 Materials and Instruments

o-Vanillin, manganese acetate tetrahydrate and sodium dicyanamide were purchased from Sigma-Aldrich and used as received without any purification. All solvents used were of reagent grade. The infrared spectrum of the complex was recorded on a Perkin Elmer Spectrum Two FTIR spectrometer in the region 4000-400 cm⁻¹. The molar conductance of 1×10⁻³ M in DMF solutions of the metal complex was measured at 25 °C using a WTW LF-330 conductivity meter with a WTW conductivity cell. The magnetic susceptibility of the powdered sample was measured at room temperature using a Johnson Matthey scientific magnetic susceptibility balance (Calibrant: Hg[Co(SCN)₄]).

## 2.2 Synthesis of the Complex [Mn₄(o-vanillin)₆(N(CN)₂)₂(MeOH)₂]

o-vanillin (0.402 g, 2.64 mmol) and Mn(OAc)₂·4H₂O (0.324 g, 1.32 mmol) were added in 100 mL flask containing 20 mL of methanol. The mixture was refluxed for one hour. Then, a 10 mL of methanolic solution of sodium dicyanamide (0.118 g, 1.32 mmol) was added. The mixture was brought to reflux for one hour. On cooling the resulting was filtered and the filtrate was left to evaporate slowly. A few days
<figure><img src="image_3.png" /><figcaption>Scheme 1. Synthetic scheme for the complex $[Mn_4(o-vanilin)_6(N(CN)_2)_2(MeOH)_2]$ (1)</figcaption></figure>

later, orange crystals suitable for X-ray diffraction were collected. Yield: 73%. Anal. Calc for $C_{54}H_{50}N_6O_{20}Mn_4$ (%): C, 49.03; H, 3.81; N, 6.35. Found: C, 49.00; H, 3.78; N, 6.32. IR (ν, cm⁻¹): 3242; 2300; 2247; 2188; 1623; 1202. Conductance Λ (S.cm².mol⁻¹): 27. μeff = 5.57 μB.

## 2.3 Determination of Crystal Structure

Crystals of complex 1 suitable for X-ray diffraction were recovered by slow evaporation of a MeOH solution of the complex. Details of the X-ray crystal structure resolution and refinement

are given in Table 1. Diffraction data were collected using an XtaLAB AFC12 (RINC) Kappa diffractometer with graphite monochromatized Mo Kα radiation (λ = 0.71073 Å). All data were corrected for Lorentz and polarization effects. No absorption correction was applied. The structure was solved with the structure solution program olex2.solve using charge flipping and refined with the SHELXL refinement package using least-squares minimization (Dolomanov et al., 2009). Molecular graphics were generated using ORTEP (Farrugia, 2012).

<table><caption>Table 1. Crystalline and structure refinement data of $[Mn_4(o-vanilin)_6(N(CN)_2)_2(MeOH)_2]$ (1)</caption><thead><tr><th colspan="2">Crystal data</th></tr></thead><tbody><tr><td>Chemical formula</td><td>C<sub>54</sub>H<sub>50</sub>Mn<sub>4</sub>N<sub>6</sub>O<sub>20</sub></td></tr><tr><td>Mr (g/mol)</td><td>1322.78</td></tr><tr><td>Temperature (K)</td><td>293(2)</td></tr><tr><td>Crystal system, space group</td><td>Triclinic, P-1</td></tr><tr><td>a (Å)</td><td>9.1014(4)</td></tr><tr><td>b (Å)</td><td>11.1169(4)</td></tr><tr><td>c (Å)</td><td>14.8923(4)</td></tr><tr><td>α (°)</td><td>93.698(3)</td></tr><tr><td>β (°)</td><td>96.960(3)</td></tr><tr><td>γ (°)</td><td>107.165(3)</td></tr></tbody></table>
<table><thead><tr><th colspan="2">Crystal data</th></tr></thead><tbody><tr><td>Volume (Å<sup>3</sup>)</td><td>1421.07(9)</td></tr><tr><td>Z</td><td>1</td></tr><tr><td>D<sub>calc</sub> (gcm<sup>-3</sup>)</td><td>1.546</td></tr><tr><td>μ (mm<sup>-1</sup>)</td><td>0.95</td></tr><tr><td>F(000)</td><td>676.0</td></tr><tr><td>Crystal size (mm)</td><td>0.10 × 0.07 × 0.06</td></tr><tr><td>λ (MoKα) (Å)</td><td>0.71073</td></tr><tr><td>θ range (°)</td><td>2.37-27.10</td></tr><tr><td>Indices h, k, l</td><td>-11 ≤ h ≤ 11, -14 ≤ k ≤ 14, -19 ≤ l ≤ 19</td></tr><tr><td>No. of measured reflections</td><td>37629</td></tr><tr><td>No. of independent reflections</td><td>6250</td></tr><tr><td>No. of observed [I > 2σ(I)] reflections</td><td>4703</td></tr><tr><td>R<sub>int</sub></td><td>0.0544</td></tr><tr><td>R[F<sup>2</sup> > 2σ(F<sup>2</sup>)]</td><td>0.035</td></tr><tr><td>wR(F<sup>2</sup>)</td><td>0.092</td></tr><tr><td>Goodness-of-fitt (Gof) on F<sup>2</sup></td><td>1.08</td></tr><tr><td>Data/restraints/parameters</td><td>6250/61/604</td></tr><tr><td>Final Indices [I > 2σ(I)]</td><td>R<sub>1</sub> = 0.0345, wR<sub>2</sub> = 0.0798</td></tr><tr><td>Final R Indices (for all data)</td><td>R<sub>1</sub> = 0.0544, wR<sub>2</sub> = 0.0918</td></tr><tr><td>Δpmax, Δpmin (eÅ<sup>-3</sup>)</td><td>0.45, -0.42</td></tr></tbody></table>

# 3. RESULTS AND DISCUSSION

## 3.1 General Study

The complex [Mn₄(o-vanillin)₆(N(CN)₂)₂(MeOH)₂] was obtained by mixing o-vanillin ligand, Mn(OAc)₂·4H₂O and sodium dicyanamide in a 2:1:1 ratio (Scheme 1). The vC=O band pointed at 1662 cm⁻¹ in the IR spectrum of the free o-vanillin is shifted to 1623 cm⁻¹ upon coordination of the o-vanillin to the Mn atom. The band at 3242 cm⁻¹ due to the vC-H of the o-vanillin is not present in the spectrum of the complex. This fact suggests that the o-vanillin acts in its deprotonated form (Gadiaga et al., 2023; Orton et al., 2023). The band due to the phenolic vC=O vibration which appears at 1253 cm⁻¹ for the free o-vanillin, appears around 1240 cm⁻¹ in the IR spectrum of the complex, suggesting the participation of the phenolic oxygen atom in the coordination. The methoxy oxygen atoms remain uncoordinated as evidenced by the non-shift of its vC=O band pointed in the two spectra around 1202 cm⁻¹. The IR spectrum of the complex exhibits also three strong bands pointed at 2300, 2247 and 2188 cm⁻¹. These bands are attributed to the dicyanamide groups which are coordinated in μ₁,₅-η¹:η¹ bridged mode (Haba et al., 2020; Zhao et al., 2016). The conductimetric measurements of the millimolar solution of the complex were recorded in millimolar DMF solution. The molar conductivity value of the fresh solution of 27 S·cm²·mol⁻¹ corresponds to a neutral electrolyte (Geary, 1971). After fifteen days, this value

remains quite-constant, indicating that the complex is stable in DMF solution.

## 3.2 Crystallographic Study of the Complex

The complex crystallizes in the triclinic system with a P-1 space group. The ORTEP diagram of the structure of the complex is illustrated in Fig. 1a, while selected bond lengths and angles of the coordination environment of the metal centers are listed in Table 2. As shown in Fig. 1 the complex is a tetranuclear manganese (II) complex. The asymmetric unit consists of four Mn²⁺ ions, six monodeprotonated o-vanillin molecules, two coordinated μ₁,₅-η¹:η¹ N(CN)₂ groups, and two coordinated methanol molecules. Thus, the complex is formulated as [Mn₄(o-vanillin)₆(N(CN)₂)₂(MeOH)₂] in which two units of [Mn₂(o-vanillin)₃(N(CN)₂)(MeOH)] are linked by two dicyanamide groups acting μ₁,₅-η¹:η¹ mode. In each dinuclear unit of [Mn₂(o-vanillin)₃(N(CN)₂)(MeOH)] the two metal centers are bridged by two phenoxo oxygen atoms from two o-vanillin molecules. The third o-vanillin molecule is exclusively coordinated to one of Mn(II) centers through its carbonyl oxygen atom and its phenoxo oxygen atom, yielding a six coordinated Mn(II) ion (Mn2). The second Mn(II) center is coordinated to methanol oxygen atom yielding a pentacoordinated Mn(II) ion (Mn1). The methoxy oxygen atoms of the o-vanillin remain uncoordinated. The Mn1 ion is coordinated by three phenolate oxygen atoms from three o-vanillin molecules, one carbonyl
<figure><img src="image_4.png" /><figcaption>Fig. 1. Infrared spectrum of the ortho-vanillin molecule</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2. Infrared spectrum of the complex</figcaption></figure>
oxygen atom, and one dicyanamide nitrogen
atom. The Mn2 ion is coordinated by two
phenolate oxygen atoms, two carbonyl oxygen
atoms from two ligand molecules, one methanol
oxygen atom, and one dicyanamide nitrogen
atom. The two metal centers have two different
environments Fig. 1b. The environment around
the pentacoordinated Mn1 ion can be determined
by the Addison parameter τ = (α - β)/60, with α
and β being the two largest angles around Mn1
(Addison et al., 1984; Ndoye et al., 2018). The τ
value of 0.305 indicates a highly distorted square
pyramidal environment (Diop et al., 2019). The
basal plane is occupied by O1, O2, O5 and N1,
while the apical position is occupied by O8. The
cissoid angle values are in the range [81.40(6)°-
105.14(8)°] while the transoid angle values are
O5—Mn1—O2 = 135.16(7)° and O1—Mn1—N1
= 164.69(8)°. Furthermore, the sum of the angles
subtended by the atoms occupying the basal
plane, which is equal to 362.61°, deviates from
the ideal value of 360° and the transoid angles
are far from the ideal value of 180°. The values
of the bond angles between the atom occupying
the apical position (O8) of the pyramid and the
atoms in the basal plane are in the range
[76.08(6)°-146.38(7)°], deviating strongly from
the ideal value of 90°. The geometry around the
hexacoordinated Mn2 ion is best described as a
distorted octahedron. The basal plane is
occupied by O5, O6, O8 and O9. The sum of the
cissoid angles subtended by these atoms is
close to the ideal value of 360° with a value of
359.55°. The apical positions occupied by O4
and N3 subtend an angle of 170.9(8)° with Mn2.
The structure displays that the atoms O5 and O8
act as bridging atoms between two Mn(II) ions
[Mn1···Mn2 distance of 3.3402(5) Å], which
creates a quite coplanar four-membered
centrosymmetric ring, with a maximum deviation
of Mn2 of 0.067 Å out of the mean plane. The
Mn—O—Mn angle values of 102.23(6)° [Mn2—
O8—Mn1] and 102.26(6)° [Mn2—O5—Mn1] are
quite-equal. The dihedral angle between the
mean planes defined by O5/Mn1/O8 and
O5/Mn2/O8 is equal to 8.94(2)°. The values of
the bond angles between the atoms occupying
the vertices O4 and N3 and the atoms in the
basal plane are respectively in the intervals
[85.53(7)°-89.44(7)°] and [86.88(8)°-100.65(8)°].
The bond lengths of the bridged phenoxo oxygen
atoms with the metal centers are 2.1790(15) Å
[Mn1—O5], 2.1106(15) Å [Mn2—O5], 2.1693(15)
Å [Mn1—O8] and 2.1214(15) Å [Mn2—O8]. The
values are comparable to the value reported for a
similar complexes in which the manganese
atoms are bridged by two phenoxo oxygen atoms

(Ding et al., 2013; Sasaki et al., 1998). The bond
lengths of the carbonyl oxygen atoms with the
manganese ions are 2.1798(17) Å [Mn1—O2],
2.1598(18) Å [Mn2—O6] and 2.1517(18) Å
[Mn2—O9] and agree with the value reported in
similar complex (Costes et al., 2018). The bond
length of the non-bridged phenoxo oxygen atom
with Mn of 2.1362(14) Å [Mn1—O1] is longer
than the value (1.907(2) Å) reported for the
complex [Mn(L)] ((H₂L = N,N'-bis(3-
methoxysalicylidene)propylene-1,3-diamine)
(Banerjee et al., 2020). The bond parameters of
the bridging region are slightly different: 2.177(2)
Å [Mn1—N1] and 2.188(2) Å [Mn2—N3],
166.2(2)° [Mn1—N1—C9], and 171.3(2)° [Mn2-
N3—C10]. The dicyanamide ligand is slightly bent
with a C9—N2—C10 angle of 121.4(3)° and two
N—C—N straight linear units with angles values
of 173.5(3)° [N1—C9—N2] and 173.0(3)° [N3—
C10—N2]. These parameters values are
comparable to the values reported for the
complex cis-[Mn(bpy)(N(CN)₂)₂]ₘ (bpy is
bipyridine) (Escuer et al., 2000). In the structure,
only the oxygen atoms from the bidentate o-
vanillin coordinated to Mn1 are involved in the
hydrogen bond. In fact, strong intramolecular
hydrogen bonds between the OH (O4—H4) of
the methanol group acting as donor and the
phenoxo oxygen atoms (O1) and the methoxy
(O3) acting as acceptors are observed
[O4(methanol)—H4···O1(phenoxo) and O4(methanol)—H4
···O3(methoxy). Additionally, weak hydrogen bond
involving the methoxy O—CH₃ as donor and the
carbonyl oxygen atom O2 as acceptors (C18—
H18C···O2(carbonyl)) is revealed. The molecules of
the complexes do not exhibit intermolecular
hydrogen bonds (Table 3, Fig. 2).

3.3 Thermogravimetric Study

Thermal decompositions of complex 1 has been
investigated using TGA (Fig. 3). The complex
was heated in temperatures ranging from 30 °C
to 800 °C with a flow rate of 10.00 °C/min under
N₂ atmosphere. The complex is stable up to 50
°C and then begins to decompose continuously
(Fig. 3). The mass loss of 4.43 % between 50°C
and 184°C corresponds to the loss of the
methanol molecule (4.84 % of the molar mass).
Between 185°C and 357°C, a mass loss of 6.92
% is noted which corresponds with the loss of
dicyanamide molecule (8.32 % of the molar
mass). In the range of 358°C - 400°C, the mass
loss of 63.75% indicates that the organic
molecule has undergone a complete degradation
(65.38 % of the molar mass). Beyond this
temperature, 24.90 % of the mass remains,
which probably corresponds to the formation of
manganese oxide. This indicates that this
complex may be used as single source precursor
for the formation of metal oxide.

<figure><img src="image_6.png" /><figcaption>Fig. 3. ORTEP representation of the structure of [Mn<sub>2</sub>(o-vanillin)<sub>3</sub>(N(CN)<sub>2</sub>)(MeOH)<sub>2</sub>·displacement ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. ORTEP representation of the three-dimensional supramolecular network of the complex</figcaption></figure>
<table><caption>Table 2. Selected geometric parameters (Å, °) for the complex</caption><tr><td>Mn1—O8</td><td>2.1693(15)</td><td>Mn2—O8</td><td>2.1214(15)</td></tr><tr><td>Mn1—O1</td><td>2.1362(14)</td><td>Mn2—O5</td><td>2.1106(15)</td></tr><tr><td>Mn1—O5</td><td>2.1790(15)</td><td>Mn2—O4</td><td>2.214(2)</td></tr><tr><td>Mn1—O2</td><td>2.1798(17)</td><td>Mn2—O9</td><td>2.1517(18)</td></tr><tr><td>Mn1—N1</td><td>2.177(2)</td><td>Mn2—O6</td><td>2.1518(18)</td></tr><tr><td>Mn1···Mn2</td><td>3.3402(5)</td><td>Mn1···Mn2</td><td>8.4530(5)</td></tr><tr><td>O8—Mn1—O5</td><td>76.08(6)</td><td>Mn2—O5—Mn1</td><td>102.26(6)</td></tr><tr><td>O8—Mn1—O2</td><td>146.38(7)</td><td>C9—N1—Mn1</td><td>166.2(2)</td></tr><tr><td>O1—Mn1—N1</td><td>164.69(8)</td><td>N1—C9—N2</td><td>173.5(3)</td></tr><tr><td>O5—Mn1—O2</td><td>135.16(7)</td><td>N3—C10—N2</td><td>173.0(3)</td></tr><tr><td>O8—Mn2—O6</td><td>162.80(7)</td><td>C10—N3—Mn2<sup>i</sup></td><td>171.3(3)</td></tr><tr><td>O5—Mn2—O8</td><td>78.57(6)</td><td>C10—N2—C9</td><td>121.4(2)</td></tr><tr><td>Mn2—O8—Mn1</td><td>102.23(6)</td><td></td><td></td></tr></table>

Symmetry code: (i) -x+2, -y+2, -z+1.

<table><caption>Table 3. Hydrogen bonds (Å, °) of the complex</caption><thead><tr><th>D—H···A</th><th>D—H</th><th>H···A</th><th>D···A</th><th>D—H···A</th></tr></thead><tbody><tr><td>C18—H18C···O2</td><td>0.96</td><td>2.64</td><td>3.190(4)</td><td>117.2</td></tr><tr><td>O4—H4···O1</td><td>0.76(3)</td><td>2.12(3)</td><td>2.827(3)</td><td>155(3)</td></tr><tr><td>O4—H4···O3</td><td>0.76(3)</td><td>2.41(3)</td><td>2.986(3)</td><td>134(3)</td></tr></tbody></table>

<figure><img src="image_8.png" /><figcaption>Fig. 5. TGA curve of complex 1</figcaption></figure>

### 3.4 Magnetic Property of 1

The temperature dependence of the magnetic susceptibility (χ) of the complex was measured in the temperature range 300 – 1.8 K under a magnetic field of 0.1 T. The curve (χ) vs 1/χ is presented in Fig. 4a and shows a classical

paramagnetic Curie C/T where C is the Curie constant. It is more suitable to show χ vs T in order to remove the 1/T dependence and focus on the temperature dependence of C (Fig. 4b). At high temperature χ is temperature independent and we can estimate the value of the spin as well as the g factor using the relation
$\frac{Na^2\mu\beta^2nS(S+1)}{3k_B}$ where Na is Avogadro Number, $\mu_B$ the Bohr magneton, $k_B$ the Boltzmann constant (both in cgs unit) and n the number of ions in the unit cell. At room temperature we find $\chi_M T = 17.52 \text{ cm}^3\text{K.mol}^{-1}$ which correspond to fours isolated Mn(II) ions (S=5/2) (Abedi et al., 2016; L. Zhang, Wang, & Xu, 2014) with g=2.01. On cooling, the $\chi_M T$ values decrease slowly reaching a value of 16 cm$^3$.K.mol$^{-1}$ at 32 K. Below this temperature, the $\chi_M T$ values decreases more rapidly reaching a value of 8 cm$^3$.K.mol$^{-1}$ at 2 K. This temperature variation of $\chi_M T$ suggest the presence of a weak antiferromagnetic interaction between the Mn$^{II}$ centers (Gómez-Saiz et al., 2008; Wałęsa-Chorab et al., 2013).

In order to estimate the intradimer and interdimer magnetic exchange interactions between the metal ions of tetrametallic complex, the magnetic susceptibility data for 1 were fit using Easyspin-Curry program (Stoll & Schweiger, 2006), with the following spin Hamiltonian

$$ \mathcal{H} = \sum_{i,j=1}^{4} J_{ij}S_{Mni}S_{Mnj} + g\mu_bH\sum_{i=1}^{4}S_{Mni} $$

The best fit shown a small antiferromagnetic intradimer coupling constant J=0.8 cm$^{-1}$ via o-vanillin bridge and no significant interdimer coupling constant via dicyanamide bridge. Ultimately, 1 can be magnetically approximated by

$$ \mathcal{H} = J(S_{Mn1}S_{Mn2} + S_{Mn3}S_{Mn4}) $$

The negligible interdimer coupling can be explained by the large distance between Mn···Mn of 8.4530(5) Å between Mn(II) bridged by dicyanamide ligand ions (van Albada et al., 2006). The small and antiferromagnetic interactions between the manganese centers in the same unit [Mn$_2$(o-Vanillin)$_3$(N(CN)$_2$)(MeOH)] [Mn1···Mn2 distance of 3.3402(5) Å] can be explained by the angle Mn–O–Mn of the super exchange path-way. That has been shown that

<figure><img src="image_9.png" /><figcaption>Fig. 6. Top (a). Plots of $\chi_M$ vs T for the tetranuclear manganese [Mn$_4$(o-Vanillin)$_6$(N(CN)$_2$)$_2$(MeOH)$_2$]. Bottom (b) Plot of the $\chi_M T$ product vs T for the tetranuclear manganese [Mn$_2$(o-vanillin)$_3$(N(CN)$_2$)(MeOH)$_2$].</figcaption></figure>
the value of the bridging angle M–O–M is an important parameter in the magneto-structural correlation. If the M–O–M angle value is less than 99°, ferromagnetic interaction is observed, while antiferromagnetic interaction is observed when the M–O–M angle value is greater than 99° (van Albada et al., 2006; Escuer et al., 1999; Moustapha-Sow et al., 2013). In the case of complex **1** the angle [Mn1–O5–M2 and Mn1–O8–Mn2 are both equal to 102.24 (7)°] is coherent with an AFM but small coupling.

## 4. CONCLUSION

The present study is carried out by synthesizing and characterizing a tetranuclear manganese complex [(Mn₂(o-vanillin)₃(N(CN)₂)(MeOH))₂] (1). The structure of complex 1 is solved by XRD. The complex is composed of two units of [Mn₂(o-vanillin)₃(N(CN)₂)(MeOH)] which are bridged by two dicyanamide groups acting in µ₁,₅-η¹,⁻·η¹ mode. In the unit [Mn₂(o-vanillin)₃(N(CN)₂)(MeOH)], the two metal centers Mn1 and Mn2 are linked by two bridged phenoxy oxygen atoms. The pentacoordinated Mn1 ion is in a highly distorted square pyramidal environment, while the hexacoordinated Mn2 ion is in an octahedral environment. TGA allowed the study of the behavior of the complex when increasing the temperature. A three-step decomposition of complex 1 is observed, and the residue correspond to the formation of manganese oxide above 400 °C. The variable temperature magnetic measurement shows that two Mn(II) ions bridged by two phenoxy oxygen atoms in the same unit display antiferromagnetic interaction with a coupling constant *J* of 0.8 cm⁻¹. No significative interaction was observed between Mn(II) ions situated in two different units bridged by dicyanamide group.

## SUPPORTING INFORMATION

CCDC-2425332 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;

## DISCLAIMER (ARTIFICIAL INTELLIGENCE)

Author(s) hereby declare that NO generative AI technologies such as Large Language Models (ChatGPT, COPILOT, etc.) and text-to-image generators have been used during the writing or editing of this manuscript.

## COMPETING INTERESTS

Authors have declared that no competing interests exist.

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