# Mono- to Tetra-nuclear Iron Dichloride Complexes Bearing N -(Fluoro)-aryl-Substituted Iminopyridine Ligands: Synthesis, Structures, Magnetic Properties, and Polymerization of Isoprene

Obaid Hashmi, Raphael Caulier, Frederic Capet, Olivier Mentré, Claire Minaud, Jean-François Meunier, Iker del Rosal, Chiara Dinoi, Marc Visseaux, Yohan Champouret

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Obaid Hashmi, Raphael Caulier, Frederic Capet, Olivier Mentré, Claire Minaud, et al. Mono- to Tetra-nuclear Iron Dichloride Complexes Bearing N -(Fluoro)-aryl-Substituted Iminopyridine Ligands: Synthesis, Structures, Magnetic Properties, and Polymerization of Isoprene. Inorganic Chemistry, 2025, 64 (29), pp.14836-14852. 10.1021/acs.inorgchem.5c01030 . hal-05163264

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Mono- to tetra-nuclear iron dichloride complexes bearing *N*-*(fluoro)-aryl-substituted iminopyridine ligands: Synthesis, structures, magnetic properties and polymerization of isoprene

Obaid H. Hashmi<sup>a§</sup>, Raphael Caulier<sup>a§</sup>, Frederic Capet<sup>a</sup>, Olivier Mentré<sup>a</sup>, Claire Minaud<sup>a</sup>, Jean-François Meunier<sup>b</sup>, Iker del Rosal<sup>c</sup>, Chiara Dinoi<sup>c</sup>, Marc Visseaux<sup>a*</sup> and Yohan Champouret<sup>a*</sup>

* Email: marc.visseaux@univ-lille.fr; yohan.champouret@univ-lille.fr

<sup>a</sup> Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181, UCCS, Unité de Catalyse et Chimie du Solide, F-59000, Lille, France

<sup>b</sup> Laboratoire de Chimie de Coordination UPR 8241, CNRS & Université de Toulouse, 31077 Toulouse, France

<sup>c</sup> LPCNO, UMR 5215, Univ. Toulouse-CNRS, INSA, UPS, 135 Avenue de Rangueil, 31077 Toulouse, France

<sup>§</sup> O.H.H and R.C contributed equally to this work

**ABSTRACT:** The reaction of $FeCl_2$ with iminopyridine ligand containing fluorinated *N*-aryl groups {2-[(Ar)N=C(R)]C₅H₄N, where Ar = 3,5-(CF₃)₂C₆H₃ and R = H (L1) or CH₃ (L3); Ar = C₆F₅ and R = H (L2) or CH₃ (L4); and iminoquinoline 2-[(3,5-(CF₃)₂(C₆H₃)N=CH)C₉H₆N (L5)} resulted in distinct iron coordination complexes whose structures were determined by single-crystal X-ray diffraction. Ligands L1 and L2 yielded mononuclear bis-ligated iron dichlorides (L1 or L2)₂FeCl₂ (complexes 1 and 2), while L3 and L5 formed dinuclear complexes (L3 or L5)₂Fe(µ-Cl₂)FeCl₂ (complexes 3 and 5). Ligand L4 produced an unprecedented tetranuclear dimer [(L4)₂FeCl(µ-Cl₂)FeCl₂]₂ (complex 4). Mössbauer spectroscopy revealed single quadrupole doublets for mononuclear complex 2, and two sets of doublets for dinuclear complexes 3 and 5 and tetranuclear complex 4, confirming Fe(II) in high-spin states. Magnetic susceptibility and Curie–Weiss analyses elucidated magnetic interactions, showing reduced effective magnetic moments correlating with increasing Fe-(µ-Cl)-Fe bridges. DFT calculations and NBO analyses supported these findings, highlighting lower charge density on bridging compared to terminal chlorides. Activated by $Al^iBu_3/[CPh_3][B(C_6F_5)_4]$, complexes 1 and 2 showed lower activities (TOF = 1 000 – 1 200 h⁻¹) than complexes 3 and 5 (TOF = 90 000 h⁻¹) for isoprene polymerization, producing polymers with enriched *cis*-1,4-*alt*-3,4 microstructures.
INTRODUCTION

Bidentate ligands containing neutral *N*-donor atoms, such as iminopyridine (ImPy) along with its derivatives, have been extensively employed as versatile platforms to accommodate a wide range of metals.¹ Their simple synthesis enables the development of a variety of ligands with diverse steric and electronic characteristics, which are also capable of exhibiting, in some cases, non-innocent redox properties upon coordination.² Meanwhile, the use of iron metal in catalysis is particularly attractive due to its abundance, low cost and diverse reactivity, making it a convincing and sustainable choice for a broad panel of applications.³ In this context, iron-based complexes bearing ImPy ligands have proved increasingly valuable as homogeneous catalysts, particularly in processes such as transfer hydrogenation,⁴ tandem C-H activation/arylation,⁵ atom transfer radical polymerization,⁶ 1,4-functionalization reaction of 1,3-dienes,⁷ cycloaddition⁸ and ethylene oligomerization.⁹ Furthermore, we and others have demonstrated the ability of iron complexes supported by ImPy ligands, when appropriately activated, to efficiently polymerize 1,3-diene monomers¹⁰ to produce materials of significant importance to the elastomer industry.¹¹

Within this framework, a search in the Cambridge Structural Database for molecular structures of $FeCl_2$ supported by an ImPy ligand, limited here to a hydrogen or methyl group on the imino-carbon (aldiminopyridine and ketiminopyridine, respectively) and without a substituent on the pyridyl ring, revealed various coordination modes. For example, the simplest molecular structure (ImPy)$FeCl_2$, in which the iron center is in a tetrahedral geometry and coordinated to two chlorides together with one ImPy ligand, has been reported with an aldiminopyridine bearing bulky *N*-alkyl¹⁰¹ or *N*-silyl group¹² and ketiminopyridine with monoalkyl-*N*-aryl substituent¹⁰c (Chart 1 - A). In contrast, for ImPy ligands with sterically hindered *N*-silyl or di-substituted *N*-aryl groups (such as the one with bulky substituents in 2- and 6-positions on the *N*-aryl group), the formation of a dimeric [(ImPy)$FeCl(\mu-Cl)]_2$ species featuring an iron center in a pentacoordinate arrangement is generally observed (Chart 1 - B).¹⁰,¹²,¹³ On the other hand, if the substituent carried by the imine nitrogen atom is sufficiently restricted, a homoleptic complex (ImPy)$_2FeCl_2$ comprising two ligands and two chlorides is generally found, with the iron center adopting a distorted octahedral geometry (Chart 1 - C).²f,⁴,¹⁰c,o,¹⁴ More recently, the group of Wang has described the uncommon coordination mode of iron dichloride with a series of ImPy ligands (Chart 1 - D), which led to the formation of unsymmetrical dinuclear iron complexes composed of high-spin (HS) and low-spin (LS) iron centers of the form (ImPy)$_2Fe(\mu-Cl)$_2FeCl$_2$.¹⁰n To our knowledge, only a few molecular structures resolved by X-ray diffraction analysis with this atypical iron dichloride coordination have been described in the literature, with the ligands di(phenylphosphino)pyridine,¹⁵ pyridine-oxazoline¹⁶ and 1-chloro-3(pyridin-2-yl)imidazo[1,5-a]pyridine.¹⁷
<figure><img src="image_2.png" /><figcaption>Chart 1. Isolated molecular structures of FeCl<sub>2</sub> coordinated to ImPy ligands resolved by X-ray diffraction measurements</figcaption></figure>

In this context, we recently introduced fluorinated *N*-aryl groups on the imine part of the ImPy ligand, from which the resulting iron complexes showed extremely high activity (TOF > 1 x 10<sup>6</sup> h<sup>-1</sup>) in isoprene polymerization when supported by the ketimino counterpart.<sup>10m</sup> We have now succeeded in determining the molecular structure of these iron complexes, which turn out to exhibit a diverse range of coordination modes at the iron center. This variation, influenced by the substituent on the imino carbon and the *N*-aryl group, results in structures such as those illustrated in Chart 1 – C and D, but also to an unprecedented coordination arrangement of the form [(ImPy)<sub>2</sub>Fe(μ-Cl)<sub>2</sub>Fe(μ-Cl)Cl]<sub>2</sub>, as shown in Chart 1 – E. Here, we present the coordination chemistry of these complexes, complemented by Mössbauer
spectral analysis, density functional theory calculation and magnetic measurements, along with further
evaluations of their catalytic performance in isoprene polymerization.

RESULTS AND DISCUSSION

**Synthesis of iminopyridine/iminoquinoline ligands L1–L5.** The aldiminopyridine ligands L1 and L2 have been briefly mentioned in the literature,¹⁸⁻²⁰ without any synthetic description/characterization (Scheme 1). In contrast, the ketiminopyridine compound L3 has recently been used as a starting material for transfer hydrogenations with ammonia borane²¹ as well as a supporting ligand, together with L4, for aluminum pre-catalysts for butadiene polymerization.²² In parallel, our group has described the synthesis of L3 and L4 to accommodate iron dichloride, after which the corresponding ketimino complexes were subsequently employed in isoprene polymerization.¹⁰m Herein, the iminopyridine compounds (L1 – L4) have been prepared in moderate to high yields (45 – 88%) by a condensation reaction of the appropriate carbonyl-substituted 2-pyridine with a slight excess of the corresponding aniline (1.1 equiv.) in the presence of a catalytic amount of *p*-toluenesulfonic acid (PTSA, 5 – 10 mol%) (Scheme 1). As previously described by us and others, we observed that the preparation of compound L4 was particularly challenging, requiring only 4 hours of reaction time, a prolonged time leading to degradation of the compound. In addition, we noted that the purification procedure used, such as recrystallisation in pentane, led to a significant loss of the compound due to hydrolysis of the imine group. Thus, we were able to isolate L4 in 45% yield, which is a significant improvement as compared to previous reports (11%),¹⁰m,²² despite the presence of a small amount of starting materials (< 10%). Nevertheless, compound L4 was used without further purification, which proved not to be an obstacle for the following steps. On the other hand, the new aldiminoquinoline L5 was prepared in high yield (80%) by condensation reaction of 2-quinolinecarboxaldehyde with 3,5-bis(trifluoromethyl)aniline, in presence of catalytic amount of formic acid and magnesium sulfate, at room temperature in dichloromethane for 18 hours.

<figure><img src="image_3.png" /><figcaption>**Scheme 1. Synthesis of L1 - L5 ligands**</figcaption></figure>

The ¹H/¹⁹F NMR spectra along with the HRMS of the known iminopyridine compounds (L1 – L4) were consistent with the data reported in the literature (Figures S1-S14 of the Supporting Information, SI).
The new **L5** ligand has been fully characterized by ¹H-, ¹³C-, ¹⁹F-NMR spectroscopies and HRMS (Figures S15-S18).

In addition, single crystals of **L4** have been grown by slow evaporation of a concentrated solution in dichloromethane, while single crystals of **L5** have been obtained by recrystallization from a concentrated acetonitrile solution that has been left to stand for two days at -20 °C. These single crystals have been subjected to X-ray diffraction studies and the ORTEP type views of the ligands **L4** and **L5** are shown in Figures 1a and 1b, with selected bond distances and angles, respectively. Crystal data and structure refinement for **L4** and **L5** are gathered in Table S1, SI section.

<figure><img src="image_4.png" /><figcaption>**Figure 1.** ORTEP-type view of single unit of a) **L4** [N2-C6 = 1.280(2) Å, N2-C6-C5-N1 = -175.6(2)°] and b) **L5** [N2-C10 = 1.2771(19) Å, N2-C10-C9-N1 = 176.6(1)°]. The ellipsoids have been drawn at 30% probability level (hydrogen atoms, except H10 for **L5**, have been omitted for clarity and only specific atoms have been labelled).</figcaption></figure>

The ligands **L4** and **L5** feature a *quasi* planar (iminoethyl)-pyridine/quinoline frameworks [torsion angles: N2-C6-C5-N1 = -175.6(2)°; N2-C10-C9-N1 = 176.6(1)°, respectively] with a conformation in which the nitrogen from the pyridyl/quinolyl and imino subunits are *s-trans* to each other, due to the repulsion of both nitrogen lone pairs. The C6-N2 (**L4**) and C10-N2 (**L5**) bond lengths were found in the typical C(sp²)-N bond range [1.280(2) and 1.2771(19) Å, respectively]. The *N*-aryl plane in **L4** is tilted by 62.16° with respect to the pyridine ring, while in **L5** these two planes (*N*-aryl/quinoline planes) are slightly less inclined (44.91°). This configuration arrangement is due to the *π* – *π* stacking of the *N*-aryl planes of two **L4** molecules since both are parallel to each other and at a distance of the centroid of 3.538 Å (Figure S19). In contrast, the presence of a weak H–F interaction (H10-F2 distance = 2.584 Å) between two molecules of **L5** in the unit cell seems to impose the orientation of both planes (Figure S20).²³

**Synthesis of iminopyridine/iminoquinoline iron dichloride complexes 1 – 5.** The iron complexes 1 – 5 have been prepared in moderate yields through the complexation reaction of **L1** – **L5** (Scheme 2), respectively, in the presence of 1 equiv. of anhydrous FeCl₂ in THF for 18 hours, followed by recrystallization of the resulting compounds. It is noteworthy that, as commonly reported in the
literature, this complexation reaction typically proceeds with a 1:1 stoichiometric ratio between the ligand and $FeCl_2$ in order to target mono-substituted complexes for subsequent involvement in diene polymerization catalysis, which requires the metal coordination sphere to be unsaturated.¹⁰ Single crystals of complexes **2** and **3** were grown from a saturated solution of the complex in $CH_3CN$ followed by slow diffusion of diethyl ether at room temperature for 2 to 4 days, while for complexes **1** and **4**, it was necessary to store the solution of $CH_3CN$, after diffusion of $Et_2O$, at -20 °C. In parallel, single crystals of complex **5** were produced from a hot concentrated $CH_3CN$ solution and placed at -20 °C for one day. In each case, we were able to collect a moderate quantity of single crystals through multiple recrystallizations, which were carried out under identical conditions for all remaining products (total yields of 30 - 55%). The resulting single crystals of complexes **1** to **5** have been characterized by ¹H NMR spectroscopy and elemental analysis (see experimental part), with subsequent X-ray diffraction studies.

<figure><img src="image_5.png" /><figcaption>**Scheme 2.** Synthesis of complexes **1-5** (yields in X-ray quality crystals in bracket)</figcaption></figure>

Elemental analyses of most complexes were consistent with the formula of each corresponding compound drawn in Scheme 2, where complexes **1** and **2** contain two ImPy ligands per iron center, in contrast to the 1:1 ligand-to-metal ratio observed in complexes **3** to **5**.
The ¹H NMR spectra of all complexes were recorded in CD₂Cl₂ and CD₃CN (Figures S21-S22) and showed very broad proton signals in their respective ¹H NMR spectra over a range from + 200 to – 25 ppm, consistent with the paramagnetic nature of these iron complexes.

**Description of the molecular structure of complexes 1-5.** Complexes 1 and 2 crystallized in the monoclinic system with space group *P*2₁/*n* and *C*2/*c*, respectively. From the ORTEP type view, complexes 1 and 2 exhibit a distorted octahedral geometry around the iron centers, coordinated by two ImPy ligands and two terminal chloride atoms (Figure 2a and 2b, Table S2, SI section). The ImPy ligands act as bis-chelates, binding through both *N*<sub>imine</sub> and *N*<sub>pyridine</sub> atoms, comparably to the structure depicted in Chart 1 – C. The similar coordination chemistry of iron dichloride within these two complexes will be described together; selected bond lengths and angles are reported in Table 1.

<figure><img src="image_6.png" /><figcaption>**Figure 2.** ORTEP-type view of single unit with ellipsoid drawn at 30% probability level with and only specific atoms labelled of a) complex 1 (hydrogen atoms, except H6 and H20, as well as two non-coordinated CH₃CN solvent molecules have been omitted for clarity) and b) complex 2 (hydrogen atoms, except H6 and H18, have been omitted for clarity)</figcaption></figure>

<table><caption>**Table 1.** Selected bond distances, angles and torsion angles for complexes 1 and 2</caption><thead><tr><th></th><th>Complex 1</th><th>Complex 2</th></tr></thead><tbody><tr><td>Fe1-Cl1</td><td>2.3696(6)</td><td>2.4358(4)</td></tr></tbody></table>
<table><tr><td>Fe1-Cl2</td><td>2.4284(6)</td><td>2.3430(4)</td></tr><tr><td>Fe1-N1</td><td>2.2217(19)</td><td>2.2202(12)</td></tr><tr><td>Fe1-N2</td><td>2.2727(19)</td><td>2.2375(12)</td></tr><tr><td>Fe1-N3</td><td>2.2209(2)</td><td>2.2295(12)</td></tr><tr><td>Fe1-N4</td><td>2.2862(19)</td><td>2.3202(12)</td></tr><tr><td>N1-Fe1-N3</td><td>169.76(7)</td><td>151.35(4)</td></tr><tr><td>N4-Fe1-Cl1</td><td>157.90(5)</td><td>162.53(3)</td></tr><tr><td>Cl2-Fe1-N2</td><td>160.03(5)</td><td>164.00(3)</td></tr><tr><td>N1-C5-C6-N2</td><td>1.4(3)</td><td>-10.3(2)</td></tr><tr><td>N3-C19(C17)-C20(C18)-N2</td><td>-6.8(3)</td><td>-5.2(2)</td></tr></table>

In complexes 1 and 2, the N<sub>pyridine</sub> atoms of the distinctive ligands bind to the six-coordinated iron center in *trans* position relative to each other, while the N<sub>imine</sub> atoms are in *trans* position with respect to the chloride atoms. The bond distances between the two Fe-Cl bonds in each complex are inequivalent, showing a difference of 0.06 – 0.09 Å, yet falling within the same range of Fe-Cl lengths in related ImPy iron dichloride complexes [ca. 2.33 – 2.44 Å]<sup>2f,4,10o</sup> As expected, the Fe-N<sub>imine</sub> bond distances [ca. 2.29 Å] in both complexes are longer than the Fe-N<sub>pyridine</sub> bond lengths [ca. 2.22 Å], due to a greater π-back-bonding ability of the coordinated N<sub>pyridine</sub> atom relative to a stronger σ-donation of the N<sub>imine</sub>. The Fe-N<sub>imine</sub> bond lengths are highly unequal in complex 2 with the longest Fe1-N4 being in *trans* position to the longest Fe-Cl1 bond length, although these Fe-N<sub>imine</sub> bond distances remain at both extremes of the range reported in the literature [Fe-N<sub>imine</sub>: 2.23 – 2.32 Å]<sup>2f,4,10o</sup> With respect to the Fe-N<sub>pyridine</sub> bond lengths, they are found in the upper range compared to the parent complexes, the order of Fe-N<sub>pyridine</sub> distances in (ImPy)<sub>2</sub>FeCl<sub>2</sub> can be summarized as: 2,3,4,5,6-pentafluoro-N-aryl ≈ 3,5-trifluoromethyl-N-aryl (2.23 – 2.22 Å) > 2,4,6-trifluoro-N-aryl (2.22 – 2.20 Å)<sup>10o</sup> > 3-nitro-N-aryl (2.20 – 2.19 Å)<sup>4</sup> > N-aryl (2.15 Å).<sup>2f</sup> This observation shows that a decrease in the electron density of the N-aryl substituent can also influence the length of the N<sub>pyridine</sub>-Fe bond and thus reduce the ability of the N<sub>pyridine</sub> atom to coordinate to the iron center in complexes 1 and 2, compared with their relatives. In addition, it is notable that the two N-aryl substituents in complex 1 deviate significantly from orthogonality with respect to their related pyridine rings (57.38° and 33.11°), while being closer to orthogonality in complex 2 (80.95° and 75.28°). The imino groups are quasi-aligned with respect to the pyridine ring for all ligands in both complexes (torsion N1-C5-C6-N2 and N3-C19(C17)-C20(C18)-N4 angles of ca. -10.3 – 1.4°).

From a partial view of the unit cell of complex 1 depicted in Figure S23 the assembly of complexes seems to be driven by the presence of weak intramolecular non-bonding F···F contacts [F-F distance = 2.904(4) Å, which is slightly inferior than the sum of their van der Waals radii (ca 2.94 Å)],<sup>24</sup> with the additional presence of weak intermolecular H···F interactions [H-F distance = 2.46 Å] between two respective molecules.<sup>23</sup> In the case of complex 2, the occurrence of π – π interactions between the pyridine rings of two separate complexes appears to govern the crystal packing (Figure S24 in the SI that shows the parallelism between two respective pyridine rings with a distance of ca 3.45 Å).<sup>25</sup>
Complexes **3** and **5** were found to exhibit crystallization in the monoclinic system, with space group *C2/c* and *P2₁/c*, respectively (Figures 3a and 3b, Tables S2-S3, SI section). Unlike complex **3**, complex **5** exists in two different units within the unit cell, with distance and angle parameters very close to each other (deviation < 1%), so that only one unit of complex **5** will be discussed in the following section. The analogous coordination chemistry of iron dichloride with ligands **L3** and **L5** will be discussed together; selected bond lengths and angles are reported in Table 2.

<figure><img src="image_7.png" /><figcaption>**Figure 3.** ORTEP-type view of single unit, with ellipsoid drawn at 30% probability and only specific atoms labelled of a) complex **3** (hydrogen atoms as well as two non-coordinated CH₃CN solvent molecules have been omitted for clarity and b) complex **5** (hydrogen atoms, except H46 and H57, have been omitted for clarity)</figcaption></figure>

<table><caption>**Table 2.** Selected bond lengths (Å), angles and torsion angles (°) for complexes **3** and **5**</caption><thead><tr><th></th><th>Complex 3</th><th></th><th>Complex 5</th><th></th></tr></thead><tbody><tr><td>Fe1-Cl1</td><td>2.4423(9)</td><td>Fe1-Cl1</td><td>2.421(2)</td><td></td></tr><tr><td>Fe1-Cl1'</td><td>2.4423(9)</td><td>Fe1-Cl2</td><td>2.450(2)</td><td></td></tr><tr><td>Fe2-Cl1</td><td>2.3927(10)</td><td>Fe2-Cl1</td><td>2.389(2)</td><td></td></tr><tr><td>Fe2-Cl1'</td><td>2.3927(10)</td><td>Fe2-Cl2</td><td>2.377(2)</td><td></td></tr><tr><td>Fe2-Cl2</td><td>2.2388(10)</td><td>Fe2-Cl3</td><td>2.242(2)</td><td></td></tr><tr><td>Fe2-Cl2'</td><td>2.2388(10)</td><td>Fe2-Cl4</td><td>2.258(2)</td><td></td></tr></tbody></table>
<table><tbody><tr><td>Fe1-N1</td><td>2.147(3)</td><td>Fe1-N1</td><td>2.223(4)</td></tr><tr><td>Fe1-N2</td><td>2.188(3)</td><td>Fe1-N2</td><td>2.195(4)</td></tr><tr><td>Fe1-N1'</td><td>2.147(3)</td><td>Fe1-N3</td><td>2.223(4)</td></tr><tr><td>Fe1-N2'</td><td>2.188(3)</td><td>Fe1-N4</td><td>2.249(4)</td></tr><tr><td>N1-Fe1-N1'</td><td>155.86(16)</td><td>N1-Fe1-N3</td><td>149.0(1)</td></tr><tr><td>N2-Fe1-Cl1</td><td>170.22(8)</td><td>N2-Fe1-Cl1</td><td>174.1(1)</td></tr><tr><td>Cl1'-Fe1-Cl1</td><td>88.69(5)</td><td>N4-Fe1-Cl2</td><td>175.3(1)</td></tr><tr><td>Cl1'-Fe2-Cl1</td><td>91.03(5)</td><td>Cl1-Fe1-Cl2</td><td>86.74(5)</td></tr><tr><td>Cl2'-Fe2-Cl2</td><td>123.52(6)</td><td>Cl2-Fe2-Cl1</td><td>89.15(5)</td></tr><tr><td></td><td></td><td>Cl3-Fe2-Cl4</td><td>123.47(7)</td></tr><tr><td>N1-C5-C6-N2</td><td>3.22</td><td>N1-C9-C10-N2</td><td>6.4</td></tr><tr><td></td><td></td><td>N3-C27-C28-N4</td><td>4.8</td></tr></tbody></table>

The ORTEP type view of complexes **3** and **5** illustrates a structure containing two Fe centers in distinctive coordination environment, in the form of a dinuclear neutral complex of the type $(L)_2Fe(\mu-Cl)_2FeCl_2$, which is rarely observed in the literature (see Introduction part, Chart 1 – D). The Fe1 metal center is hexa-coordinated with two ImPy chelate ligands and two chloride atoms bridging with Fe2, the latter being tetra-coordinated with two terminal chlorides. The Fe1 center is in a distorted octahedral environment while the other Fe2 is in a distorted tetrahedral geometry. The arrangement of the two ImPy ligands around Fe1 is similar to that observed with the aldiminopyridine analogues **1** and **2**. The distances between the Fe1-Cl bridging bonds of the hexa-coordinated iron center $[ImPy_2Fe1 - \mu-Cl \approx 2.45 - 2.40 \AA]$ were found to be slightly higher than the Fe2-Cl distances $[Cl_2Fe2 - \mu-Cl \approx 2.38 - 2.40 \AA]$, but much longer than the two terminal Fe-Cl bond lengths $[(\mu-Cl)_2Fe2 - Cl \approx 2.24 - 2.26 \AA]$, due to the higher coordination number of Fe1. The additional methyl substituent on the carbon of the imino group in complex **3**, compared to the related aldimino complex **1**, led to a sharp decrease of the Fe1-Nimino bond distances (Fe1-Nimino = 2.19 Å, $\Delta$ = 0.09 Å compared to complex **1**) as well as for the Fe- Npyridine bond lengths (Fe1-Npyridine = 2.15 Å, $\Delta$ = 0.06 – 0.07 Å with respect to complex **1**). This observation could be due to an increase of $\sigma$-donation of the Nimino atoms of L3 along with the presence of bridging chloride in complex **3** compared to **1**. The extension of pyridine ring to quinoline in complex **5** led to Fe-Nquinoline/imine bond distances much closer than in the case of ImPy iron complexes, with the Fe1-Nquinoline bond lengths (ca. 2.22 Å) being, this time, in the range of the Fe1-Nimino bond distances (ca. 2.20 – 2.25 Å), probably due to the extension of the aromaticity. In both complexes, the torsion angle N1-C5-C6-N2 was found to be 3.22° in complex **3** and 6.4°/4.8° for its analog N1-C9-C10-N2 and N3-C27-C28-C4, respectively, in complex **5**, indicating that the imino group is almost planar to the pyridine (or quinoline) ring. In addition, the Fe-Fe interatomic distances (3.42 Å in complex **3** and 3.47 Å in complex **5**) were found to be near to that observed for Wang's complex $[3.51 \AA]^{10n}$ but much longer than Mak's complex $[2.42 \AA]$, since the latter contains a phosphine oxide arm instead of an imine.¹⁵

The crystal packing of complex **3** (Figure S25) shows a similar face-to-face arrangement of the two pyridines of the ligand L3 as observed previously with complex **2**. This arrangement again suggests a
possible occurrence of $\pi-\pi$ interactions between the two pyridine rings resulting from a pseudo-centroid contact (Centroid1···Centroid2) of ca 3.53 Å, which is similar in the case of complex 5, where $\pi-\pi$ interactions of two quinoline rings from two independent complexes are observable (Figure S26), with a pseudo-centroid contact of ca 3.45 Å.²⁵

The structure of complex 4 represents an orthorhombic crystal system with space group *Pbca* that corresponds to the point group *D*₂h. Complex 4 shows a centrosymmetric dinuclear coordination complex somewhat like 3 and 5 but, surprisingly, it exists in the form of an unprecedented dimeric structure of the type [(L4)₂Fe(μ-Cl)₂Fe(μ-Cl)Cl]₂·(CH₃CN)₂ (Figure 4, selected angle and bond distances are presented in Table 3, crystal data and structure refinement are gathered in Table S3, SI section).

<figure><img src="image_8.png" /><figcaption>**Figure 4.** ORTEP-type view of single unit of complex 4 with ellipsoid drawn at 30% probability level (hydrogen atoms as well as two non-coordinated CH₃CN solvent molecules have been omitted for clarity and only specific atoms have been labelled)</figcaption></figure>

<table><caption>**Table 3.** Selected bond lengths (Å), angles and torsion angles (°) for complex 4</caption><tr><td>Fe1-Cl1</td><td>2.4796(6)</td><td>Cl1-Fe1-Cl2</td><td>88.00(2)</td></tr><tr><td>Fe1-Cl2</td><td>2.3886(6)</td><td>Cl1-Fe2-Cl2</td><td>85.53(2)</td></tr><tr><td>Fe2-Cl1</td><td>2.3882(7)</td><td>Cl3-Fe2-Cl3'</td><td>86.03(2)</td></tr><tr><td>Fe2-Cl2</td><td>2.5884(7)</td><td>Cl1-Fe2-Cl3</td><td>110.96(2)</td></tr><tr><td>Fe2-Cl3</td><td>2.3447(6)</td><td>Cl2-Fe2-Cl4</td><td>90.33(2)</td></tr><tr><td>Fe2-Cl3'</td><td>2.5902(7)</td><td>Cl2-Fe2-Cl3'</td><td>177.63(2)</td></tr><tr><td>Fe2-Cl4</td><td>2.2748(6)</td><td></td><td></td></tr><tr><td>Fe1-N1</td><td>2.1644(16)</td><td>N1-C5-C6-N2</td><td>1.6</td></tr><tr><td>Fe1-N2</td><td>2.2212(17)</td><td>N3-C18-C19-N4</td><td>4.5</td></tr><tr><td>Fe1-N3</td><td>2.1552(17)</td><td></td><td></td></tr><tr><td>Fe1-N4</td><td>2.2114(17)</td><td></td><td></td></tr></table>
At the centroid of the rhombus Fe2Cl3Fe2'Cl3' lies an inversion center of symmetry, which generates the whole symmetric dimer from the asymmetric monomer unit [(L4)2Fe(µ-Cl)2Fe(µ-Cl)Cl], whose characteristics are similar to the related unsymmetrical dinuclear coordination complexes 3 and 5. Complex 4 consists of four iron centers, with the two Fe1 centers at either end of the structure possessing two ImPy L4 ligands and sharing two bridging chloride atoms (Cl1 and Cl2), with the two remaining Fe2 at the center of the structure. In addition, the two Fe2 centers are linked by two bridging chloride atoms (Cl3), which are further bound to a terminal chloride (Cl4) to complete the coordination sphere. Like complexes 3 and 5, this structure also consists of two different types of iron centers, Fe1 being in a distorted octahedral environment whereas the geometry at the Fe2 centers can be best described as slightly distorted trigonal bipyramidal (geometric parameter = 0.79),²⁶ with the Cl2-Fe2-Cl3' angle closed to linearity [177.63(2)°]. The distances between all Fe-Cl bonds in the complex are clearly not equivalent, with the Fe2-Cl2 and Fe2-Cl3' bond, in *trans* position, being the two longest [2.5884(7) Å and 2.5902(7) Å, respectively]. The bridged Cl1 atom between Fe1 and Fe2 reveals that the Fe1-Cl1 bond distance is longer than the Fe1-Cl2 [2.4796(6) Å and 2.3886(6) Å, respectively], resulting in the other bridge Cl2 atom being further away from Fe2 than Cl1 [Fe2-Cl1: 2.3882(7) Å], while the third couple of bridged Cl3' and Cl3 atoms feature a bond distance Fe2-Cl3 of 2.3447(6) Å and Fe2-Cl3' of 2.5902(7) Å. As expected, the shortest Fe-Cl bond length is found for the terminal Fe2-Cl4 bond [2.2748(6) Å]. The position of the two ImPy ligands surrounding Fe1 in complex 4 is identical to that of the previously described complexes with the N<sub>pyridine</sub> atoms of the ImPy ligands in *trans* to one another. As found in complex 3, the Fe1-N<sub>imino</sub> bond distances (ca. 2.21 – 2.22 Å) are longer than those of the Fe1-N<sub>pyridine</sub> bonds (ca. 2.15 – 2.16 Å), and the N1-C5-C6-N2 and N3-C18-C19-N4 torsion angles are 1.6° and 4.5°, respectively, revealing that the imino group is *quasi* planar to the pyridine ring for the two coordinated L4 ligands. The Fe1-Fe2 interatomic distance of 3.58 Å is longer than those observed for the monomeric analog complexes 3 and 5 [3.42 – 3.47 Å], while Fe2-Fe2' are separated by 3.61 Å. As seen previously, the alignments of complexes within the crystal packing could also result from the presence of weak non-bonding F···F contacts of 2.94(2) Å and H···F interactions of ca 2.67 Å. In addition, one can also observe a similar face-to-face arrangement of the two pyridines as found for complexes 2, 3 and 5, resulting to π–π interactions between two pyridine rings of two independent complexes along the centroid pseudo-contact (Centroid1···Centroid2) of ca 3.99 Å (Figure S27).

**Mössbauer spectroscopy studies.** Zero-field ⁵⁷Fe Mössbauer spectra of complexes 2 - 5 were recorded at 80 K to determine the oxidation and spin states of the iron centers. The spectrum of the mono-nuclear complex 2, where Fe adopts a highly distorted octahedral geometry, exhibits an isomer shift (IS) of δ = 1.1576(25) mm/s and a quadrupole splitting (QS) of ΔE<sub>q</sub> = 3.0223(50) (Figure 5.a and Table 4). These values are characteristic of Fe(II) in a HS state²⁷ and closely match those reported for analogous mono-nuclear iron complex (Chart 1 – C), such as the (ImPy)₂FeCl₂ complex described by Ghosh *and* coll. (δ = 1.096 mm/s and ΔE<sub>q</sub> = 3.055 mm/s).²f
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>**Figure 5.** Zero-field ⁵⁷Fe Mössbauer spectra recorded at 80 K and spectral fits for a) complex 2, b) complex 3, c) complex 4 and d) complex 5. Black circles indicate experimentally obtained data; the pink line refers to the fit of the quadrupole doublet data for complex 2; the blue and red lines refer, for complexes 3 to 5, to the fit of the quadrupole doublet data for sites 1 and 2, respectively, with the total fit represented by a green line. Note that for complexes 3 and 5, a minor contribution (site 3, < 8%) of Fe³⁺ is visible as a dotted gray line, arising from slight oxidation of the sample prior to analysis.</figcaption></figure>

<table><caption>**Table 4.** Selected Zero-Field ⁵⁷Fe Mössbauer parameters for complexes 2 to 5 recorded at 80 K</caption><thead><tr><th rowspan="2">Complex</th><th colspan="3">Site 1</th><th colspan="3">Site 2</th></tr><tr><th>δ<br>(mm/s)</th><th>ΔE<sub>q</sub><br>(mm/s)</th><th>site population<br>(%)</th><th>δ<br>(mm/s)</th><th>ΔE<sub>q</sub><br>(mm/s)</th><th>site population<br>(%)</th></tr></thead><tbody><tr><td><b>2</b></td><td>1.1576(25)</td><td>3.0223(50)</td><td>100</td><td>n.a.</td><td>n.a.</td><td>n.a.</td></tr><tr><td><b>3</b></td><td>1.1597(60)</td><td>3.021(12)</td><td>47.7(36)</td><td>1.0422(75)</td><td>2.640(17)</td><td>44.1(4)</td></tr><tr><td>Calculated 3*</td><td>0.8731</td><td>3.691</td><td></td><td>0.8300</td><td>2.395</td><td></td></tr><tr><td><b>4</b></td><td>1.039(11)</td><td>3.064(25)</td><td>44.0(72)</td><td>0.9470(94)</td><td>2.713(22)</td><td>49.7(74)</td></tr><tr><td><b>5</b></td><td>1.1506(56)</td><td>3.097(19)</td><td>52(12)</td><td>1.1129(93)</td><td>2.770(54)</td><td>48(13)</td></tr></tbody></table>

After deconvolution, the spectra of the di-nuclear complexes **3** and **5** (Figures 5.b and 5.d, respectively) reveal two overlapping quadrupole doublets in equal proportion (see Figures S28-S29 and Tables S4-S5 in the SI for details), alongside a minor contribution of Fe³⁺ (< 8%), due to sample handling in air that led to a partial oxidation. Complex **3** displays δ = 1.1597(60) mm/s and ΔE<sub>q</sub> = 3.021(12) mm/s for site 1, which closely resembles the values found for complex **2**. Site 2 shows a slightly smaller IS and QS, with δ = 1.0422(75) mm/s and ΔE<sub>q</sub> = 2.640(17) mm/s, respectively, yet both values remain consistent with Fe(II) in a HS state. A similar trend is observed for complex **5**, where site 1 exhibits a

<footnote>*Calculated Mössbauer parameters obtained from DFT (B3PW91/6-311G(d,p)) geometry optimization of complex 3, assuming a high-spin (S = 4, multiplicity = 9) state.</footnote>
slightly higher IS and QS values of $\delta = 1.079(13)$ mm/s and $\Delta E_q = 2.967(20)$ mm/s, compared to site 2 ($\delta = 0.904(10)$ mm/s and $\Delta E_q = 2.785(19)$ mm/s). As reported in the literature, six-coordinated Fe(II) centers in octahedral geometries exhibit higher IS values than tetrahedral environments, due to the lower *s*-character of the metal orbitals in *O*<sub>*h*</sub> coordination (d<sup>2</sup>sp<sup>3</sup> hybridization) compared to *T*<sub>*d*</sub> (sp<sup>3</sup> hybridization).<sup>28</sup> Based on these trends, site 1 in complexes **3** and **5** is assigned to an Fe(II) HS center in a pseudo-octahedral coordination, while site 2 corresponds to an Fe(II) HS center in a pseudo-tetrahedral environment.

The spectrum of the tetra-nuclear iron complex **4** (Figure 5.c) also reveals two Fe(II) HS sites in a 1:1 ratio, similar to complexes **3** and **5**. Site 1 exhibits $\delta = 1.1506(56)$ mm/s and $\Delta E_q = 3.097(19)$ mm/s, while site 2 presents $\delta = 1.1129(93)$ mm/s and $\Delta E_q = 2.770(54)$ mm/s. The smaller difference in IS and QS between sites 1 and 2 in complex **4** compared to complexes **3** and **5** suggests a more uniform electronic environment, likely due to the distorted trigonal bipyramidal geometry, which has an intermediate *s*-character between octahedral and tetrahedral coordination.

These findings contrast with those described by the group of Wang, who reported mixed HS/LS Fe(II) states in related complexes.<sup>10n</sup> In contrast, our data are comparable to those found in the case of di-nuclear (['PrOH]<sub>4</sub>Fe(μ-I<sub>2</sub>)FeI<sub>2</sub> complex described by the group of Soares.<sup>29</sup> While the observed Mössbauer parameters provide strong evidence for exclusively Fe(II) in HS states across all complexes studied, we could not propose an explanation for the differences observed with those described in the literature.

**Density Functional Theory (DFT) Calculations.** The electronic structure of complexes **1**, **2**, **3** and **4** were further investigated through computational studies at the DFT B3PW91/6-311G(d,p) and wB97XD/Def2SVP level of theory (see SI for computational details, Tables S6 to S13 and Figures S30-S43). For both functionals, the calculated molecular parameters (bond distances and angles) were found to be in good agreement with the experimental X-ray values (Tables S6 to S8 and Figures S30 to S32) except for complex **4** (Table S12 and Figure S43) for which wB97XD method could not reproduce the experimental structure. Thus, only the results obtained with the B3PW91 method will be discussed. The computed structures of the LS (*S* = 0 : singlet), HS (*S* = 2 : quintet) and *S* = 1 (triplet) spin states of **1** and **2** reveal that these two complexes have a quintet ground state (*2*S* + 1 = 5*) (Complex **1**, Figure 6.a), which is energetically favored by 20.7 and 22.6 kcal.mol<sup>-1</sup> over the singlet and triplet states, respectively for complex **1** (Figure S33) and by 23.3 and 21.3 kcal.mol<sup>-1</sup> for complex **2** (Figure S34). Similarly, for complexes **3** and **4**, the forms in which each iron atom adopts a high-spin ground state (*S*<sub>Fe</sub> = 2 for each iron atom) are those with the lowest energy (Figures S35-S36 and S37-S38, respectively). The resulting spin density population on the iron centers for all complexes are consistent with Mössbauer spectroscopy results, indicating approximately four unpaired electrons per iron center (calculated for complexes **1** to **4**: ≈ 3.77, see SI, Figures S33-S34 and S36-S38, respectively). For complex **3**, as proposed by Wang
and coll., we studied the possibility of having a septet state (2S + 1 = 7) with a hexa-coordinated Fe(II) HS and a tetra-coordinated Fe(II) LS. This spin state although possible is 18.2 kcal/mol less stable than the ground state. Interestingly, for complexes **3** and **4**, the computed ground states correspond to 2S + 1 = 9 and 2S + 1 = 17 spin configurations, respectively (Figures 6.b and 6.c), which are degenerate with the corresponding open-shell singlet states (Figures S35 and S37).

<figure><img src="image_13.png" /><figcaption>**Figure 6.** Computed spin density population for the ground states (in black, bold) and the Natural Bond Orbital (NBO) charges for chlorides (green in bracket) and iron atoms (red in bracket) of complexes a) **1**, b) **3** and c) **4**</figcaption></figure>

Taking into account that the distinction between the 2S + 1 = 9 (**3**) and 2S + 1 = 17 (**4**) spin states and the corresponding open-shell singlet states lies in the spin coupling between the iron centers (ferromagnetic coupling of high-spin Fe(II) atoms for the 2S + 1 = 9 and 2S + 1 = 17 and antiferromagnetic for the open-shell singlet), the small energy difference computed here indicates that there is no strong coupling between the iron centers in complexes **3** and **4**. The Natural Bond Orbital (NBO) analysis revealed that the positive charges are consistently localized at the iron centers, while the negative charges are situated on the nitrogen and chloride atoms (Figure S39). Interestingly, in complexes **3** and **4**, the bridging chloride atoms exhibit a lower negative charge density than the terminal chloride atoms (Figure 6).

**Magnetic Properties Measurements.** The plots of the thermal evolution of the magnetic susceptibility, $\chi(T)$, at $H = 0.1$ T (assumed as M/H) and magnetization versus field $M(H)$ at 2 K are depicted in Figure 7.a and 7.b for all compounds in the title series. For comparison, the magnetic behavior of the well-known $FeCl_2$ compound under identical experimental conditions is also presented (weak antiferromagnetism between ferromagnetic (FM) coupled layers). These results were analyzed on the basis of the prior results of Mössbauer spectroscopy and DFT studies, i.e., assuming high-spin $Fe^{2+}$ ions ($S = 2$) in all available Fe sites. The $\chi(T)$ plots were fitted to the Curie-Weiss (CW) law: $\chi = \chi_0 + C/(T-\Theta)$, where the temperature independent $\chi = \chi_0$ term is mainly assigned to a diamagnetic contribution. The values of C, $\Theta$, and the deduced effective moment per Fe atoms are listed in Table 5, together with the maximal Fe moment values from M(H) plots at $T = 2$ K and $H = 9$ T. Additionally, the table includes estimates of the average magnetic exchanges for complexes **3**, **4** and **5**, deduced from the mean field approximation: $J/K_b = z S(S+1)/3$, where $S = 2$ and $z$ corresponds to the average number
of Fe neighbors around a central one (i.e., z = 1 for the dimeric **3** and **5** complexes, and z = 1.5 for the tetrameric **4** complex).

<table><caption>**Table 5.** All parameters extracted from our Curie-Weiss fits and magnetization plots with the Fe/T indexes for each compound</caption><thead><tr><th>Complex</th><th>C<br>(emu.K.mol<sup>-1</sup>)</th><th>Θ<sub>CW</sub><br>(K)</th><th>J/K<sub>b</sub><br>(K)</th><th>χ<sub>0</sub><br>(mol.emu<sup>-1</sup>)</th><th>μ<sub>eff</sub><br>(μ<sub>B</sub>/Fe)</th><th>nCl<sub>T</sub><br>/nFe<sup>a</sup></th><th>M<sub>max</sub><br>(μ<sub>B</sub>/f.u.)</th></tr></thead><tbody><tr><td>FeCl<sub>2</sub></td><td>3.69</td><td>26.6</td><td></td><td>0</td><td>5.44</td><td>-</td><td>3.67</td></tr><tr><td>1</td><td>5.02</td><td>0</td><td>0</td><td>-7.0(2) x 10<sup>-4</sup></td><td>6.34</td><td>2</td><td>4.47</td></tr><tr><td>2</td><td>5.08</td><td>0</td><td>0</td><td>-9.0(3) x 10<sup>-4</sup></td><td>6.37</td><td>2</td><td>4.68</td></tr><tr><td>3</td><td>7.03</td><td>4.49</td><td>2.245</td><td>-1.43(4) x 10<sup>-3</sup></td><td>5.3</td><td>1</td><td>3.1</td></tr><tr><td>4</td><td>10.39</td><td>2.89</td><td>0.963</td><td>-8(4) x 10<sup>-5</sup></td><td>4.56</td><td>½</td><td>2.4</td></tr><tr><td>5</td><td>6.51</td><td>4.35</td><td>2.175</td><td>-1.20(4) x 10<sup>-3</sup></td><td>5.10</td><td>1</td><td>2.8</td></tr></tbody></table>

<figure><img src="image_14.png" /><figcaption>**Figure 7.** a) $\chi^{-1}(T)$ plots for $FeCl_2$ and complexes **1** to **5**, after normalization for one Fe atom formulae, b) magnetization plots at 2 K for the same compounds.</figcaption></figure>

$FeCl_2$ displays the typical expected behavior built on weakly-coupled ferromagnetic triangular layers, with evidence of a spin-flip transition at ~1.2 T. The effective magnetic moment, $\mu_{eff} = 5.44 \mu_B/Fe$, exceeds the spin-only value $2\sqrt{S(S+1)}$ of 4.9 $\mu_B$, in good agreement with the strong anisotropy of $Fe^{2+}$ single ions in $FeCl_2$, due to unfrozen orbital contribution. The main results for our series are rationalized as follows: firstly, the $\chi(T)$ curves slightly deviate at low temperature from linear CW laws, which indicate very weak magnetic exchange interactions, even in the multinuclear complexes. Apart for complex **4**, the $\mu_{eff}$ values exceed the spin-only $S = 2$ values, which suggests a significant orbital contribution to the local magnetic moments, consistent with previous observations in halide-bridged

<footnote>a number of terminal chloride ions compared to number of Fe</footnote>
dimeric Fe complexes.³⁰ Secondly, there is a significant trend of decreasing μeff values as the number of Fe-(μ-Cl)-Fe bridges increases, which is in line with the increasing of the maximal moments per Fe (at 2 K and 9 T) for each complex (see Table 5). Given the various FeNₓClᵧ coordination environment in complexes 1-5, one can expect sizeable mean orbital contributions on each Fe. However, the significant decreasing of the averaged μeff may suggests other effects. In a first approximation, one could also propose more delocalized magnetic moments within these bridges, but DFT calculations indicate similar local Fe magnetic moments (~3.7 μB) and similar natural charges (~0.46 q) for all bridging chlorides, thereby minimizing this effect. Here, the analysis of the charges on all Cl⁻ ions show larger electronic density on the terminal ones (~0.55 q), revealing a plausible enhanced electronegative effect on terminal Fe-Cl sites. In such case, the local distortion of the electronic cloud from partial oxidation from Fe²⁺ to Fe(2+δ)⁺ is expected to increase the spin-contribution to μeff. It is in tune with the clear correlation between μeff and the number of terminal Cl⁻ ions per Fe, labeled as nClₜ/nFe in Table 5. Conversely, an increase in the ratio of bridging to terminal chlorides within these complexes leads to a decrease in the effective magnetic moment.

Finally, the estimation of the magnetic exchanges afore-mentioned suggest weak ferromagnetic interactions in the multi-nuclear complexes, consistent with the positive exchange interactions predicted by Kanamori-Goodenough rules for 90° Fe²⁺-X-Fe²⁺ super-exchanges. All bridging Fe-(μ-Cl)-Fe angles are close to 90° within complexes 3 to 5, and resemble the FM-layer case well known for FeCl₂.³⁰ In such case, the FM component of the exchange involves Fe<sub>eg</sub>-Cl<sub>pσ</sub>-Cl<sub>pσ'</sub>-Fe<sub>eg</sub> double correlation. Note that this ~90° exchange is FM for all d<sup>n</sup>-X-d<sup>n</sup> cations.

**Polymerization of isoprene.** The ketiminopyridine iron complexes **3** and **4** have previously demonstrated high activity in isoprene polymerization (TOF > 300 000 h⁻¹), after activation with 10 equivalents of Al¹Bu₃ and 1 equivalent of [CPh₃][B(C₆F₅)₄].¹⁰m To extend this study, we evaluated the catalytic performance of complexes **1–5** under modified conditions with a ratio Fe/Al¹Bu₃/[CPh₃][B(C₆F₅)₄]/isoprene = 1/3/1/500 in toluene at room temperature (Scheme 3), the results are gathered in Table 6.

<figure><img src="image_15.png" /><figcaption>**Scheme 3.** Isoprene polymerization with complexes **1–5**</figcaption></figure>
<table><caption>Table 6. Polymerization results of isoprene with complexes 1 to 5</caption><thead><tr><th>Entry<sup>a</sup></th><th>Complex</th><th>Time</th><th>Yield (%)</th><th>M<sub>n</sub> (g/mol)<sup>b</sup></th><th>D<sup>b</sup></th><th>cis-1,4 (%)<sup>c</sup></th><th>3,4 (%)<sup>c</sup></th></tr></thead><tbody><tr><td>1</td><td>1</td><td>30 min</td><td>95</td><td>204 000</td><td>2.2</td><td>50</td><td>50</td></tr><tr><td>2</td><td>2</td><td>25 min</td><td>85</td><td>166 400</td><td>3.6</td><td>53</td><td>47</td></tr><tr><td>3</td><td>3</td><td>&lt; 20 sec</td><td>95</td><td>135 000</td><td>3.2</td><td>60</td><td>40</td></tr><tr><td>4</td><td>4</td><td>&lt; 20 sec</td><td>95</td><td>189 000</td><td>2.3</td><td>50</td><td>50</td></tr><tr><td>5</td><td>5</td><td>2 h</td><td>&lt; 1</td><td>nd</td><td>nd</td><td>nd</td><td>nd</td></tr></tbody></table>

All complexes were capable of converting 500 equivalents of isoprene after activation, apart from complex 5, likely due to steric congestion hindering monomer coordination, as found for related iron complex bearing N-phenyl-1-(quinolin-2-yl)methanimine ligand.<sup>10f</sup> The aldiminopyridine iron complexes 1 and 2 displayed moderate activity (TOF = 1 000 – 1 200 h<sup>-1</sup>), reaching almost full conversion in 30 min (Table 6, entries 1 and 2), while their ketimine counterparts 3 and 4 exhibited significantly higher activity (TOF = 90 000 h<sup>-1</sup>), reaching complete conversion in less than 30 seconds Table 6, entries 3 and 4), consistent with previous report.<sup>10m</sup> Although the presence of uncoordinated FeCl<sub>2</sub> species in complexes 3 and 4 cannot be entirely ruled out as contributing to the polymerization under the conditions employed, it is important to note that FeCl<sub>2</sub> alone, when activated, has been reported to exhibit no catalytic activity in isoprene polymerization.<sup>31</sup> Therefore, we consider it unlikely that such species contribute significantly to the polymerization process or act as chain transfer agents under the conditions used in this study.

Given that all precatalysts (complexes 1–4) feature iron centers coordinated by two bidentate ligands, the metal is initially saturated and lacks vacant sites for monomer binding. Upon activation, partial ligand dissociation likely occurs, generating a mono-substituted cationic iron–alkyl species with an open site for isoprene coordination. This dissociation may take place during alkylation or upon monomer approach, with the second ligand displaced due to its lower affinity under catalytic conditions. Such activation pathways are consistent with those reported for related iron complexes bearing bidentate nitrogen ligands.<sup>31,32</sup>

The M<sub>n</sub> values obtained for each entry (M<sub>n,exp</sub> = 135 000 – 200 000 g/mol) were well above the expected theoretical one (M<sub>n,theo</sub> = 34 000 g/mol), suggesting a propagation rate much higher than the initiation rate or incomplete activation of all iron centers. Despite these high M<sub>n</sub> and slightly broad dispersities (Table 6, entries 1–4, D = 2.2 – 3.6), the SEC traces showed monomodal Gaussian distribution (Figures S46, S49, S52 and S55), indicating single-site catalysis with negligible degradation of the active species.

<footnote><sup>a</sup>Conditions: Fe<sub>cat</sub>/Al<sup>i</sup>Bu<sub>3</sub>/[CPh<sub>3</sub>][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] = 1/3/1 with Fe<sub>cat</sub> = 10 µmol; Isoprene = 5 mmol, 500 equiv./Fe<sub>cat</sub>, volume toluene = 5 mL, [C]<sub>isoprene</sub> = 1 mol/L; <sup>b</sup> determined by size-exclusion chromatography (SEC), calibrated with polystyrene standard; <sup>c</sup> determined by <sup>1</sup>H and <sup>13</sup>C NMR; nd = not determined. M<sub>n,theo</sub> = 500 * M<sub>isoprene</sub> = 500 * 68.12 = 34 060 g/mol.</footnote>
The ¹H and ¹³C NMR spectra of all resulting polymers revealed key differences in polymer microstructure (recorded in CDCl₃, Figure S44-S45, S47-S48, S50-S51, S53-S54). Carbon assignments, based on literature data,³³ confirmed the absence of *trans*-1,4 units in all polymer samples (Table 6, entries 1–4), only the characteristic signals of *cis*-1,4 and 3,4 units are noticeable.

Thanks to a comprehensive and detailed study by Ricci's group on the ¹³C NMR attribution of the olefinic carbons with respect to the distribution of *cis*-1,4 and 3,4 units in the polyisoprene chain, we can now qualitatively determine the enchainment of these units in our samples (Figure 8).¹⁰c,³⁴

<figure><img src="image_16.png" /><figcaption>**Figure 8.** ¹³C NMR spectra (75 MHz, CDCl₃, 25 °C) of polyisoprenes obtained with complexes **1** - **4** (Table 6).</figcaption></figure>

Polymers obtained from complex **4** exhibited four distinct olefinic carbon signals (δ = 111.5, 126.5, 133.9 and 147.7 ppm, Figure 8, Entry 4), consistent with a highly alternating *cis*-1,4-*alt*-3,4 sequence, a result that is identical to that found with the catalytic system CoCl₂(P(alkyl)Ph₂)₂/MAO.²⁹a Similar selectivity was observed for the polymer issued from complex **2** (Figure 8, Entry 2), indicating a strong preference for alternating enchainment. In contrast, polymers obtained from complexes **1** and **3** (Figure 8, Entry 1 and 3, respectively) displayed additional signals at δ = 135 and 125 ppm, suggesting a less ordered insertion pattern, with some deviation from strictly alternated enchainment, such as found in the polymer obtained from monoalkyl-*N*-aryl-substituted ImPy iron chloride complexes.¹⁰c
These data gathered in the olefinic carbon region of the ¹³C NMR spectra are also apparent in the aliphatic carbon region, where the regularity of the *cis*-1,4-*alt*-3,4 sequence is clearly altered for the polymers issued from entries 1 and 3 compared to entries 2 and 4 (Figures S45, S48, S51, S54).

Overall, the *N*-aryl substituent of the ImPy ligand has a more pronounced impact on catalyst selectivity for isoprene polymerization, whereas the methyl substituent on the imino-carbon notably enhances the catalytic activity. This is evident from the observation that both the aldimine complex **2** and the related ketimine complex **4**, which contain the *N*-C₆F₅ substituent, lead to highly enriched *cis*-1,4-*alt*-3,4 polyisoprene. In contrast, complexes **1** and **3**, bearing the *N*-3,5-(CF₃)₂C₆H₃ substituent, lead to a statistical distribution of *cis*-1,4 and 3,4 units.

Previous work by the group of Ricci for a series of related monoalkyl-*N*-aryl-substituted ImPy iron chloride complexes suggested that the steric congestion caused by the alkyl group in the *ortho* position of the *N*-aryl substituent influences the alternating sequence of *cis*-1,4 and 3,4 units.¹⁰c Specifically, smaller substituents tend to produce enriched *cis*-1,4-*alt*-3,4 polyisoprene, while larger groups lead to short blocks of *cis*-1,4 units within the alternating sequence of the polymer chain. The authors proposed a possible scheme of formation of enriched *cis*-1,4-*alt*-3,4 polyisoprene that considers not only the steric effects of the catalyst but also the mode of coordination of isoprene to the iron center. Our results add a new dimension to this proposition by showing that, for iron complexes **1–4**, steric hindrance is not necessarily the main factor in determining catalyst selectivity for alternating *cis*-1,4 and 3,4 insertions. In particular, the presence in ImPy ligand of a pentafluoro-phenyl group on the *N*-imine substituent can also induce this selectivity, independently of the nature of the imino-carbon substituent. This contrasts with previous reports on bis- and tris-alkyl-*N*-aryl substituted ImPy iron complexes, which displayed both lower activity and lower selectivity, including *trans*-1,4 units.¹⁰j,m This suggests that selectivity can also be affected by the electronic nature of the catalyst, adding a new layer of complexity to our understanding of the polymerization mechanisms of isoprene that have been proposed to date.³⁵

## CONCLUSIONS

This study highlights the diverse coordination chemistry and catalytic performance of iron complexes supported by iminopyridine (ImPy) and iminoquinoline ligands, focusing on their application in isoprene polymerization. The introduction of fluorinated *N*-aryl groups and variations from aldimine or ketimine substituents led to the formation of unique iron complexes with diverse coordination modes. Mononuclear iron species were obtained with aldimine-pyridine ligands **L1** and **L2**, while the corresponding ketimine-pyridine ligands led to dinuclear (with N-3,5-(CF₃)₂C₆H₃) or tetranuclear (with N-C₆F₅) complexes. Interestingly, replacing the pyridine by a quinoline scaffold in the aldimine series still resulted in a dinuclear complex when bearing the N-3,5-(CF₃)₂C₆H₃ group. These findings confirm that ligand design plays a crucial role in determining nuclearity. However, at this stage, it remains difficult to predict the nuclearity of the resulting complexes. This is particularly evident when
considering the work by Wang, where the use of aldimino-pyridine ligands bearing methylene (substituted)-diphenyl groups also led to dinuclear species,<sup>10n</sup> despite markedly different steric and electronic profiles from our systems. Mössbauer spectroscopy, density functional theory (DFT) calculations, and magnetic measurements confirmed that the iron centers, whether coordinated by an ImPy ligand or simply in the form of a bridging FeCl<sub>2</sub> unit, exhibit high-spin Fe(II) states across all complexes. The magnetic measurements show that the effective magnetic moment decreases as the proportion of bridging to terminal chlorides increases. Weak ferromagnetic interactions were observed in multinuclear complexes, suggesting that terminal chloride ions influenced the degree of magnetic delocalization. The ketimine iron complexes demonstrated exceptional catalytic activity in isoprene polymerization upon activation with Al<sup>i</sup>Bu<sub>3</sub>/[CPh<sub>3</sub>][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>], when compared to their aldimine counterparts, with the resulting polymers displaying a range of microstructures. Complexes with pentafluorophenyl N-aryl substituents produced highly enriched *cis*-1,4-*alt*-3,4 polyisoprene, highlighting the significant role of electronic effects on selectivity. Conversely, complexes with trifluoromethyl N-aryl substituents yielded a statistical distribution of *cis*-1,4 and 3,4 units, indicating that both steric and electronic factors influence polymerization outcomes. This work advances the understanding of the coordination chemistry of iron complexes with ImPy ligands and provides new insights into the factors governing isoprene polymerization. These findings contribute to the rational design of catalysts tailored for specific polymerization applications, paving the way for sustainable processes in the elastomer industry.

## EXPERIMENTAL SECTION

**General methods and equipment:** all complexation reactions were performed in the absence of air using Schlenk techniques under argon atmosphere, and the resulting complexes were stored in a glove box after recrystallization (O<sub>2</sub> < 1 ppm, H<sub>2</sub>O < 1 ppm, Jacomex, France). All solvents were dried through an alumina column (Mbraun, Mérignac, France) deoxygenated by the freeze-pump-thaw method and stored in the glovebox on 4 Å molecular sieves. 2-Pyridinecarboxaldehyde, 2-acetylpyridine, 2-quinolinecarboxaldehyde, 2,3,4,5,6-pentafluoraniline, 3,5-bis(trifluoromethyl)aniline, anhydrous FeCl<sub>2</sub>, [CPh<sub>3</sub>][B(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>] and Al<sup>i</sup>Bu<sub>3</sub> were purchased from Merck, TCI Chemicals or Strem Chemicals and used as received. Isoprene was purchased from TCI Chemicals, dried over CaH<sub>2</sub> for 4 days and distilled under argon, then stored at -20 °C in the glove box after being deoxygenated. NMR spectra were recorded on a Bruker Advance 300 instrument at 300 K. The chemical shifts (indicated in ppm) were determined using the residual signal of the deuterated solvent in accordance with the literature.<sup>36</sup> Elemental analyses were carried out by Céline Delabre on an Elementar Vario Cube instrument at UCCS, Université Lille Nord de France.

Mössbauer spectroscopy analyses were performed at the LCC Laboratory in Toulouse, France. Spectra were collected on a constant-acceleration conventional spectrometer with a 0.30 GBq source of <sup>57</sup>Co
(Rh matrix) at 80 K, using an OXFORD cryostat. The thermal scanning was monitored by a temperature controller LAKESHORE 335. The absorber was a sample of about 100 mg of powder that was enclosed in a 20 mm diameter cylindrical plastic sample holder. Mössbauer's parameters were fitted to a Lorentzian model using the Recoil software. The isomer shift values are given with respect to metallic iron at room temperature. Magnetic properties were measured with a Physical Properties Measurement System (PPMS) DynaCool 9 T from Quantum Design at the Université de Lille. The Field-Cooled (FC) susceptibility versus temperature were measured at 0,1 T between 350K and 2K, the magnetization was collected at 2 K between +/-9 T.

X-ray measurements for L4, L5 and complexes 1 to 5 were carried out at a temperature of 100 K, using a Bruker Apex II DUO CCD 4K diffractometer with a wavelength (λ) of 0.71073 Å. Crystal structures were determined using SHELXT, then refined by F2-based least-squares procedures using SHELXL.37 Bond distances and angles were determined using the Olex2 or Mercury program, while the representation of each molecular structure was carried out using Mercury program. Detailed information on the refinement data can be found in Tables S1, S2 and S3 in the supplementary information.

Size exclusion chromatography (SEC) analyses of the samples were carried out in THF (+ 0.1% toluene) as eluent at 35 °C (1 mL/min) using an Agilent 1260 pump, a Wyatt Optilab refractometer (Refractive Index, RI) and Waters Styragel columns (HR1, HR3 and HR4) calibrated with polystyrene standards.

All DFT calculations were carried out with the Gaussian 09 suite of programs.38 Geometries were fully optimized in gas phase without symmetry constraints, employing the B3PW91 functional.39 The nature of the extrema was verified by analytical frequency calculations. The calculation of electronic energies and enthalpies of the extrema of the potential energy surface (minima and transition states) were performed at the same level of theory as the geometry optimizations. IRC calculations were performed to confirm the connections of the optimized transition states. Iron atoms were treated with an effective core potential (10 MDF), associated with its adapted basis set40 augmented with a polarization function (ζ<sub>f</sub> = 2.462).41 For the other elements (H, C, N, Cl and F), Pople's triple-ζ basis set 6-311G(d,p) was used.42 The electronic charges (at the DFT level) were computed using the natural population analysis (NPA) technique.43

General Procedure for the Synthesis of Ligands L1–L4

A solution of 2-pyridinecarboxaldehyde (or 2-acetylpyridine) (9 mmol) in toluene (50 mL) and the
corresponding aniline (9.5 mmol, 1.05 equiv.) in the presence of a catalytic amount of *p*-toluenesulfonic
acid (PTSA, 5 mol%) was prepared in a round-bottomed flask fitted with Dean Stark apparatus. The
mixture was refluxed in an oil bath set to 130 °C in open air for the desired time. The solution was
cooled to room temperature, filtered and concentrated under reduced pressure to obtain a viscous crude
product, which was diluted in pentane and filtered over celite. The filtrate was evaporated under reduced
pressure and the residue was dried under high vacuum for a prolonged time to obtain a crystalline
product, which was subsequently used for the complexation reaction.
*N-(3,5-bis(Trifluoromethyl)phenyl)-1-(pyridin-2-yl)methanimine (L1):* reaction time = 1 h, orange crystals. Yield: 60%. ¹H NMR (300 MHz, C₆D₆, 25 °C) δ (ppm) = 8.47 (dd, ³JHH = 4.8 Hz, ⁴JHH = 1.7 Hz, 1H, Hₐ), 8.27 (s, 1H, Hb), 8.08 (d, ³JHH = 7.8 Hz, 1H, Hc), 7.56 (s, 1H, Hd), 7.24 (s, 2H, Hε), 7.08 (ddd, ³JHH = 7.8, 7.8 Hz, ⁴JHH = 1.7 Hz, 1H, Hf), 6.66 (dd, ³JHH = 7.8, 4.8 Hz, 1H, Hg) (Fig. S1, SI). ¹⁹F NMR (282 MHz, C₆D₆, 25 °C) δ (ppm) = -62.61 (s, 6F) (Fig. S2, SI). ¹³C NMR (75 MHz, C₆D₆, 25 °C) δ (ppm) = 164.3, 154.5, 152.8, 148.2, 136.8, 136.4, 136.3, 125.6, 123.5, 121.6, 121.4 (Fig. S3, SI). IR (cm⁻¹) = 1620 ν(C=N). HRMS-ESI (m/z): [M + H]⁺ calcd for C₁₄H₈F₆N₂, 319.0672; found, 319.0679 (Fig. S4, SI).

*N-(Perfluorophenyl)-1-(pyridin-2-yl)methanimine (L2):* reaction time = 1 h, brown crystals. Yield: 49% ¹H-NMR (300 MHz, C₆D₆, 25 °C) δ (ppm) = 8.66 (s, 1H, Hₐ), 8.41 (dd, ³JHH = 4.8 Hz, ⁴JHH = 1.7 Hz, 1H, Hb), 8.09 (d, ³JHH = 7.8 Hz, 1H, Hc), 6.97 (ddd, ³JHH = 7.8 Hz, 7.8 Hz, ⁴JHH = 1.7 Hz, 1H, Hd), 6.59 (dd, ³JHH = 7.8, 4.8 Hz, 1H, Hε) (Fig. S5, SI). ¹⁹F NMR (282 MHz, C₆D₆, 25 °C) δ (ppm) = -153.4 (dd, J = 21.4, 6.4 Hz, 2F, Fmeta), -160.3 (t, J = 21.4 Hz, 1F, Fpara), -163.5 (dd, J = 21.4, 6.4 Hz, 2F, Fortho) (Fig. S6, SI). ¹³C NMR (75 MHz, C₆D₆, 25 °C) δ (ppm) = 170.1, 154.2, 150.1, 142.1, 140.1, 138.5, 136.3, 125.9, 121.6 (Fig. S7, SI). IR/cm⁻¹ = 1628 ν(C=N). HRMS-ESI (m/z): [M + H]⁺ calcd for C₁₂H₅N₂F₅, 273.0451; found, 273.0453 (Fig. S8, SI).

*N-(3,5-bis(Trifluoromethyl)phenyl)-1-(pyridin-2-yl)ethan-1-imine (L3):* reaction time = 4 h, dark yellow crystals. Yield: 88%. ¹H NMR (300 MHz, CD₂Cl₂, 25 °C, δ): = 8.67 (dd, ³JHH = 4.8 Hz, ⁴JHH = 1.7 Hz, 1H, Hₐ), 8.25 (d, ³JHH = 7.8 Hz, 1H, Hc), 7.83 (dd, ³JHH = 7.8, 7.8 Hz, 1H, Hd), 7.65 (s, 1H, Hb), 7.42 (dd, ³JHH = 7.8, 4.8 Hz, 1H, Hε), 7.30 (s, 2H, Hf), 2.37 (s, 3H, Hg) (Fig. S9, SI). ¹⁹F NMR (282 MHz, CD₂Cl₂, 25 °C) δ (ppm) = -63.32 (s, 6F) (Fig. S10, SI). IR/cm⁻¹ = 1649 ν(C=N). HRMS-ESI (m/z): [M + H]⁺ calcd for C₁₅H₁₀N₂F₆, 333.0826; found, 333.0838 (Fig. S11, SI).

*N-(Perfluorophenyl)-1-(pyridin-2-yl)ethan-1-imine (L4):* reaction time = 4 h, orange-brownish crystals. Yield: 45%. ¹H NMR (300 MHz, C₆D₆) δ (ppm) = 8.35 (dd, ³JHH = 4.8 Hz, ⁴JHH = 1.7 Hz, 1H, Hₐ), 8.25 (d, ³JHH = 7.8 Hz, 1H, Hb), 7.03 (ddd, ³JHH = 7.8, 7.8 Hz, ⁴JHH = 1.7 Hz, 1H, Hc), 6.63 (dd, ³JHH = 7.8 Hz, ³JHH = 4.8 Hz, 1H, Hd), 2.20 (s, 3H, Hε) (Fig. S12, SI). ¹⁹F NMR (282 MHz, C₆D₆, 25 °C) δ (ppm) = -152.5 (d, J = 23.8 Hz, 2F, Fmeta), -163.1 (t, J = 21.7 Hz, 1F, Fpara), -163.6 (dd, J = 23.8, 21.7 Hz, 2F, Fortho) (Fig. S13, SI). IR/cm⁻¹ = 1647 ν(C=N). HRMS-ESI (m/z): [M + H]⁺ calcd for C₁₃H₇N₂F₅, 287.0608; found, 287.0617 (Fig. S14, SI).

*N-(3,5-bis(Trifluoromethyl)phenyl)-1-(quinolin-2-yl)methanimine (L5):* To a solution of 2-quinolinecarboxaldehyde (m = 500 mg, n = 3.18 mmol) in CH₂Cl₂ (15 mL) was added 3,5-bis(trifluoromethyl)aniline (m = 481 mg, n = 1 equiv.) followed by three drops of formic acid and magnesium sulfate (500 mg). The solution was stirred at room temperature for 18 h and concentrated under vacuum to yield an orange oil. The product was recrystallized at -20 °C in pentane to form red crystals of L5. Yield: 80%. ¹H NMR (300 MHz, C₆D₆, 25 °C) δ (ppm) = 8.43 (s, 1H, Hₐ), 8.32 (d, ³JHH = 4.8 Hz, ¹H, Hb), 7.08 (dd, ³JHH = 7.8 Hz, ⁴JHH = 1.7 Hz, 1H, Hc), 7.56 (s, 1H, Hd), 7.24 (s, 2H, Hε), 7.08 (ddd, ³JHH = 7.8, 7.8 Hz, ⁴JHH = 1.7 Hz, 1H, Hf), 6.66 (dd, ³JHH = 7.8, 4.8 Hz, 1H, Hg) (Fig. S15, SI).
= 8.2 Hz, 1H, Hb), 8.26 (d, $^3J_{HH}$ = 8.6 Hz, 1H, Hc), 7.64 (d, $^3J_{HH}$ = 8.6 Hz, 1H, Hd), 7.57 (s, 1H, He), 7.42-7.35 (m, 2H, Hfg), 7.29 (s, 2H, Hh), 7.20 (d, $^3J_{HH}$ = 8.2 Hz, 1H, Hi) (Fig. S15, SI). $^{19}$F NMR (282 MHz, C$_6$D$_6$, 25 °C) δ (ppm) = -62.54 (s, 6F) (Fig. S16, SI). $^{13}$C NMR (75 MHz, C$_6$D$_6$, 25 °C) δ (ppm) = 164.5, 154.5, 152.7, 148.7, 136.7, 132.8, 132.4, 130.6, 130.3, 129.3, 125.5, 121.9, 121.5, 119.8, 118.6 (Fig. S17, SI). IR/cm$^{-1}$ = 1620 ν(C=N). HRMS-ESI (m/z): [M + H]$^+$ calcd for C$_{18}$H$_{10}$N$_2$F$_6$, 369.0826; found, 369.0845 (Fig. S18, SI).

**General Procedure for the Synthesis of Complexes 1-5**

The corresponding ligand (3.5 mmol) and anhydrous FeCl₂ (3.5 mmol, 1 equiv.) were added to a Schlenk flask under an argon atmosphere followed by the addition of dry THF (32 mL). The mixture was stirred overnight at room temperature. After 18 hours, the solution was then concentrated under reduced pressure to about 10 mL and dry pentane (30 mL) was added under argon to precipitate the complex. The supernatant was removed with a filter cannula and the residual solid was washed twice with 30 mL of pentane before being dried under vacuum. All complexes were recrystallized using suitable solvent mixtures to yield single crystals, as described below and in the article. The ¹H NMR spectra were recorded in CD₂Cl₂ and CD₃CN and are displayed in Figs. S21 and S22 in the SI, respectively.

**Complex 1 [L1]₂FeCl₂:** single blue crystals of complex 1 were collected from a saturated CH₃CN solution of the complex, followed by slow diffusion of diethyl ether for 1 day, and then stored at -20 °C for a few days. Yield: 30%. ¹H NMR (300 MHz, CD₂Cl₂, 25 °C, δ): 147.3 (Δν₁/₂ = 614 Hz), 67.5 (Δν₁/₂ = 930 Hz), 51.8 (Δν₁/₂ = 154 Hz), -3.1 (Δν₁/₂ = 62 Hz), -8.3 (Δν₁/₂ = 560 Hz), -19.6 (Δν₁/₂ = 124 Hz). The ¹H NMR spectrum reveals six resonances whereas seven protons were expected; the signal could be too broad and lost in the baseline. ¹H NMR (300 MHz, CD₃CN, 25 °C, δ): 174.8 (Δν₁/₂ = 1268 Hz) and 173.8 (Δν₁/₂ = 674 Hz), 61.2 (br, Δν₁/₂ = 560 Hz), 49.2 (Δν₁/₂ = 405 Hz), 23.3 (Δν₁/₂ = 624 Hz), 5.37 (Δν₁/₂ = 446 Hz), 1.13 (Δν₁/₂ = 152 Hz). The ¹H NMR spectrum reveals seven resonances as expected. IR/cm⁻¹ = 1597 ν(C=N). Anal. Calcd. for C₂₈H₁₆Cl₂F₁₂FeN₄ (%): C 44.07, H 2.11, N 7.34; found C 43.70, H 2.085, N 6.69.

**Complex 2 [L2]₂FeCl₂:** single violet crystals of complex 2 were grown from a saturated solution of the complex in CH₃CN followed by slow diffusion of diethyl ether at room temperature for 4 days. Yield: 35%. ¹H NMR (300 MHz, CD₂Cl₂, 25 °C, δ): 202.5 (Δν₁/₂ = 1020 Hz), 149.4 (Δν₁/₂ = 1670 Hz), 125.8 (Δν₁/₂ = 1050 Hz), 91.2 (Δν₁/₂ = 1530 Hz), 63.4 (Δν₁/₂ = 420 Hz), 51.9 (Δν₁/₂ = 223 Hz), 50.7 (Δν₁/₂ = 234 Hz), -11.3 (Δν₁/₂ = 327 Hz). The ¹H NMR spectrum in CD₂Cl₂ reveals eight resonances whereas five protons were expected; a rearrangement of the complex in solution could explain this additional number of signals. ¹H NMR (300 MHz, CD₃CN, 25 °C, δ): 162.2 (Δν₁/₂ = 1850 Hz), 129.3 (Δν₁/₂ = 2400 Hz), 54.0 (Δν₁/₂ = 650 Hz), 46.3 (Δν₁/₂ = 916 Hz), 8.3 (Δν₁/₂ = 323 Hz). In contrast, the ¹H NMR spectrum in CD₃CN reveals five resonances as expected. IR/cm⁻¹ = 1597 ν(C=N). Anal. Calcd. for C₂₄H₁₀Cl₂F₁₀FeN₄ (%): C 42.95, H 1.50, N 8.35; found C 39.11, H 1.554, N 7.79. Despite several attempts, single crystals of
complex 2 were the only ones that failed to give satisfactory results, showing discrepancies of up to 5% between the theoretical and experimental carbon values. For complex 2, the difference between theoretical and experimental values on the analysis of several samples was $ΔC_{theo/exp} = 3 – 5\%$, $ΔH_{theo/exp} = 0.02 – 0.8\%$ and $ΔN_{theo/exp} = 1 – 3\%$. The challenge of determining the CHN values of fluorine-containing compounds is well known and is attributed to fluorine's tendency to attack the quartz tube during pyrolysis, which can lead to misleading results.⁴⁴

**Complex 3 [(L3)₂Fe(μ-Cl₂)FeCl₂]:** single red/pink crystals of complex 3 were grown from a saturated solution of the complex in CH₃CN followed by slow diffusion of diethyl ether at room temperature for 2 to 4 days. Yield: 45%. ¹H NMR (300 MHz, CD₂Cl₂, 25 °C, δ): 72.7 (Δν₁/₂ = 420 Hz), 70.3 (Δν₁/₂ = 92 Hz), 55.21 (Δν₁/₂ = 214 Hz), 49.5 (Δν₁/₂ = 270 Hz), 48.9 (Δν₁/₂ = 150 Hz), -12.6 (Δν₁/₂ = 34 Hz), -13.9 (Δν₁/₂ = 370 Hz), -18.5 (Δν₁/₂ = 78 Hz), -23.7 (Δν₁/₂ = 474 Hz). ¹H NMR (300 MHz, CD₃CN, 25 °C, δ): 68.0 (Δν₁/₂ = 163 Hz), 65.5 (Δν₁/₂ = 103 Hz), 50.1 (Δν₁/₂ = 128 Hz), 47.9 (Δν₁/₂ = 185 Hz), 37.7 (Δν₁/₂ = 514 Hz), 14.6 (Δν₁/₂ = 270 Hz), -8.2 (Δν₁/₂ = 43 Hz), -9.8 (Δν₁/₂ = 60 Hz). The ¹H NMR spectra reveal more resonances than expected in both solvents; a rearrangement of the complex in solution could explain this additional number of signals. IR/cm⁻¹ = 1597 ν(C=N). Anal. Calcd. for C₃₀H₂₀N₄F₁₂Fe₂Cl₄ + two molecules of CH₃CN (%): C, 40.83; H, 2.62; N, 8.40. Found: C, 41.17; H, 2.77; N, 8.72. When crystals are grinded and vacuum dried for a prolonged time (> 24 h), Anal. Calcd. for C₃₀H₂₀N₄F₁₂Fe₂Cl₄ (%): C₃₀H₂₀N₄F₁₂Fe₂Cl₄: C, 39.25; H, 2.20; N, 6.10. Found: C, 39.44; H, 2.34; N, 5.79.

**Complex 4 [(L4)₂Fe(μ-Cl)₂Fe(μ-Cl)Cl]₂:** single violet crystals of complex 4 were grown from a saturated CH₃CN solution of the complex, followed by slow diffusion of diethyl ether for 1 day, and then stored at -20 °C for a few days. Yield: 30%. ¹H NMR (300 MHz, CD₂Cl₂, 25 °C, δ): 70.2 (Δν₁/₂ = 82 Hz), 55.5 (Δν₁/₂ = 135 Hz), 48.9 (Δν₁/₂ = 76 Hz), 47.8 (Δν₁/₂ = 269 Hz), -9.1 (Δν₁/₂ = 326 Hz), -12.4 Δν₁/₂ = 30 Hz - 13.5 (Δν₁/₂ = 331 Hz), -18.2 (Δν₁/₂ = 74 Hz). The ¹H NMR spectrum reveals eight resonances whereas five signals were expected (some small peaks can also be noticeable in the spectra); a rearrangement of the complex in solution could explain this additional number of signals. ¹H NMR (300 MHz, CD₃CN, 25 °C, δ): 130.1 (Δν₁/₂ = 1560 Hz), 64.8 (Δν₁/₂ = 220 Hz), 48.2 (Δν₁/₂ = 257 Hz), 10.2 (Δν₁/₂ = 680 Hz). The ¹H NMR spectrum reveals four resonances whereas five signals were expected; the last signal could be too broad and lost in the baseline. IR/cm⁻¹ = 1595 ν(C=N). Anal. Calcd. for C₅₂H₂₈N₈F₂₀Fe₄Cl₈ + two molecules of CH₃CN (%): C, 38.79; H, 1.98; N, 8.08. Found: C, 38.54; H, 1.91; N, 8.06.

**Complex 5 [(L5)₂Fe(μ-Cl₂)FeCl₂]:** single dark green crystals of complex 5 were produced from a hot concentrated CH₃CN solution and placed at -20 °C for 1 day. Yield: 55%. ¹H NMR (300 MHz, CD₂Cl₂, 25 °C, δ): 93.6 (Δν₁/₂ = 260 Hz), 52.5 (Δν₁/₂ = 73 Hz), 15.5 (Δν₁/₂ = 36 Hz), 6.0 (Δν₁/₂ = 34 Hz), -4.75 (Δν₁/₂ = 30 Hz), -17.3 (Δν₁/₂ = 77 Hz), -20.2 (Δν₁/₂ = 675 Hz), -24.5 (Δν₁/₂ = 50 Hz). ¹H NMR (300 MHz, CD₃CN, 25 °C, δ): 95.5 (Δν₁/₂ = 1760 Hz), 49.7 (Δν₁/₂ = 1030 Hz), 18.33 (Δν₁/₂ = 410 Hz), 5.5 (Δν₁/₂ = 152 Hz).
Hz), –3.8 (Δν₁/₂ = 400 Hz), –16.1 (Δν₁/₂ = 1020 Hz). Both ¹H NMR spectra reveal less resonances than expected; the missing signal could be too broad and lost in the baseline. Anal. Calcd. for C₃₇H₂₄N₄F₁₂Fe₂Cl₄ (%): C, 44.17; H, 2.40; N, 5.57. Found: C, 43.83; H, 2.002; N, 5.47.

**Polymerization of isoprene with complexes 1 to 5:**

In a glovebox, 10 µmol of complex is dissolved in 5 mL of toluene, then 30 µmol of Al¹Bu₃ is added to the solution, followed immediately by addition of 10 µmol of [CPh₃][B(C₆F₅)₄]. The solution is left stirred less than 15 seconds and 5 mmol of isoprene is added. Once a strong mass setting is observed, polymerization is quenched with toluene acidified with HCl, and the polymer is precipitated in 250 mL ethanol containing a small amount of 2,6-di-tert-butyl-4-methylphenol. The resulting solid is separated from the solvent, dissolved in chloroform and the solvent evaporated. The solid obtained is dried under high vacuum prior to NMR and SEC analysis.

**ASSOCIATED CONTENT**

**Supporting Information:** ¹H and ¹³C NMR spectra and HRMS data of all ligands; crystal data and structure refinement details of L4, L5 and complexes, additional information on Mössbauer spectroscopy and DFT analysis, NMR spectra and SEC data of resulting polymers, and Cartesian coordinates of computed complexes (PDF).

References for the Supporting Information are included in the regular reference list of the manuscript. Full crystallographic data are available free of charge from the Cambridge Crystallographic Data Centre (CCDC).

**Accession Codes**

CCDC 2421238-2421244 contain the supplementary crystallographic data for this manuscript. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

**AUTHOR INFORMATION**

**Corresponding Authors**

**Marc Visseaux** – Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181, UCCS, Unité de Catalyse et Chimie du Solide, F-59000, Lille, France; https://orcid.org/0000-0002-2060-9433; Email: marc.visseaux@univ-lille.fr

**Yohan Champouret** – Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181, UCCS, Unité de Catalyse et Chimie du Solide, F-59000, Lille, France; https://orcid.org/0000-0002-4277-7470; Email: yohan.champouret@univ-lille.fr
## Authors

**Obaid H. Hashmi** – *Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181, UCCS, Unité de Catalyse et Chimie du Solide, F-59000, Lille, France*; https://orcid.org/0000-0001-7636-698X

**Raphael Caulier** – *Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181, UCCS, Unité de Catalyse et Chimie du Solide, F-59000, Lille, France*; https://orcid.org/0009-0000-9528-2183

**Frederic Capet** – *Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181, UCCS, Unité de Catalyse et Chimie du Solide, F-59000, Lille, France*; https://orcid.org/0000-0002-8832-5381

**Olivier Mentré** – *Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181, UCCS, Unité de Catalyse et Chimie du Solide, F-59000, Lille, France*; https://orcid.org/0000-0002-1822-6003

**Claire Minaud** – *Univ. Lille, CNRS, Centrale Lille, Univ. Artois, UMR 8181, UCCS, Unité de Catalyse et Chimie du Solide, F-59000, Lille, France*; https://orcid.org/0000-0002-5750-4854

**Jean-François Meunier** – *Laboratoire de Chimie de Coordination UPR 8241, CNRS & Université de Toulouse, 31077 Toulouse, France*; https://orcid.org/0000-0001-9007-473X

**Iker del Rosal** – *LPCNO, UMR 5215, Univ. Toulouse-CNRS, INSA, UPS, 135 Avenue de Rangueil, 31077 Toulouse, France*; https://orcid.org/0000-0001-6898-4550

**Chiara Dinoi** – *LPCNO, UMR 5215, Univ. Toulouse-CNRS, INSA, UPS, 135 Avenue de Rangueil, 31077 Toulouse, France*; https://orcid.org/0000-0002-2097-9696

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

The authors sincerely thank the University of Lille for financially supporting Obaid H. Hashmi and the National Research Agency (ANR) for funding Raphael Caulier and this project (ANR FEATHER, ANR-23-CE06-0023). The authors also acknowledge Aurélie Malfait and Maxence Espinat for SEC analyses, Dr. Eric Buisine for his valuable assistance with NMR spectroscopy, and Céline Delabre for performing elemental analyses. The authors acknowledge the HPCs CALcul en Midi-Pyrénées (CALMIP-EOS grant 1415).

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✓ Nuclearity of Fe complexes governed by ligand properties

<figure><img src="image_17.png" /></figure>

<figure><img src="image_18.png" /></figure>

<figure><img src="image_19.png" /></figure>

✓ X-ray
strucures

<figure><img src="image_20.png" /></figure>

✓ Mössbauer,
magnetism,
DFT

✓ Stereo-/regio-selective isoprene polymerization