# Revisiting [Fe(bipy)2Et2]: electronic structure and new considerations on the thermal decomposition mechanism

Rinaldo Poli

► To cite this version:

Rinaldo Poli. Revisiting [Fe(bipy)2Et2]: electronic structure and new considerations on the thermal decomposition mechanism. Journal of Organometallic Chemistry, 2025, 1039, pp.123787. 10.1016/j.jorganchem.2025.123787. hal-05218836

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# Revisiting [Fe(bipy)₂Et₂]: electronic structure and new considerations on the thermal decomposition mechanism

Rinaldo Poli<sup>a,b,*</sup>

<figure><img src="image_3.png" /></figure>

## ARTICLE INFO

**Keywords:**

Iron

2,2-Bipyridine

Non-innocent ligands

Ligand exchange

Spin crossover reactivity

DFT calculations

## ABSTRACT

The present computational investigation discredits a previously proposed mechanism for the thermal decomposition of the title compound in THF, according to which β-H elimination from one Et ligand would occur after an initial rearrangement of one bipy ligand from κ² to κ¹. It is alternatively proposed that the initial step is full replacement of one bipy ligand with one solvent molecule, to yield 5-coordinate [(bipy)(THF)FeEt₂]. The title compound, the 5-coordinate [(κ²-bipy)(κ¹-bipy)FeEt₂], the 5-coordinate [(bipy)(THF)FeEt₂] and the 6-coordinate [(bipy)(THF)₂FeEt₂] complexes have been analyzed in all possible spin states. While [(bipy)₂FeEt₂] is diamagnetic, but suggested by the calculation to rather adopt an open-shell (broken-symmetry) singlet with a low spin (S = ½) Fe³⁺ and non-innocent bipy ligands, [(bipy)₂]⁺, the other three systems have a broken-symmetry triplet ground state (S = 1 overall) with an intermediate spin (S = 3/2) Fe³⁺. The low computed Gibbs energy cost for the bipy/THF substitution (5.6 kcal mol⁻¹) makes [(bipy)(THF)FeEt₂] a kinetically competent intermediate. Full bipy dissociation prior to the β-H elimination step is also consistent with the previously observed dependence of the decomposition rate on the solvent donor properties.

## 1. Introduction

Compound [(bipy)₂FeEt₂] (bipy = 2,2'-bipyridine) was first described in 1968 by Yamamoto et al [1] and used as a butadiene cyclodimerization catalyst [1], as an initiator for the polymerization of various monomers such as acetylene, acrylonitrile, methyl methacrylate, acrylamide, and others [1–5], and more recently as an additive in dual coordinative chain transfer olefin polymerization catalysis [6–8]. Its sharp ¹H NMR spectrum demonstrates its diamagnetism, while X-ray crystallography has established its cis-octahedral coordination geometry [9]. It is therefore a textbook example of a low-spin octahedral d⁶ system, as expected on the basis of the strong-field ethyl ligands. The π-accepting ability of the bipydine ligands may also contribute to keep the system diamagnetic, since complex [Fe(bipy)₃]²⁺ is also a low-spin system [10], whereas many other octahedral Fe¹¹ complexes, e.g [FeCl₂(bipy)₂], adopt a high-spin (S = 2) ground state and several other similar complexes exhibit spin-crossover behavior [11,12]. However, a recent study by Chirik *et al.* on the closely related [Fe(c-C₄H₈)(phen)₂] complex, based on a combination of X-ray diffraction, Mössbauer spectroscopy and DFT calculations, demonstrated the non-innocent behavior of the phenantroline ligands, pointing to the presence of a

low spin (S = ½) Fe³⁺ center antiferromagnetically coupled to one delocalized electron on partially reduced and equivalent phen ligands, [(phen)₂]⁺ [13]. On the basis of this result and on DFT calculations presented in the same contribution [13] for the related [FeMe₂(bipy)₂], the title complex was suggested to adopt an electronic structure identical to that of [Fe(c-C₄H₈)(phen)₂]. Indeed, the non-innocent behavior of the bipy ligand is well-known [14–22], a relevant related example being [(bipy)(bipy·)ₚCr³⁺Cl₂] [17].

Another point of interest for the title compound is the mechanism of its thermal decomposition. Detailed investigations reported in a seminal contribution by Kochi *et al.* [9] showed that it slowly decomposes at 50 °C in THF to selectively yield the disproportionation products C₂H₆ and C₂H₄, with <0.1 % of the coupling product n-butane. This reaction was proposed to proceed *via* a classical β-H elimination/reductive elimination mechanism, whereby an open coordination site is generated by partial dissociation of a bipy ligand, allowing the generation of a hydride-ethylene intermediate (Scheme 1). This pathway was supported by the observed retardation effect of added free bipyridine, which led the authors to suggest competitive coordination to the unsaturated 5-coordinate intermediate with generation of a saturated [Fe¹¹(κ²-bipy)(κ¹-bipy)₂Et₂] complex (Scheme 1). In the present contribution, on the

<footnote><sup>a</sup> CNRS, LCC (Laboratoire de Chimie de Coordination), Université de Toulouse, UPS, INPT, 205 Route de Narbonne, BP 44099, F-31077 Toulouse Cedex 4, France</footnote>

<footnote><sup>b</sup> Institut Universitaire de France, 1 rue Descartes, 75231 Paris Cedex 05, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: rinaldo.poli@lcc-tdhouse.fr.</footnote>
basis of DFT calculations, this decomposition mechanism is found to be sterically prohibited and an alternative mechanism, involving full dissociation of one bipy ligand, is proposed as the initial step.

## 2. Computational details

The computational work was carried out using the Gaussian09 suite of programs [23]. Gas-phase geometry optimizations were performed without any symmetry constraint using the B3PW91* functional, which is a reparametrized version of B3PW91 with the same parameters previously optimized for B3LYP [24], and the 6-311G(d,p) basis functions for all light atoms (H, C, N, O), whereas the Fe atom was treated with the LANL2DZ basis set augmented by an f polarization function ($α = 2.462$) [25]. The unrestricted formulation was used for open-shell molecules. All final geometries were characterized as local minima by verifying that all second derivatives of the energy were positive. Corrections for dispersion were carried during the B3PW91* geometry optimizations using Grimme's D3 empirical method (BPW91*-D3), using SR6 and S8 parameters identical to those optimized for B3PW91 [26]. Thermochemical corrections were obtained on the fixed optimized geometries at 298.15 K on the basis of frequency calculations, using the standard approximations (ideal gas, rigid rotor and harmonic oscillator). A correction for solvation effects in THF solution was calculated by treating the solvent medium as a polarizable continuum with the SMD approach [27], using the standard parameters and the dielectric constant ($ε = 7.4257$) of the Gaussian program package. A further correction of 1.95 kcal/mol was applied to bring the G values from the gas phase (1 atm) to the solution (1 mol/L) standard state [28].

## 3. Results and discussion

The choice of functional for the DFT investigation of first row metal complexes with different spin isomers at close energies is particularly delicate. This has been highlighted in particular for Fe spin crossover complexes, for which a reparametrized version of B3LYP with a reduced $c_3$ coefficient was shown to best fit the experimental behavior [24, 29–31]. When dealing with bond breaking processes, the inclusion of dispersion effects is also important. I have previously used a reparametrized B3PW91 functional, with correction for dispersion effects by Grimme's D3 method (B3PW91*-D3) [26], to study a number of Fe systems. This approach generally gave good quantitative agreement with a number of experimentally available energy data [32–34]. Therefore, this functional has also been selected for the present system, but the relative energies of spin isomers for the title compound were also assessed at other levels (vide infra). However, I wish to underline that the insights provided by the obtained results go beyond the absolute values of the calculated thermodynamic parameters, thus extensive benchmarking of various computational approaches against the few available

experimental data would not be justified. A solvation correction was also applied in THF, which was the solvent used in the experimental investigation [9]. All optimized local minima are described by their relative Gibbs energy values in THF in the standard state (298 K, 1 M concentration).

### (a) Electronic structure of [(bipy)$_2$FeEt$_2$]

Octahedral $Fe^{II}$ complexes can in principle exist in the singlet, triplet and quintet states [35]. As stated in the Introduction, complex [(bipy)$_2$FeEt$_2$] is known to be diamagnetic. In agreement with the previously reported DFT investigation of the closely related [(bipy)$_2$FeMe$_2$] and [(phen)$_2$Fe(c-C$_4$H$_8$)] complexes [13], the broken-symmetry (bs) singlet solution was found more stable than the restricted closed-shell singlet (the Gibbs energy advantage for the bs solution is 3.0 kcal mol$^{-1}$). Therefore, the molecule is best described as a low-spin $Fe^{III}$ complex ($S = 1/2$) with antiferromagnetic coupling of the metal electron with an unpaired electron delocalized on the non-innocent [(bipy)$_2$]* ligand system (formal $-0.5$ charge for each bipy ligand, since the two bipy ligands are equivalent in the optimized structure, see Table 1). The spin density distribution in the bs singlet structure is shown in greater detail in Fig. 1a. The spin density on each H atoms (not shown in the Figure) is <0.01 (maximum 0.008, minimum $-0.002$, average 0.001, sum 0.032).

Within the present investigation, the higher spin states have also been considered. There are several possible electronic structures, depending on the spin density distribution. Using the ($S_{Fe}$, $S_{bipy}$) symbolism to represent them, the two singlets can be identified as (0,0) for the restricted singlet, [(bipy)$_2$Fe$^{II}$Et$_2$], and (1/2, -1/2) for the bs singlet, [(bipy)(bipy*)$^{II}$Fe$^{III}$Et$_2$]. There are therefore three possible triplet ($S = 1$) states with different electronic distributions: (1,0), (1/2,1/2) and (3/2, -1/2); and three possible quintet ($S = 2$) states: (2,0), (3/2,1/2) and (5/2, -1/2). There is also an additional possibility with a (5/2,1/2) electronic distribution, for which the overall spin state is septet. All systems where $S_{bipy}$ is either 1/2 or $-1/2$ are formally $Fe^{III}$ complexes in the low (1/2), intermediate (3/2) or high (5/2) spin state, whereas those with $S_{bipy} = 0$ are formally $Fe^{II}$ complexes in the low (0), intermediate (1) or high (2) spin state. All possibilities were explored and the results are reported in Table 1.

The freely optimized triplet and quintet states spontaneously converged to electronic distributions indicating $Fe^{III}$ complexes, namely (1/2,1/2) and (3/2,1/2), respectively, rather than $Fe^{II}$ complexes, confirming the preference of this ligand environment to oxidize the metal center and the non-existence of the (1,0) and (2,0) spin distributions. This is most clearly indicated by the spin density on the (bipy)$_2$ ligands (Table 1), which is close to 1, whereas the spin density on the Fe atom is much lower than expected for the $Fe^{II}$ formulation (1.29 for the triplet and 3.34 for the quintet). A small spin density is also present on the Et ligands, but the majority is in all cases shared by the Fe center and the two bipy ligands. Application of the broken symmetry approach to the

<figure><img src="image_4.png" /><figcaption>Scheme 1. Proposed decomposition mechanisms for [(bipy)$_2$FeEt$_2$] [9].</figcaption></figure>
<caption>**Table 1**</caption>

Bond distances (Å), angles (°), relative standard Gibbs energies in THF (ΔG<sub>298,THF</sub>, kcal mol<sup>-1</sup>), <S<sup>2</sup>> at convergence, atomic or group Mulliken charges (q) and spin densities (s) for the optimized geometries of all cis-[(bipy)<sub>2</sub>FeEt<sub>2</sub>] spin isomers<sup>a</sup>.

<table><thead><tr><th rowspan="2">Parameter</th><th rowspan="2">X-ray<sup>b</sup></th><th colspan="6">DFT</th></tr><tr><th>Singlet (0,0)</th><th>Singlet (1/2, -1/2)</th><th>Triplet (1/2,1/2)</th><th>Triplet (3/2, -1/2)</th><th>Quintet (3/2,1/2)<sup>c</sup></th><th>Septet (5/2,1/2)</th></tr></thead><tbody><tr><td>Fe-C</td><td>2.063(2), 2.068(2)</td><td>2.043, 2.061</td><td>2.012, 2.035</td><td>2.002, 2.016</td><td>2.015, 2.049</td><td>2.011, 2.030</td><td>2.090, 2.104</td></tr><tr><td>Fe-N (trans to Et)</td><td>1.976(2), 1.985(2)</td><td>1.948, 1.957</td><td>1.993, 2.004</td><td>2.051, 2.076</td><td>2.047, 2.206</td><td>2.111, 2.228</td><td>2.304, 2.326</td></tr><tr><td>Fe-N (trans to N)</td><td>1.937(2), 1.943(2)</td><td>1.927, 1.940</td><td>1.956, 1.956</td><td>1.946, 1.951</td><td>2.192, 2.235</td><td>2.195, 2.242</td><td>2.193, 2.196</td></tr><tr><td>C-C (interpyridine)</td><td>1.456(3), 1.451(3)</td><td>1.451, 1.454</td><td>1.445, 1.453</td><td>1.440, 1.442</td><td>1.450, 1.461</td><td>1.444, 1.457</td><td>1.452, 1.453</td></tr><tr><td>C-Fe-C</td><td>85.0(1)</td><td>83.7</td><td>86.1</td><td>88.6</td><td>85.2</td><td>86.4</td><td>103.6</td></tr><tr><td>C-Fe-N (trans)</td><td>170.4(1), 171.7(1)</td><td>169.0, 173.3</td><td>171.8, 172.8</td><td>169.8, 175.6</td><td>166.8, 172.6</td><td>163.8, 173.7</td><td>156.0, 166.1</td></tr><tr><td>C-Fe-N (cis), Et<sup>1</sup></td><td>89.2(1), 91.9(1), 92.3(1)</td><td>87.2, 94.4, 95.0</td><td>88.6, 95.6, 99.1</td><td>87.8, 94.1, 94.6</td><td>90.5, 96.6, 101.7</td><td>91.4, 95.5, 103.7</td><td>97.6, 97.6, 98.6</td></tr><tr><td>C-Fe-N (cis), Et<sup>2</sup></td><td>91.1(1), 91.7(1), 91.9(1)</td><td>86.5, 92.4, 92.7</td><td>88.0, 93.2, 93.7</td><td>89.4, 96.8, 100.0</td><td>91.3, 98.6, 102.2</td><td>95.0, 99.3, 104.2</td><td>87.4, 94.7, 102.0</td></tr><tr><td>N-Fe-N (same bipy)</td><td>80.4(1), 80.9(1)</td><td>81.0, 81.2</td><td>80.7, 80.7</td><td>80.5, 81.1</td><td>72.4, 76.2</td><td>71.8, 75.5</td><td>71.0, 71.6</td></tr><tr><td>N-Fe-N (cis, ≠ bipy)</td><td>95.6(1), 95.6(1), 95.9(1)</td><td>93.9, 95.3, 98.0</td><td>87.2, 92.1, 95.7</td><td>83.9, 90.0, 93.2</td><td>83.2, 88.2, 95.2</td><td>79.3, 86.8, 91.1</td><td>73.4, 85.8, 88.6</td></tr><tr><td>N-Fe-N (trans)</td><td>174.7(1)</td><td>174.7</td><td>172.2</td><td>169.4</td><td>159.8</td><td>156.7</td><td>153.2</td></tr><tr><td>ΔG<sub>298,THF</sub></td><td></td><td>3.0</td><td>0.0</td><td>3.1</td><td>-1.9</td><td>0.8</td><td>15.0</td></tr><tr><td>&lt;S<sup>2</sup>&gt;</td><td></td><td>0</td><td>0.685</td><td>2.047</td><td>2.900</td><td>6.147</td><td>12.017</td></tr><tr><td>q/s (Fe)<sup>d</sup></td><td></td><td>0.60/0.00</td><td>0.60/0.86</td><td>0.58/1.29</td><td>0.69/3.10</td><td>0.67/3.34</td><td>0.83/4.33</td></tr><tr><td>q/s (Et)<sub>2</sub><sup>d,e</sup></td><td></td><td>-0.45/0.00</td><td>-0.37/-0.09</td><td>-0.22/-0.17</td><td>-0.25/-0.33</td><td>-0.17/-0.34</td><td>-0.49/0.43</td></tr><tr><td>q/s (bipy)<sub>2</sub><sup>d,e</sup></td><td></td><td>-0.15/0.00</td><td>-0.23/-0.77</td><td>-0.36/0.88</td><td>-0.44/-0.77</td><td>-0.50/1.00</td><td>-0.34/1.23</td></tr></tbody></table>

<sup>a</sup> The spin state is indicated as (S<sub>Fe</sub>,S<sub>bipy</sub>), as suggested by analysis of the Mulliken spin distribution.

<sup>b</sup> From ref [9].

<sup>c</sup> The antiferromagnetic (5/2, -1/2) quintet spin isomer optimized to the same minimum as the regular quintet.

<sup>d</sup> q = Mulliken charge; s = Mulliken spin density.

<sup>e</sup> Cumulative values for all atoms in the ligands.

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** (a) Spin density distribution in the optimized *bs* singlet structure of [(bipy)<sub>2</sub>FeEt<sub>2</sub>]. (b) Gibbs energies (kcal mol<sup>-1</sup>), relative to the *bs* singlet, of all spin isomers of cis-[(bipy)<sub>2</sub>FeEt<sub>2</sub>]. The BP86, B3LYP* and B3LYP values were obtained by single-point calculations and using thermal corrections from the B3PW91*-optimized geometries. The <S<sup>2</sup>> values are in angle brackets.</figcaption></figure>

triplet and quintet states did produce a new solution for the triplet state, namely (3/2, -1/2), whereas the initial (5/2, -1/2) guess for the quintet state converged again to the same (3/2,1/2) solution. With the exception of the septet, which is much less stable, the energies of all spin isomers are in a relatively small 5 kcal mol<sup>-1</sup> range. The calculations predict the (3/2, -1/2) triplet solution to be the ground state, in clear disagreement with the experimental evidence.

Obviously, the selected B3PW91* functional must overestimate the stability of the (3/2, -1/2) triplet by at least 2 kcal mol<sup>-1</sup>. A rapid foray into other functional was carried out on the single-point B3PW91*-optimized geometries, yielding the results shown in Fig. 1b The B3LYP* functional, previously shown to afford the best results for a large collection of Fe<sup>II</sup> spin crossover complexes [24,29–31], expectedly yields results very similar to those of B3PW91*. The application of the standard

B3LYP led, as expected, to greater stabilization of the higher spin states relative to the reparametrized functionals. On the other hand, as also expected, the pure BP86 functional destabilizes the higher spin states and places the restricted singlet solution as the lowest energy. It is also interesting to note that BP86 leads to extensive quenching of the spin density for the two *bs* solutions (<S<sup>2</sup>> = 0.032 for the *bs* singlet and 2.351 for the *bs* triplet), i.e. close to the values of the (0,0) and (1/2,1/2) solutions, respectively, whereas all other functionals yield consistently higher values, see Fig. 1b A full geometry optimization of the *bs* singlet with BP86 led to convergence to the restricted singlet solution. Further exploration of other functionals was not carried out. Clearly, the appropriate description of this organometallic Fe<sup>II</sup> complex is a delicate exercise. The remainder of the investigation was continued with the B3PW91* functional, keeping in mind that, in all evidence, it slightly
overestimates the stability of the higher spin state solutions.

A comparison of the experimentally determined X-ray structure of the title compound with those optimized for the various spin states (Table 1) may help in the assessment of the ground state (the nature of the ground state as a singlet is undebatable on the basis of the NMR spectrum, but does the complex contain Fe<sup>II</sup> or Fe<sup>III</sup>?). The interpyridine C—C distance has often been taken as an indicator of the bipy ligand electronic state because it tends to lengthen upon reduction [15,36], but strong back-bonding without formal electron transfer was shown to produce the same effect [37]. The calculated interpyridine C—C distances in the two singlet spin isomers (Table 1) are practically identical and both are consistent with those experimentally determined by X-ray diffraction [9]. Other differences between the two optimized singlet geometries are too subtle and both geometries are in comparably good agreement with the experimental X-ray structure. The most notable difference is perhaps exhibited by the Fe-C distances, which are slightly shorter in the *bs* singlet, as well as in all other higher-spin isomers with the only exception of the septet state, relative to the restricted singlet. This phenomenon is consistent with metal oxidation, since Fe<sup>III</sup> has a smaller ionic radius than Fe<sup>II</sup> and promotes greater bond covalency. On the other hand, promotion of electrons from the non-bonding t<sub>2g</sub> orbitals to the metal-ligand antibonding *e*<sub>g</sub> orbitals is expected to lengthen the metal-ligand bonds. Thus, only high-spin (t<sub>2g</sub>e<sub>g</sub><sup>0</sup>) Fe<sup>III</sup> leads to longer Fe-Et bonds, whereas low-spin (t<sub>2g</sub>e<sub>g</sub><sup>0</sup>) Fe<sup>III</sup> and even intermediate-spin (t<sub>2g</sub>e<sub>g</sub><sup>1</sup>) Fe<sup>III</sup> yield shorter Fe-Et bonds than low-spin (t<sub>2g</sub>e<sub>g</sub><sup>0</sup>) Fe<sup>II</sup>. While the experimental Fe-C bond lengths match better those optimized for the restricted singlet, the difference between the two singlet isomers is quite small (< 0.04 Å). Therefore, the comparison of experimental (X-ray) and optimized structural parameters does not allow a clear-cut electronic structure assignment. On the basis of the electronic similarity between this complex and the [(phen)<sub>2</sub>Fe(c(C<sub>4</sub>H<sub>8</sub>)] complex, for which the *bs* singlet structure appears in greater agreement with the Mössbauer spectrum [13], the open-shell antiferromagnetic singlet solution is likely also the ground state of [(bipy)<sub>2</sub>FeEt<sub>2</sub>].

The trans-[(bipy)<sub>2</sub>FeEt<sub>2</sub>] stereoisomer was also calculated in the restricted singlet state and found less stable by 9.6 kcal mol<sup>-1</sup> than the *bs* singlet *cis* isomer. A broken symmetry approach led, in this case, to the same geometry and energy as the spin restricted optimization, with zero spin density on all atoms. The higher energy of this isomer can be attributed to the combined effect of the mutual *trans* influence of the two Et ligands (the Fe-C distance is on average 2.02 Å in the *cis* structure and 2.11 Å in the *trans* structure) and of the steric repulsion between the two bipy ligands, which cannot adopt a coplanar arrangement, see Figure S1.

### (b) Exploration of the β-H elimination in [(bipy)<sub>2</sub>FeEt<sub>2</sub>]</

According to the mechanistic proposal made by Yamamoto *et al.* [4], by Kochi *et al* [9] and later by other authors [5,13,38], complex *cis*-[(bipy)<sub>2</sub>FeEt<sub>2</sub>] starts its decomposition with the partial dissociation of one bipy ligand (Scheme 1), in order to liberate the needed vacant coordination site for the β-H elimination process. The geometry optimization of *cis*-[(κ<sup>2</sup>-bipy)(κ<sup>1</sup>-bipy)FeEt<sub>2</sub>] gave a stable minimum located at 18.7 kcal mol<sup>-1</sup> above the starting octahedral complex as a restricted singlet, or 15.4 kcal mol<sup>-1</sup> as a broken-symmetry singlet. The two spin isomers exhibit an essentially identical geometry (Fig. 2) and do not contain a vacant coordination site. The position left vacant by the dissociated 1'-N atom is occupied by the 5'-CH moiety in an agostic interaction.

The rigid nature of the bipy ligand, which limits the possible rearrangements only to rotations around the Fe-N(1) and C(2)-C(2') bonds, makes it impossible to move the dissociated pyridine ring much further away from the metal center. A 180° rotation around the Fe-N bond would result in a clash of the dissociated pyridine ring with the ethyl ligand. Under these circumstances, it is impossible to envisage the addition of a third bipy ligand to produce the previously proposed (Scheme 1) [(κ<sup>2</sup>-bipy)(κ<sup>1</sup>-bipy)<sub>2</sub>FeEt<sub>2</sub>] intermediate. Indeed, use of a molecular builder shows unbearable steric clashes when attempting to accommodate the third bipy ligand in a monodentate fashion to the

<figure><img src="image_6.png" /><figcaption>Fig. 2. Optimized geometry of [(κ<sup>2</sup>-bipy)(κ<sup>1</sup>-bipy)FeEt<sub>2</sub>] (S = 0) with selected bond distances (Å) in blue.</figcaption></figure>

position occupied by the agostic interaction of Fig. 2. Geometry optimization of a [(κ<sup>2</sup>-bipy)(κ<sup>1</sup>-bipy)<sub>2</sub>FeEt<sub>2</sub>] structure was nevertheless attempted, from a guess geometry with Fe-N distances of 2.15 Å for the two κ<sup>1</sup>-bipy ligands, but the optimization process led to the reestablishment of the Fe-(5'-CH) agostic interaction with one of the two ligands and to expulsion of the other one (Figure S2). Hence, the observed retardation of the decomposition rate by addition of bipy cannot be attributed to the formation of this intermediate. Furthermore, an attempted optimization of the putative β-H elimination product, [(bipy)<sub>2</sub>Fe(H)(Et)(C<sub>2</sub>H<sub>4</sub>)], did not provide a stable minimum for this chemical composition, but rather led to a high-energy configuration of [(κ<sup>2</sup>-bipy)(κ<sup>1</sup>-bipy)FeEt<sub>2</sub>] in which the 6th coordination site of the metal ion establishes an agostic interaction with the β-C-H bond of one Et group (at 15.7 and 28.1 kcal mol<sup>-1</sup> for the broken-symmetry singlet and restricted singlet, respectively, from the starting [(bipy)<sub>2</sub>FeEt<sub>2</sub>]), see Figure S3.

### (b) Bipy/THF exchange in [Fe(bipy)<sub>2</sub>Et<sub>2</sub>]</

In one of their previous contributions [3], Yamamoto *et al.* demonstrated that the [(bipy)<sub>2</sub>FeEt<sub>2</sub>] decomposition is highly solvent-dependent, higher rates being observed in solvents with greater donor power (MeCN > DMF > acetone > THF), whereas decomposition was not detected in Et<sub>2</sub>O and n-hexane at room temperature. It is thus logical to envisage that the required first step is full dissociation of one bipy ligand and coordination of one or two monodentate ligands.

The bipy substitution reaction is indeed computed as a low energy-cost process, see Fig. 3. Partial dissociation produces the already mentioned [(κ<sup>2</sup>-bipy)(κ<sup>1</sup>-bipy)FeEt<sub>2</sub>], but the most stable spin isomer of this species is actually the antiferromagnetic triplet state [(κ<sup>2</sup>-bipy*)<sub>2</sub>(κ<sup>1</sup>-bipy)Fe<sup>III</sup>Et<sub>2</sub>] in which the Fe<sup>III</sup> ion has an intermediate spin configuration, namely (3/2, -1/2). Contrary to the same spin isomer of the [(κ<sup>2</sup>-bipy)<sub>2</sub>FeEt<sub>2</sub>] precursor, the ligand unpaired electron is now entirely confined within the κ<sup>2</sup>-bipy ligand, with negligible spin density on the κ<sup>1</sup>-bipy ligand. In agreement with the selective reduction of the κ<sup>2</sup>-bipy ligand, a significant negative charge is borne by this ligand (-0.426), whereas the κ<sup>1</sup>-bipy ligand has a small positive one (0.004). All other spin isomers of [(κ<sup>2</sup>-bipy)(κ<sup>1</sup>-bipy)FeEt<sub>2</sub>] are much higher in energy, particularly the singlets. The closest one, at 4.0 kcal/mol from the (3/2, -1/2) ground state, is the quintet state (3/2,1/2), the relative stability of which may be overestimated, relatively to the lower-spin states, by the B3PW91* functional, even though the over-stabilisation may depend more on the local spins rather than on the overall multiplicity. Hence, the nature of the ground state for this intermediate seems unambiguous. Selected geometrical parameters, relative energies and charge and spin density distributions for all spin isomers are collected in Table S1. Like for the 18-electron [(κ<sup>2</sup>-bipy)<sub>2</sub>FeEt<sub>2</sub>] precursor, only one quintet solution with the (3/2,1/2) spin distribution could be obtained. Views of all optimized structures are available in Figure S4 (the two singlets are also shown in Fig. 2). The “agostic” Fe···H—C interaction is present only in the two singlets and in the ferromagnetic (1/2,1/2) triplet, which adopt
<figure><img src="image_7.png" /><figcaption>Fig. 3. Gibbs energy profiles for the bipy ligand substitution by THF.</figcaption></figure>

a pseudo-octahedral coordination geometry, whereas the (3/2, -1/2) triplet and the (3/2,1/2) quintet exhibit a trigonal bipyramidal structure and the septet is square pyramidal. Obviously, only 16-electron Fe<sup>II</sup> complexes with S = 0 and 15-electron Fe<sup>III</sup> complexes with S = 1/2 can offer an empty *d* metal orbital to establish a 3c-2e interaction with the β-CH bond of the Et ligand, whereas all higher spin states have all *d* orbitals at least half-filled [35].

The (3/2, -1/2) ground state of the 16-electron [(κ²-bipy)(κ¹-bipy)FeEt₂] intermediate is slightly more stable (-0.2 kcal mol⁻¹) than the 18-electron *bs* singlet complex, but less so than the 18-electron (3/2, -1/2) isomer. The ground state of the molecule must obviously be the singlet [(κ²-bipy)₂FeEt₂] precursor, but partial dissociation of one bipy ligand is predicted to be a very facile process and occurs with a change of spin state.

As already argued in the previous section, the rigidity of the bipy
ligand in [(κ²-bipy)(κ¹-bipy)FeEt₂] does not allow the formation of a
ligand adduct, hence donor solvent molecules can only coordinate by
substituting the (κ¹-bipy) ligand. The thermodynamics of the bipy ligand
substitution by THF for all spin states is illustrated in Fig. 3. Note that the
neat solvent ([THF] = 12.3 M) was used as standard state for the TS
correction. The exchange is uphill in all cases, indicating that THF has
lower coordinating power than (κ¹-bipy). The residual (κ²-bipy) ligand
is still non-innocent, yielding spin isomers with very similar coordina-
tion geometries and electron density distributions as the parent [(κ²-
bipy)(κ¹-bipy)FeEt₂] system (Table S2 and Figure S4). The largest dif-
ference is observed for the septet system, which rearranges from a
square pyramid (axial Et) to a trigonal bipyramid (axial THF and one
bipy N) as a result of the ligand substitution. Another difference con-
cerns the antiferromagnetic quintet (5/2, -1/2): while [(bipy)₂FeEt₂]
and [(κ²-bipy)(κ¹-bipy)FeEt₂] converged to the same ferromagnetic (3/
2,1/2) solution, the THF complex yielded a new minimum, which cor-
responds however to a high spin [(κ²-bipy)(THF)Fe¹¹Et₂] complex with
an “innocent” bipy ligand, i.e. (2,0). The spin density distribution in this
spin isomer (Table S2) features essentially 4 unpaired electrons on the
metal atom and little residual spin density on the ligands. The other
quintet corresponds again to the ferromagnetic (3/2,1/2) solution. The
lowest energy isomer remains the antiferromagnetic triplet state, with a
free energy cost of only 5.6 kcal mol⁻¹ for the ligand exchange from the
starting [(bipy)₂FeEt₂] complex.

Further addition of a second THF molecule yields 18-electron [(bipy)(THF)₂FeEt₂], the ground state of which is again the antiferromagnetic triplet (3/2, -1/2). In this case, the guess (5/2, -1/2) quintet converged again to the (3/2,1/2) solution. The (0,0) and (1/2,1/2) spin isomers exhibit a pseudo-octahedral coordination geometry, whereas one of the

two THF ligands is only loosely bonded to the metal center in the other
spin isomers and minimally perturbs the coordination geometry of the
[(bipy)(THF)FeEt₂] precursor. The charge and spin density distributions
are quite close to those of [(bipy)(THF)FeEt₂] for all spin isomers
(Table S3 and Figure S4). The THF addition process is slightly endoergic
(except for the restricted singlet and (1/2,1/2) triplet. In conclusion, the
bipy ligand substitution process by THF appears to yield the 5-coordi-
nate mono-THF complex with a broken-symmetry triplet configuration
(3/2, -1/2) as the most stable intermediate.

It is interesting to comment on the different electronic structures of the ground states in the four different stoichiometries examined above: [(bipy)₂FeEt₂], [(κ²-bipy)(κ¹-bipy)FeEt₂], [(bipy)(THF)FeEt₂] and [(bipy)(THF)₂FeEt₂]. The first one prefers the singlet state, while all others are more stable as triplets. The most important difference is the presence of two κ²-coordinated bipy ligands in the first one, while there is only one in the other three. Thus, the first complex enjoys a stronger ligand field, possibly associated to the increased π-acidic nature, favoring the lower spin state. Thus, the κ¹-bipy ligand behaves similarly to THF, since it is “innocent” in all different spin configurations of [(κ²-bipy)(κ¹-bipy)FeEt₂], and the coordination geometries of each pair of [(κ²-bipy)(κ¹-bipy)FeEt₂] and [(bipy)(THF)FeEt₂] with the same spin state are identical. However, the presence of just one κ²-coordinated bipy ligand is sufficient to yield non-innocence, resulting in a Fe¹¹¹ system.

The less crowded coordination sphere in [(bipy)(THF)FeEt₂] should permit β-H elimination, ultimately leading to the formation of ethylene and ethane. It should be underlined, however, that this process requires the availability of an empty orbital and no such orbital is available for the Fe¹¹¹ ion in the broken-symmetry (3/2, -1/2) triplet configuration of the [(bipy)(THF)FeEt₂] complex. Thus, the continuation of this process may involve additional spin state changes. The remainder of the decomposition mechanism is currently under scrutiny and will be reported in due course.

**4. Conclusions**

The most important result of the present investigation is to show that
the thermal decomposition of the title compound in THF cannot take
place via the previously proposed mechanism, according to which the
retardation effect by excess bipy was attributed to competitive coordi-
nation of an additional bipy ligand to a 5-coordinate [(κ²-bipy)(κ¹-bipy)
FeEt₂] intermediate to yield 6-coordinate [(κ²-bipy)(κ¹-bipy)₂FeEt₂]
(Scheme 1). Such a coordination is sterically impossible, while β-H
elimination from the Et ligand on this 5-coordinate intermediate stops at
a high-energy β-Et agostic structure. On the other hand, the experi-
mentally proven dependence of the decomposition rate on the donating
properties of the solvent molecules suggests that the reactions starts with
full replacement of one bipy ligand with THF. The resulting 5-coordinate
[(bipy)(THF)FeEt₂], in its broken-symmetry triplet ground state (quartet
Fe<sup>III</sup> antiferromagnetically coupled to a (bipy)₂ ligand system) has
indeed a kinetically accessible energy and thus represents the likely
intermediate in which the follow-up β-H elimination occurs, though the
intimate nature and spin state of all subsequent intermediates remain to
be established. The present investigation also suggests that [(bipy)₂
FeEt₂], like the previously reported [Fe(c-C₄H₈)(phen)₂] [13], is not a
Fe<sup>II</sup> complex but rather an open-shell complex of Fe<sup>III</sup> (low spin, S = 1/2)
antiferromagnetically coupled to one unpaired electron on (bipy)₂, to
yield a broken-symmetry singlet.

CRediT authorship contribution statement

**Rinaldo Poli:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Conceptualization.

Declaration of competing interest

The authors declare the following financial interests/personal re-
lationships which may be considered as potential competing interests:

Rinaldo Poli reports a relationship with University of Toulouse that
includes: non-financial support.

Acknowledgements

The CALMIP mesocenter of the University of Toulouse is gratefully
acknowledged for the allocation of computational resources.

Supplementary materials

Supplementary material associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2025.123787.

Data availability

Data will be made available on request.

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