# Bioinspired bi- or trinuclear FeMoS model complexes for the partial region of FeMo cofactor of nitrogenase

Maxime Laurans, Philippe Schollhammer

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Maxime Laurans, Philippe Schollhammer. Bioinspired bi- or trinuclear FeMoS model complexes for the partial region of FeMo cofactor of nitrogenase. Coordination Chemistry Reviews, 2026, 548 (Part 1), pp.217128. 10.1016/j.ccr.2025.217128. hal-05291105

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Submitted on 12 Dec 2025

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# Bioinspired bi- or trinuclear FeMoS model complexes for the partial region of FeMo cofactor of nitrogenase

Maxime Laurans<sup>a, *,</sup> , Philippe Schollhammer<sup>b, *</sup>

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

ARTICLE INFO

Keywords:

N₂ reduction

Nitrogenase

Bio-inspired models

FeMoS complexes

A B S T R A C T

Sulfur-rich {FeMo} complexes of low nuclearity ({Fe₂Mo}, {FeMo}) can be considered as simple truncated models of the cofactor of Mo-nitrogenase. They are suitable to address questions concerning the role of the still unclear FeMo cooperativity in the biological N₂-reduction process performed by this metallo-enzyme. This review affords a state of the art of the chemistry of di- and tri-heterometallic {FeMoS} complexes. Although well-defined synthetic procedures have been evidenced for the design of such compounds, very few investigations on their activity towards N₂ and related substrates have been reported. This shows the interest to develop new light bio-inspired {FeMoS} molecules and to study their reactivity, as well as their redox behaviours, to help to better understand the biological N₂-fixation. This approach involving di- and tri- heterometallic {FeMoS} species is a promising field of investigation.

1. Introduction

The development of renewable fuels and chemicals may provide promising solutions to the global climate change and to the increasing energy demand. In this context, ammonia is envisaged as a potential green energy storage for the implementation of a sustainable energy economy [1,2]. However, its conventional production, through the well-known Haber-Bosch process (eq.1), represents 1–2 % of global energy consumption and 1 % of the global CO₂ emission. Steam reforming produced H₂ and N₂ are reacted at high temperature (450–600 °C) and

$$
\mathbf{N_2} + 3\mathbf{H_2} \xrightarrow[\substack{\text{450-600° C} \\ \text{100-250 bars}}]{\text{Fe or Ru based cat.}} 2\mathbf{NH_3} \quad (1)
$$

$$
\begin{align}
& \mathbf{N_2} + 8\mathbf{H}^+ + 16\text{MgATP} + 8\mathbf{e}^- \xrightarrow[\substack{\text{atmospheric pressure} \\ \text{room temperature}}]{\text{Mo-nitrogenase}} 2\mathbf{NH_3} \tag{2} \\
& \qquad + \mathbf{H_2} + 16\text{MGADP} + 16\mathbf{P_i} \nonumber
\end{align}
$$

high pressure (100–250 bars) in the presence of an iron/ruthenium-
based catalyst to cleave the thermodynamically stable N≡N triple
bond (944 KJ/mol) [1,3]. For these reasons, ammonia is the highest

carbon footprint chemical [4]. 80 % of ammonia production is employed within the fertilizers industry to ensure food production [1,5,6]. A moderate prediction of future ammonia needs by 2050 anticipates that its production through the Haber-Bosch process will emit more than twice of the current released amount of carbon dioxide [6]. Although improvements of the Haber-Bosch process should help to reach a less emissive global production of ammonia [7], the development of sustainable alternatives is essential and will constitute a breakthrough [8–14].

The biological incorporation of nitrogen is ensured by nitrogenases found in bacteria and archea [15]. These enzymes reduce atmospheric nitrogen into ammonia at ambient pressure and temperature (eq.2). Three types of enzymes are defined depending on their elemental composition: Mo-, V- and Fe-dependent nitrogenases. They combine two proteins among which, one holds the catalytic centre. The catalytic protein encloses two metallo-clusters: an electron relay, the P cluster (8Fe—7S), and the active site named the cofactor FeMco (M = Mo, V or Fe), where the N₂ fixation occurs. Great efforts have recently resulted in the full characterization of FeMoco, FeVco and FeFeco underlying structural differences and similarities (Fig. 1-left) [16–24]. The FeMco can be depicted as two cubane clusters [Fe₄S₃C] and [Fe₃MS₃C] (M = Mo, V or Fe) sharing a hexavalent µ₆-C. The two metallic fragments are

<footnote><sup>a</sup> UMR CNRS 8180 Institut Lavoisier de Versailles, Université de Versailles Saint-Quentin-en-Yvelines, Université Paris-Saclay, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France</footnote>

<footnote><sup>b</sup> UMR CNRS 6521 Chimie, Electrochimie Moléculaires et Chimie Analytique, Université de Bretagne Occidentale, UFR Sciences et Techniques, 6 Avenue Victor le Gorgeu, CS 93837, Brest-Cedex 3, 29238, France</footnote>

<footnote>* Corresponding authors.</footnote>

<footnote>E-mail addresses: maxime.laurans@uvsq.fr (M. Laurans), schollha@univ-brest.fr (P. Schollhammer).</footnote>
linked by three bridging ligands that differ depending on the hetero-metal (three sulfides when M = Mo or Fe and two sulfides and one carbonate if M = V). Finally, homocitrate and histidine residues surround the apical heterometal.

The FeMoco is the most studied active site of nitrogenases because of its early and full characterization as well as of its highest catalytic efficiency [15,25]. Since the first FeMoco N₂-reduction cycle proposed by Lowe-Thorneley in 1984, significant advances in the knowledge of this process have been made, especially during the last decade [15,26–58]. However, N₂ reduction mechanism elucidation at FeMoco is still in debate and has stimulated further investigations.

Indeed, a crucial intermediate, called “Janus”, existing in two forms with either two bridging hydrides or with bound N₂ following H₂ displacement has been assigned to the state E₄, which arises after the accumulation of 4H⁺,4e⁻ (Fig. 1-right) [30–34]. Thus, mechanistic studies suggest that the release of one equivalent of H₂ is required to promote N₂ fixation at the multi-iron centre and it is proposed that the sulfido bridges act as proton relays which may facilitate the reductive elimination of H₂ and the N₂ binding (Fig. 2) [35–37]. The binding mode of N₂ at this stage and its further reduction into NH₃ as well as the role of the labile sulfur atoms remains an open question and numerous mechanistic possibilities involving stabilized bound diazene N₂H₂ have been considered [38–72], mainly through theoretical studies [40–54] and via experimental studies of stabilized diazene at iron complexes [55–72].

Despite these significant advances, the role and the presence of the apical molybdenum atom is still not fully understood but it might be crucial in the catalytic activity rate of the enzyme. [73–78].

Within this context, several mono and multimetallic molecular models have been considered both to support the structural and spectroscopic characterization of the FeMoco and to understand its functioning, with the hope to ensure N₂ cleavage under mild conditions [79–81].

More and more N₂-complexes turn out to be suitable for N₂ activation and for its functionalization [82–86], but steric and electronic tunings and mechanistic investigations remain still necessary to rationalize the parameters that favour N₂-fixation.

Bio-inspired mononuclear Mo-based complexes have been studied since the seventies and important insights into the mechanism of the reduction of N₂ at a single metal centre (Chatt and Schrock cycles) have been achieved. Recently, monometallic Mo-systems, which act through a transitory dimerization via an end-on N₂-coordination, were described among the most efficient catalyst for N₂ reduction [87]. Significant advances showing a moderate activity of N₂-mononuclear species, often in harsh reactional conditions, strongly suggest that the inherent metal-metal synergy in polymetallic active-sites, such as the FeMoco, may be essential to reach a higher catalytic efficiency for N₂ reduction. Indeed, polymetallic FeMoco catalytic mechanism implies metal-metal cooperativity to ensure the multistep N₂ reduction to NH₃.

The benefits of metallic synergy in the context of molecular catalysis have recently regained interest as a promising field to achieve innovative and challenging chemical transformations as N₂ cleavage [88–91]. For this reason, the activity of polynuclear complexes were examined [92,93]. Most of them have been prepared in order to reproduce the structure of the complete active site and they are based on related cubane clusters [94–97]. The design of such compounds with a sulfur environment having a catalytic activity towards N₂-reduction is still a challenge [98,99] that requires a fine control of steric and electronic properties [100–102]. Since the late sixties, in a comprehensive structural approach, sulfido {FeMo} clusters, with high nuclearity [95–97], structurally very close to the FeMoco were reported even before its full characterization [17]. It is worth noting that the chemistry of sulfur-based {FeMoS}-polymetallic complexes of low nuclearity (two and three metal centres), related to FeMoco, has been poorly developed while they allow to model the behaviour of one or two iron and a molydenum centres in proximity.

In this review, we aim to highlight the synthetic developments in sulfur-rich heterometallic {FeMoS} complexes based on {FeMo₂}, {Fe₂Mo} and {FeMo} cores. {Fe₂Mo} ('linear'/bent) and {FeMo} frameworks can be considered as the simplest models of truncated moieties of the Mo-nitrogenase cofactor (Fig. 3) with structural minimal requirements.

## 2. FeMo₂ complexes

Although, one may consider that {FeMo₂} fragment is not directly structurally related to FeMoco, the dinuclear sulfur [Mo₂]-complexes appealing reactivity towards N₂-Reduction Substrates (NRS) prompted us to include {FeMo₂} clusters within the scope of this review [103–105]. A great diversity of sulfur-containing trimetallic [FeMo₂]-complexes has been reported [94,106,107].

### 2.1. Complexes with quasi-linear (μ-S)ₓ{FeMo₂}-core

#### 2.1.1. Sulfido-bridged [FeMo₂]-complexes

The preparation of sulfido trimetallic {FeMo₂}-complexes principally follows a building-up approach involving the monometallic anion [MoS₄]²⁻ and various iron precursors in different stoichiometry (Fig. 4) [106].

Sulfido [FeMo₂]-complexes prepared from [MoS₄]²⁻ feature a quasi-linear {Mo-Mo-Mo} arrangement with a central iron atom whereas no {Fe-Mo-Mo} linkage has been reported to date. [NH₄]₂[MoS₄] reacts with [NH₄]₂Fe(SO₄)₂·6H₂O or FeSO₄·7H₂O salts in water to form the anion [Ph₄P]₂[FeMo₂S₄(µ₂-S)₄] ([Ph₄P]₂I) by precipitation with Ph₄PCl but that was not isolated pure [108,109]. Bubbling NO gas into a CH₂Cl₂ solution of [Ph₄P]₂I yields the amorphous complex ([FeMo₂S₄(NO)(µ₂-S)₄]²⁻)ₙ (2) proposed to arise from the polymerization process of an

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Left: Representation of the nitrogenase cofactor FeMoco (M = Mo, V, or Fe). Right: Representation of the simplified Lowe-Thorneley cycle encompassing the “Janus” state E₄.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 2. Schematic view of the proposed “Janus” state.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 3. Representation of truncated {FeMo} (a) and {Fe₂Mo} (b and c) moieties of the Mo-nitrogenase cofactor.</figcaption></figure>

intermediate [FeMo₂S₄(NO)(µ₂-S)₄]²⁻ [109].

The reduced discrete form of 1²⁻, [NR₄]₃[FeMo₂S₄(µ₂-S)₄] ([NR₄]₃, R = Me, Et), is prepared by the addition of [NR₄]₂[MoS₄] salts in DMF to a MeCN solution of the complex [NR₄][Fe(S₂COC₂H₅)₃] in which the xanthate ligand was proposed to act as a reducing agent (Fig. 4) [110]. [NEt₄]₃ is also obtained by reacting the dithiocarbamate precursor [Fe(S₂CN(CH₂)₅)₂] with two equivalents of [NEt₄]₂[MoS₄] in CH₂Cl₂ [111]. Mössbauer and crystallographic investigations suggested that the reduction is Mo-centered and cyclic voltammetry of [NEt₄]₂³⁻ showed that its oxidation is irreversible [110,111]. Interestingly and contrary to 1²⁻, bubbling NO in a DMF solution of [N(PPh₃)₂]₂[Et₄N]₃ leads to its fragmentation into [MoS₄]²⁻ and the heterobimetallic complex [N(PPh₃)₂]₂[FeMoS₂(NO)₂(µ₂-S)₂] ([N(PPh₃)₂]₂₄) (see Section 4.1) [112]. Spectroscopic evidences of 3³⁻ buried into the Orange Protein (ORP) have been recently reported [113]. Protein-assisted synthesis of metallic complexes allows the in-situ generation of thioclusters [MMo₂S₄(µ₂-S)₄]³⁻ (M = Fe, Cu, Co Ni or Cd) by combination of the monomeric salts [NH₄]₂[MoS₄] and MSO₄ with the apo-ORP in aqueous medium. Such artificial synthetic proteins revealed to be active towards light-driven hydrogen evolution in the presence of a photosensitizer, such as [Ru(bpy)₃]²⁺ and a sacrificial electron donor (ascorbic acid). The most

active assembly is the modified protein with [FeMo₂] cluster. Although the study was led under N₂ atmosphere, it did not demonstrate any evidence for N₂ reduction activity [114].

The complex [NH₄]₂[Fe(NO)(S₂MoS₂)₂] ([NH₄]₂₅) is a rare example of a nitroso [FeMo₂]-complex (Fig. 4). [NH₄]₂₅ was characterized by EPR spectroscopy among a mixture of nitroso complexes from the reaction of different polynuclear iron precursors (Roussin's 'esters' [Fe₂(µ₂-SR')₂(NO)₄] (R'' = Me, Et, nPr, tPr, nBu, tBu, nC₅H₁₁) and [NH₄]₂[MoS₄] (excess, "Mo:Fe" ratios 5:1 or 10:1) (Fig. 4) [115]. The formation of the anionic heterobimetallic [FeMo]-complex [FeMoS₂(NO)₂(µ₂-S)₂]⁻ (6⁻) was also detected (see Section 4.1) when "Mo:Fe" ratio is 1:1.

The derivative [Me₄N]₃[FeMo₂((CH₂S)₂)₂(S)(µ₂-S)₄] ([Me₄N]₃₇) is the first reported heterotrimetallic complex that displays a square pyramidal geometry at the Mo centre (Fig. 4). Its [Me₄N]⁺ salt is obtained through a one-pot reaction of [NH₄]₂[MoS₄] with an excess of sodium ethane-1,2-dithiolate, [Me₄N]Br and FeCl₃ in a DMF/Acetonitrile/MeOH mixture [116]. The analogous salts [R₄N]₃₇ (R = Et, tBu) were also obtained following the same synthetic procedure [117]. The two terminal Mo centres are coordinated to ethane-1,2-dithiolate ligands and they adopt a square-pyramidal geometry. Unlike the previous
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Synthetic routes of quasi-linear sulfido [FeMo₂]-complexes. The yields are indicated into brackets when available. For 5²⁻ and 6⁻ iron precursors, R'' = Me, Et, ⁴Pr, ⁴Pr, ⁴Bu, ⁴Bu, ⁴C₅H₁₁.</figcaption></figure>

sulfido [FeMo₂]-complexes, the short FeMo distances hint significant metal-metal interactions. Magnetic and Mössbauer studies revealed oxidation states of V and III for the Mo and Fe centres, respectively, which differs from the Fe(II) and Mo(V or VI) [117] atoms formally proposed in 1²⁻. A quasi-reversible monoelectronic reduction wave was detected in the CV of 7³⁻, which suggests a relative stability of the reduced species. The use of this complex as a building block was proposed for the preparation of clusters of higher nuclearity but no reactivity towards NRS was reported.

### 2.1.2. Thiolato-bridged [FeMo₂]-complexes

The compound [NEt₄]₂[FeMo₂(CO)₈(µ₂-SR)₄] ([NEt₄]₂8) (R = Ph or Bn) (Fig. 5) was prepared according to three synthetic pathways: by reacting [NEt₄]₂[Mo(SR)₂(CO)₄] with [NEt₄]₂[FeCl₄] (R = Ph) or FeBr₂ (R = Bn) or [Mo(CO)₄(C₇H₈)] with [NEt₄]₂[Fe(SPh)₄] as well as through the recombination of [NEt₄]₂[Mo₂(SR)₂(CO)₈] in the presence of FeBr₂ [118].

The first route systematically leads to [NEt₄]₂8 even when employing a Mo:Fe molar ratio 1:1. The norbornadiene pathway is described at room temperature only for R = Ph. Finally, the third pathway is a straight and easy way to prepare [NEt₄]₂8 except when R = ⁴Bu. Interestingly, the one-pot reactional mixture of [Mo(CO)₆], [NEt₄]₂[FeCl₄] and NaSPh, with the appropriate stoichiometry, in acetonitrile, also affords [NEt₄]₂8 (71 % yield). Spectroscopic characterization indicated

the presence of a central Fe(II) atom surrounded by Mo(0) atoms and the absence of metal-metal bonds. The reaction of [NEt₄]₂8 at low temperature with bipyridine or o-phenanthroline leads to the [FeMo]-complex [FeMo(CO)₄L₂(µ₂-SPh)₂] (9) (L = bipyridyne or o-phenanthroline) by the substitution of a {Mo(CO)₄(SPh)₂} fragment. The high affinity of bipyridine and o-phenanthroline with Fe(II) ion explained the instability of 9, that tends to form [FeL₃]²⁺ in solution at room temperature. Moreover, when R = Ph, [NEt₄]₂8 yields [NEt₄]₂[FeMo₂(CO)₈(R'NC)₂(µ₂-SPh)₄] ([NEt₄]₂10) by reacting with the corresponding isocyanates at low temperature (12 % yield for R' = ⁴Bu or 24 % yield for R' = Cy). The cyclic voltammetry of 8 exhibited a partially reversible two-electron oxidation in acetonitrile at low temperature. The oxidation of 8 affords the neutral complex [FeMo₂(CO)₈(µ₂-SPh)₄] (11), in which Mo—Fe distances are shortened, as it was ascertained by the comparison of X-ray data of single crystals of 8 and 11 [119]. Normal electron-counting rules and the values of ν(CO) bands in the IR spectrum suggest an 18-electron configuration of the oxidized Mo(I) centres (Fig. 5). The CV of 8 showed on the reverse scan several reduction processes, which suggests intricate evolution involving solvolysis of the complex. 11 could also be prepared via the chemical oxidation of 8 by action of I₂. It is reversibly reduced in the presence of [NEt₄]BH₄.

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Synthesis pathways of compounds [NEt₄]₂8 and reversible structural changes upon redox process of complexes 8²⁻ and 11 (for R = Ph). The yields are indicated into brackets when available.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 6. Representation and synthesis pathway of complex 12[I] (with Cp' = η⁵-C₅H₄Me and Cp = η⁵-C₅H₅).</figcaption></figure>

## 2.2. Non-linear (μ₃-S) [FeMo₂]-complexes

Two preparative strategies can be distinguished for the synthesis of (μ₃-S)[FeMo₂]-clusters: by building the {FeMo₂}-core using dinuclear {Mo₂}- and mononuclear {Fe}-synthons or by displacement of a metallic fragment in a preformed polymetallic precursor.

### 2.2.1. Combination of mononuclear {Fe} and dinuclear {Mo₂} moieties

The reaction of [MoCp'(μ₂-S)(μ₂-SH)]₂ with [FeICp(CO)₂] (with Cp' = η⁵-C₅H₄Me and Cp = η⁵-C₅H₅) in refluxing toluene leads to the air stable trinuclear cation [FeMo₂CpCp'(μ₂-S₂)(μ₃-S₂)]I (12[I]) (Fig. 6) [120].

12⁺ presents a reactive persulfido {Mo₂(S₂)} site towards substrates such as MeI or alkynes after its reduction by potassium graphite. No investigation with NRS is mentioned. 12[I] displays an irreversible reduction process and further studies of the chemically reduced species with alkene or alkyne suggested that the elimination of the {Fe}-moiety yields a bis(alkene/alkyne) homobimetallic molybdenum complex.

Computational studies rationalized the weaker σ-interaction of the

{CpFe⁺} moiety with the {Mo₂S₄} core compared to that of a methylene group. Moreover, the association of the {Fe} fragment with the {Mo₂(μ₂-S)₂(μ₂-S₂)} framework, compared to that of an ethylene fragment, lowers the reactivity of the disulfido bridge towards olefins and dihydrogen [121].

### 2.2.2. Substitution of metal moieties in a trinuclear cluster

The preparation of symmetrical and unsymmetrical complexes of general formula [FeMo₂Cp'Cp''(CO)₇(μ₃-S)] (13) (with Cp' ≠ Cp'' or Cp' = Cp'') by a sequential isolobal displacement of two {M(CO)₃} (M = Co, Fe) moieties in the trimetallic cluster [Fe₂CoH(CO)₉(μ₃-S)] have been reported (Fig. 7). The first step is the replacement of one {Fe(CO)₃} fragment through an 'electrophilic addition-elimination' in the presence of [Mo(CO)₃Cp'Cl] (Cp' = η⁵-C₅H₅, η⁵-C₅H₄Me, η⁵-C₅H₄CO₂Me or η⁵-C₅H₄CO₂Et), which yields the intermediate [FeMoCoCp'(CO)₈(μ₃-S)] (14). In some cases, 14 is obtained within a mixture containing the corresponding complex 13 (Fig. 7) [122–124]. This step is followed, in the presence of an additional equivalent of [MoClCp''(CO)₃] (Cp'' = η⁵-C₅H₅, η⁵-C₅Me₅), by the formation of 13 with the loss of the {Co(CO)₃}

<figure><img src="image_10.png" /><figcaption>Fig. 7. Synthetic pathways of μ₃-S [Fe₂Mo]-complexes by metal fragment replacement. The yields are indicated into brackets when available.</figcaption></figure>
fragment [125]. Similarly, **14**, with Cp' = η⁵-C₅H₄CO₂Me or η⁵-C₅H₄CO₂Et, could be generated by displacing a cobalt moiety {Co(CO)₃} with Na[MoCp'(CO)₃] in the precursor [FeCo₂(CO)₉(µ₃-S)] [126] or following other procedures with the precursor [Mo{As(CH₃)₂}Cp'(CO)₃] (22 % yield) [127]. The reactivity of the tungsten analogous of **14** with various permethylated or functionalized cyclopentadienyl molybdenum synthons, such as Na[MoCp''(CO)₃] (Cp'' = η⁵-C₅H₅, η⁵-C₅H₄C(O)Me or η⁵-C₅H₄CO₂Me), allowed the formation of various compounds **13** with a {FeMo₂} core (Cp'' = η⁵-C₅H₅ (14 % yield), η⁵-C₅H₄C(O)Me (5 % yield) and η⁵-C₅H₄CO₂Me (6 % yield)) [128]. **13** (Cp' = Cp'' = η⁵-C₅H₄CO₂Me (4 % yield) or η⁵-C₅H₄CO₂Et (9 % yield)) was also identified as a side-product in the synthesis of [Mo₄Cp'₄(µ₃-S)₂(µ₂-CO)₄] by reacting the corresponding complex [Mo₂Cp'₂(CO)₄] and with [Fe₂(CO)₆(µ₂-SR)₂] (R = Et, pH) [129,130]. The formation of air-stable crown ether trinuclear complexes [FeMo₂(CO)₇(µ₃-S)(µ₂-Cp')] (Cp' = {η⁵-C₅H₄CH₂(CH₂OCH₂)ₙ/₂}₂ (n = 1-4) (15) or {η⁵-C₅H₄C(O)CH₂}₂ (16)) have also been described (Fig. 7) [131–134]. Further investigations of the properties of **15** highlighted their potential for the elaboration of redox-switching devices as the first oxidation shifts significantly towards less positive potentials in the presence of alkaline salts. The treatment of **16** with NaBH₄ leads to the complex [Mo₂Fe(CO)₇(µ₃-S)(µ₂-Cp')] (Cp' = {η⁵-C₅H₄C(OH)CH₂}₂) (34 % yield) in which the C=O bonds have been reduced into CHOH group.

### 3. Fe₂Mo complexes

#### 3.1. Quasi-linear (µ-S)ₓ[Fe₂Mo]-complexes

[Fe₂Mo]-complexes with a sulfur environment can act as interesting molecular stoichiometric models of a truncated portion of FeMoco. [Fe₂Mo]-complexes preparation follows three main strategies: (i) a building-up based on smaller nuclearity precursors, (ii) the scission of high nuclearity clusters and (iii) an isolobal metal fragment substitution.

##### 3.1.1. Sulfido-bridged [Fe₂Mo]-complexes

The preparation of trimetallic sulfido [Fe₂Mo]-complexes from the monomeric anion [MoS₄]²⁻ and iron/diiron synthons results in symmetrical {Fe-Mo-Fe} or dissymmetrical {Fe-Fe-Mo} frameworks depending mainly on the nuclearity of the iron precursors (Fig. 8).

The one-pot reaction of [Ph₄P]₂[MoS₄] with two equivalents of FeCl₂·4H₂O in acetonitrile quantitatively forms the compound [Ph₄P]₂[Fe₂MoCl₄(µ₂-S)₄] ([Ph₄P]₂**17**) [135]. This latter could also be formed by the addition of FeCl₃·6H₂O to [Ph₄P]₂[FeMo(S)₂(SPh)₂(µ₂-S)₂] ([Ph₄P]₂**18**) with a concomitant release of Ph₂S₂ [136,137]. In polar solvents, [Ph₄P]₂**17** would evolve into the dinuclear compound [Ph₄P]₂[FeMoCl₂(S)₂(µ₂-S)₂] (19) and FeCl₂. The presence of [NEt₄]₂**19** was also detected in the cyclic voltammetry of [NEt₄]₂**17** recorded in DMF [138]. [NEt₄]₂**17** exhibits a partially reversible monoelectronic reduction which results in the formation of [Fe₂MoCl₄(S)₄]³⁻. The decomposition of this reduced species affords a product, that was tentatively assigned, by spectroelectrochemistry and Raman spectroscopy studies, to the cubane-like Mo₂Fe₂S₄ species [{Fe₂Mo₂Cl₂(S)₈} {(FeCl₂)₂}]⁴⁻ whose the formation is inhibited in the presence of an excess of chloride [137–139].

The reaction between [MoS₄]²⁻ and the di-iron species [NEt₄]₂[Fe₂(SAr)₄(µ₂-S)₂] in acetonitrile affords the complex [NEt₄]₃[Fe₂Mo(S)₂(SAr)₂(µ₂-S)₄] ([NEt₄]₃**20**) with a {FeIII·FeII·MoVI} linkage, which differs from that in **17** (FeII⁵MoVI⁵FeII⁵), but has a connection with the electronic structure MoIII⁵FeIII⁵FeII⁵, settled by Mössbauer studies and DFT calculations, of the FeMoco resting state E₀ [15,140–142]. **20** is the first example of a complex bearing two neighbouring Fe centres with two different oxidation states that have been spectroscopically characterized. More recently, the complex [PPh₄]₂[Fe₂Mo(SSiPh₃)₄(µ₂-S)₄] (21) has been prepared by reacting [PPh₄]₂[MoS₄] with [Fe(SSiPh₃)₂(CH₃CN)₂] [143]. The crystal structure of **21** displays a linear {Fe-Mo-Fe} core similar to that described in **17**.

##### 3.1.2. Thiolato-bridged [Fe₂Mo]-complexes

The bridging-tetrakis(thiolato) complex [Fe₂Mo(CO)₇(µ₂-S'Bu)₄] (22) is a rare example of Fe₂Mo cluster featuring a quasi-linear {Fe₂Mo} structural arrangement supported by thiolato bridges (Fig. 9). It is prepared by the reaction of [Mo(S'Bu)₄] in the presence of three equivalents of [Fe₂(CO)₉] in toluene [144]. Lowering the excess of the di-iron precursor decreases the yields in **22**.

#### 3.2. Non-linear (µ₃-S)[Fe₂Mo]-complexes

As their quasi-linear analogues, non-linear heterometallic [Fe₂Mo]-

<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Synthetic routes of quasi-linear sulfido [Fe₂Mo]-complexes. The yields are indicated into brackets when available.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>Fig. 9. Synthetic route of quasi-linear thiolato-bridged complex 22.</figcaption></figure>

complexes with sulfido bridges are directly related to the FeMoco structure and they can be considered as molecular models of the truncated apex of the FeMoco.

### 3.2.1. Building-up approach

The di-anion $[Fe_2(CO)_6(\mu_2-S)_2]^{2-}$ is an appealing precursor for the elaboration of non-linear trimetallic bridging sulfido clusters in reason of a large range of reactivity, especially with monometallic complexes

<figure><img src="image_13.png" /><figcaption>Fig. 10. Preparation of µ₃-S [Fe₂Mo]-complexes through a building-up approach using a di-iron precursor. The yields are indicated into brackets when available.</figcaption></figure>
[145]. Despite the fact that its reactivity with MoIV/V halides may afford mixtures of clusters with high nuclearity, which are difficult to isolate [146], in general the combination of [Fe₂(CO)₆(µ₂-S)₂]²⁻ with high-oxidation-state molybdenum precursors leads to trinuclear [Fe₂Mo]-complexes featuring a butterfly {Fe₂S₂} core without Fe···Mo interaction. With low-oxidation-state molybdenum precursors, a planar {Fe₂S₂} core is linked to the apical molybdenum atom through Fe—Mo bonds (Fig. 10).

The hydrazido species [MoCl(NNRR')₂(PPh₃)₂]Cl (R = R' = Me or R = Me, R' = Ph) reacts with Li₂[Fe₂(CO)₆(µ₂-S)₂] to yield the air-stable bis-hydrazido(2-) complex [Fe₂Mo(CO)₆(NNRR')₂(PR'₃)(µ₃-S)₂] (R'' = Ph, Me) (23) [147,148], which is one example of a (µ₃-S)ₘ[Fe₂Mo] cluster bearing a diazo pattern. Further studies on the reduction of such hydrazido ligand at a Fe₂Mo core is not reported unfortunately. Unlike the structure of 23, the complex [Et₄N][Fe₂Mo(CO)₈(S₂CNEt₂)(µ₃-S)₂] ([Et₄N]24) consists in a quasi-planar {Fe₂S₂} geometry with two Fe(II)-Mo(0) bonds and no Fe(II)-Fe(II) interaction [149]. [Et₄N]24 would result, in the presence of low oxidation state molybdenum precursors, from the disproportionation of the dianion [Fe₂(CO)₆(µ₂-S)₂]²⁻ into the species [Fe₂(CO)₆S₂]⁴⁻ and [Fe₂(CO)₆(µ₂-S)₂]. This latter species would be the intermediate affording the heterotrimetallic cluster [Et₄N]24. According to the general strategy, the compound [Fe₂Mo(CO)₁₀(µ₃-S)₂] (25), which features also a planar {Fe₂S₂} core, was prepared by action of [Mo(CO)₅(THF)] with [Fe₂(CO)₆(µ₂-S₂)], in THF, at room

temperature [150,151] in order to explore its activity as an oxo-transfer catalyst. With a different goal concerning thin film deposition, the complex [Fe₂Mo(CO)₆Cp₂(µ₃-S)₂] (26) was prepared from the reaction of [Fe₂(CO)₆(µ₂-S₂)] with [Mo(H)₂Cp₂], in THF, at low temperature [152].

### 3.2.2. Cluster fragmentation

The fragmentation of {FeₓMoᵧSz} clusters of higher nuclearity leads to [Fe₂Mo]-products. Despite low yields, this pathway may constitute a route for the formation of hetero-trimetallic systems [Fe₂Mo(µ₃-Sₓ)] with a bidentate O-based ligand bonded on the molybdenum atom, which mimics the homocitrate group coordinated to the molybdenum centre in the Mo-nitrogenase cofactor (Fig. 11).

The compound [Fe₂Mo(CO)₄(PEt₃)₂(Cl₄-cat)₂(µ₃-S)₂] (Cl₄-cat = tetrachlorophenol-1,2-ate) (27) was isolated in traces when the cluster [Et₄N]₂[Fe₃Mo(Cl₄-cat)(Cl)₃(MeCN)(µ₃-S)₄] is treated with PEt₃ in the presence of NaBPh₄ under CO pressure (34 atm) [153]. Under the same conditions of CO pressure, the complexes [Fe₂Mo(CO)₄(PR₃)₂(Cl₄-cat)₂(O)(µ₃-S)₂] (28) (R = Et, nPr) and [Fe₂Mo(CO)₅(PEt₃)(Cl₄-cat)₂(O)(µ₃-S)₂] (29) result from the cleavage of the precursors [(Cl₄-cat)Mo₂Fe₆S₈(PR₃)₆] (R = Et, nPr) while the compound [Fe₂Mo(CO)₂(depe)₂(Cl₄-cat)₂(O)(µ₃-S)₂] (30) (depe = 1,2-bis(diethylphosphino)ethane) arises from the decomposition of the cluster [Fe₃Mo(CO)₄(PEt₃)₂(Cl₄-cat)₂(µ₃-S)₃] in the presence of depe [154]. Their

<figure><img src="image_14.png" /><figcaption>Fig. 11. Preparation of µ₃-S [Fe₂Mo] clusters by fragmentation pathways.</figcaption></figure>
crystal structures display a “butterfly” shaped {Fe₂S₂} core with Fe—Mo distances suggesting metal-metal interactions. These crystallographic data combined with Mössbauer studies and magnetic moment measurement suggested Mo(V)Fe(I)Fe(I) or Mo(V)Fe(I)Fe(II) oxidation states for the metal centres in 29.

### 3.2.3. Metallic moiety replacement

Similarly to the preparation of [FeMo₂]-complexes presented before, the replacement with an isolobal metallic moiety of a {Co(CO)₃H} fragment in FeMoCo clusters, allowed to obtain [Fe₂Mo]-systems. The treatment of 14 (Cp' = η⁵-C₅H₅, η⁵-C₅H₄C(O)Me, η⁵-C₅H₄CO₂Me) with Na[Fe(CO)₄H] in refluxing THF followed by acidification leads to the formation of the hydride complex [FeHFeMoCp'(CO)₈(µ₃-S)] (31) (Fig. 12-top) [155]. Likewise, the air-stable [Fe₂MoCp'(CO)₆(µ₃-S)(µ₂-η²(S):η²(C)CH₂Ph)] (Cp' = η⁵-C₅H₅ or η⁵-C₅Me₅), (32) was prepared by photolytic metal fragment substitution in [Fe₃(CO)₉(µ₃-S)₂] in the presence of acetylide precursor [Mo(C≡CPh)Cp'(CO)₃] [156]. 32 exhibits short Fe—Mo bonds, a bridging sulfido group and a µ₂-η²(S):η²(C)-(R-C-S) linkage (Fig. 12-top). This Fe-C-Mo linkage is also related to the dinuclear species [FeMo(CO)₅(L)(µ₂-η²:η²-SCR)] (50 or 51) (see in [FeMo]-complexes section 4.2) (Fig. 17).

### 3.2.4. Sulfur insertion

The sulfido [Fe₂Mo(CO)₆Cp(µ₂-CR)(µ₂-CO)(µ₃-S)] (33) and the thioacylo [Fe₂MoCp(CO)₆(µ₂-CO)₂(µ₃-η¹(C):η²(S)-SCR)] (34) (R = Tol) complexes have similar structural features than 31 and 32. They were obtained, as a mixture, by treating the complex [Fe₂MoCp(CO)₈(µ₃-CR)(µ₂-CO)] with elemental sulfur in THF at room temperature (Fig. 12-bottom) [157]. 34 arises from the coupling of a sulfur atom with an alkylidene bridge. It is worth noting that 34 is converted into 33 upon heating in toluene.

## 4. Heterodinuclear [FeMo]-complexes

Heterobimetallic [FeMo]-complexes with a sulfur environment are the simplest molecular models of the FeMoco apex. These derivatives are perfectly suited to examine iron-molybdenum centres cooperativity in regard to the FeMoco structure. However, it is only recently that some few dithiolato and dithiolene complexes have been examined as models for a comprehensive approach of FeMoco intimate mechanisms.

### 4.1. Sulfido-bridged [FeMo]-complexes

The main examples of sulfido-bridged [FeMo]-complexes will be considered in this section [94,106,107]. Similar synthetic approaches

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Synthetic pathways for the preparation of [Fe₂Mo]-complexes by metal fragment replacement (top) and by sulfur atom insertion (bottom). The yields are indicated into brackets when available.</figcaption></figure>
<figure><img src="image_16.png" /><figcaption>Fig. 13. Preparation of sulfido [FeMo]-complexes and representation of the FeMoco. The yields are indicated into brackets when available.</figcaption></figure>

than those described for the preparation of hetero-trimetallic were employed for the synthesis of hetero-bimetallic anions (Fig. 13). The latter were originally considered as useful building blocks for the synthesis of clusters with higher nuclearity [106].

The elaboration of bimetallic sulfido [FeMo]-complexes principally follows a building-up approach combining two mononuclear synths of molybdenum and iron. The anion [Et₄N]₂[MoS₄] is combined with [PPh₄]₂[Fe(SPh)₄] to afford the complex [Et₄N]₂[FeMo(S)₂(SPh)₂(µ₂-S)₂] ([Et₄N]₂**18**) [158]. The use of analogous dithiolato compounds, such as [Et₄N]₂[Fe(S₂-o-Xyl)₂]²⁻ (S₂-o-Xyl = α,α'-(SCH₂)₂C₆H₄), leads to [Et₄N]₂[FeMo(S₂-o-xyl)-S₂(µ₂-S)₂] ([Et₄N]₂**35**) [159]. The treatment

of [Et₄N]₂[MoS₄] with [FeCl₂] gives rise to the chloro derivative [Et₄N]₂[FeMoCl₂(S)₂(µ₂-S)₂] ([Et₄N]₂**19**), which can be transformed into **18**²⁻ by action of PhSH/Et₃N (quantitative interconversion in solution) [158,160]. Conversely, in the presence of a chloride source, such as benzoyl chloride, **18**²⁻ quantitatively regenerates **19**²⁻ in solution. The extension of this reactivity to alkoxy derivatives, such as [Et₄N]₂[-FeMo(OR)₂(S)₂(µ₂-S)₂] (R = OPh, [Et₄N]₂**36**), can be performed by action of alcoholate salts [158,161]. Different combinations of ligands could be obtained as it has been exemplified with the compound [FeMo(S)₂(BPA)(DMF)(µ₂-S)₂] (BPA = bis(pyridyl-2-methyl)amine) (37) using this strategy with the [Et₄N]₂[MoS₄] brick [162].

<figure><img src="image_17.png" /><figcaption>Fig. 14. Synthesis pathways of bridging-sulfido oxo [FeMo]-complexes.</figcaption></figure>
[Et₄N]₂¹⁸ and the analogous compound [Et₄N]₂[FeMo (S)₂(STol)₂(µ₂-S)₂] [[Et₄N]₂³⁸] are also prepared through the cleavage of the trimetallic anion [Et₄N]₃[FeMo₂S₄(µ₂-S)₄] (3) by treatment with a thiol C₆H₅SH (PhSH), p-CH₃C₆H₄SH (TolSH)) and triethylamine (yield ~50 %) [160].

The reactivity of [PPh₄]₂¹⁸ with a trisulfide RSSSR (R = C₇H₇) allows the formation of the original polysulfido complex [PPh₄]₂[FeMo (S)₂(S₅)(µ₂-S)₂] [[PPh₄]₂³⁹] [163]. The two nitrosyl complexes [N(PPh₃)₂]₂[FeMo(S)₂(NO)₂(µ₂-S)₂] ([N(PPh₃)₂]₂⁴) and [NH₄]₂[FeMo (S)₂(NO)₂(µ₂-S)₂]⁻ ([NH₄]₆) are until now the rare examples of sulfur-rich [FeMo]-complexes featuring small diatomic nitrogen-based ligand. [N(PPh₃)₂]₂⁴ is obtained by the evolution of [NET₄][N(PPh₃)₂]³ upon NO bubbling [112]. The reaction of the Roussin's ester [Fe₂(NO)₄(µ₂-SR'')₂] (R'' = Me, Et, nPr, tPr, nBu, tBu, nC₅H₁₁) with the dianion [NH₄]₂[MoS₄] affords the dinitrosyl complex [NH₄]₆ together with [NH₄]₂[Fe(NO)(S₂MoS₂)₂] ([NH₄]₂⁵) and [NH₄]₂[Fe(NO)₂(SR)₂] as side-products [115].

More recently, the oxo complex [PPh₄]₂[FeMoCl₂(O)(η²-S₂)(µ₂-S)₂] ([PPh₄]₂⁴⁰) has been obtained by reacting the anion [MoOS₃]²⁻ with FeCl₂ and S₈. Its treatment with alkyne (CO₂MeCCCO₂Me) or dithiol (toluene-3,4-dithiol) affords the complexes [PPh₄]₂[FeMoCl₂(O)(η²-S₂(CCO₂Me)₂)(µ₂-S)₂] [[PPh₄]₂⁴¹] and [PPh₄]₂[FeMoCl₂(O)(η²-S₂-tol)(µ₂-S)₂] ([PPh₄]₂⁴²), respectively (Fig. 14) [164,165]. Similarly to [Et₄N]₂¹⁹, the substitution of the chloro groups in [PPh₄]₂⁴², in the presence of thiophenol and trimethylamine, produces [PPh₄]₂[FeMo (SPh)₂(O)(η²-S₂)(µ₂-S)₂] ([PPh₄]₂⁴³).

The reactivity of heterobimetallic complexes with a {FeMoS₂(µ₂S)₂} core, such as [PPh₄]₂¹⁹, has been explored towards ligands exchange reactions (Fig. 15) [166]. Fast kinetic studies, in acetonitrile, of the mechanism of substitution of a chloride ligand by a thiolate p-RC₆H₄S⁻ (R = H, Cl, Me or OMe) in [PPh₄]₂¹⁹ suggests that the binding of a first thiolate substrate at the molybdenum atom is required in a first step and then the replacement of the chloride with an incoming RS⁻ group arises and follows a dissociative or associative pathway depending on the nature of the R group (Fig. 15-i). Electron-donor substituents favour the dissociative process while the associative mechanism is observed with electron withdrawing groups. Such process was also studied, in acidic medium, in the presence of pyridinium acid [pyrH]⁺ and reveals that the protonation of a bridging sulfur atom is a key step during this process [167] (Fig. 15-ii), mimicking thus the biological processes [36,37].

Preliminary binding of the thiophenolate to the Mo atom in [PPh₄]₂¹⁹ facilitates the protonation by a factor of ca. 29 when comparing the protonation kinetic constants of [PPh₄]₂¹⁹ and of [FeMoCl₂S₂(C₆H₅S)(µ₂-S)₂]³⁻. These studies also show that a µ₂-S atom presents similar proton affinities than a µ₃-S centre. Apart these works, the reactivity and the behaviour of {FeMoS₂(µ₂-S)₂} based complexes have been little described [94,158].

## 4.2. Thiolato-bridged [FeMo]-complexes

Early in the seventies, bis(µ₂-thiolato) [FeMo]-complexes [FeMoCl₂Cp₂(µ₂-SR)₂] (Cp = η⁵-C₅H₅, R = Me (44), nBu(45)) have been described. 44 and 45 are obtained by the reaction of iron halide with the bis-thiolato precursor [Mo(SR)₂Cp₂] (Fig. 16-i) [168,169]. Although, the complex 44 is barely soluble and it degrades in DMSO, the crystal structure determination of 45 is achieved showing that it exhibits a tetrahedral geometry around the iron atom [170].

Unlike [MoS₄]²⁻ salts, the thiolato complex [Mo(S'Bu₄) does not react unequivocal with iron (II) halide salts [171]. However, the complex [Mo(S'Bu₂)(RNC)₄] reacts with FeX₂ to form the heterobimetallic species [FeMoX₂(RNC)₄(µ₂-S'Bu₂)] (46) (with R = Cy or tBu and X = Cl or Br) (Fig. 16-ii) [171]. The electrochemical study of the compound [FeMoBr₂(tBuNC)₄(µ₂-S'Bu₂)] shows a reversible monoelectronic oxidation and a Mo-centred irreversible reduction in dichloromethane. The irreversibility of the reduction of these bis-thiolato suggests the instability of their reduced forms similarly to their sulfido analogs. The versatility of this pathway has been demonstrated with the preparation of the compound [FeMo(CO)₄(CyNC)₄(µ₂-SPh)₂] (47) by reacting the thiolato iron species [Fe(SPh)₂(CyNC)₄] with [Mo(CO)₄(C₇H₈)] (Fig. 16-iii) [172]. Spectroscopic studies suggest that the Fe(II) and Mo(0) centres does not interact through a metal-metal bond.

The air-stable complex [NET₄][FeMo(SPh)(CO)₆(µ₂-SPh)₂] ([NET₄] 48) is obtained in moderate yields in a one-pot reaction of [FeBr₂(CO)₄] with [Mo(CO)₄(C₇H₈)] (< 34 %) or [Mo(MeCN)₃(CO)₃] (34 % yield) in the presence of thiophenolate salt at low temperature (Fig. 16-iv) [173]. It is proposed that a reactive thiolato iron intermediate is formed, which then reacts readily with the molybdenum precursor. Unlike 45, for which a d(Fe—Mo) of 3.66 Å has been determined, and 47, the 18 electrons counting rule and the crystallographic distance d(Fe—Mo) (2.7–2.8 Å) strongly suggest a metal-metal bond in the “butterfly”-

<figure><img src="image_18.png" /><figcaption>**Fig. 15.** Proposed reactivity pathways of ¹⁹²⁻ towards thiolates (with R = H, Cl, Me or OMe) in the absence (i) and in the presence (ii) of pyridium acid [pyrH]⁺ [166,167].</figcaption></figure>
<figure><img src="image_19.png" /></figure>

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

<figure><img src="image_21.png" /></figure>

<figure><img src="image_22.png" /></figure>

<figure><img src="image_23.png" /><figcaption>Fig. 16. Preparation of bridging thiolato [FeMo]-complexes.</figcaption></figure>

shaped anion **48** [170,173]. Electrochemical investigations by cyclic voltammetry in acetonitrile at -30 °C have shown that **48** undergoes reductive and oxidative irreversible processes.

The bridging mono-thiolato complex [FeMoCp(CO)₇(µ₂-SPh)] (Cp = η⁵-C₅H₅) (49) is prepared by the reaction of [Fe(SPh)Cp(CO)₂] and [Mo(CO)₅(THF)] in THF (Fig. 16-v). In the presence of triphenylphosphine, 49 is cleaved into two mononuclear complexes [Fe(SPh)(CO)₂Cp] and [Mo(CO)₅(PPh₃)] [174].

Similarly to the formation of **33** and **34**, the reaction of elemental sulfur with the bridging-alkylidyne compounds [FeMoTp(CO)₅(µ₂-CR)] (Tp = hydrotris(pyrazol-1-yl)borate) or [FeMoCp(CO)₃(µ₂-CR)] (x = 5 or 6, Cp = η⁵-C₅H₅) in THF, at room temperature, leads to the formation of a C—S bond with the alkylidyne bridge and affords the thioacyl derivatives [FeMoCp(CO)₅(µ₂-η²:η²-SCR)] (Cp = η⁵-C₅H₅) (50) (R = p-C₆H₄CH₃ (Tol), 2,6-C₆H₃(CH₃)₂ (Xyl)), (51) (R = Tol, N<sup>i</sup>Pr₂) (Fig. 17) [157,175–178]. The association of [MoTp(CO)₂(κ²-η¹:η²-SCR)] (R =
<figure><img src="image_24.png" /><figcaption>**Fig. 17.** Syntheses of thioacyl [FeMo]-complexes. The yields are indicated into brackets when available.</figcaption></figure>

Tol) with [Fe₂(CO)₉] in THF also leads to **51** (L = Tp; R = Tol) (44 % yield) [179,180]. However, when R = NMe₂, the mononuclear precursor [MoTp(CO)₂(κ²-η¹:η²-SCR)] does not react with [Fe₂(CO)₉] [178,180]. Side-products, such as [FeMoCp(CO)₅(µ₂-η²:η²-SC(O)C(Tol))] (Cp = η⁵-C₅H₅) (52) [157] or [FeMoTp(CO)₅(µ₂-η²:η²-SC(O)C(Tol))] (53) [181], with a thiocarboxylate bridge, arising from a CO insertion into the thi-oalkylidyne bridge, are identified. Interestingly, similarly to **32** and **34**, **50–51** and **52–53** exhibit a S-Fe-C linkage. Further reactivity of **50** (R = Xyl) towards the bis-(diphenylphosphino)methane (dppm) results in the substitution of two CO, which affords the compound [FeMoCp(CO)₂(µ₂-CO)(µ₂-dppm)(µ₂-η²:η²SC(Xyl))] (Cp = η⁵-C₅H₅) (54) (94 % yield) [177]. Furthermore, the alkylidyne species [FeMoCp(CO)₂(µ₂-CO)(µ₂-dppm)(µ₂-C(Xyl))] (Cp = η⁵-C₅H₅) reacts with elemental sulfur providing **54** in moderate yield [177].

Novel bridging dithiolato bimetallic [FeMo]-complexes have been recently prepared but their reactivity is still poorly developed [182–184]. The reaction of [Fe(CO)₂(κ²-dppe)(κ²-pdt)] (dppe = bis (diphenylphosphino)ethane and pdt = 1,2-propanedithiolate) with [Mo(CO)₃(MeCN)₃] yields cleanly to the complex [FeMo(CO)₅(κ²-dppe)(µ₂-pdt)] (55) (Fig. 18-i) [182]. Although alkyldithiolato bimetallic complexes are common, **55** is the only example of a bridging alkyl dithiolato [FeMo]-complex [185–188]. **55** was described as a Fe(II)Mo(0) system on the basis of Mössbauer and DFT studies [182]. Electrochemical investigations under N₂ saturated solution does not show any change that can be related to N₂ activation after the reduction of **55** or its oxidation. The reductive process is accompanied by a concomitant elongation of

the Fe—Mo distance also observed previously for FeMo compounds such as 8²⁻ [119,189]. The multi-electron reduction occurs at the iron centre and induces the partial decoordination of the dppe ligand concomitantly with the shift of the bridging carbonyl group to a terminal position [189].

The use of the 1,2-benzene-dithiolato molybdenum precursor [Mo(bdt)(CO)₂(PMe₃)₂] (bdt = 1,2-benzenedithiolate) with [Fe(CO)₅] and Me₃NO in cold THF extends the series of heterobimetallic FeMo complexes with [FeMo(CO)₅(PMe₃)₂(µ₂-bdt)] (56) and [FeMo(CO)₆(PMe₃)(µ₂-bdt)] (57) (Fig. 18-ii) [183]. The benzenedithiolato bridge maintains a shorter Fe—Mo distance in **56** and **57** (2.6885(6) and 2.6936(4) Å respectively) compared to **55** (2.7795(8) Å). The redox state of the metal atoms in **57** were determined by Mössbauer spectroscopy and X-ray photoemission spectroscopy, which suggest an Fe(0) and Mo(II). The iron atom ensures the electron donation to Mo atom to complete its electronic stability. In the presence of hydrazine monohydrate, **56** and **57** decomposes and the planar homodimetallic compound [Fe₂(CO)₄(PMe₃)₂(µ₂-η²:η¹-bdt)₂] is isolated. The molybdenum moiety was not identified.

A similar synthetic approach allows to prepare the complex [FeMoCl₂Cp'₂(µ₂-η⁴:η²-bdt)] (58) (Cp' = η⁵-C₅Me₅) through the combination of [MoCl₂Cp'*(κ²-bdt)] and [FeCp'(MeCN)₃][PF₆], in the presence of one equivalent of [CoCp₂] (Fig. 18-iii) [184]. The reaction of **58** with TMSN₃ (TMS = tetramethylsilyl) leads to the formation of a bridged nitrido complex [FeMoClCp'₂(µ₂-N)(µ₂-η²:η²-bdt)] (59), in which a chloro ligand is lost, the Fe—Mo distance is shortened and the
<figure><img src="image_25.png" /><figcaption>Fig. 18. Dithiolato [FeMo]-complexes synthetic pathways (Cp' = η⁵-C₅Me₅).</figcaption></figure>

coordination mode of the benzenedithiolato ligand is modified [184].

The reactivity of **59** has been investigated upon addition of protons in the presence of a reducing agent (Fig. 19) [184]. Each chemical step was achieved independently and each intermediate has been isolated and characterized. The addition of HPF₆ to **59** leads to a µ₂-imido species that is then reduced into its µ₂-amino form through a PCET process (Proton Coupled Electron Transfer) in the presence of one equivalent of 1,2-diphenylhydrazine (DPH). Then the reduction of the µ₂-amino complex with one equivalent of [CoCp₂], followed by the addition of one equivalent of HPF₆ leads to the formation of an amine derivative. Concomitantly to this transformation, the decrease of the Fe—Mo interaction arises together with the weakening of the µ₂-η⁴:η² coordination of the benzenedithiolate group. Finally, the release of ammonia occurs by substitution with an acetonitrile molecule and **58** is regenerated by the action of TMSCl in reducing conditions. The ability of the FeMo to accommodate different bridging mode, µ₂-η²:η² ↔ µ₂-η⁴:η², of the benzenedithiolato ligand promotes the flexibility of {FeMoS₂} core and its ability to induce the transformation FeMo≡N → FeMo=NH → FeMo=NH₂ → FeMo-NH₃. These results strongly recall the works reported in the early 2000 concerning the conversion of dinitrogenous substrates into NH₃ at a {Mo₂S₂} core [103–105]. Overall, this study demonstrates the interest to study bridged dithiolate [FeMo]-complex as FeMoco apex molecular model.

Very recently, the preparation and the reactivity of [FeMo] phosphinothiolato-bridged complexes have been reported [190,191].

The action of KHBPh₃ on the µ₂-η²:η²-acetylene complex [FeMoCp'₂(µ₂-η²:η²-C₂H₂)(µ₂-SC₆H₄PPh₂)][BAr₄F] (BAr₄F = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate and Cp' = η⁵-C₅Me₅, 60[BAr₄F]) leads to the formation of the µ₂-η¹:η²-vinylic derivative [FeMoCp'₂(µ₂-η¹:η²-CH=CH₂)(µ₂-SC₆H₄PPh₂)] (61) (Fig. 20) [190]. 61 undergoes H-migration catalyzed by H₂O to yield the µ-carbyne compound [FeMoCp'₂(µ₂-CCH₃)(µ₂-SC₆H₄PPh₂)] (62), which is then protonated and affords the hydrido complex [FeMoCp'₂(H)(µ₂-CCH₃)(µ₂-SC₆H₄PPh₂)][BAr₄F] (63[BAr₄F]).

Furthermore, the complex [FeMoCp'(µ₂-PSS)₂] (64) (with Cp' = η⁵-C₅Me₅ and PSS = PhP(o-C₆H₄S⁻)₂) has been prepared and its reactivity towards the reduction of nitrite into nitric oxide has been described (Fig. 21) [191].

64 is obtained by adding [Mo(PSS)₂]Na(THF)₃ to a solution of [FeCp'Cl]₂ in THF. The chemical oxidation of **64** with Fe*·BF₄ in a CH₂Cl₂/MeCN mixture leads to [FeMoCp'(MeCN)(PSS)(µ₂-PSS)]BF₄ (65 [BF₄]) which is efficient for the selective electrocatalytic conversion of nBuN₄NO₂ into NO in the presence of PhCOOH (Turnover Frequency ~10³ s⁻¹ and faradic efficiency ~ 94(±5)%). Thorough mechanistic studies and the isolation of the nitroso complex [FeMoCp'(MeCN)(NO) (PSS)(µ₂-PSS)]BF₄ (66[BF₄]) revealed the preferential binding of NO to the molybdenum atom, the iron centre acting as an acceptor for oxygen atom transfer.

In conclusion, these recent studies highlight the significance of FeMo systems for the activation of small molecules such as HCCH and NO₂.
<figure><img src="image_26.png" /><figcaption>**Fig. 19.** Transformation of a nitrido complex 59 upon sequential protonation and reductive steps (DPH = diphenylhydrazine).</figcaption></figure>

<figure><img src="image_27.png" /><figcaption>**Fig. 20.** Synthetic pathway and reactivity of complex 60[BAr₄F] (Cp' = η⁵-C₅Me₅).</figcaption></figure>

<figure><img src="image_28.png" /><figcaption>**Fig. 21.** Synthetic pathway for complexes 64, 65[BF₄] and 66[BF₄].</figcaption></figure>
## 5. Conclusions

The state of the art of the chemistry of di- and tri-nuclear hetero-metallic [Mo₂Fe]-, [Fe₂Mo]_ and [FeMo]-complexes featuring a sulfur environment has been presented in this review. [Fe₂Mo] and [FeMo] derivatives afford an original approach for modelling the key-truncated {FeₓMoS} (x = 1 or 2) moiety of the active site of Mo-nitrogenase. It is obvious that the activity of such [FeMoS]-species towards N₂ and N₂-Reduction Substrates as well as their redox properties have been understudied despite the fact, that strategies for their syntheses can be well identified. Very recent interest in their chemistry and further developments should contribute to better describe the behaviours of {FeS}/ {Fe₂S} sites into the vicinity of a molybdenum centre, and reciprocally the role of this latter within such a metal-cooperativity. Furthermore, both the electrochemical and the catalysis studies of these complexes are scarce. They would be of prior interest to help to understand the still unclear role of the FeMo apex. Finally, this approach involving heterometallic FeMo species still remains an open and promising avenue.

## Authors contribution

M.L. and P.S. contributed equally to the writing of the manuscript.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:P SCHOLLHAMMER reports financial support was provided by University of Western Brittany. P SCHOLLHAMMER reports financial support was provided by National Centre for Scientific Research. M LAURANS reports financial support was provided by European Commission. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

CNRS (Centre National de la Recherche Scientifique) and the Université de Bretagne Occidentale (Brest) are acknowledged for financial support. This project has received funding from the European Union's Horizon 2020 research and programme under the Marie Skłodowska-Curie grant agreement No 899546.

## Data availability

Data will be made available on request.

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