# Synthesis and Reactivity of an Iron–Tin Complex with Adjacent Stannylidyne and Ferriostannylene Units

Yang Liu, Franz F. Westermair, Isabelle Becker, Sebastian Hauer, Michael Bodensteiner, Christoph Hennig, Gábor Balázs, Franc Meyer, Ruth M. Gschwind, Robert Wolf

► To cite this version:

Yang Liu, Franz F. Westermair, Isabelle Becker, Sebastian Hauer, Michael Bodensteiner, et al., Synthesis and Reactivity of an Iron–Tin Complex with Adjacent Stannylidyne and Ferriostannylene Units. Journal of the American Chemical Society, 2025, 147 (8), pp.7083–7093. 10.1021/jacs.4c18423. [hal-05122139]

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# Synthesis and Reactivity of an Iron–Tin Complex with Adjacent Stannylidyne and Ferriostannylene Units

Yang Liu, Franz F. Westermair, Isabelle Becker, Sebastian Hauer, Michael Bodensteiner, Christoph Hennig, Gábor Balázs, Franc Meyer, Ruth M. Gschwind, and Robert Wolf*

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Cite This: J. Am. Chem. Soc. 2025, 147, 7083–7093

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## ACCESS

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Supporting Information

**ABSTRACT:** Heavier transition metal carbyne analogs hold significant potential for cooperative activation of small molecules. However, complexes containing more than one heavier tetrylidyne ligand RE (E = Si, Ge, Sn, Pb) are rare due to the high oligomerization tendency of RE ligands. In this study, we describe the complex [Fe(SnAr')₂] (1; Ar' = 2,6-Dipp₂-C₆H₃, Dipp = 2,6-Pr₂-C₆H₃), which features adjacent Fe–Sn single and double bonds. Complex 1 exhibits versatile reactivity with transition metal and main group compounds. Treatment of complex 1 with Ni(COD)₂ (COD = 1,5-cyclooctadiene) yields the tetranuclear complex [Fe(μ-SnAr')₂Ni] (2), characterized by an unusual “push–pull” interaction between nickel(0) and the two coordinat-

ing Sn atoms, as revealed by quantum chemical studies. The reaction of complex 1 with AlBr₃ results in Al–Br bond cleavage and Ar' migration to aluminum. CH₃I adds oxidatively to the Sn atom that is singly bonded to Fe, while PMe₃ coordinates to Fe, inducing reversible cleavage of the Fe=Sn double bond. In addition, complex 1 activates inorganic molecules. CO₂ undergoes disproportionation to produce a carbonate-bridged Ar'Sn(μ–OCO₂)SnAr' ligand, whereas CS₂ is reductively coupled to form an ethylene tetrathiolate ligand ([C₂S₄]⁴⁻). The reaction with white phosphorus (P₄) generates an unusual Ar'P₄Sn₂Ar' ligand. This multifaceted reactivity illustrates the behavior of the Fe and Sn sites in complex 1, suggesting that complexes of this type are promising reagents for small molecule activation.

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## INTRODUCTION

Terminal carbyne complexes typically possessing a metal–carbon triple bond M≡CR (Figure 1a, I), have been widely investigated ever since the pioneering works by Fischer and Schrock in the 1970s,¹,² unfolding their broad applications in materials science and organic transformations, e.g., alkyne metathesis.³ In contrast, metal complexes with monosubstituted heavier tetrylidyne ligands RE (E = Si, Ge, Sn, Pb) are much less explored. These heavier carbyne homologues exhibit M–ER bonding interactions and reactivities that differ significantly from their carbon counterparts.⁴

The heavier group 14 elements, Si–Pb, show a strong reluctance to engage in s/p valence orbital hybridization, resulting in a decreased tendency for heavier tetrylidynes RE to form multiple bonds.⁵ Nevertheless, a few examples of M≡ER triple bonds, similar to metal carbyne complexes (Figure 1a, I), and metallotetrylenes showing M–ER single bonds (Figure 1a, III) have been reported.⁴,⁶,⁷,⁸ The singly bonded RE ligands (M–ER) are usually amphiphilic due to a chemically active electron lone pair and vacant p orbital(s) on the p-block element (Figure 1a, III).⁹ Additionally, the Wesemann and Tilley groups reported several cationic metal complexes [M=ER]⁺ (M = Ti, Ir, Co; E = Ge, Sn) bearing M=E

double bonds recently.¹⁰,¹¹ In these complexes, the positive charge is largely localized on the p-block atom which also possesses an empty p-orbital to avoid the otherwise reactive RE radical (Figure 1a, II). Noteworthily, [Ir=ER]⁺ (E = Ge, Sn) complexes have shown pronounced reactivity in cooperative X–H bond activation (X = N, O, H).¹⁰ᵇ

Overall, the unique bonding characteristics, coupled with the cooperativity between the metal and p-block centers, endow [M(ER)] complexes with distinctive reactivity, positioning them at the forefront of synthetic chemistry in the quest for innovative molecular activation methodologies. The application of [M(ER)] (E = Si–Pb) complexes in cooperative small molecule activation, such as H₂,¹⁰ᵇ,¹² NH₃,¹⁰ᵇ,¹³ CO₂¹⁰ᶜ and aldehydes/ketones,¹⁴ as well as in catalysis⁹ᶜ has been demonstrated, although the number of successful examples remains relatively limited.

Received: December 23, 2024

Revised: January 25, 2025

Accepted: January 28, 2025

Published: February 11, 2025

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<figure><img src="image_14.png" /><figcaption>**Figure 1.** (a) Bonding modes between transition metal and monosubstituted tetryl element ligand. (b) $M_2E_2$ clusters. (c) Ferriobis(plumbylene) with two PbAr\* units. (d) Iron-stannilyldyne complex featuring Fe=Sn double bond. (e) The FeSn₂ cluster 1.</figcaption></figure>

Inspired by the rapidly growing family of multimetallic systems that exhibit enhanced reactivity due to cooperative effects,¹⁵ we sought to further develop this strategy by incorporating two heavier RE units into a transition metal complex. Specifically, we targeted a rare heterotrimetallic $M_2E_2$ core with two-coordinate p-block metal centers. Compared to the ubiquitous heterobimetallic ME core, such a system would be of fundamental interest not only in general synthetic chemistry but also in molecular activation, as it offers potentially greater flexibility for substrate binding in terms of available reactive sites from multiple M/E centers, and hence greater scope of reactivity.

However, the synthesis of type $[M(ER)_2]$ ($E = Si-Pb$) complexes with adjacent RE ligands is challenging due to the inherent propensity of RE moieties toward dimerization to form a ditetrylyne unit REER.¹⁶ Power and coworkers have described heterotetrametallic complexes $[M(ER)]_2$ ($M = Cr, Mo, W, Fe, Co; E = Ge, Sn$; Figure 1b) featuring rhomboid $M_2E_2$ cores.¹⁷ Each RE ligand bridges two metal atoms in those complexes, resulting in three-coordinate E centers with suppressed electrophilicity and nucleophilicity due to the additional $M \rightarrow E/E \rightarrow M$ interactions, reducing their reactivity. Nevertheless, the $Co_2Sn_2Ar'_2$ (Ar' = 2,6-Dipp₂-C₆H₃, Dipp = 2,6-Pr₂-C₆H₃) cluster activated white phosphorus ($Pd_4$), affording an unusual ternary complex with a $Co_2Sn_2P_4$ core.¹⁷c To our knowledge, the only well-defined complex containing more than one coordinating RE unit with two-coordinate group 14 atoms is [(CO)₄Fe(PbAr\*)₂] (Ar\* = 2,6-Trip₂-C₆H₃, Trip = 2,4,6-Pr₃-C₆H₂; Figure 1c), synthesized by the reaction of a diplumbyne with $Fe_2(CO)_9$.¹⁸ The two Fe-Pb-Cipso linkages are bent and feature Fe-Pb single bonds (type IIIa). However, its reactivity was not reported. Additionally, complexes featuring Fe-Sn multiple bonds are rare, with most known examples being iron-stannylene complexes [Fe-(SnR₂)].¹¹,¹⁹ Tilley's cationic complex $[Cp\*(Pr₂MeP)Fe-(SnAr\*)]⁺$ (Figure 1d) and the related hydride $[Cp\*- (Pr₂MeP)Fe(H)(SnAr\*)]⁺$ (Ar\# = 2,6-Mes₂-C₆H₃, Mes =

2,4,6-Me₃-C₆H₂) appear to be the only iron-stannilyldyne complexes reported so far. According to structural and quantum chemical studies, these complexes show a type II Fe=Sn double bond.¹¹ Remarkably, the reduction of this complex by a single electron led to intramolecular C-H bond activation, which was proposed to proceed through a neutral iron-stannilyne intermediate featuring an Fe=Sn double bond. However, isolable neutral complexes featuring M=ER double bonds are still unknown to date.

Here, we report the synthesis and characterization of the neutral FeSn₂ cluster 1 bearing two adjacent Ar'Sn ligands (Figure 1e). The bonding situation and electronic structure of 1 have been established spectroscopically and computationally, revealing an Fe-Sn single bond and an Fe=Sn double bond within the same molecule. The reactivity of this heterotrimetallic system toward Lewis acid (AlBr₃), Lewis base (PMe₃), and small inorganic molecules such as $Pd_4$, $CS_2$ and $CO_2$ was investigated. These studies illustrate a highly versatile and cooperative reactivity of the amphiphilic FeSn₂ core of 1.

## ■ RESULTS AND DISCUSSION

**Synthesis and Characterization.** Treatment of $[Ar'SnCl]_2$ with one equivalent of the Fe²⁻ synthon [Li(DME)]₂[Fe(COD)₂] (COD = 1,5-cyclooctadiene, DME = 1,2-dimethoxyethane) in toluene at 60 °C (3 days) leads to the formation of the title complex $[Fe(SnAr')_2]$ (1) in 73% yield as dark brown crystals after workup (Scheme 1). The reaction of

<figure><img src="image_15.png" /><figcaption>**Scheme 1.** Synthesis of Complex 1</figcaption></figure>

$[Ar'SnCl]_2$ with two equivalents of the Fe⁻ synthon [K(18-c-6)][Fe(η⁴-anthracene)₂] (18-c-6 = 18-crown-6) still yields 1, showing the preference for a single Fe site of this system. Complex 1 is stable at room temperature (r.t.) in an inert atmosphere, both in the solid state and in common organic solvents (e.g., benzene, toluene and THF). Heating a solution of 1 in $C_6D_6$ to 80 °C for several days does not result in any noticeable decomposition as evidenced by NMR monitoring experiments. However, 1 decomposes rapidly in air to give intractable mixtures.

The molecular structure of 1 was determined by single-crystal X-ray crystallography (Figure 2) and further supported by multinuclear NMR spectroscopy, HR-LIFDI-mass spectrometry, inductively coupled plasma-optical emission spectroscopy (ICP-OES) and elemental analysis. The molecular structure of 1 features a unique FeSn₂ core where the iron atom bridges the two Ar'Sn moieties in an asymmetric fashion and is η⁶-coordinated by a flanking Dipp-aryl ring of one Ar' substituent (Figure 2a). The overall structure of compound 1 resembles an unknown metal η²-distannyne complex. Notably, in metal η²-disilyne²⁰ and η²-digermyne¹⁶c,²¹ complexes, $[M(η²-R'EER')]$ (M = Ni, Pd, Pt, Ag; E = Si, Ge; R' = alkyl, aryl or silylene substituent), the $M_2$ rings are perpendicular to the C-E-E-C arrays so that the metal centers can interact with the $E \equiv E$ π-out orbitals in a classic way that is analogous to metal η²-alkyne complexes. However, the FeSn₂ plane in 1 is
a

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b

<figure><img src="image_17.png" /><figcaption>**Figure 2.** Molecular structure of complex **1** with 50% probability ellipsoids: front view (a) and side view without Dipp groups (b). Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Fe1–Sn1 2.2954(7), Fe1–Sn2 2.6207(7), Sn1–C1 2.135(2), Sn2–C1' 2.243(2), Sn1···Sn2 3.0595(6); Sn1–Fe1–Sn2 76.66(2), Fe1–Sn1–C1 176.60(6), Fe1–Sn2–C1' 91.10(5).</figcaption></figure>

coplanar with the C1'-Sn1···Sn2–C1 array (Figure 2b). Given that the slipped Sn–Sn $\pi_{in}$ orbital in the distannyne compound [Ar'SnSnAr'] is reluctant to engage in bonding, a distinct Sn–Fe–Sn bonding mode is expected for **1** (vide infra).²²

The Fe1–Sn2 bond length of 2.6207(7) Å and the bent geometry at Sn2 (∠Fe1–Sn2–C1': 91.10(5)°) in **1** signify a metallostannylene structural motif of the Fe1Sn2Ar' unit with a type III Fe1–Sn2 single bond (Figure 1a). Compared to the structures of known ferriostannylene complexes (Fe–Sn: 2.56–2.61 Å and ∠Fe–Sn–C: 106.7–118.2°),¹¹,¹⁷b,²³ the Fe1–Sn2 bond length is slightly longer, and the Fe1–Sn2–C1' angle is more acute, probably due to the strain induced by the coordination of the arene moiety to iron. In distinct contrast, the Fe1–Sn1 bond length of 2.2954(7) Å is significantly shorter than the Fe1–Sn2 bond, and the geometry at Sn1 is nearly linear (∠Fe1–Sn1–C1: 176.60(6)°). These metrical parameters are comparable with those in [Cp*('Pr₂MeP)Fe(SnAr#)]⁺ (2.2889(6) Å and 169.85(9)°; Figure 1d)¹¹ and complex **4** (2.2960(4) Å and 175.45(8)°; vide infra), indicating the Fe1=Sn1 double bond character in **1**. The Sn1···Sn2 distance of 3.0595(6) Å is substantially longer than those of the relevant distannyne compound [Ar'SnSnAr'] (2.667(5) Å)¹⁶a and complex [Co(SnAr')]₂ (2.8700(5) Å),¹⁷c as well as a regular Sn–Sn single bond (ca. 2.80 Å),²⁴ but shorter than the sum of the van der Waals radii of tin (Σr<sub>vdW</sub> = 4.84 Å).²⁵

The ¹H NMR spectrum of **1** (C₆D₆, 298 K) shows three sets of signals attributed to the Dipp flanking groups with an integral ratio of 1:1:2, agreeing with the asymmetric coordination pattern (Figure S1). Neither broadening nor coalescence of these signals was observed at 341 K, indicating the absence of dynamic behavior, i.e., exchange of the coordinating aryl groups on the NMR time scale (Figure S4). The ¹¹⁹Sn{¹H} NMR spectrum of **1** shows two singlets with remarkably different chemical shifts at +466 ppm and +3229 ppm, which are assigned to Sn1 and Sn2, respectively. This assignment is supported by DFT calculations (Table

S19). The downfield-shifted signal of Sn2 lies within the range of 2515–3762 ppm, characteristic of ferriostannylenes (type IIIa)¹¹,²³ and other metallostannylenes with two-coordinate Sn centers.¹⁰b,¹²a The signal of Sn1 is markedly upfield shifted relative to Sn2, which indicates a strong Fe → Sn back-donation, viz. the formation of a multiple bond, and is similar to that of the doubly bonded Sn1 in 4 (+768 ppm; vide infra). By contrast, the cationic complex [Cp*('Pr₂MeP)Fe(SnAr#)]⁺ shows a ¹¹⁹Sn{¹H} NMR resonance at +1024 ppm.¹¹ The zero-field ⁵⁷Fe Mössbauer spectrum of **1**, recorded at 80 K for a solid sample, displays a doublet with the isomer shift value of δ = 0.61 mm s⁻¹ and quadrupole splitting value of |ΔE<sub>Q</sub>| = 0.66 mm s⁻¹. Although the relationship between the δ/ΔE<sub>Q</sub> values and oxidation states of iron complexes featuring low-coordinate and low-valent Fe centers remains ambiguous,²⁶ the isomer shift of **1** is positively shifted compared with known low-valent, η⁶-arene-coordinate Fe⁰ and Fe⁻¹ systems, such as [bis(NHC)Fe⁰(η⁶-arene)] (0.43 mm s⁻¹; NHC = N-heterocyclic carbene),²⁷ [(η⁶-arene)Fe⁰Ge(N''')'(NHC)]⁺ (0.47 mm s⁻¹; N'' = {[Ph₂PCH₂Si('Pr)₂](Dipp)N},¹²b and [(η⁶-arene)Fe⁻¹Sn(N''')'(NHC)] (0.52 mm s⁻¹).¹²b The higher isomer shift of **1** may therefore indicate a comparatively high d-electron density at the Fe atom, which shields the s-electrons from the nucleus. The UV-vis spectrum of **1** (toluene, r.t.) shows a prominent band at 450 nm and two weak, broad bands at 590 and 730 nm. These absorption features are assigned to transitions primarily from the highest occupied molecular orbitals (HOMO and HOMO-1) to the lowest unoccupied molecular orbitals (LUMO, LUMO+1, and LUMO+2), based on time-dependent DFT calculations (Figure S61).

**Electronic Structure and Bonding.** To gain more insight into the electronic structure of **1**, quantum chemical calculations were performed at different levels of theory (see Supporting Information for details). The optimized structure agrees very well with the X-ray crystallographic data (Figure S60 and Table S7). The HOMO (−4.07 eV) represents the Fe1–Sn2 σ-bond with significant contribution from the electron lone pair on Sn2 (31% Fe, 46% Sn2; Figure 3a). HOMO–1 (−4.44 eV) corresponds to the Fe1–Sn1 out-of-plane π-bond with minor delocalization toward Sn2, forming a 3-center-2-electron (3c2e) π-interaction over the FeSn₂ core (37% Fe1, 30% Sn1, 13% Sn2; Figure 3b). The Fe1–Sn1 σ-bond is much lower in energy (HOMO–16, −7.27 eV; Figure S62). The LUMO and LUMO+2 mainly consist of the Sn-centered p orbitals, specifically the orthogonally oriented Sn2 p<sub>out</sub> and Sn1 p<sub>in</sub> orbitals, while the LUMO+1 is mainly contributed by the Fe 3d orbitals (Figure S62). The bonding modes between Fe and Sn atoms in **1** are further illustrated by electron localization function (ELF)²⁸ analysis, which exhibits the electron distribution patterns consistent with an Fe1–Sn1 double bond, an Fe1–Sn2 single bond with weak π-interaction, and relatively low ELF value between the two Sn atoms (Figure S64). The Mayer bond order (MBO) of Fe1–Sn1 (1.50) and Fe1–Sn2 (0.94) bonds suggests their double and single bond character, respectively, which is in line with the canonical MO and ELF analyses. The MBO of the Sn1···Sn2 (0.59) unit is relatively low, suggesting a rather weak Sn···Sn interaction in **1**. An independent gradient model based on Hirshfeld partition (IGMH)²⁹ analysis also reveals that the Sn···Sn interaction is much weaker compared to the Fe–Sn interactions (Figure S65).

Intrinsic bond orbital (IBO)³⁰ analysis provides further insights into the bonding situation of **1**. The Fe1–Sn1 σ-
<figure><img src="image_18.png" /><figcaption>**Figure 3.** (a,b) Selected MOs of **1** (Isovalue = 0.04). (c–f) Selected IBOs of **1**. The bond type and percentage of the electron density on Fe and Sn are given. (g) Plot of the Laplacian of the electron density on the FeSn₂ plane of **1** (BCP: blue dots, RCP: brown dots, bond paths: green lines).</figcaption></figure>

bonding interaction consists mainly of the Sn1-based lone pair, thus representing a dative bond (Figure 3c). The out-of-plane Fe1–Sn1 π-bond extends to the adjacent Sn2 atom, which aligns with the HOMO–1 of **1** (Figure 3d). A nonbonding electron lone pair resides exclusively at Sn2 and lies in the FeSn₂ plane (Figure 3e). The Fe1–Sn2 σ-bond shows strong polarization toward Fe1 (Figure 3f). The natural charge distribution (Fe1: –0.65, Sn1: +1.04, Sn2: +0.92; Table S8) reveals an electron-rich iron center, which is consistent with the high positive isomer shift value obtained from the ⁵⁷Fe Mössbauer measurement of **1** (vide supra), and the positive charge accumulation of Sn centers. Furthermore, substantial net electron transfer from the Ar'Sn1 moiety (+0.52 au) to the Ar'Sn2Fe unit is calculated, implying a zwitterionic character of **1** (Table S8). Quantum theory of atoms in molecules (QAIM)³¹ analysis reveals two Fe–Sn bond critical points (BCP; Figure 3g). However, neither a BCP between Sn1 and Sn2 atoms nor a ring critical point (RCP) within FeSn₂ unit

could be located, indicating the absence of a Sn1–Sn2 bond. The energy density ($H(r)$) and ellipticity of electron density ($\epsilon(r)$) at the BCP of Fe1–Sn1 bond ($H(r)$: –0.031 $e.a_0^{-3}$; $\epsilon(r) = 0.15$) are larger than those of the Fe1–Sn2 bond ($H(r)$: –0.015 $e.a_0^{-3}$; $\epsilon(r) = 0.09$), providing additional evidence for the multiple bond character of the Fe1–Sn1 bond.³² The delocalization index (DI)³³ values of the basins between Fe1 and Sn1 (1.41)/Sn2 (0.94), and Sn1 and Sn2 (0.46) agree well with the MBO.

An energy decomposition analysis coupled with natural orbitals for chemical valence (EDA-NOCV)³⁴ of the Fe1=Sn1 bond provides a consistent bonding scenario with the above analyses. The smallest orbital interaction energy $|\Delta E_{\text{orb}}|$ (139.62 kcal mol⁻¹) was found between the [Ar'Sn2Fe]⁻ anion and the [Ar'Sn1]⁺ cation fragments, both in the singlet state, which is significantly smaller than those of other states (e.g., S = 1/2–4; 198.59–331.16 kcal mol⁻¹; Table S17 and Figure S81 and see SI for details). These data further corroborate the zwitterionic character of **1** and indicate the presence of a σ-donor-π-acceptor type double bond between Fe1 and Sn1.

Overall, the bonding of the FeSn₂ core in **1** can be best described as containing an Fe1=Sn1 double bond, comprising one σ- and one π-bond, and a type IIIa Fe1–Sn2 single bond, with little or negligible Sn1···Sn2 interaction. It is worth noting that the ligand constraint might play a crucial role in the formation of such a bonding situation. For example, the complex [(η²-RSiSiR)Fe(η⁶-C₆H₆)] (R = κ²-amidinate),³⁵ shows a three-membered FeSi₂ metallacyclic structure. Additionally, complex **1** has a zwitterionic character with the positive charge residing primarily at the doubly bonded Sn1 atom. Noteworthy, a few cationic transition metal complexes with M=ER double bonds have been reported (e.g., [(PMe₃)₃(H)M(EAr*)]⁺ (M = Ir, Co; E = Ge, Sn)¹⁰ᵇ,c and [Cp*(Pr₂MeP)Fe(SnAr#)]⁺,¹¹ Figure 1d). To our knowledge, **1** represents the first isolable neutral iron-stannylidyne complex featuring an Fe=Sn double bond.

**Reactivity Study.** To test if the lone pair on Sn2 and the p-vacant orbital on Sn1 could potentially form “push-pull” interactions, we performed the reaction of **1** with Ni(COD)₂. This led to the formation of complex **2** as dark green crystals in 69% yield (Scheme 2). The solid-state molecular structure of **2** (Figure 4a) reveals a planar, quasi-rhomboid FeNiSn₂ core structurally similar to the Co₂Sn₂ core in complex [Co(SnAr')]₂.¹⁷c The Ni1–Sn1 (2.7018(24) Å) and Ni1–Sn1' (2.4328(22) Å) bond lengths are comparable to the bis-(stannylene)nickel complexes reported by Wesemann and coworkers, where two stannylene (SnR₂) ligands serve as L- and Z-type ligands.³⁶ The Fe1–Sn1 (2.3775(22) Å) and Fe1–Sn1' (2.4431(23) Å) bonds are elongated and shortened, respectively, compared to those in **1** (Table 1).

Albeit isoelectronic, complex **2** shows a distinct electronic structure from [Co(SnAr')]₂.¹⁷c The IBO analysis of **2** reveals that the electron lone pair from Sn1' coordinates to Ni forming a highly polarized Ni1–Sn1' σ-bond (0.29 Ni1/0.53 Sn1'), while the Ni1–Sn1 bond is predominantly contributed by Ni 3d orbitals (0.90 Ni1/0.13 Sn1; Figure S68). This suggests a Sn1' → Ni → Sn1 "push-pull" interaction, showcasing the electron donating and accepting properties of Sn2 and Sn1 in **1**, respectively. In addition, IBO and MBO analyses of the Fe–Sn bonds in **2** (IBOπ<sub>Sn1–Fe1–Sn1</sub>: 0.29 Sn1/0.44 Fe1/0.20 Sn1'; MBO<sub>Fe1–Sn1</sub>: 1.26, MBO<sub>Fe1–Sn1</sub>: 1.22; Figure S68 and Table S10), indicate a π-electron flow from Sn1 to Sn2 (via the
<figure><img src="image_19.png" /><figcaption>Scheme 2. Reactivity Study of Complex 1</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Figure 4.** Molecular structures of 2 (a), 3 (b), 4 (c), 5 (d), 6 (e), 7 (f), and 8 (g), showing 50% probability thermal ellipsoids. Hydrogen atoms, solvent molecules, and the ¹Pr substituents are omitted for clarity.</figcaption></figure>

central Fe) upon the addition of Ni. Despite the contraction of the Sn1···Sn1' distance to 2.8860(3) Å in 2 (vs 3.0595(6) Å in 1), QAIM analysis suggests no Sn1–Sn1' bond (Figure S69). The ¹H NMR spectra of 2 recorded at 298 and 341 K are nearly identical, displaying four sets of signals corresponding to the Dipp groups. This observation is consistent with the asymmetric structure observed in the solid state and indicates no dynamic behavior in solution on the NMR time scale (Figure S8). Due to the increase in the coordination number of Sn1', a considerable upfield shift of its ¹¹⁹Sn{¹H} NMR resonance signal (+1701 ppm vs +3229 ppm in 1; Table 1)

was observed,³⁷ while the signal at +369 ppm was assigned to Sn1 (Table S19). The zero-field ⁵⁷Fe Mössbauer spectrum (80 K, solid state) shows a doublet with $\delta = 0.55$ mm s⁻¹ and $|\Delta E_Q| = 0.76$ mm s⁻¹. The smaller $\delta$ value of 2 compared to 1 (0.66 mm s⁻¹; Table 1) suggests a decrease of electron density on Fe1, which is consistent with the change of its natural charge (Fe1: −0.46 in 2 vs −0.65 in 1; Table S8).

To probe the potential amphiphilic reactivity of the Sn centers in 1, as predicted by the canonical MO analysis, we investigated the reactivity of 1 toward the Lewis acid AlBr₃, the electrophile CH₃I, and the nucleophile Me₃P. At −30 °C, 1
<table><caption>Table 1. Selected Bond Lengths (Å) and Angles (Deg) and Experimental <sup>119</sup>Sn NMR and <sup>57</sup>Fe Mössbauer Data of Complexes 1–8</caption><thead><tr><th></th><th>Fe-Sn1 (Å)</th><th>Fe-E (Å)</th><th>Fe1-Sn1-C, Fe1-E-C (deg)</th><th><sup>119</sup>Sn NMR δ<sub>Sn1</sub>, δ<sub>E</sub> (ppm)<sup>a</sup></th><th><sup>57</sup>Fe Mössbauer δ, |ΔE<sub>Q</sub>| (mm s<sup>-1</sup>)</th></tr></thead><tbody><tr><td>1, E=Sn2'</td><td>2.2954(7)</td><td>2.6207(7)</td><td>176.60(6), 91.10(5)</td><td>466, 3229</td><td>0.61, 0.66</td></tr><tr><td>2, E=Sn1'</td><td>2.3775(22)</td><td>2.4431(23)</td><td>165.29(7), 95.98(7)</td><td>369, 1701</td><td>0.55, 0.76</td></tr><tr><td>3, E=Sn2</td><td>2.3768(9)</td><td>2.5249(6)</td><td>158.02(9)</td><td>15, 2393</td><td></td></tr><tr><td>4, E=Sn2</td><td>2.2960(4)</td><td>2.5188(5)</td><td>175.45(8), 97.45(8)</td><td>768, 107<sup>b</sup></td><td></td></tr><tr><td>5, E=Sn2</td><td></td><td>2.4864(7)</td><td>99.78(2)</td><td>2403, 1456</td><td>0.50, 1.17</td></tr><tr><td>6, E=Sn2</td><td>2.4136(8)</td><td>2.7925(8)</td><td>154.92(5), 86.14(5)</td><td>-124, 655<sup>c</sup></td><td></td></tr><tr><td>7, E=Sn2/Sn2'</td><td></td><td>2.5421(5)/2.5874(6)</td><td>96.50(7), 132.50(7)</td><td>264, 343<sup>c</sup></td><td></td></tr><tr><td>8, E=Sn2</td><td></td><td>2.7892(7)</td><td></td><td>-455, -366<sup>d</sup></td><td></td></tr></tbody></table>

<sup>a</sup>Signals were assigned based on DFT calculations (Table S19). <sup>b</sup>Signals were assigned based on a comparison with literature data. <sup>c</sup>Signals were not assigned. <sup>d</sup>Multiplets due to the Sn–P coupling.

reacts readily with AlBr₃, affording complex **3** as maroon crystals in 84% yield (Scheme 2). The solid-state molecular structure of **3** (Figure 4b) shows that, instead of a donor–acceptor adduct, Al–Br bond cleavage along with Ar' migration occurred. The Ar' substituent migrates from Sn2 to Al, while one Br migrates from Al to bridge the two Sn centers with 2.6875(5) Å (Sn1–Br3) and 2.8765(8) Å (Sn2–Br3) bond lengths. The Sn1 center adopts a nearly T-shaped geometry with Fe1–Sn1–C1 angle of 158.02(9)° and with the Fe1–Sn1 bond length of 2.3768(9) Å, which lies within the range of the reported iron-stannylene complexes (2.37–2.48 Å).<sup>11,38</sup> The Sn2 center has a pyramidal environment, which is typical for three-coordinate Sn(II) centers with a stereochemically active electron lone pair. The Fe1–Sn2 (2.5249(6) Å) bond is considerably shorter than those of ferriostannylene complexes (2.56–2.60 Å).<sup>11,17b,23</sup> IBO analysis corroborates the presence of Fe1–Sn1 and Fe1–Sn2 single bonds and the nonbonding electron lone pair on Sn2 (Figure S71). The <sup>119</sup>Sn{¹H} NMR spectrum of **3** displays two singlets at +15 ppm and +2393 ppm, which were assigned to Sn1 and Sn2, respectively, based on DFT calculations (Table S19). The high-frequency chemical shift for Sn2 indicates a low coordination number and is in agreement with the <sup>119</sup>Sn{¹H} NMR chemical shifts reported for ferriostannylene complexes.<sup>11,23</sup> These structural features allow several possible Lewis-type descriptions of **3** (Scheme S1).

The reaction of **1** with CH₃I afforded complex **4**, resulting from an oxidative addition of CH₃I to Sn2 (Scheme 2). Complex **4** was isolated as dark red crystals in 67% yield. The solid-state molecular structure of **4** reveals a Sn(IV) center in tetrahedral geometry (Figure 4c). The Fe1–Sn2 bond length of 2.5188(5) Å lies in the expected range for a Fe–Sn(IV) single bond.<sup>39</sup> The Sn1···Sn2 distance of 3.3804(5) Å is much longer than that in **1** (3.0596(6) Å), indicating no interaction between the Sn atoms (Figure S75). Notably, the Fe1–Sn1 bond length of 2.2960(4) Å and the Fe1–Sn1–C1 bond angle of 175.45(8)° are almost identical to those in **1** (2.2954(7) Å and 176.60(6)°; Table 1), indicating a similar bonding scenario. Indeed, the presence of an Fe1=Sn1 double bond and the zwitterionic character of **4**, which includes an [Ar'Sn1]<sup>+</sup> moiety, are supported by the canonical MO, IBO, NPA and EDA-NOCV analyses (Figures S73, S74, and S82, Tables S8 and S18), as well as the MBO (MBO<sub>Fe–Sn1</sub>: 1.68; Table S13). Hence, the Fe1–Sn1 bond is essentially unaffected by the oxidative addition of CH₃I to Sn2. The <sup>119</sup>Sn{¹H} NMR resonances of **4** were detected at +768 ppm (Sn1) and +107 ppm (Sn2), which is consistent with an Fe=SnAr' moiety and a tetrahedral Sn(IV) center, respectively.<sup>11,39</sup> This reaction

highlights the amphiphilic character of Sn2 in **1** and demonstrates the potential of **1** to form other complexes with Fe–Sn multiple bonds via the derivatization of the ferriostannylene moiety.

Treatment of **1** with excess PMe₃ resulted in the clean conversion into complex **5**, which was isolated as dark red crystals in quantitative yield (Scheme 2). Intriguingly, PMe₃ was found to coordinate to the Fe center, inducing the cleavage of the Fe1=Sn1 double bond and the formation of a Sn1–Sn2 bond, giving rise to a distannyne (Ar'SnSnAr') ligand (vide infra). Monitoring the stoichiometric reaction of **1** with PMe₃ by ¹H NMR spectroscopy uncovered that the reaction is reversible. A van 't Hoff analysis based on ¹H NMR data at various temperatures provides the thermodynamic parameters: ΔH° = -14.77 kcal mol⁻¹, ΔS° = -32.53 cal mol⁻¹ and ΔG° (298 K) = -5.08 kcal mol⁻¹ (ΔG°<sub>calc.</sub> = -5.34 kcal mol⁻¹; Figure S77 and see SI for details). The K<sub>eq</sub>(298 K) was calculated to be 5.3 × 10³ M⁻¹, indicating that the formation of **5** is favored.

The solid-state molecular structure of **5** shows the Ar'SnSnAr' ligand coordinating to the iron center via one η¹-Sn atom (Sn2) and one η⁶-Dipp group (Figure 4d). The three-coordinate Sn2 center adopts a planar geometry with a large Fe1–Sn2–Sn1 angle of 160.12(1)° and a Fe1–Sn2 bond length of 2.4864(7) Å. This Fe1–Sn2 bond is considerably shorter than the Fe1–Sn2 single bond in **1** (2.6207(7) Å; Table 1) and lies slightly above the range expected for iron-stannylene complexes (2.37–2.48 Å).<sup>11,38</sup> In line with this relatively short Fe1–Sn2 bond, IBO and MBO (MBO<sub>Fe–Sn1</sub>: 1.34) analyses suggest double bond character, comprising both σ and π components (Figure S78 and Table S15). The zero-field <sup>57</sup>Fe Mössbauer spectrum (80 K, solid state) of **5** reveals a doublet with δ = 0.50 mm s⁻¹ and |ΔE<sub>Q</sub>| = 1.17 mm s⁻¹, which is slightly lower than those of **2** (0.55 mm s⁻¹) and **1** (0.66 mm s⁻¹; Table 1). The <sup>119</sup>Sn{¹H} NMR spectrum of **5** exhibits two singlets at +2403 ppm and +1456 ppm, for Sn1 and Sn2, respectively, which is consistent with the presence of two-coordinate stannylene (Sn1) and metal-stannylene (Sn2) units.

The coordination of Lewis basic ligands, such as isocyanides,<sup>40</sup> amines,<sup>41</sup> imines<sup>42</sup> and NHCs<sup>43</sup> to distannynes, forming L → Sn σ-bonds, has been well established. By contrast, the coordination of distannynes to transition metals is underexplored. So far, only two distannyne-iron carbonyl complexes were reported, where the distannynes are intra-molecularly stabilized by amine ligands and act as electron donors, forming Sn → Fe σ-bonds.<sup>44</sup> Notably, the bonding of the distannyne in **5** is distinct, featuring an Fe1=Sn2 double
bond. Moreover, the reversible cleavage and formation of an M–E multiple bond, as observed for S in solution, represents a new reaction pattern for heavier tetrylidyne complexes.

The reactivity of **1** toward small molecules, such as carbon disulfide (CS₂), carbon dioxide (CO₂), and white phosphorus (P₄), was also investigated. The addition of one equivalent of CS₂ to **1** in toluene at -30 °C immediately gave a red solution, from which complex **6** was isolated as red crystals in 58% yield (Scheme 2). The solid-state molecular structure of **6** shows the reductive coupling of two CS₂ molecules, resulting in a C₂S₄ unit which bridges two FeSn₂ clusters in a μ–κ²(S¹,S¹):κ²(S²,S²) fashion (Figure 4e). One pair of the S atoms (S1 and S3) is each bonded to a Sn atom, while the other pair (S2 and S4) bridges between Sn and Fe atoms. The C₂S₄ unit is planar, with a C3–C4 bond length of 1.355(3) Å, which is close to a typical C=C double bond.²⁴b The C–S bond lengths (1.762(2) Å to 1.786(2) Å) are characteristic of C–S single bonds.²⁴ These structural features suggest the presence of an ethylene tetrathiolate (C₂S₄)⁴⁻ ligand in **6**. The ¹H NMR spectrum (C₆D₆, r.t.) displays only one set of signals that can be assigned to two Ar' groups. Additionally, its ¹¹⁹Sn{¹H} NMR spectrum (C₆D₆, r.t.) exhibits only two singlets at +655 ppm and -124 ppm, supporting a C₂ symmetry of **6** in solution on the NMR time scale. A few p-block element-mediated CS₂ reductive coupling reactions have been reported,⁴⁵ including three examples with stannylenes.⁴⁵b–d In these cases, the (C₂S₄)⁴⁻ ligands were proposed to be formed via carbene intermediates and favor a μ–κ²(S¹,S²):κ²(S¹',S²') coordination mode (Figure S59). The formation of **6** is proposed to proceed through an initial [3 + 2] cycloaddition of CS₂ to the Fe1=Sn1 double bond of **1**, assisted by Sn2, generating a carbene species, which then dimerizes to afford **6** (Scheme S2). This mechanism is underpinned by the reactivity of a disilene with CS₂, where a [3 + 2]-cycloadduct with a bridging (C₂S₄)⁴⁻ unit was obtained.⁴⁶

The ability of **1** to reduce CS₂ motivated us to explore its reactivity toward CO₂, a much less reactive inorganic molecule. Exposure of a benzene solution of **1** (dark brown) to a CO₂ atmosphere (4 bar) led to an orange solution, from which complex **7** was isolated as orange crystals in 30% yield (Scheme 2). According to the solid-state molecular structure of this complex, **1** reacts with two equivalents of CO₂, forming a CO and a CO₃²⁻ ligand. The CO ligand coordinates to Fe, while the carbonate ligand bridges the two Sn atoms in a μ–κ¹(O):κ²(O',O'') fashion. Two of these units are held together via a planar, centrosymmetric Fe₂Sn₂ ring (Figure 4f). Recently, Wesemann and coworkers reported the CO₂ reduction with a cobalt-stannylidyne complex [(Me₃P)₃CoSnTbb] (Tbb = 4'-Bu-2,6-(CH(SiMe₃)₂)₂-C₆H₂),¹⁰c which is proposed to proceed via an initial [2 + 2]-cycloaddition of CO₂ to the Co≡Sn triple bond. The Aldridge group has reported distannyne-mediated reductions of CO₂, giving [Ar⁴Sn(μ–OCO₂)SnAr⁴] (Ar⁴ = NCN pincer ligand) as products, where a bis(stannylene)oxide [(Ar⁴EtSn)₂O] intermediate was identified.⁴⁷ Given the simultaneous existence of a metal–tin multiple bond, the electron-rich nature of the Fe center, the electrophilic Sn centers, and the closely positioned Ar'Sn moieties in **1**, we propose that the first step in the reduction of CO₂ by **1** occurs through a [2 + 2]-cycloaddition of CO₂ to the Fe=Sn double bond, assisted by the second Sn atom (Scheme S3). This is followed by the splitting of the C=O bond, resulting in the formation of a carbonyl iron intermediate with a (Ar'Sn)₂O

ligand which then inserts a second equivalent of CO₂ to yield the carbonate-bridged ligand Ar'Sn(μ–OCO₂)SnAr'. Dimerization yields the final product 7.

The distinct reactivity of **1** toward CO₂ and CS₂ is likely caused by the different polarity of C=O and C=S bonds; the more polarized C=O bond favors a [2 + 2]-cycloaddition with the Fe=Sn double bond of **1**, while CS₂ favors [3 + 2]-cycloaddition. By contrast, the digermyne compound [(Ar-(Me₃Si)N)Ge]₂ (Ar = 4-Me-2,6-(Ph₂CH)₂-C₆H₂) has been shown to reduce both CO₂ and CS₂ in the same manner.⁴⁸

Complex **1** also activates the unpolar bonds of white phosphorus. The reaction of **1** with P₄ proceeds smoothly in toluene at room temperature and furnishes complex **8**, which was isolated as orange crystals in quantitative yield (Scheme 2). The solid-state molecular structure of **8** reveals a P₄Sn five-membered ring in an envelope conformation, coordinating η⁴ to iron via three P and one Sn atoms (Figure 4g). The (η⁶-arene)Ar' substituent migrates from Sn2 to P2, and the Ar'Sn1 unit bridges two P atoms (P1 and P4) with bond lengths of 2.6657(5) Å (P1–Sn1) and 2.8070(6) Å (P4–Sn1). The P–P bond lengths in **8** are very similar and range from 2.1313(7) to 2.1666(7) Å, being typical for P–P single bonds.²⁴ Both Sn centers adopt a trigonal pyramidal geometry with a Sn1···Sn2 distance of 3.1262(2) Å. The ³¹P{¹H} NMR spectrum of **8** shows four multiplets of an AEMX spin system, with Jₚₚ coupling constants ranging from 490 to 3.0 Hz. ¹¹⁷/¹¹⁹Sn satellites were detected for these multiplets with coupling constants ranging from 753 to 107 Hz (Figures S31 and S33, Table S1). In the ¹¹⁹Sn{¹H} NMR spectrum, two multiplets due to the Sn–P coupling were detected at –454 ppm (Sn1) and –366 ppm (Sn2; Figures S32 and S34, Table S19). P₄ activation by heavier trytel elements remains limited.⁴⁹ To our knowledge, only two examples of low-valent tin compounds, the distannyne [(DippNC(CH₃))₂C₆H₃Sn]₂⁴² and the stannylene [Ar#Sn(Si⁴Bu₃)],⁵⁰ have been reported to react with P₄ to give butterfly like P₄ units via cleavage of one P–P bond. By contrast, the reaction of the Co₂Sn₂ cluster [Co(SnAr')₂ with P₄ results in a P₄ chain,¹⁷c while a heterobimetallic CoSn cluster reacting with P₄ gave the triple-decker cobalt complex [(tBu₂C₂P₂)₂Co₂(μ,η⁵:η⁵'-SnP₄)] with a planar P₄Sn unit.⁵¹

## ■ CONCLUSION

In summary, we synthesized and characterized [Fe(SnAr')₂] (1), the first complex to feature both ferriostannylene and stannylidyne functionalities. The combination of iron and two monosubstituted tin centers, along with the constrained ligand structure, forms an Fe–Sn single bond and an Fe=Sn double bond (σ+π). This arrangement results in a positively charged Sn center and an electron-rich Fe center. The unusual coordination environment effectively reduces the tendency of the two aryl-tin moieties to dimerize, thereby maintaining the high reactivity of the two-coordinate Sn centers. Additionally, the proximity of the metal centers enhances cooperative reactivity.

Comprehensive spectroscopic characterization and theoretical calculations support the existence of an amphiphilic FeSn₂ core with multiple reactive sites. This was demonstrated in the reactions of **1** with Ni(COD)₂, AlBr₃, CH₃I, and PMe₃. The lone-pair electrons and vacant p-orbital of the two Sn atoms in **1** create “push-pull” interactions with nickel due to their optimal distance and orientation. In the reaction with AlBr₃, the cleavage of the Al–Br bonds was observed, accompanied by the migration of Ar'. The formation of a 1·AlBr₃ adduct may
initiate this reaction. The oxidative addition of CH₃I leads to the formation of a Sn(IV) center, illustrating the amphiphilic nature of the ferriostannylene site. Interestingly, PMe₃ binds to the iron center, resulting in the cleavage of the Fe≡Sn double bond and the formation of a Sn−Sn single bond. Notably, this process is reversible. Furthermore, compound **1** activates CS₂ and CO₂ through different reaction pathways, and P₄, which leads to the cleavage of multiple P−P bonds.

Our results emphasize the significant synthetic potential of complexes that feature multiple coordinatively unsaturated sites from group 14 elements. Complexes similar to **1** should be easily obtainable using comparable strategies with other low-valent transition metalates⁵² and appropriate main group precursors. This bodes well for future investigation pursuing further challenging synergistic bond activation reactions. Studies in this direction are in hand.

■ ASSOCIATED CONTENT

Supporting Information

The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.4c18423.

Experimental details and synthetic procedures, further spectroscopic and analytical data, crystallographic information, and details of DFT calculation (PDF)

Accession Codes

Deposition Numbers 2406557, 2406570–2406571, 2406573–2406574, 2406598, 2407837, and 2410942 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via the joint Cambridge Crystallographic Data Centre (CCDC) and Fachinformationszentrum Karlsruhe Access Structures service.

■ AUTHOR INFORMATION

Corresponding Author

Robert Wolf - Institute of Inorganic Chemistry, University of Regensburg, Regensburg 93040, Germany; ● orcid.org/0000-0002-4066-6483; Email: robert.wolf@ur.de

Authors

Yang Liu - Institute of Inorganic Chemistry, University of Regensburg, Regensburg 93040, Germany; ● orcid.org/0000-0002-2486-8520

Franz F. Westermair - Institute of Organic Chemistry, University of Regensburg, Regensburg 93040, Germany;
● orcid.org/0000-0002-0197-1664

Isabelle Becker - Institute of Inorganic Chemistry, University of Göttingen, Göttingen 37077, Germany

Sebastian Hauer - Institute of Inorganic Chemistry, University of Regensburg, Regensburg 93040, Germany; ● orcid.org/0009-0009-9963-4751

Michael Bodensteiner - Institute of Inorganic Chemistry, University of Regensburg, Regensburg 93040, Germany;
● orcid.org/0000-0002-1850-5192

Christoph Hennig - European Synchrotron Radiation
Facility, Rossendorf Beamline (BM20-CRG), Grenoble
38043, France; Institute of Resource Ecology, Helmholtz-
Zentrum Dresden-Rossendorf, Dresden 01314, Germany;
● orcid.org/0000-0001-6393-2778

Gábor Balázs - Institute of Inorganic Chemistry, University of Regensburg, Regensburg 93040, Germany

Franc Meyer - *Institute of Inorganic Chemistry, University of Göttingen, Göttingen 37077, Germany*; ● orcid.org/0000-0002-8613-7862

Ruth M. Gschwind - Institute of Organic Chemistry, University of Regensburg, Regensburg 93040, Germany;
● orcid.org/0000-0003-3052-0077

Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.4c18423

Funding

European Synchrotron Radiation Facility (ESRF), ROBL beamline (BM20), DFG, RTG 2620 IonPairs in Reaction Project 426795949.

Notes

The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

We thank Dr. Christian M. Hoidn for preliminary studies on the synthesis of complex **1**, Vanessa Tomanek and Martina Amann (University of Regensburg) for assistance with ICP-OES and IR measurements, and Serhiy Demeshko (University of Göttingen) for the discussion of the ⁵⁷Fe Mössbauer results. I.B. thanks the Fonds der Chemischen Industrie (FCI) for a Kekulé fellowship. We gratefully acknowledge the European Synchrotron Radiation Facility (ESRF) for the provision of synchrotron radiation facilities under the proposal number CH-6760 by using the ROBL beamline (BM20). F.F.W. and R.M.G. would like to thank the Deutsche Forschungsgemeinschaft for funding (DFG, RTG 2620 IonPairs in Reaction Project 426795949). This work is dedicated to Professor Philip P. Power, a pioneer in main group and transition metal coordination chemistry, on the occasion of his retirement.

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