Ammonia activation using a heteroleptic stannylene and lithium stannylenoid formation facilitated by hemilabile iminophosphorane-based ligands

Abstract

Heteroleptic stannylenes, featuring pendant hemilabile iminophosphorane functionalities and kinetically stabilizing terphenyl ligands, were synthesized straightforwardly through formal C–H activation. Subsequently, they were investigated for their ability to activate ammonia through N–H bond scission. By combining synthetic modifications of the ancillary ligand framework and computational analyses, detailed insights into the mechanism of NH₃ activation by these systems were obtained, highlighting an activation pathway at tin without a change in oxidation state. Additionally, an observed by-product during these studies underscores the non-innocence of a lithium salt in the synthesis of the stannylene starting materials, providing access to a novel lithium stannylenoid.

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Introduction

The activation of small molecules like ammonia (NH₃) to construct complex nitrogen-containing value-added products is a significant and dynamic area of study, particularly considering both their widespread use as feedstocks due to large-scale synthesis in industrial processes and that a majority of homogeneous catalytic processes involves these molecules.¹ In this context, the direct hydroamination of non-activated alkenes with NH₃ has emerged as one of the ‘holy grails’ in catalysis.² Whilst transition metal complexes have traditionally been the focus of small molecule activation and catalysis, they face challenges in activating the N–H bond of ammonia, which has a high gas-phase bond dissociation energy of over 99 kcal mol⁻¹, and tend to form robust Werner type complexes.³ In this context, the selective formation of monomeric transition metal amido hydride complexes through the oxidative addition of NH₃ has only been accomplished using two iridium-based pincer derivatives.⁴ Main group species are increasingly gaining traction in this field, following the groundbreaking discoveries of NH₃ activation by cyclic (alkyl)(amino)carbenes (Scheme 1, I) and heavier alkyne analogues (Scheme 1, III).^5–7 More recently, advancements in NH₃ activation by main group compounds include thermoneutral and reversible ammonia splitting protocols using geometrically constrained phosphines,⁸ activation via single-electron transfer protocols employing either a carbene-stabilized dithiolene zwitterion or a bismuth(II) complex,^9,10 and the pioneering report on catalytic NH₃ activation and transfer by an aluminium–carbon-based ambiphile.¹¹ While a considerable number of Group 14-based systems (Si, Ge, Sn) have been developed to activate the N–H bond of ammonia,^5,6,12–14 further advancements are required to achieve the catalytic functionalization of unsaturated substrates by these systems. Thus, it is crucial to maintain a delicate balance in transferring the activated small molecule to a substrate while preserving the ability to activate the next small molecule (i.e. to form a catalytic cycle).

Scheme 1 Top: First reports on ammonia activation by Group 14 compounds I and III; bottom: Oxidative addition and reductive elimination sequence of ammonia by a bis(boryl)stannylene V (Dipp = 2,6-ⁱPr-C₆H₃; ^DippTer = 2,6-(2,6-ⁱPr-C₆H₃)₂C₆H₃).

Given this consideration, heavier low-valent Group 14 compounds such as stannylenes show great promise due to their inherently weak M–E bonds (M = heavier tetrel element; E = H, N) and an accessible Sn^II/Sn^IV redox couple. However, only one example of oxidative addition of NH₃ by a stannylene (V) has been reported, capitalizing on strongly σ-donating boryl ligands (Scheme 1, V).¹⁴ Decomposition from the Sn^IV intermediate VI to N-borylated products and reduced tin clusters indicates that further development in ligand design must be explored to tackle the challenge of instability and aggregation of the corresponding complexes after substrate activation. This is especially true for the heavier tetrel elements, such as tin, for which the low-valent form and, consequently, the +II oxidation state are thermodynamically favoured.

In this study, we introduce a direct synthetic approach for a range of stannylenes, bolstered by ancillary hemilabile¹⁵ iminophosphorane ligands via an intermolecular C–H activation pathway. This strategy was designed to streamline activation processes at Sn^II, meanwhile furnishing a comprehensive mechanistic account of subsequent NH₃ activation within these frameworks. Furthermore, we have devised a straightforward protocol for the preparation of a lithium stannylenoid. Initially detected as a by-product during the synthesis of heteroleptic stannylenes featuring pendant iminophosphorane functionalities, this method offers a reliable route to its controlled synthesis.

Results and discussion

Synthesis and characterisation of heteroleptic stannylenes with pendant iminophosphorane functionalities

Reactions of the heteroleptic terphenyl-/amido stannylene ^MesTerSn{N(SiMe₃)₂} (^MesTer = 2,6-(2,4,6-CH₃-C₆H₂)₂C₆H₃) (1) with freshly prepared imino-phosphoranes Me₃PNR (R = 2,6-ⁱPrC₆H₃ (Dipp) (2a), R = 3,5-CH₃C₆H₃ (Xyl) (2b), R = 1-adamantyl (Ad) (2c))¹⁶ in C₆D₆ at room temperature lead to the formation of bis(trimethylsilyl)amine (HN(SiMe₃)₂), as evident from its characteristic ¹H NMR chemical shift (δ¹H = 0.10 ppm), and overall clean formation of the base-stabilized stannylenes ^MesTerSnCH₂PMe₂NR (3a–c), each of which featuring a four-membered Sn,C,P,N heterocycle (Scheme 2A).

Scheme 2 (A) Reactivity of stannylene 1 towards iminophosphoranes 2a–c to give stannylenes 3a–c. (B) Molecular structure of ^MesTerSnCH₂PMe₂NDipp (3a) determined by single crystal X-ray crystallography. Anisotropic displacement parameters are drawn at the 50% probability level (hydrogen atoms (except for H25A and H25B) and lattice solvent have been omitted for clarity). Selected bond lengths (Å) and angles (°): Sn1⋯N1 2.2877(14), Sn1–C1 2.2527(16), Sn1–C25 2.3220(17), P1–N1 1.6143(13), P1–C25 1.7503(17), P1–C26 1.8001(17), P1–C27 1.8050(17), N1–C28 1.426(2), C1–Sn1–C25 106.05(6), P1–C25–Sn1 91.90(7), C25–P1–N1 100.92(7), P1–N1–C28 127.42(11). (C) Computed mechanism for the formation of 3a–c from 1 and 2a–c (BP86-D3BJ/Def2-TZVP/Benzene(PCM)).

The formation of 3a–c can be straightforwardly monitored by ³¹P NMR spectroscopy, which shows the formation of new singlet signals with tin satellites. These signals are shifted to lower-field when compared to the iminophosphoranes 2a–c (for an overview of characteristic NMR data of 2a–c and 3a–c, see Table 1).

Table 1 Selected characteristic NMR data of 2a–c and 3a–c^a

Compound	δ ^31P, J119/117Sn,P^c	δ ^119Sn, ^2JSn,P	δ ^1H(PCH2)	δ ^1H(P(CH3)n)^b
a δ values are given in ppm and J values in Hz. The data is given in benzene-d6 as solvent and at room temperature. b n = 3 for 2a–c and n = 2 for 3a–c. c Average value given. d The data is given in toluene-d8 as solvent and at 243 K.
2a	−8.2	—	—	0.93
2b	1.4	—	—	0.95
2c	−14.5	—	—	1.00
3a	40.4	422.0	0.24	0.72
	J 119/117Sn,P = 163.6	^2 J Sn,P = 165.9
3a	—	—	0.14 and 0.28	0.58
3b	42.1	217.6	0.39 and 0.47	0.63 and 1.00
	J 119/117Sn,P = 142.3	^2 J Sn,P = 145.7
3c	33.8	213.3	0.39 and 0.62	0.62 and 0.74
	J 119/117Sn,P = 137.6	^2 J Sn,P = 138.6

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Notably, whilst conversion to 3b and 3c is quantitative within hours at room temperature, the formation of 3a is significantly slower. This can be mitigated by performing the reaction at 70 °C, leading to completion within 16 h. Overall, the multinuclear NMR spectroscopic data imply identical structures in solution, however, the Dipp-substituted derivative 3a shows signal broadening at room temperature and the ¹¹⁹Sn signal is low-field shifted, in comparison to 3b and 3c, by over 200 ppm.

The mechanisms for the formation of 3a–c were investigated using Density Functional Theory (DFT) at the BP86-D3BJ/Def2-TZVP/Benzene(PCM) level of theory (Scheme 2C), finding that in all cases the reactions are exergonic (ΔG_298K: 3a, −2.8 kcal mol⁻¹; 3b, −10.0 kcal mol⁻¹; 3c −10.1 kcal mol⁻¹; Scheme 2C). Comparison of the transition states (ΔG^‡_298K: 3a, +31.6 kcal mol⁻¹; 3b, +17.1 kcal mol⁻¹; 3c, +19.1 kcal mol⁻¹) demonstrates the effect of higher steric bulk in 2a, where steric restriction prohibits the formation of a stabilizing Sn–N interaction, resulting in a higher activation energy when compared to 2b and 2c. This correlates with the experimental findings, where heating is required to obtain a similar rate of reaction for the formation of 3a, but not 3b and 3c (vide supra).

To elucidate any fluctional behaviour on the NMR timescale, a variable-temperature (VT) ¹H NMR (600 MHz) experiment of compound 3a in toluene-d₈ was performed (Fig. S20†), showing that the signals with the best resolution are observed at 243 K (Fig. S23†).¹⁷ At that temperature, the same signal pattern as for 3b and 3c is observed (e.g., one signal each for the –CH₂PMe₂– moiety, three signals for the terphenyl methyl groups etc.). In the temperature range 193–353 K, the respective ³¹P{¹H} and ¹¹⁹Sn{¹H} NMR signals shift by Δδ³¹P{¹H} = 4.7 and Δδ¹¹⁹Sn{¹H} = 49.2 ppm, respectively. This leads to the assumption that in solution more pronounced dative interactions between the nitrogen functionalities and the tin atoms in 3b,c when compared to 3a are responsible for the high-field shift in the ¹¹⁹Sn{¹H} NMR spectra. Notably, compared to true two-coordinate stannylenes, the observed ¹¹⁹Sn{¹H} NMR chemical shifts of 3a–c are significantly shifted upfield. For instance, the starting material 1 exhibits a ¹¹⁹Sn{¹H} NMR chemical shift of δ¹¹⁹Sn{¹H} = 1192.3 ppm (Fig. S83†).¹⁷

3a–c were purified by crystallization from aliphatic hydrocarbons and in the case of 3a and 3c, crystalline material suitable for single crystal X-ray diffraction (SCXRD) analysis was obtained, with the molecular structure of 3a being shown in Scheme 2B.¹⁷ The dative interaction between Sn1–N1 is suggested by relatively long distances (2.2877(14) Å (3a) and 2.269(2) Å (3c)), both significantly exceeding the respective single bond radii of the respective atoms (2.11 Å) by at least 0.15 Å.¹⁸ This dative interaction and the formation of respective four-membered Sn,C,P,N rings is further supported by almost perpendicular Sn1–C25–P1 angles of 91.90(7)° (3a) and 91.86(9)° (3c), respectively. By comparing the structures of the herein obtained free iminophosphoranes 2a,c with 3a,c, the respective nitrogen–phosphorus bond lengths are elongated by approximately 0.06 Å (1.6143(13) Å (3a) and 1.6116(17) Å (3c) vs. 1.5569(17) Å (2a) and 1.558(4) Å (2c)¹⁹), indicative of reduced charge transfer from the nitrogen atom to phosphorus but increased to the tin atom.²⁰ The structural data is in good agreement to a previously reported five-membered Sn,C,C,P,N complex which was obtained through the reaction of the respective Sn–P Lewis pair with an organic azide (N–P 1.613(5)/1.608(4) Å; Sn–N 2.305(4)/2.292(4) Å).²¹

To obtain insights into the bonding within the core of 3a–c, DFT and Natural Bond Orbital (NBO) calculations were performed.

Inspection of the Kohn–Sham frontier molecular orbitals of 3a–c shows major contribution from the tin atomic orbitals, with a lone pair of electrons and orthogonal p-orbital in the HOMO and LUMO respectively (Fig. S84–S86†), which is typical for divalent Group 14 atoms.¹⁷ Further analysis of the Lewis structure by NBO 7.0²² reveals a lone pair of electrons (s^0.88p^0.12 hybridised, 1.95 e⁻) in addition to two covalent, polar Sn–C single bonds around the tin atom (polarised 78% and 83% towards C), with no NBO describing a covalent Sn–N interaction, as expected from the elongated Sn⋯N interatomic distances found in the crystal structures (Fig. S89–S91†).¹⁷ Second Order Perturbation Theory analysis revealed a significant donor–acceptor interaction between the Sn and N atoms, with the nitrogen lone pair (LP) datively bonding into a valence lone vacant (LV) orbital at Sn with an energy of 40.7 kcal mol⁻¹. This energy increases to 53.2 and 49.8 kcal mol⁻¹ when the less sterically demanding xylyl or adamantyl substituents are present in 3b and 3c, respectively, aligning well with the observations from ¹¹⁹Sn NMR spectroscopy. In addition, the phosphorus bears four single bonds to each of its surrounding carbon and nitrogen neighbours, in line with the description of a zwitterionic ^–N–P⁺ moiety. The drain of N-charge density to the tin atom causes an elongation of the P–N bonds in comparison to the parent iminophosphoranes 2a–c.

Based on both the crystallographic and theoretical insights, it is most appropriate to describe this ligand moiety as zwitterionic, bonding covalently only via the carbon atom.²⁰

Investigations on hemilability of the iminophosphorane-based ligand

To investigate the potential replacement of the nitrogen donor functionality with stronger Lewis bases and experimentally probe the hemilability of the selected ligand framework, we examined the reactivity of 3a–c towards the N-heterocyclic carbene (NHC) 1,3,4,5-tetramethyl-2-imidazol-2-ylidene (IMe₄). Accompanied by subtle yet noticeable colour changes in the reaction mixtures, we observed the clean formation of the corresponding NHC-stabilized stannylenes ^MesTerSn(IMe₄)CH₂PMe₂NR (4a–c) (Scheme 3A). Due to the coordination of the NHC to the tin atoms in 4a–c, the ³¹P and ¹¹⁹Sn resonances are observed at higher field (e.g., δ³¹P{¹H} = 5.6 ppm; δ¹¹⁹Sn{¹H} = −175.1 ppm for 4a; cf.Table 1).

Scheme 3 (A) Reactivity of 3a–c towards IMe₄ to give the NHC-stabilized stannylenes 4a–c. (B) Molecular structure of ^MesTerSn(IMe₄)CH₂PMe₂NDipp (4a) determined by single crystal X-ray crystallography. Anisotropic displacement parameters are drawn at the 50% probability level (hydrogen atoms, except for H25A and H25B, and second molecule have been omitted for clarity). Selected bond lengths (Å) and angles (°): Sn1⋯N1 3.571(3), Sn1–C1 2.269(3), Sn1–C25 2.263(3), Sn1–C40 2.287(3), N1–P1 1.573(3), P1–C25 1.766(3), C1–Sn1–C25 104.09(12), C1–Sn1–C40 92.65(11), C25–Sn1–C40 90.49(11).

The molecular structure was verified crystallographically in 4a (Scheme 3B), revealing no tin–nitrogen donor–acceptor interaction (Sn1⋯N1 > 3.5 Å). The tin–carbene separation (Sn1–C40 2.287(3) Å) aligns with that observed in other stannylene carbene adducts.¹⁶ Consequently, the P1–N1 bond length of 1.573(3) Å in 4a is shorter than that observed in starting material 3a (1.6143(13) Å). The maintained electron density at the nitrogen atom now reinforces the P^δ+–N^δ− bond to almost the value in the free iminophosphorane 2a (1.5569(17) Å).

The electronic structure of the 3-coordinate tin compound 4a was investigated by NBO analysis, revealing a lone pair of electrons (s^0.81p^0.19 hybridized, 1.93 e⁻) situated at the Sn^II atom, in addition to three polar, covalent Sn–C single bonds (Fig. S92†). The IMe₄ moiety adopts the imidazolium resonance form, characterized by one bonding (BD) NBO describing an external Sn–C σ-bond. NBO analysis further gives a 3-center-4-electron hyperbond between N97, C93, and N94 (across the NCN interface of the NHC), combining two NBOs (shown In Fig. S92†) in a ratio of 49.3 to 50.7 and a total electron count of 3.86.

In addition, as in the case of 3, the P–N functionality is zwitterionic, with the phosphorus atom forming four single bonds to each of its surrounding bonding partners, characterized by four bonding NBOs.

Ammonia activation with 3a–c

Comparison of the singlet–triplet energy difference and the magnitude of the HOMO–LUMO gap of divalent Group 14 atoms has been shown to be indicative for the tendency of such systems to undergo E–H oxidative addition (E = H, NR₂, OR).²³ Furthermore, considering the kinetics of E–H oxidative addition, it has been reported that the energy gap between the metallylene singlet ground state and the triplet excited state is inversely correlated with reactivity.²³ Additionally, σ-donating substituents based on electropositive elements, in relation to the heavier tetrel elements, such as carbon, silicon, and boron have been demonstrated to reduce the HOMO–LUMO gap through destabilization of the HOMO.^6,14,23,24 The boryl-substituted stannylene V and its NH₃ adduct reported by Aldridge et al.,¹⁴ which enables the N–H oxidative addition of ammonia, has a DFT calculated singlet–triplet difference and HOMO–LUMO gap of −11.45 kcal mol⁻¹ and −1.12 eV respectively (recalculated at the BP86-D3BJ/def2-TZVP level of theory to align with our methodology). 3a–c, on the other hand, bear carbon-based ligands, and possess comparatively large singlet–triplet energy differences and HOMO–LUMO gaps (3a: −35.0 kcal mol⁻¹, +2.67 eV; 3b: −34.0 kcal mol⁻¹, +2.98 eV; 3c: −41.7 kcal mol⁻¹, +2.97 eV) (Fig. S84–S86†).¹⁷ This renders the oxidative addition of E–H-containing substrates such as ammonia a challenging task with these systems.

We hypothesized however, that the pendant, hemilabile, iminophosphorane-based ligand might facilitate ammonia activation by combining a hydrogen bonding N_ammonia–H⋯N_Ligand interaction and a donor–acceptor N_ammonia → Sn interaction to form a six-membered transition state. This would thus allow for the oxidation-state-neutral NH₃ activation with respect to the tin ambiphile.

Accordingly, 3a–c were reacted with 1.2 bar NH₃ in C₆D₆ at room temperature (Scheme 4A). To our delight, clean reactions were observed by immediately monitoring the reaction progress using ¹H and ³¹P NMR spectroscopy. Interestingly, these reactions varied depending on the substitution pattern at nitrogen. The Ad-substituted derivative 3c reacts cleanly within 30 minutes, and the formation of the free iminophosphorane 2c was observed by its characteristic ³¹P NMR chemical shift at δ³¹P = −14.5 ppm, with no detectable intermediates (Fig. S71†).¹⁷ In contrast, the reaction of the Xyl-substituted derivative 3b results in the consumption of the starting material and the observed signal broadening of the ³¹P NMR signal corresponding to 3b over the course of 24 hours at room temperature, leading to the formation of iminophosphorane 2b (Fig. S69†).¹⁷ The Dipp-substituted derivative 3a, under the same reaction conditions, exhibits a broad ³¹P resonance at δ³¹P = 16.3 ppm in addition to the observed ³¹P NMR signal resembling the free iminophosphorane 2a, which gradually decreased over approximately 16 hours to yield 2a as the sole P-containing species (Fig. S70†).¹⁷ In all cases, the tin containing product was identified to be the literature-known NH₂ bridged dimer [^MesTerSn(μ-NH₂)]₂ (5) as verified by SCXRD and multinuclear NMR spectroscopy (Scheme 4A and Fig. S73–S75†).^17,25

Scheme 4 Mechanistic investigation of ammonia activation at complexes 3a and 3b. (A) Reactivity of 3a,b towards ammonia to give the bridged tin amide [^MesTerSn(μ-NH₂)]₂ (5) and free iminophosphoranes 2a,b. (B) Calculated mechanisms (BP86-D3BJ/def2-TZVP) for the formation of 5 from 3a (red) or 3b (blue); An energy difference of 7.2 kcal mol⁻¹ is an effect of the different Ar group used.

To test our initial hypothesis and gain insight into the mechanism for the N–H activation of ammonia by aryl-substituted 3a and 3b, DFT investigations were conducted (Scheme 4B). Contrary to their formation, they were found to be almost identical. Two barrierless pathways were found for the approach of NH₃ towards Sn, resulting in two different encounter complexes as possible intermediates (A). Whilst the intermediate A2, which features an intramolecular hydrogen bond, was found to be the energetically most favourable of the two (ΔG₂₉₈: −7.5 kcal mol⁻¹; +1.1 kcal mol⁻¹), no pathway towards a lower energy activation product was found, and so it is most likely in equilibrium with the starting materials. This reflects the experimental data, where an intermediate with ¹H and ³¹P NMR signals consistent with an ammonia complex is initially observed for 3a + NH₃, whereas in the case of 3b + NH₃, only some signal broadening can be seen.

Following the barrierless formation of encounter complex A1 (ΔG₂₉₈: +0.6 kcal mol⁻¹; +7.2 kcal mol⁻¹) however, the ammonia N–H can then undergo metathesis across the Sn–C bond (ΔG^‡₂₉₈ +14.4 kcal mol⁻¹; +19.4 kcal mol⁻¹), in a similar concerted transition state to that in the formation of 3b (Scheme 2C) and its cooperative nature is also reminiscent of that found recently for the N–H metathesis of ammonia across a P–N bond.²⁶

This produces the base-stabilized terphenyl Sn^II amido intermediate B (ΔG₂₉₈: −3.0 kcal mol⁻¹; +5.1 kcal mol⁻¹) which undergoes a barrierless disassociation of 2, to form intermediate C (ΔG₂₉₈: +1.8 kcal mol⁻¹; +9.0 kcal mol⁻¹), which finally dimerises to the crystallographically and NMR spectroscopically characterized 5, thus providing the thermodynamic driving force for the overall reaction (ΔG₂₉₈ −13.3 kcal mol⁻¹; −6.1 kcal mol⁻¹). Due to the observed formation of an intermediate during the reaction of 3a with NH₃ and to evidence our hypothesis of ammonia–adduct formation, a sample of 3a in toluene-d₈ was reacted with 1.2 bar of NH₃ whilst keeping the sample at low temperature (−80 °C) for its in situ characterization by multinuclear NMR spectroscopy.¹⁷ Indeed, the corresponding ammonia adduct ^MesTerSn(NH₃)CH₂PMe₂NDipp (A2a) was successfully identified, and the observed signal pattern perfectly matched those of the stannylene–carbene adducts 4a–c reported in this study (Scheme 5).¹⁷ It is noteworthy that 4a–c did not react with ammonia even under harsher conditions (2 bar NH₃, prolonged heating up to 100 °C), likely due to the strong coordination of the NHC to the tin atom, persistently blocking Lewis base coordination of NH₃.

Scheme 5 A: Synthesis of intermediate A2a at −80 °C in toluene-d₈; B: Stacked ¹H NMR spectra of 3a and A2a at −80 °C in toluene-d₈.

Synthesis and characterization of the lithium stannylenoid 7

During the investigations of ammonia activation by 3a, a by-product (7) (up to 10%) was occasionally detected by ³¹P NMR spectroscopy at a chemical shift of δ³¹P = 30.5 ppm during its synthesis. We found that this occurred when 3a–c were synthesized from dried mother liquors of 1, rather than crystalline material, presumable due to small amounts of lithium bis(trimethylsilyl)amide (LiN(SiMe₃)₂) being carried through from its synthesis (Sn{N(SiMe₃)₂} + ^MesTerLi) reacting with 2a, generating a reactive intermediate.¹⁶

To confirm this experimentally, we targeted independent synthetic routes to synthesize 7.

By reacting 2a with equimolar amounts of LiN(SiMe₃)₂ in benzene, complete consumption of 2a was observed, resulting in the formation of apparently two new species in a 3 : 1 ratio according to ³¹P NMR spectroscopy: one broad singlet at δ³¹P = 11.3 ppm and one sharp singlet at δ³¹P = 25.9 ppm. Through analysis of the multinuclear NMR data and employing ECC-DOSY NMR spectroscopy (with adamantane as the internal standard), we identified the minor component of the product mixture as the lithium salt “LiCH₂PMe₂NDipp” (6).^17,27,28 Despite its heterogeneity, this mixture could still serve as a readily accessible source of “LiCH₂PMe₂NDipp” (6) for the synthesis of compound 7.

By either reacting 1 with two equivalents of 6 or reacting 3a with one equivalent of the 6, clean conversions towards the species with the phosphorus resonance at δ³¹P = 30.5 ppm are observed (see Scheme 6A).

Scheme 6 (A) Synthesis of lithium stannylenoid 7. (B) Molecular structure of ^MesTerSn(CH₂PMe₂NDipp)₂Li (7) in the crystal. Anisotropic displacement parameters are drawn at the 50% probability level (hydrogen atoms, except for H25A, H25B, H28A and H28B, have been omitted for clarity). Selected bond lengths (Å) and angles (°): Sn1–C1 2.242(3), Sn1–C25 2.293(3), Sn1–C28 2.288(3), N1–P1 1.598(2), N2–P2 1.596(2), Sn1⋯Li1 2.913(5), Li1–N1 1.921(5), Li1–N2 1.924(6), P1–C25 1.766(3), P2–C28 1.773(3), P1–C26 1.822(3), P1–C27 1.800(3), P2–C29 1.807(3), P2–C30 1.804(3), N1–C31 1.429(4), N2–C43 1.428(3), C1–Sn1–C25 106.51(10), C1–Sn1–C28 100.13(10), C25–Sn1–C28 91.72(11), N1–P1–C25 108.65(13), N2–P2–C28 109.55(13), N1–Li1–N2 147.1(3).

Recrystallization from n-hexane at −30 °C clearly confirms the compound to be the lithium stannylenoid (lithium stannate(II)) [^MesTerSn(CH₂PMe₂NDipp)]₂Li (7), which was isolated as a colourless crystalline material (see Scheme 6B).

The molecular structure of 7 reveals the tin atom to be three-coordinate (Σ∢Sn = 298.4(3)°), with all tin–carbon bond lengths (Sn1–C1 2.242(3) Å, Sn1–C25 2.293(3) Å, Sn1–C28 2.288(3) Å) falling within the same range, typical of tin–carbon single bonds (single bond covalent radii: 2.15 Å (ref. 18)), and in good agreement with the molecular structures of 3a,c, and 4a reported herein. NBO analysis found a lone pair of electrons situated at the Sn^II atom (s^0.81p^0.19 hybridized, 1.91e⁻), in addition to three polar, covalent tin–carbon single bonds (Fig. S93†).¹⁷ The interatomic distances P1–N1 1.598(2) Å and P2–N2 1.596(2) Å fall between those of 3a and 4a. Accordingly, 7 bears two zwitterionic iminophosphine arms, which have similar electronic structures to those described for 3a (vide supra). The lithium–nitrogen bond lengths (mean value 1.923(6) Å) fall within the same range as observed for other lithium stannylenoids, e.g., in the lithium bis(imino)stannylenoid ClSn(NIPr)₂Li (1.946(9) Å and 2.004(9) Å; NIPr = bis(2,6-(diisopropylphenyl)imidazolin-2-imino)).²⁹ A lone pair of electrons on each nitrogen atom directs towards the lithium cation situated in the centre; however, as expected, no significant donor–acceptor interaction was found by Second Order Perturbation Theory analysis. The Sn1⋯Li1 distance of 2.913(5) Å clearly exceeds the sum of covalent radii (2.73 Å (ref. 18)) and accordingly, no bonding interaction between the lithium and the tin atom is detected.³⁰

The ¹¹⁹Sn{¹H} NMR spectrum displays a doublet resonance for the trigonal pyramidal coordinated tin atom due to coupling to phosphorus (²J_Sn,P = 167.0 Hz) at δ¹¹⁹Sn{¹H} = 437.9 ppm, placing it in the same range as observed for 3a (Table 1). Additionally, a singlet is observed in the ⁷Li{¹H} NMR spectrum at δ⁷Li{¹H} = 1.3 ppm.

7 joins the ranks of scarce examples of structurally characterized and largely unexplored stannylenoids,³¹ thus representing a heavier congener of a carbenoid.³² Its synthesis has been proven to be straightforward, starting from the respective stannylenes 1 or 3a, respectively.

Conclusions

In conclusion, we present a comprehensive investigation into the activation of ammonia by heteroleptic stannylenes featuring pendant hemilabile N-donor iminophosphorane functionalities. These stannylenes are accessed via formal C–H activation at a C(sp³)–H moiety adjacent to the phosphorus atom in the employed free iminophosphoranes in reactions with the heteroleptic amido-/terphenyl-stannylene precursor. Our findings reveal that these systems react with ammonia, cleaving one N–H bond in a redox-neutral manner, thus tin maintains in its +II oxidation state throughout the process. Depending on the N-substituent, spectroscopic characterization of an ammonia–stannylene adduct is possible, and the mechanism of ammonia activation is rationalized computationally. Furthermore, we discovered the non-innocence of lithium hexamethyldisilazide, originating from the synthesis of the stannylene starting materials, leading to the development of a straightforward synthetic protocol for a novel lithium stannylenoid.

Author contributions

D. M. J. K. and M. F. carried out the experimental work. N. G., R. H.-I., and D. S. were responsible for the SCXRD experiments. O. P. E. T. carried out the quantum chemical calculations and wrote the initial draft of the computational part of the manuscript. M. F. was responsible for the conceptualization, supervision of the experimental investigations, and wrote the initial draft of the manuscript. All authors contributed to the finalization of the manuscript and agreed to the submitted content.

Data availability

The data that support the findings of this study are available in the ESI† of this manuscript or by contacting the authors. Included are synthetic and characterizing data, representative spectra, details of the X-ray crystal structures and quantum chemical calculations. X-ray data is further available through the CCDC. The authors have cited additional references within the ESI.†^33–49

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

Financial support (VCI Liebig fellowship for M.F., Alexander von Humboldt post-doctoral fellowship for O.P.E.T.) is gratefully acknowledged. M. F. further wishes to acknowledge the Georg-August-Universität Göttingen for financial support and Prof. Inke Siewert for her continuous support and guidance. The research group led by Prof. Sven Schneider, with special mention to Marc Neben, Felix Fuchs and Sven Rosendahl, is warmly acknowledged for granting access to their ammonia line and for their assistance in its utilization. We thank Franziska Rüttger (research group led by Prof. Dietmar Stalke) for assistance with the interpretation of the DOSY NMR data.

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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures, spectroscopic data, computational and crystallographic details. CCDC 2356897 (2a), 2356898 (2c), 2356899 (3a), 2356900 (3c), 2356901 (4a) and 2356902 (7). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi02202e

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