Crystalline Keteniminyl Anions

doi:10.21203/rs.3.rs-5364043/v1

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Crystalline Keteniminyl Anions

https://doi.org/10.21203/rs.3.rs-5364043/v1

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The synthesis, characterization, and reactivity of keteniminyl anions [R¹C = C = NR²]⁻, a hitherto uncharted functional group, are the primary focus of this study. Our comprehensive analyses, including NMR spectroscopy, X-ray crystallography, and density functional theory calculations, have illuminated the distinct electronic characteristics of these anions. They are characterized by their nucleophilic/basic anionic carbon and π electrons, which are extensively delocalized along the PCCN chain. These anions undergo a range of facile reactions, such as protonation, alkylation, silylation, and metalation at the carbon site, leading to various ketenimine derivatives. They also participate in hydroamination reactions, yielding amino enamide functional groups. Additionally, the phosphino substituent in keteniminyl anions acts as a previously underappreciated weak π-electron acceptor when the phosphorus atom is in a pyramidalized state, thereby facilitating the stabilization of the electron-rich anionic carbon. The isolation of phosphino and thiophosphino keteniminyl anions not only represents a significant synthetic achievement but also heralds the potential for the future isolation of novel electron-rich species featuring phosphino substituents.

Keteniminyl

Ligand Exchange

Phosphino

For centuries, the scientific community has meticulously explored a wide range of functional groups, particularly those centered around carbon, nitrogen, and oxygen — the cornerstone elements of organic matter. While the understanding of many such entities is robust, recent revelations have illuminated the properties of diazoalkenes^1–4 and ketenyl anions.^5–9 Initially considered fleeting and unstable, these compounds have now been shown to possess stability, allowing for their crystallization and detailed study, akin to other well-established chemical entities. Nonetheless, despite these strides, certain compounds remain elusive, largely due to their high reactivity and the challenges in developing efficient synthetic pathways.

In the study of heteroallenes, ketenes (R¹R²C = C = O) and ketenimines (R¹R²C = C = NR³) have proven to be particularly crucial, acting as versatile precursors for the construction of intricate molecular architectures.^10–15 Ketenyl anions have been recently acknowledged for their effectiveness in producing a range of ketene derivatives.^5–9 However, the challenge in isolating the keteniminyl anion [R¹C = C = NR²]⁻ I (Fig. 1a), as well as its resonance counterpart, the ynamide anion [R¹C ≡ C − NR²]⁻ II, results in many facets of these species remaining unexplored.^10–15 To date, reports are scarce regarding instances where these anions have formed complexes with aluminum, transition metals, and rare-earth metals, as seen in the formations of R¹C(M) = C = NR² (M = Al¹⁶, Ti,¹⁷ Os,¹⁸ Ru,¹⁹ Nb,²⁰ Fe,²¹ Co,²¹ Au²²,) and R¹C ≡ C − N(M)R² (M = Sc,²³ Y,²³ Sm,²³ V,²⁴). Notably, complexes of the keteniminyl anion with palladium^25–28 and copper^29–39 have been hypothesized as pivotal intermediates in various organic reactions, though conclusive evidence remains sparse.

As reactive intermediates typically present in reaction mixtures at low temperatures,^{40–42,43,44} keteniminyl anions are swiftly utilized in chemical syntheses, being depleted before comprehensive characterization is feasible. This rapid consumption underscores the difficulties in thoroughly investigating their properties and roles within diverse chemical reactions. Consequently, the direct examination of these anions via spectroscopic or crystallographic methods continues to be formidable. Previous trapping experiments provided some insights into keteniminyl anions, notably the postulated lithium/potassium salts A (Fig. 1b),^{40–42,43,44} which are conjectured to be promising starting points for preparing a range of valuable organic compounds. Up to now, the related lithium ynamide complex B, characterized by a nearly linear Si − C−C − N chain (classified as type II), has been serendipitously synthesized via the C − C coupling of two isonitriles, facilitated by the use of a (tris(trimethylsilyl)silyl)lithium reagent.⁴⁵ Despite these advances, the pursuit of mild, general routes for isolating keteniminyl anions remains a significant scientific challenge, with the full potential of their reactivity yet to be realized.

In 1969, Boyer⁴⁶ and Green⁴⁷ independently made a seminal contribution to the field by demonstrating the formation of ketenimines. They utilized an approach involving the photolysis of diphenyldiazomethane in the presence of isonitriles, paving the way for subsequent explorations into the interactions between isonitriles and diazo compounds. Over the years, this area of study has been developed, often leveraging metal catalysts or light-driven processes to achieve significant advancements.^48–51 A critical aspect of many such processes is the elimination of N₂ from diazo compounds, resulting in the formation of carbene intermediates — a step frequently requiring metal catalysts or specialized conditions.

In a departure from reliance on catalysts, Hansmann introduced a process where an N₂/isonitrile exchange occurs at a vinylidene carbon center, leading to the production of vinylidene ketenimines.^1,52 Shortly thereafter, Gessner revealed a phosphine/CO exchange reaction that enabled the production of isolable ketenyl anions.⁵ In parallel, we have isolated stable ketenyl anions through an N₂/CO exchange at an anionic carbon center.⁶ Building upon these foundational efforts and other ligand exchange reactions involving low-valent main group elements,^9,53–56 we envisioned the potential to generate keteniminyl anions via a direct N₂/isonitrile exchange anchored on a diazomethyl anion. In the present work, we report the synthesis, characterization and reactivity of the inaugural class of isolable, crystalline keteniminyl anions, employing phosphino and thiophosphino substituents (Fig. 1c). Additionally, we have documented the carbon-centered reactivities of these novel compounds, exploring reactions such as protonation, alkylation, silylation, and metalation, thereby substantiating their distinct keteniminyl anion nature.

Synthesis and Characterization of Keteniminyl Anion Salts.

Initially, we synthesized the monophosphino-substituted diazomethane 1 (³¹P NMR: 112.1 ppm) via the methanolysis of its precursor, (phosphino)(silyl)diazomethane.⁵⁷ Subsequent in situ deprotonation of 1 and the addition of equimolar quantities of 2,6-xylyl isonitrile (XylNC) at ambient temperature yielded the potassium keteniminyl anion salts 2a (³¹P NMR: 75.4 ppm), 2b (complexed with 18-C-6, ³¹P NMR: 81.7 ppm) and 2c (complexed with [2,2,2]-cryptand, ³¹P NMR: 83.2 ppm) (Fig. 2).

Further explorations were directed towards thiophosphino keteniminyl anions to probe the influence of substituents on the structural attributes of keteniminyl anions. The oxidation of 1 to 3 (³¹P NMR: 65.6 ppm) was achieved using 1/8 equivalents of S₈ in benzene (Fig. 2). This was followed by in situ deprotonation and subsequent addition of equimolar quantities of XylNC at ambient temperature, leading to the synthesis of potassium keteniminyl anion salts 4a (³¹P NMR: 68.4 ppm) and 4b (complexed with 18-C-6, ³¹P NMR: 45.8 ppm).

The infrared (IR) spectra of 2a-c and 4a-b display CCN stretching frequencies ranging from 1997 to 2043 cm^− 1 (Table 1). Except for 4a, which exhibits a stretching frequency at 2043 cm^− 1, these frequencies are generally lower than those observed in B (2032 cm^− 1).⁴⁵ The ¹³C NMR spectra of 2 and 4 show that C(1) manifests as a doublet across the series, with specific chemical shifts and coupling constants recorded as 56.8 ppm (¹J_P−C = 59.6 Hz) for 2a, 55.8 ppm (¹J_P−C = 46.0 Hz) for 2b, 56.9 ppm (¹J_P−C = 32.3 Hz) for 2c, 47.0 ppm (¹J_P−C = 228.5 Hz) for 4a, and 46.7 ppm (¹J_P−C = 193.9 Hz) for 4b. Additionally, C(2) also appears as a doublet with shifts of 140.9 ppm (²J_P−C = 12.5 Hz) for 2a, 142.4 ppm (²J_P−C = 12.4 Hz) for 2b, 143.2 ppm (²J_P−C = 11.7 Hz) for 2c, 136.8 ppm (²J_P−C = 4.5 Hz) for 4a, and 138.6 ppm (²J_P−C = 4.6 Hz) for 4b. When compared to structurally similar compounds such as B (47.9 ppm),⁴⁵ Ph₃P = C = C = NPh (5.8 ppm),⁵⁸ Ph₃PC(AuCl) = C = NPh (33.4 ppm),⁵⁹ and M(Ph₃PC − C ≡ N) (M = Li, Na, K) (− 5.4–5.4 ppm),⁶⁰ the C(1) resonances exhibit a significant high-frequency shift. Although the C(2) signal for B was not assigned,⁴⁵ preventing a direct comparison, the C(2) signals in our systems are comparable to those found in Ph₃P = C = C = NPh (136.4 ppm) and M(Ph₃PC − C ≡ N) (127.7–142.5 ppm), albeit displaying a lower frequency shift relative to Ph₃PC(AuCl) = C = NPh (170.1 ppm).

X-ray crystallographic analysis unambiguously established the molecular structures of 2a-c and 4a-b (Fig. 3). Compound 2a crystallizes as a tetrameric arrangement (Fig. 3a), consisting of two dimeric subunits linked through K(2)−η⁶-Xyl interactions. Within each dimeric subunit, K(1) is coordinated by P(1), C(1), C(2), C(3), C(4), N(2), and the Dipp group from the diazaphosphinyl moiety, while the coordination sphere of K(2) includes C(2), N(1), C(3), C(4), and N(2). In 2b, the [K(18-C-6)] cation preferentially interacts with N(1) rather than C(1) (Fig. 3b), likely influenced by steric hindrance from the diazaphosphino group. For 2c, the potassium ions are fully encapsulated by cryptands, resulting in no direct contact with the “naked” keteniminyl anions (Fig. 3c). Similar to 2a, compound 4a crystallizes as a tetrameric structure (Fig. 2d). Most notably, in 4b, the [K(18-C-6)] cation forms a monomeric complex by chelating C(1) and S(1) atoms (Fig. 3e), closely resembling the structure of Gessner’s ketenyl anion potassium salt [Ph₂P(S)C = C = O][K(18-C-6)].⁵ It is noteworthy that 4b represents the first crystallographically characterized α-alkali-metallated keteiminyl anion.

Table 1

Selected bond lengths and angles as well as IR frequencies of the C − C−N vibration of **2a-c**, **4a-b** and B.
	2a	2b	2c	4a	4b	B
C − C (Å)	1.231(2) 1.247(3)	1.242(2)	1.232(6)	1.249(2) 1.245(3)	1.284(4)	1.221(5)
C − N (Å)	1.306(2) 1.291(2)	1.2927(18)	1.295(5)	1.265(2) 1.272(3)	1.275(4)	1.316(4)
C − C−N (°)	177.1(2) 167.8(2)	175.25(14)	172.5(4)	168.3(2) 170.9(2)	170.4(3)	175.3(3)
P − C (Å)	1.749(2) 1.747(2)	1.7395(14)	1.731(5)	1.693(1) 1.698(2)	1.692(3)	1.796(3)^[a]
P − C−C (°)	164.3(2) 146.0(2)	154.24(12)	162.7(4)	154.6(2) 151.6(2)	152.2(2)	171.6(3)^[b]
C − C−N (cm^− 1)	2016	2006	2010	2043	1997	2032

[a] Si − C bond length. [b] Si − C−C bond angle.

The P − CCN units in these compounds demonstrate notably distinctive bond angles and lengths compared to related compounds (Table 1). Specifically, the P − C−C bond angles in 2a-c range from 146.0(2) to 164.3(2)°, and in 4a, they are 151.6(2) and 154.6(2)°, all of which are significantly more acute than the Si − C−C in B (171.6(3)°).⁴⁵ The C − C bond lengths in 2a-c (1.231(2) to 1.247(3) Å) and 4a (1.245(3) and 1.249(2) Å) sit between typical double and triple bond lengths,⁶¹ and are slightly longer than those in B (1.221(5) Å). Conversely, the C − N bond lengths in 2a-c (1.291(2) to 1.306(2) Å) and 4a (1.265(2) and 1.272(3) Å) are intermediate between typical single and double bond lengths,⁶¹ and slightly shorter than those in B (1.316(4) Å). These structural features demonstrate an intermediate bonding situation that leans more towards a keteniminyl character than that observed in B, likely due to the weaker negative hyperconjugation effect arising from a phosphino substituent compared to a silyl substituent. It is particularly noteworthy that the acute P − C−C bond angle of 152.2(2)°, extended C − C bond length of 1.284(3) Å, and shortened C − N bond length of 1.275(3) Å in 4b distinctly showcase the most pronounced keteniminyl character among the series.

The phosphorus atoms in 2a-c exhibit pyramidalized geometries, akin to the structural features observed in the phosphinoketenyl anion⁶ and the rhodadiphosphinocarbene.⁶² This geometry suggests that the phosphino groups in 2a-c do not act as π-donors; typically, when a phosphino group serves as a π-donor, a planar configuration of the phosphorus atom is observed (Fig. 4a), as evidenced in monophosphino-substituted carbenes.⁶³ However, the P − C bond lengths in 2a-c—measured at 1.749(2) and 1.747(2) Å for 2a, 1.7395(14) Å for 2b, and 1.731(5) Å for 2c—lie between typical single (1.86 Å) and double (1.69 Å) bond lengths,⁶¹ indicative of P − C multiple bond character. This indicates that π-interactions, specifically involving the π electrons on C(1) and the low-lying N − P σ* antibonding orbitals, contribute to the molecular structure of 2a-c (Fig. 4a) (vide infra). Indeed, the P − C bond lengths in 4a-c—recorded at 1.693(1) and 1.698(2) Å for 4a, and 1.692(3) Å for 4b—are much closer to typical P = C double bond lengths, pointing to a greater π-accepting capability of the diazathiophosphinoyl substituent compared to the diazaphosphinyl substituent.

In our pursuit to unravel the electronic intricacies of the keteniminyl anions of 2, we have employed analyses based on intrinsic bond orbitals (IBOs)^64,65 (Figure S67) and natural bond orbital (NBO) methods, utilizing the geometry optimized at the BP86/def2-TZVP theoretical level. The highest occupied molecular orbital (HOMO) of 2 comprises an out-of-plane P(1) − C(1) − C(2) three-center-two-electron (3c-2e) π bonding orbital, which interacts with the out-of-plane p-orbital at N(1) (Fig. 4b). The HOMO − 1 is characterized by an in-plane C(1) − C(2) π bonding orbital, accompanied by a lone pair at N and a lone pair at P. Moreover, electron localization function (ELF)⁶⁶ calculations indicate a more localized electron density at C(1) in 2 than in B (Figs. 4c and 4d). This observation explains the more acute P − C−C bond angle relative to the Si − C−C bond angle in B, thereby underscoring the pronounced keteniminyl character of 2.

Natural population analysis (NPA) of 2 elucidates negative charges localized at C(1) (− 0.78 a.u.) and N(1) (− 0.54 a.u.) (Figure S68). The Wiberg bond indices (WBIs) for the bonds P(1) − C(1), C(1) − C(2), and C(2) − N(1) register at 1.11, 2.19, and 1.56, respectively. Particularly notable is the relatively short P(1) − C(1) bond in 2, which measures between 1.731(4) and 1.749(2) Å and exhibits a high WBI. This, along with a 3c-2e P(1) − C(1) − C(2) π bond, robustly underscores the π-acceptor nature of the diazaphosphinyl group, especially when the phosphorus atom adopts a pyramidalized configuration.

Reactivity of the Keteniminyl Anion Salt 2b.

The notable contribution of C(1) to both the HOMO and HOMO − 1 in 2, combined with the substantial negative charge localized at C(1), leads us to hypothesize that keteniminyl anions preferentially undergo selective reactions at the carbon atom, rather than at the nitrogen end. Building on this, we examined the reactivity of derivative 2b. Introduction of 2b to a mild Brønsted acid Et₃N•HCl, rapidly yielded ketenimine 5 (Fig. 5), which was characterized by a ³¹P NMR resonance at 105.1 ppm. The ¹H NMR spectrum of 5 displayed a doublet at 4.30 ppm with a two-bond phosphorus-proton coupling constant of 10.9 Hz, integrative of a single proton. X-ray diffraction techniques later confirmed the structure of 5 as a hydrogen-substituted ketenimine (Fig. 6a).

Further investigations of 2b with electrophiles, namely MeI, TMSCl, and IDippCuI (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), smoothly led to the formation of ketenimine 6 (³¹P NMR: 121.1 ppm), silyl ketenimine 7 (³¹P NMR: 120.7 ppm), and copper-keteniminyl complex 8 (³¹P NMR: 103.5 ppm), respectively (Fig. 5). X-ray crystallography confirmed the solid-state structures of 6–8 (Figs. 6b-d), verifying the bond formations at the C(1) position.

Notably, the C(1) − C(2) bond lengthens from 1.242(2) Å in 2b to 1.319(3) Å in 5, 1.307(5) Å in 6, 1.311(3) Å in 7, and averages 1.294(3) Å in 8, while the C(2) − N(1) distances decrease (from 1.2927(18) Å in 2b to 1.216(2) Å in 5, 1.227(5) Å in 6, 1.224(3) Å in 7, and an average of 1.239(4) Å in 8). This alteration in bond lengths can be attributed to the disappearance of the ynamide anion resonance form upon formation of the neutral ketenimines 5–8. The straightforward formation of these derivatives emphasizes the crucial role of the keteniminyl resonance in 2 and its remarkable potential for synthesizing diverse ketenimine derivatives.

Upon stirring a THF solution of 2b with diphenylamine, a new compound 9 was formed (³¹P NMR: 119.0 ppm) (Fig. 5). Mirroring the characteristics of 5, the ¹H NMR spectrum of 8 displayed an alkene resonance at 4.13 ppm, indicative of a single proton. The solid-state structure of 9 (Fig. 6e), is identified as an amino enamide salt, which is the result of a 1,2-addition of diphenylamine to the C(1) − C(2) bond in 2b. This discovery implies that keteniminyl anions can serve as powerful synthons for a variety of functionalized compounds. Furthermore, chemical modification of the phosphino group in 2b has been achieved. In particular, selective oxidation of the phosphino group with Ph₂CN₂ leads to the formation of phosphazene 10 (³¹P NMR: 15.3 ppm), while maintaining the structural integrity of the keteniminyl moiety (Figs. 5 and 6f).

Reactivity of the Copper Keteniminyl Complex 8.

It is important to note that previous studies had indicated the transient nature of copper-keteniminyl complexes, elusive in isolation but implicated in various organic transformations.^29–39 Chang’s group previously reported the synthesis of ketenimines through the copper-catalyzed azide-alkyne cycloaddition, followed by ring-opening rearrangement and nitrogen release.^34,35 The in situ generated copper-keteniminyl species are theorized to couple with diverse nucleophiles, yielding functionalized carbonyl derivatives. Yet, these proposed copper-keteniminyl intermediates have not been directly observed.

Remarkably, we now have 8, the first isolable copper-keteniminyl complex. This spurred our investigation into whether 8 acts as a key intermediate in such transformations. Intriguingly, the addition of excess Nu − H (MeOH or EtNH₂) to a THF solution of 8 led to the formation of imidate 11 (³¹P NMR: 112.0 ppm) and amidine 12 (³¹P NMR: 117.1 ppm), respectively. We presume these reactions initially undergo a protodecupration to yield the neutral ketenimine 5, which is then followed by a 1,2-addition of Nu − H across the C = C double bond.⁶⁷ However, our experiments showed that 5 did not react with MeOH at room temperature. The reaction of 5 with an equimolar amount of IDippCuOMe resulted in a mixture of 11 (³¹P NMR: 112.0 ppm), 8 (³¹P NMR: 103.4 ppm), and two additional species (³¹P NMR: 106.4 ppm and 99.6 ppm, Figure S42). Adding an excess amount of MeOH into such a mixture led to the clean formation of 11 over 13 hours. Under the influence of a catalytic amount of IDippCuOMe (30 mol%), the reaction between 5 and MeOH slowly yielded 11 at room temperature (Figure S43-45). Indeed, IDippCuOMe, released from the reaction of 8 with methanol, serves a catalytic function, facilitating the transformation of 5 into 11, a mechanism supported by our computational studies (Figure S69). These findings substantiate the hypothesis that copper-keteniminyl complexes play a crucial role in Chang’s copper-catalyzed multi-component cascade reactions, with copper acting as a key mediator in enabling the interaction between ketenimine and nucleophiles.

We present a notable development in isolating stable keteniminyl anion salts, featuring a phosphinyl or thiophosphinoyl substituent. These keteniminyl anions readily undergo protonation, alkylation, silylation, and metalation, yielding a variety of ketenimine derivatives, and demonstrating a propensity for hydroamination, leading to an amino enamide functional group. Our lab is actively exploring the stabilization of other unknown anions using the phosphino group and further investigating the potential of keteniminyl anions.

Competing interests

The authors declare no competing interests.

Author contributions

L.L.L. conceptualized and supervised the project. X.F.W., R.W. and Q.L. performed the experimental work. X.F.W., R.W., Q.L. and C.H. performed the computational work. X.F.W., R.W. and Q.L. performed the X-ray crystallographic analyses. L.L.L. and Q.L. wrote the paper with the input from all authors. All authors discussed the results in detail and commented on the paper.

Acknowledgement

We gratefully acknowledge financial support from the National Natural Science Foundation of China (22350004, 22271132, 22101114, 22401133), Shenzhen Science and Technology Innovation Program (20231120110042001, JCYJ20220530114806015) and Guangdong Innovation and Entrepreneurial Research Team Program (2021ZT09C278). We also acknowledge the assistance of SUSTech Core Research Facilities. The theoretical work was supported by the Center for Computational Science and Engineering at SUSTech.

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The authors declare no competing interests.

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Crystalline Keteniminyl Anions

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