Accessing phosphonioacetylide chemistry: isolable alkali metal precursors for rod-shaped carbon donor complexes

Abstract

Rod-shaped neutral carbon ligands such as carbon monoxide and isocyanides play a central role in organometallic chemistry but are relatively weak electron donors compared to N-heterocyclic carbenes. Phosphonioacetylides (R₃PCC) have been proposed as promising candidates for stronger electron donation within the class of sp-hybridized carbon ligands. However, their high reactivity has significantly limited their exploration and isolation. In this study, we report the synthesis and characterization of a novel phosphonioacetylide, MeR₂PCC (R = 1,3-di-tert-butylimidazolidin-2-ylidenamino). While it is highly labile at ambient temperature, the compound is stable at −40 °C and can be isolated and stored in the form of alkali metal complexes. The latter undergo efficient transmetallation reactions with a variety of electrophiles, enabling the formation of Lewis base adducts with both main group elements and transition metals, including tungsten(0), nickel(0), and rhodium(I). These results demonstrate that alkali metal phosphonioacetylides serve as versatile precursors for introducing this highly donating and structurally unique ligand into coordination complexes.

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Introduction

Scientific advances in the field of coordination chemistry, material science and catalysis in recent decades have been driven by the development of neutral carbon ligands. New impetus has come especially from the development of N-heterocyclic carbenes (NHCs), which possess superior σ-donor properties compared to classical N- and P-donor ligands.^1–8 In light of the success story of carbene ligands, it is somewhat surprising that sp-hybridized carbon donor ligands with comparable donor properties have remained relatively underexplored. Typical sp-hybridized ligands such as carbon monoxide and isocyanides^9–17 are key components in organometallic chemistry, yet they exhibit weaker σ-donor strength than their sp²- or sp³-hybridized counterparts.^18–20 In this context, phosphonioacetylides, featuring a linear C₂ moiety bound to a phosphonium center, represent a promising and conceptually intriguing extension of the family of neutral, rod-shaped carbon ligands (Fig. 1). The modular nature of these species, particularly through variation of substituents at the phosphorus center, offers the potential to fine-tune their donor strength and vary the ambiphilic character of the terminal carbon atom.

Fig. 1 Selection of sp-hybridized neutral carbon ligands and their resonance structures.

The first phosphonioacetylide complex (A) was reported in 1970, synthesized via deoxygenation of a carbonyl ligand in [Mn(CO)₅Br] using carbodiphosphorane Ph₃PCPPh₃ (Fig. 2).^21,22 In the following years, various phosphonioacetylide complexes were prepared through ligand transformations at metal centers.^23–36 Seeking a more general and direct route to such ligands, Bestmann and co-workers attempted the synthesis of a free phosphonioacetylide in 1998. They generated triphenylphosphonioacetylide (Ph₃PCC) by desilylation of the cationic precursor [Ph₃PCCSiMe₃]⁺ with a fluoride source at −78 °C and characterized the fleeting intermediate by NMR spectroscopy (Fig. 2), which could not be isolated because it proved to decompose rapidly at ambient temperature.^37,38 In 2009, Hill and co-workers seized on this work and investigated the reactivity and ligand properties of Ph₃PCC (B). The comparison of the frontier molecular orbital energies indicate that Ph₃PCC has similar π-acceptor properties but is a significantly better σ- and π-donor than methyl isocyanide.³⁹ The first room temperature stable phosphonioacetylide (C) was reported in 2021 by Zhao, Ong, and Frenking (Fig. 2). Electron-donating bulky N-heterocyclic imine (NHI) substituents bearing 2,6-diisopropylphenylgroups (dipp) at the nitrogen atoms of the heterocycle were used to stabilize the reactive PC₂ unit.⁴⁰

Fig. 2 Milestones of phosphonioacetylides: the first phosphonioacetylide complex A (left), the first free phosphonioacetylide identified spectroscopically at −78 °C (B, middle), the first room-temperature stable free phosphonioacetylide C (right).

As part of our ongoing efforts to develop electron-rich ligands, including phosphines,^41–45 pyridines,^46,47 and porphyrins,⁴⁸ we utilized the π-donating power and tunability of NHI substituents^49–52 to enhance their electron-donating capabilities. Recently, our focus shifted towards stabilizing neutral carbon ligands, such as carbenes and cumulenes.^53–56 In this study, we report on our efforts to expand the class of isolable phosphonioacetylides, as the structural factors governing their stability remain poorly understood. Addressing these knowledge gaps could unlock broader applications of phosphonioacetylides in coordination chemistry and catalysis. To this end, we investigated how introducing small NHI substituents at the phosphonium center influences the stability and ligand properties of phosphonioacetylides.

Results and discussion

In analogy to the previously reported synthetic route,⁴⁰ ethynyl phosphine 1 was synthesized using HCCMgCl for alkynylation of our previously reported phosphenium ion, bearing tert-butyl groups at the N-heterocyclic nitrogens of the NHI substituents.^57,58 Subsequent methylation with either methyl iodide or methyl triflate afforded the phosphonium salts [2]X (X = I, OTf) in good yields. Attempts to deprotonate [2]X using various inorganic bases, including nBuLi, potassium tert-butoxide (KOtBu), potassium hexamethyldisilazide (KHMDS) or neutral bases including the phosphonium ylide Ph₃PCH₂ and the superbasic phosphine P(tmg)₃,⁵⁹ generally led to unselective decomposition upon warming to ambient temperature, as indicated by the presence of multiple phosphorus-containing species observed by ³¹P NMR spectroscopy (see the SI). One notable decomposition pathway appeared to involve cleavage of the C₂ unit, as evidenced by resonance at 69.1 ppm corresponding to phosphine MePR₂.⁵⁴ Through extensive screening of different base/phosphonium salt combinations, we found that the well-defined lithium complex [R₂MePCCLi(thf)₂I] (3) and the labile potassium complex [R₂MePCCKOTf] (4) are both isolable and remain stable at ambient temperature when stored under an inert atmosphere.

Treatment of [2]I with nBuLi in THF at −40 °C, followed by immediate evaporation of the volatiles, afforded the lithium complex 3 as a white solid in very good yield (Fig. 3). Compound 3 is soluble in both polar and nonpolar solvents, including THF and toluene. However, 3 decomposes in chloroform, DCM and ACN. Successful deprotonation is confirmed by the disappearance of the alkynyl proton resonance in the ¹H NMR spectrum and the appearance of a quartet at −41.0 ppm (²J_PH = 14 Hz) in the ³¹P NMR spectrum. In the ¹³C{¹H} NMR spectrum, the α and β alkynyl carbon atoms of 3 exhibit significant shifts to higher frequency compared to the phosphonium precursor [2]⁺ (3: 106.1 ppm (C_α), 170.6 ppm (C_β); [2]⁺: 81.3 ppm (C_α), 91.9 ppm (C_β)). In contrast, the terminal carbon atoms in free phosphonioacetylides B (228.9 ppm)^37,38 and C (208.5 ppm in THF-d₈, 218.1 ppm in C₆D₆)⁴⁰ resonate at even higher frequency, suggesting lithium coordination in 3. Notably, similar trends have been observed for lithium complexes of other carbon nucleophiles, such as Bertrand's cyclopropenylidene and allenylidene.^60,61 Further evidence for the formation of a lithium-stabilized phosphonioacetylide complex is provided by the detection of a distinct ⁷Li{¹H} NMR signal at 1.4 ppm. The ¹H NMR spectrum also indicates coordination of the lithium cation by two additional THF molecules. Single-crystal X-ray diffraction (SCXRD) analysis revealed that 3 adopts a dimeric structure in the solid state (Fig. 3d). Each lithium cation is tetrahedrally coordinated by two THF molecules and two bridging phosphonioacetylide ligands, resulting in a four-membered Li₂C₂ ring with iodide as counterion. The C1–C2 bond length of 1.220(2) Å is consistent with a typical triple bond (1.21 Å; cf. double bond: 1.33 Å),⁶² the P1–C2 bond length with 1.742(14) Å is between typical phosphorus carbon single and double bonds (single bond: 1.87 Å, double bond: 1.67 Å)⁶³ and the P–C–C bond angle approaches linearity at 176.5(13)°.

Fig. 3 (a) Synthesis of the alkali metal phosphonioacetylide complexes 3–6. Solid-state structures of (b) 1, (c) [2]OTf, (d) 3 and (e) 6′. Hydrogen atoms except the hydrogen attached at C_β and counter ions (triflate for [2]OTf, iodide for 3) are omitted for clarity. Ellipsoids are displayed at 50% probability. Selected structural data is shown in Table 1.

A second viable synthetic route to an isolable phosphonioacetylide alkali metal complex was identified via deprotonation of the triflate salt [2]OTf using a slight excess of KHMDS. Treatment of [2]OTf with KHMDS in THF afforded the phosphonioacetylide complex 4, which was isolated as a white solid in 67% yield after workup. The potassium complex exhibits good solubility in THF, toluene and benzene, and its spectroscopic features are consistent with the presence of the PC₂ moiety. Notably, the ³¹P NMR spectrum displays a signal at −43.8 ppm (quartet, ²J_PH = 14 Hz), while the ¹³C NMR spectrum shows a resonance at 105.2 ppm corresponding to C_α, closely resembling the data obtained for the lithium complex 3. The ¹³C NMR signal for the terminal carbon atom (C_β) was observed at 201.0 ppm in deuterated THF. Compared with the lithium complex 3 (170.6 ppm), this resonance is significantly shifted to higher frequency suggesting a much weaker carbon-potassium interaction. Compound 4 can be stored as a solid under an inert atmosphere. Notably, the isolated material consistently retained sub-stoichiometric amounts of Et₂O, THF and HMDS, which could not be completely removed even after extended drying under reduced pressure. Consequently, compound 4 was typically generated in situ for subsequent ligand transfer reactions (vide infra).

Liberation of the free phosphonioacetylide was attempted via sequestration of the lithium and potassium metal ions using 12-crown-4 and [2.2.2]cryptand, respectively. Treatment of 3 with stoichiometric amounts of 12-crown-4 resulted in the formation of the lithium complex 5. The analysis of the NMR data reveals a shift in the ¹³C resonance of the terminal β carbon to 183.5 ppm, suggesting a weakening of the coordinative bond. Furthermore, a significant shift to lower frequency in the ⁷Li NMR to −0.3 ppm is observed, consistent with the replacement of THF by 12-crown-4 in the lithium coordination sphere.⁶⁴ Complex 5 proved to be highly labile, as the removal of solvent followed by redissolution led to partial decomposition. Moreover, the addition of more than one equivalent of 12-crown-4 resulted in the complete decomposition of the phosphonioacetylide. This behaviour suggests successful cleavage of the lithium-carbon bond, as lithium ions are known to coordinate with two crown ether molecules.⁶⁵ Similarly, the addition of stoichiometric amounts of [2.2.2]cryptand to a THF solution of complex 4 caused the ¹³C resonance of the β carbon to shift to higher frequency (215.6 ppm). This chemical shift falls within the range observed for B (228.9 ppm) and C (208.5 ppm), suggesting that the liberation of the phosphonioacetylide was either successful or that the C terminus remains weakly coordinated to the potassium ion within the cryptand cage. The latter possibility aligns with previous observations by Rosokha and coworkers for other rod-shaped ligands.⁶⁶ Supporting this interpretation, attempts to separate the free phosphonioacetylide by benzene extraction were unsuccessful. However, when the reaction mixture was stored at −40 °C, two distinct types of crystals formed: colorless blocks and brown needles. Structural analysis via SCXRD identified these as the salt [K(cryptand)]OTf and the free phosphonioacetylide 6′, respectively (Fig. 3e). Notably, the C1–C2 bond length of 6′ (1.203(5) Å) is slightly shorter than in complex 3, while the P1–C1 bond length is elongated to 1.765(2) Å.

Collectively, the deprotonation and sequestration experiments demonstrate that the generation of the phosphonioacetylide requires stabilization through the formation of alkali metal complexes, which prevent decomposition via reaction with the phosphonium precursor. Furthermore, the liberation of the phosphonioacetylide by sequestration of the alkali metals is achievable. While the free ligand is stable at low temperatures, it appears to be transient and unstable at room temperature.

Next, the hydrolytic stability of 3 was investigated. Treatment of 3 in THF with ten equivalents H₂O quantitively yielded the precipitation of the protonated alkynyl salt [2]I, which already suggests that the free phosphonioacetylide is highly basic, as lithium hydroxide is generated in the reaction.

Treatment of 3 with the Lewis acid B(C₆F₅)₃ led to the corresponding borane adduct 7, which was isolated as a white solid in 50% yield. The carbon signal of C_β in the ¹³C NMR spectrum (123.0 ppm) is shifted to lower frequency by 47.6 ppm compared to that of 3. In addition, the boron resonance at −21.5 ppm in the ¹¹B{¹H} NMR spectrum confirms the formation of a borate species, as this chemical shift is significantly shifted to lower frequency compared to free B(C₆F₅)₃ (60 ppm).⁶⁷ Further evidence for the adduct formation comes from the ³¹P{¹H} NMR spectrum, which shows distinct boron-phosphorus coupling, observed as a doublet with a coupling constant of ³J_BP = 4 Hz. These spectroscopic features are consistent with alkynylphosphonium borates previously reported by Bestmann, Stephan and Erker.^{32,33,37,38,68,69} Notably, the ⁷Li NMR spectrum of the isolated solid of 3 indicated the presence of trace amounts of lithium iodide, even after extraction and recrystallization, although an SCXRD study confirmed the successful separation (vide infra). This observation indicates a limitation of using 3 in transmetallation reactions, as the nitrogen atoms of the NHI groups can act as chelating ligands for lithium salts, thereby making the separation more difficult. Moreover, an additional potentially undesired reaction pathway was observed when 3 was treated with [Rh(cod)Cl]₂. This reaction produced a mixture of rhodium complexes [Rh(CCPMeR₂)(cod)Cl] (11) and [Rh(CCPMeR₂)(cod)I] (11′) due to anion scrambling (Fig. S73). Anion scrambling, particularly with halogens, is a well-documented issue.^70,71 It is typically mitigated by avoiding reactants with different halogens or by replacing chloride in [Rh(cod)Cl]₂ with silver triflate in the presence of coordinating solvents such as acetonitrile or THF. However, anion scrambling between halogens and triflates is uncommon due to the much weaker coordination properties of the latter. Therefore, we used the triflate complex 4 in subsequent transmetallation studies, which was generated in situ via deprotonation of [2]OTf with suitable potassium bases.

The reaction with TMSCl and the transition metal complexes [W(CO)₅(thf)] and [Rh(cod)Cl]₂ resulted in complete conversion to the corresponding terminal phosphonioacetylide complexes 8, 9 and 11 (Fig. 4a, right). These complexes were isolated in moderate yield after workup and separation of the potassium salts. In contrast, the reaction with [Ni(CO)₄] led to the formation of a side product, which complicated the separation process and thus gave 10 in low yield. ³¹P NMR spectra of the phosphonioacetylide complexes show resonances as quartets with coupling constants of ²J_PH = 14–15 Hz, and their chemical shifts fall within a very similar range (Table 1). For complex 11, an additional coupling constant of ³J_PRh = 4 Hz was observed, consistent with the terminal coordination of the C₂ unit in this complex. An overview of the ¹³C NMR resonances for the acetylide units in the phosphonioacetylide complexes is provided in Table 1. The C_α and C_β signals were assigned based on their distinct coupling constant to the phosphorus nucleus. These signals follow a trend of shifting to lower frequency as the covalency of the carbon–element bond increases, whereas they appear at higher frequency for more ionic carbon–element bonds.

Fig. 4 (a) Transmetallation reactions using either the lithium salt 3 or the potassium salt 4. The latter was generated in situ by deprotonation of [2]OTf with KHMDS or KOtBu. Solid-state structures of (b) 7, (c) 9, (d) 10 and (e) 11′. Hydrogen atoms and solvent molecules are omitted for clarity. Ellipsoids are displayed at 50% probability. Selected structural data is shown in Table 1. R = di-tert-butylimidazolidin-2-ylidenamino.

Table 1 Selected solution NMR data and structural parameters of the solid-state structures of compounds 1–10

Compound	^31P NMR shift in ppm	^13C NMR shift of Cα in ppm	^13C NMR shift of Cβ in ppm	C≡C bond length in Å	P–C bond length in Å	P–C–C bond angle in °	C–C–E bond angle in °	%Vbur ^f
a Recorded in C6D6. b Recorded in CD3CN. c Recorded in CD2Cl2. d Recorded in THF-d8. e  ^31P{^1H} NMR shift. f Bondi radii scaled by 1.17, sphere radius = 3.5, Cβ selected to coordinate to centre of the sphere with distance of 2.0, mesh spacing for numerical integration = 0.1, H atoms not included.
1	53.2 (s)^a	93.3 (d, ^1JCP = 46 Hz)^a	85.4 (d, ^2JCP = 5 Hz)^a	1.180(3)	1.8138(19)	170.35(19)	—	—
2	−41.7 (p, JPH = 15 Hz)^b	81.4 (d, ^1JCP = 190 Hz)^b	91.9 (d, ^2JCP = 34 Hz)^b	1.185(3)	1.769(2)	174.4(2)	—	—
3	−41.0 (q, ^2JPH = 14 Hz)^a	106.1 (d, ^1JCP = 164 Hz)^a	170.6 (br)^a	1.220(2)	1.7417(14)	176.52(13)	136.06(14) (X = Li1), 151.85(15) (X = Li1′)	18.6
4	−43.9 (q, ^2JPH = 14 Hz)^a	104.3 (d, ^1JCP = 159 Hz)^d	201.0 (d, ^2JCP = 9 Hz)^d	—	—	—	—	—
5	−42.9 (q, ^2JPH = 14 Hz)^a	101.8 (d, ^1JCP = 170 Hz)^a	183.5 (br)^a	—	—	—	—	—
6	−48.8 (q, ^2JPH = 14 Hz)^d	103.7 (d, ^1JCP = 155 Hz)^d	215.6 (d, ^2JCP = 15 Hz)^d	—	—	—	—	—
6′	—	—	—	1.203(5)	1.765(2)	174.0(3)	—	18.9
7	−39.6 (m)^c/−39.6 (d, ^3JBP = 4 Hz)^c^,^e	90.0 (d, ^1JCP = 197 Hz)^c	123.0 (br)^c	1.206(3)	1.7437(18)	168.02(17)	175.44(19)	18.2
8	−41.4 (q, ^2JPH = 15 Hz)^b	102.2 (d, ^1JCP = 175 Hz)^b	111.1 (d, ^2JCP = 23 Hz)^b	—	—	—	—	—
9	−42.3 (q, ^2JPH = 14 Hz)^a	104.9 (d, ^1JCP = 197 Hz)^a	151.7 (d, ^2JCP = 20 Hz)^a	1.214(4)	1.727(3)	175.5(3)	178.0(3)	19.8
10	−41.5 (q, ^2JPH = 15 Hz)^a	104.6 (d, ^1JCP = 193 Hz)^a	170.5 (d, ^2JCP = 15 Hz)^a	1.220(2)	1.7300(15)	160.66(14)	176.07(14)	20.5
11/11′	−40.7 (qd, ^2JPH = 14 Hz, ^3JPRh = 4 Hz)^a	—	—	1.2196(18)	1.7314(13)	156.95(12)	175.78(11)	19.3

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SCXRD studies of complexes 7, 9, 10 and 11′ confirmed the formation of terminal phosphonioacetylide complexes (Fig. 4). Selected structural parameters are summarized in Table 1. The observed C≡C and the P–C bond lengths are comparable to those in the lithium complex 3. However, when compared to the phosphonium cation 2, the C≡C bonds are elongated, and the P–C bonds are shortened. This suggests a significant allenylidene-type character of the PCC moiety due to π-backbonding from the anionic acetylide moiety into the σ* orbitals of the phosphonium unit. Further supporting the presence of allenylidene character, the PCC angles deviate more significantly from linearity than the CCE bond angles. This systematic deviation is thus unlikely to arise from crystal packing effects.

With complex 10 in hand, the donor strength of the new phosphonioacetylide ligand was determined using the Tolman electronic parameter (TEP)⁷² The symmetric A₁ carbonyl stretching frequency of complex 10 was experimentally observed in DCM at 2050.1 cm⁻¹. Notably, a significantly lower TEP value of 2023 cm⁻¹ was reported for phosphonioacetylide C, which, however, was converted from the corresponding rhodium complex [Rh(CO)₂Cl(C)].⁷³ To investigate this discrepancy, DFT calculations were performed to compute the TEP values for both phosphonioacetylides, following the method reported by Gusev.⁷⁴ The calculated TEP derived from complex 10 (2055.0 cm⁻¹) and from [Ni(CO)₃C] (2051.1 cm⁻¹) are in close agreement with the experimentally observed A₁ stretching frequency of 10, highlighting the new phosphonioacetylide ligand as a strong donor with an overall donor ability comparable to that of classical N-heterocyclic carbenes.^75,76 Additionally, for [Ni(CO)₃B], a TEP value of 2068.3 cm⁻¹ was calculated, demonstrating a significant impact of NHI group substitution at the phosphonium on the ligand's donor properties. These findings are consistent with the well-established influence of substituents on phosphines, as described by Tolman's substituent parameter, χ_i.⁷⁷ The steric bulk of the new phosphonioacetylide ligand was evaluated by calculating the buried volume (%V_bur)^78–80 based on the solid-state structures of the corresponding complexes (Table 1). Due to the rod-shaped structure of the ligand, the bulky NHI groups have minimal influence on the coordination sphere around the metal centre, confining the 3.5 Å sphere to a %V_bur value of 18.2–20.5%. This analysis indicates that the new phosphonioacetylide ligand exhibits steric properties comparable to those of triphenylphosphonioacetylide (%V_bur = 17.5 for complex [W(CCPPh₃)(CO)₅]),³⁶ but it is significantly less sterically demanding than phosphonioacetylide C (%V_bur = 32.4 for complex [Au(C)₂]⁺).⁷³

The reactivity studies presented here highlight the high nucleophilicity and basicity of the new phosphonioacetylide 6′, as well as its ability to function as a strong donor ligand. Like other common neutral C- and P-donor ligands, such as NHCs and phosphines, 6′ readily forms transition metal complexes. However, its unique rod-shaped structure allows it to form Lewis adducts even with bulky Lewis acids, such as tris(pentafluorophenyl)borane, which typically resist adduct formation with bulky phosphines or NHCs, often resulting in frustrated Lewis pair systems.^81–83

Density functional theory calculations at the B3LYP/6-311G(d) level were carried out to gain an insight into the ligand properties of phosphonioacetylide 6′ and investigate how these are influenced by phosphorus substituents and interactions with alkali metal cations. As demonstrated by Wagler and coworkers, the frontier orbital energies relevant for σ-, π-donor and π-acceptor interactions reveal that common sp-hybridized ligands, such as carbon monoxide and isocyanides, are significantly weaker σ and π donors compared to phosphonioacetylide B (Fig. 5). The introduction of NHI substituents destabilizes both donor and acceptor orbitals by approximately 1 eV, leading to superior donor and weaker π-acceptor ability in the case of 6′. This trend aligns with natural bond orbital (NBO) analysis, which shows that NHI substitution increases the negative charge on the C₂ unit by 0.11 eV, with the most pronounced increase at C_β (Table 2). Simultaneously, Wiberg bond indices indicate reduced backbonding from the C₂ unit to phosphorus. The phosphorus atom carries a large positive charge, which is further amplified by the electronegative nitrogen atoms of the NHI substituents. The charge distribution supports the resonance structure shown in Fig. 5, with the NHI substituents promoting increased donation of the C_α negative charge toward C_β. This is further corroborated by Wiberg bond indices, which show a higher C_α–C_β bond order for 6′ (2.59) compared to B (2.53). Notably, coordination to alkali metals significantly reduces the polarity of the C₂ unit by redistributing charge toward C_β and increasing the C_α–C_β bond order, while the overall negative charge of the C₂ unit increases only slightly. This effect is more pronounced with the strongly polarizing lithium cation. These results support experimental observations that 6′ is effectively stabilized by coordination to alkali metal ions, particularly lithium. Furthermore, they suggest that lithium complexes of other phosphonioacetylides, such as B, should also be isolable and stable under ambient conditions.

Fig. 5 Relative energies of the frontier orbital relevant for σ-, π-donor and π-acceptor interactions calculated with B3LYP/6-311G(d).

Table 2 Selected calculated natural atomic charges and Wiberg bond indices with B3LYP/6-311G(d). R = di-tert-butylimidazolidin-2-ylidenamino

Compound	P in e	Cα in e	Cβ in e	Li/K in e	P–Cα	Cα–Cβ	Cβ–Li/K
Ph3PCC-Li^+	1.57	−0.66	−0.42	0.95	1.01	2.80	0.09
Ph3PCC-K^+	1.57	−0.76	−0.29	0.97	1.03	2.74	0.01
Ph3PCC (B)	1.57	−0.97	0.00	—	1.11	2.53	—
R2MePCC-Li^+	1.98	−0.65	−0.53	0.94	0.90	2.84	0.11
R2MePCC-K^+	1.98	−0.75	−0.40	0.95	0.93	2.79	0.09
R2MePCC (6′)	1.98	−0.98	−0.10	—	1.02	2.59	—

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Experimental section

All experimental details are included in the SI.

Conclusions

In summary, a new rod-shaped phosphonioacetylide ligand is reported including its synthesis, properties, and coordination chemistry. By employing less bulky NHI groups at the phosphonium centre compared to the isolable phosphonioacetylide C, we successfully tuned the steric and electronic properties of this largely unexplored ligand class. Due to the better accessibility of the reactive C₂ unit, the free ligand is highly labile at ambient temperature but sufficiently stable at −40 °C to allow its structural characterization. In contrast, the corresponding lithium and potassium complexes (3 and 4) can be stored at ambient temperature, making them suitable for further reactivity studies. These alkali metal complexes readily undergo transmetallation with various electrophiles, providing straightforward access to terminal phosphonioacetylide complexes.

The new ligand exhibits high basicity and nucleophilicity, clearly outperforming common sp-hybridized neutral carbon ligands such as carbon monoxide and isocyanides in terms of donor strength. Its donor ability is comparable to that of classical N-heterocyclic carbenes, while its low steric bulk enhances its versatility. With this unique combination of strong donor properties and minimal steric hindrance, the rod-shaped ligand is expected to be of significant interest in coordination chemistry, particularly in systems requiring multiple strong donor ligands.

Author contributions

Synthetic experimentation was performed by F. B. (2–11) and P. L. (1). The project was directed by F. D. Computations were performed by F. B. SCXRD studies were performed by P. L. (1), K. W. ([2]I, [2]OTf, 3, 7, 10, 11′) and S. H. (refinement of 9). The manuscript was written and edited by F. B. and F. D. All authors have given approval for the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI.

Supplementary information including experimental and computational details is available See DOI: https://doi.org/10.1039/d5qi01286d.

CCDC 2456627–2456634 and 2472604 contain the supplementary crystallographic data for this paper.^84a–i

Acknowledgements

F. B. gratefully acknowledges financial support by a doctoral scholarship of the Universität Innsbruck. P. L. acknowledges the support of the DFG (IRTG 2027). We thank Priv.-Doz. Dr Alexander Pöthig and the Catalysis Research Center at Technische Universität München for providing access to the XRD instrument for the SCXRD study of 9. We thank Lena Lezius for her help with DFT calculations and Dr Michael Seidl for editing of the SCXRD part of the SI and SCXRD studies of 6′. The computational results presented here have been achieved in part using the LEO HPC infrastructure of the University of Innsbruck.

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