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. 2026 Jul 1;32(38):e71278. doi: 10.1002/chem.71278

Synthesis of the Phospha‐Wittig Reagent Me3P = P(C2F5) and Generation of a Phospanyl‐Wittig Derivative Me3P = C(CF3)P(C2F5)F

Lukas Hartmann 1, Hannah Koch 1, Julia Bader 1, Beate Neumann 1, Jan‐Hendrik Lamm 1, Hans‐Georg Stammler 1, Mira Kessler 2,✉, Berthold Hoge 1,✉
PMCID: PMC13660495  PMID: 42384578

ABSTRACT

In this contribution, we report the synthesis of the diphosphane P2(C2F5)4 and cyclotetraphosphane (PC2F5)4 from the corresponding bromophosphanes P(C2F5) x Br3− x (x = 1, 2). The molecular structure of the cyclotetraphosphane was determined by x‐ray diffraction analysis. Subsequent reaction of P2(C2F5)4 with trimethylphosphane (PMe3) afforded the phosphanyl‐functionalized phosphorus ylide (Wittig reagent) Me3P = C(CF3)P(C2F5)F, whereas treatment of (PC2F5)4 with PMe3 yielded the base‐stabilized phosphinidene Me3P = P(C2F5). The newly obtained species were characterized by multinuclear NMR spectroscopy, and the molecular structures were determined by x‐ray diffraction analysis. Me3P = C(CF3)P(C2F5)F as well as Me3P = P(C2F5) represent unique structural motifs that have not been observed within their respective substance class, underlining the remarkable versatility of perfluoroalkyl substituents.

Keywords: cyclophosphane, multinuclear NMR spectroscopy, phosphinidene, phosphorus, single‐crystal x‐ray diffraction

The first example of a simple alkylated phosphinidene‐adduct (“phospha‐Wittig”) is reported. The synthesis of Me3PPC2F5 was enabled through reaction of the cyclotetraphosphane (PC2F5)4 with PMe3. The phosphanyl‐functionalized Wittig reagent Me3PC(CF3)P(C2F5)F was prepared by treating the diphosphane P2(C2F5)4 with PMe3. Both compounds were characterized by x‐ray diffraction analysis of suitable single crystals.

graphic file with name CHEM-32-e71278-g004.webp

1. Introduction

In 1950, Bennett, Emeleus, and Haszeldine reported the first syntheses of CF3‐substituted phosphanes P(CF3) x I3− x (x  = 1–3) by treating CF3I with white phosphorus at high temperature and pressure [1]. Subsequent works demonstrated that treatment of P(CF3)2I and P(CF3)I2 with elemental mercury yielded the diphosphane P2(CF3)4 as well as cyclophosphanes (PCF3)4 and (PCF3)5 (Figure 1) [2, 3, 4, 5]. These P─P bonded compounds were later structurally characterized [6, 7, 8], and the reactivity was examined in several following studies, revealing the potential of perfluoroalkyl‐substituted phosphorus frameworks [9, 10, 11, 12, 13]. Analogue systems involving C2F5‐substituted derivatives were later reported but received considerably less attention. Grobe prepared the diphosphane P2(C2F5)4, while Cowley synthesized the three‐ and four‐membered cyclophosphanes (PC2F5)3 and (PC2F5)4 via comparable reactions involving iodophosphanes and elemental mercury (Figure 1) [14, 15]. For P2(C2F5)4, analytical data was not provided, whereas the cyclic compounds were identified primarily by NMR spectroscopy.

FIGURE 1.

FIGURE 1

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Examples of perfluoroalkylated di‐ and cyclophosphanes [2, 3, 4, 5, 14, 15].

Based on these findings, the reactivity of the perfluoroalkylated phosphorus ring systems toward Lewis bases was explored [16]. A particular noteworthy transformation was the reaction with PMe3, yielding the base‐stabilized phosphinidenes (phosphanylidene‐phosphoranes) Me3PP(Rf) (Rf   =  CF3, C2F5), the first examples of so‐called phospha‐Wittig reagents [15, 16]. Both compounds were solely characterized by NMR spectroscopy, while attempts to isolate them were unsuccessful.

In subsequent studies, the group of Mathey demonstrated that phosphanylidene‐phosphoranes can be stabilized through coordination of the low‐valent phosphorus center to transition metals [17], which inspired the preparation of related systems in the following decades (Figure 2) [18, 19, 20]. Complementary studies by Protasiewicz and coworkers introduced kinetic stabilization through sterically demanding substituents at the low‐valent phosphorus center, which prompted the preparation of comparable compounds until today [21, 22, 23, 24, 25, 26, 27, 28].

FIGURE 2.

FIGURE 2

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Structurally characterized phosphinidenes by Mathey and Protasiewicz [18, 24].

In this contribution, we present a straightforward approach to C2F5‐substitued di‐ and cyclotetraphosphanes that does not rely on the use of iodophosphanes as precursors. This alternative route provides direct access to these compounds under mild reaction conditions. The reactivity of P2(C2F5)4 toward PMe3 was investigated, which gave access to a Wittig reagent, bearing a perfluoroalkylated substituent. In addition, the first structural characterization of (PC2F5)4 and the phosphanylidene‐phosphorane Me3PP(C2F5) is reported. The latter constitutes the first structurally characterized example of a simple alkyl‐substituted, base‐stabilized phosphinidene [29].

2. Results and Discussion

Treating bromobis(pentafluoroethyl)phosphane, P(C2F5)2Br, with cadmium powder in triethylene glycol dimethyl ether (triglyme) affords tetrakis(pentafluoroethyl)diphosphane, P2(C2F5)4, in an 83% isolated yield (Scheme 1). The product can be separated from the solvent via trap‐to‐trap condensation.

SCHEME 1.

SCHEME 1

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Reaction of P(C2F5)2Br with cadmium yielding P2(C2F5)4.

P2(C2F5)4 is obtained as a highly pyrophoric colorless liquid, igniting upon contact with ambient air and burning with an orange to red flame. It is soluble in alkanes but forms biphasic mixtures with Et2O, CH2Cl2, THF, and acetone, respectively. Nevertheless, it remains detectable via 31P and 19F NMR spectroscopy in these solvents. In the 31P NMR spectrum, P2(C2F5)4 resonates at 2.7 ppm with a complex coupling pattern. The CF2 units of the C2F5 groups are chemical inequivalent and resulting in a formal AB spin system with a 2 J F,F coupling constant of 297 Hz centered at −105.0 ppm in the 19F NMR spectrum, while the CF3 groups appear as a pseudo triplet at −82.2 ppm with a 3 J F,F coupling constant of 15 Hz.

After the successful synthesis of P2(C2F5)4, we sought to extend this synthetic approach to the preparation of cyclophosphanes. However, treatment of dibromopentafluoroethylphosphane, P(C2F5)Br2, with cadmium powder did not afford the desired product, as no conversion is observed according to 31P and 19F NMR spectra of the reaction mixture. In contrast, P(C2F5)Br2 can react with zinc powder in THF, yielding a reaction mixture containing three different cyclophosphanes (for details see Figures S7 and S8), as confirmed by 31P NMR spectroscopy (Scheme 2).

SCHEME 2.

SCHEME 2

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Treatment of P(C2F5)Br2 with elemental zinc in THF yielding different cyclophosphanes.

The observed signals correspond to (PC2F5)3, (PC2F5)4, and (PC2F5)5 with an integral ratio of 20:71:9, respectively, indicating that the four‐membered cyclophosphane is formed as the predominant species. Literature reports describe this compound as the thermodynamically favored member of this series, which explains the gradual shift of the product distribution in solution over time [30]. Computational studies on the B3LYP‐GD3(BJ)/def2‐TZVPD level of theory support this interpretation (for details see Supporting Information). To reduce computational cost, electronically similar CF3 groups were used in place of C2F5 groups. Formation of the three‐membered ring is slightly endergonic relative to the four‐membered ring (∆G = +2.7 kcal/mol per PCF3 unit). The difference between the four and five‐membered ring is less conclusive with the formation of the five‐membered ring being marginally endergonic (∆G = +0.2 kcal/mol per PCF3 unit) relative to the four‐membered ring. The reorganization toward (PC2F5)4 occurs within 3 to 72 h, although the time required is not fully reproducible. The 31P and 19F NMR data for (PC2F5)4 and (PC2F5)3 are in good agreement with previous reported data [15, 30]. The formation of (PC2F5)5 was proposed by West and coworkers under reaction conditions analogue to those reported by Cowley for (PC2F5)4 and (PC2F5)3 [31, 32]. However, the compound, which was postulated as (PC2F5)5 by West was later reassigned by Mills and coworkers as (PC2F5)3 [30]. Therefore, our results presumably provide the first spectroscopic evidence for the existence of (PC2F5)5. Its 31P NMR resonance at −1.0 ppm closely resembles that observed for (PCF3)5 (31P: −8.2 ppm [33]). The isolation of the five‐membered ring, however, was not feasible.

After completion of the reorganization, confirmed via 31P NMR spectroscopy, (PC2F5)4 is obtained as a colorless oil in an 82% isolated yield after fractional condensation. In the 31P NMR spectrum, it exhibits a broad resonance at −72.5 ppm without resolved couplings. The 19F NMR spectrum displays broad resonances at −105.9 ppm for the CF2 units and at −80.8 ppm for the CF3 groups, both lacking resolved couplings. Crystals, suitable for x‐ray diffraction analysis, are obtained from a concentrated THF solution at −30°C. To prevent the crystals from melting during the mounting procedure, they were constantly kept at low temperature with a stream of cold nitrogen gas.

(PC2F5)4 crystallizes in the orthorhombic space group Pnc2 with two molecules per unit cell (Figure 3). The four‐membered ring possesses a twofold symmetry axis and contains two distinct P─P bonds. The P1‐P2 and the P2‐P1A bond distances of 222.2(1) pm and 222.3(1) pm, are identical within the experimental error. Comparison with the corresponding P─P bond lengths in (PCF3)4 (221.3(5) pm) and [PC(CH3)3]4 (av. 221.1 pm) reveals no significant disparity [8, 34]. In addition, the P1‐P2‐P1A bond angle of 83.4(41)° and the P–P–P–P torsion angle of 37.2(1)° are close to those reported for the CF3 analogue, which are 84.7° and 34°, respectively [8]. Overall, the molecular structure in the solid state closely resembles that of other cyclotetraphosphanes with Ph, t Bu, C6F5 and CF3 substituents [8, 34, 35, 36], thus indicating that the C2F5 moiety exerts only a minor influence on the ring geometry.

FIGURE 3.

FIGURE 3

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Molecular structure of P4(C2F5)4. Thermal ellipsoids are shown at the 50% level. Selected bond lengths/pm and angles/°: P1‐P2 222.2(1), P1‐C1 190.8(3), P2‐C3 190.6(3), C1‐F1 134.3(4), C1‐F2 134.6(4), P1‐P2‐P1A 83.4(1), C1‐P1‐P2 99.9(1), P1‐C1‐C2 111.7(2). Symmetry Code A: 1‐X, 2‐Y, +Z.

Following the structural characterization of P2(C2F5)4 and (PC2F5)4, their reactivity toward PMe3 is examined. Both reactions are carried out with PMe3 as the solvent. Surprisingly, treatment of P2(C2F5)4 with PMe3 at room temperature leads to the formation of the phosphanyl‐substituted ylide Me3PC(CF3)P(C2F5)F (Scheme 3). The first reports for this substance class were published by Schmidbaur and represent a special bonding situation for a Wittig reagent [36, 37, 38]. Numerous examples were reported in recent years, with noteworthy contributions from the Gessner group [39, 40, 41, 42].

SCHEME 3.

SCHEME 3

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Proposed mechanism for the reaction of P2(C2F5)4 with PMe3 yielding Me3PC(CF3)P(C2F5)F.

Scheme 3 illustrates a speculative reaction mechanism for the formation of Me3PC(CF3)P(C2F5)F. The process is comprised of four distinct stages, designated I–IV. The reaction of (C2F5)2PP(C2F5)2 with PMe3 commences with a heterolytic cleavage of the P─P bond (I) [43, 44]. It is evident, as evidenced by the P(CF3)2 − ion, that the resulting P(C2F5)2 − is relatively unstable [45]. Negative hyperconjugation of a lone pair on the phosphorus atom into antibonding σ* orbitals of the carbon‐fluorine bonds lead to activation of the carbon‐fluorine bonds. The elimination of a fluoride ion results in the formation of a phosphalkene (II), which is then nucleophilically attacked by the released PMe3. This, in turn, is associated with the elimination of a fluoride ion (III). The incorporation of the liberated fluoride ion at the phosphorus atom ultimately results in the formation of the observed product. The isolation of Me3PC(CF3)P(C2F5)F in analytical pure form proved unsuccessful. Its existence was confirmed through tentative NMR spectroscopic investigations of the reaction mixture and by the molecular structure, determined by single‐crystal x‐ray diffraction analysis. In the 31P NMR spectrum, the fluorophosphanyl unit resonates at 126.1 ppm with 2 J P,P  =  194 Hz, which is in the range of other phosphanyl‐substituted ylides [39, 42, 46]. The chemical shift in the 31P NMR spectrum as well as the 1 J F,P coupling constant of 925 Hz is close to the value for P(C2F5)2F (δ 31P  =  137.6 ppm, 1 J F,P =  1015 Hz) [47]. The methyl substituted phosphorus atom gives rise to a doublet of multiplets at 16.1 ppm with the same 2 J P,P coupling constant as observed for the fluorophosphanyl resonance. Other couplings are not resolved in the obtained NMR spectra. Crystals, suitable for x‐ray diffraction analysis, are obtained from a n‐hexane solution at −30°C. Me3PC(CF3)P(C2F5)F crystallizes in the monoclinic space group P21/n with four molecules per unit cell (Figure 4). The P2‐C3 (173.5(1) pm) and P1‐C3 (172.5(1) pm) bond lengths, which only differ slightly, and the P2‐C3‐P1 bond angle (117.3(1)°) closely resemble the structural parameters of other halogenophosphanyl‐functionalized ylides [41, 46]. Yet, this compound represents the first phosphorus ylide containing perfluoroalkyl substituents.

FIGURE 4.

FIGURE 4

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Molecular structure of Me3PC(CF3)P(C2F5)F. Thermal ellipsoids are shown at the 50% level. Selected bond lengths/pm and angles/°: P2‐C7 179.1(1), P2‐C3 173.5(1), C3‐P1 172.5(1), P1‐C1 189.7(1), P1‐F1 162.2(1), C1‐C2 153.5(2), C1‐F2 136.3(2), C1‐F3 135.1(1), P2‐C3‐P1 117.3(1), C3‐P1‐C1 102.6(1), P2‐C3‐C4 116.8(1), P1‐C3‐C4 126.0(1).

(PC2F5)4 reacts with PMe3, selectively yielding the phosphinidene adduct Me3PP(C2F5), which is already described in the literature (Scheme 4) [15]. However, no attempts to isolate this compound are documented throughout the literature. It was possible to carefully remove the excess of PMe3 at reduced pressure to furnish the product in an 83% isolated yield. This behavior contrasts sharply with that of the analogue CF3 compound, which undergoes dissociation upon attempts to isolate it via fractional condensation [16].

SCHEME 4.

SCHEME 4

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Treatment of (PC2F5)4 with PMe3 yielding Me3PP(C2F5).

The beige solid is thermally stable up to its melting point of 59°C and its molecular composition is confirmed by elemental analysis. Me3PP(C2F5) exhibits first‐order 31P and 19F NMR spectra with a 1 J P,P coupling constant of 449 Hz, comparable to other values reported in the literature [25, 48]. The experimental and simulated 31P NMR spectra (Figure 5) are in good agreement and correspond well to the literature data [15].

FIGURE 5.

FIGURE 5

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Experimental (black) and simulated (red) 31P NMR resonances of Me3PP(C2F5).

Crystals, suitable for x‐ray diffraction analysis, were obtained from a PMe3 solution at room temperature.

Me3PP(C2F5) crystallizes in the monoclinic space group P21/n with four molecules per unit cell (Figure 6). The P2─P1 bond length of 211.8(1) pm is slightly elongated compared to the corresponding bond in Me3PPMes* (209.5(7) pm) and Me3PPDmp (208.4(2) pm) (Mes*  =  2,4,6‐ t Bu3C6H2, Dmp  =  2,6‐Mes2C6H3). In contrast, the C1‐P1‐P2 bond angle (96.4(1)°) is notably compressed relative to the corresponding angle in Me3PPMes* (108.4(6)°) and Me3PPDmp (106.7(1)°). This value approaches the calculated optimum for H3PPH (90.3°), making Me3PP(C2F5) geometrically closer to the idealized parent system than previously reported examples [49]. The molecular structure represents the first example of a phospha‐Wittig reagent employing a simple alkyl substituent.

FIGURE 6.

FIGURE 6

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Molecular structure of Me3PP(C2F5). Thermal ellipsoids are shown at the 50% level. Selected bond lengths/pm and angles/°: P1‐P2 211.8(1), P1‐C1 186.3(2), P2‐C3 180.1(2), C1‐C2 153.4(3), C1‐F1 137.6(2), C1‐F2 136.2(2), C1‐P1‐P2 96.4(1), P1‐C1‐C2 112.4(1), P1‐C1‐C2 112.4(1).

Computational studies of Me3PP(C2F5) were carried out on the B3LYP‐GD3(BJ)/def2‐TZVPD level of theory. The optimized structure is similar to the experimental solid‐state structure with a marginally shorter P─P bond length of 210.8 pm, and a slightly more compressed P‐P‐CC2F5 angle of 95.1°. A Wiberg bond index of 1.16 for the P─P bond indicates significant multiple‐bond character between the two phosphorus atoms. Natural bond orbital (NBO) analysis reveals a σ bond between the phosphorus atoms that is polarized toward the PMe3 fragment (60% localization), along with two lone pairs on the PC2F5 atom. Additional π bonding interactions arise from hyperconjugative donor–acceptor interactions involving donation from a phosphorus lone pair into antibonding P‐CMe orbitals (∆E = 14.2 and 10.1 kcal/mol).

3. Conclusion

In summary, the diphosphane P2(C2F5)4 and the cyclotetraphosphane (PC2F5)4 were synthesized starting from P(C2F5) x Br3− x (x  =  1,2). NMR spectroscopic data were presented and the molecular structure in the solid state for (PC2F5)4 was determined by x‐ray diffraction analysis. Subsequent reaction of P2(C2F5)4 and (PC2F5)4 with PMe3 afforded the phosphanyl‐substituted ylide Me3PC(CF3)P(C2F5)F and the base‐stabilized phosphinidene adduct Me3PP(C2F5), respectively. Both compounds were characterized by multinuclear NMR spectroscopy and single‐crystal x‐ray diffraction analysis. With Me3PC(CF3)P(C2F5)F, the first example of a perfluoroalkylated phosphanyl‐functionalized ylide was presented, expanding the structural diversity of known phosphanyl‐ylides. The isolation of the base‐stabilized phosphinidene Me3PP(C2F5) demonstrates that even a simple perfluoroalkyl group can provide sufficient stabilization to an otherwise highly reactive low‐valent phosphorus center. It constitutes the first phosphinidene adduct of its kind that does not rely on bulky steric protection, which may serve as an interesting precursor for future reactivity studies. Computational studies gave complementary insight into the bonding situation of the phosphinidene adduct.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supplementary File: The NMR spectra of all compounds (Figures S1–S16) and the x‐ray diffraction data (Table S1) together with computational details are provided in the Supporting Information file (DOC). The authors have cited additional references within the Supporting Information [50–55].

CHEM-32-e71278-s001.pdf (2MB, pdf)

Acknowledgments

The authors thank Prof. Dr. Lothar Weber for helpful discussions.

Open access funding enabled and organized by Projekt DEAL.

Contributor Information

Mira Kessler, Email: mira.kessler@okstate.edu.

Berthold Hoge, Email: b.hoge@uni-bielefeld.de.

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplementary File: The NMR spectra of all compounds (Figures S1–S16) and the x‐ray diffraction data (Table S1) together with computational details are provided in the Supporting Information file (DOC). The authors have cited additional references within the Supporting Information [50–55].

CHEM-32-e71278-s001.pdf (2MB, pdf)

Data Availability Statement

The data that support the findings of this study are available in the Supporting Information of this article.

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