Bifunctional Fluorophosphonium Triflates as Intramolecular Frustrated Lewis Pairs: Reversible CO₂ Sequestration and Binding of Carbonyls, Nitriles and Acetylenes

Abstract

Electrophilic fluorophosphonium triflates bearing pyridyl (3[OTf]) or imidazolyl (4[OTf])‐substituents act as intramolecular frustrated Lewis pairs (FLPs) and reversibly form 1 : 1 adducts with CO₂ (5 ⁺ and 6 ⁺). An unusual and labile spirocyclic tetrahedral intermediate (7 ²⁺) is observed in CO₂‐pressurized (0.5–2.0 bar) solutions of cation 4 ⁺ at low temperatures, as demonstrated by variable‐temperature NMR studies, which were confirmed crystallographically. In addition, cations 3 ⁺ and 4 ⁺ actively bind carbonyls, nitriles and acetylenes by 1,3‐dipolar cycloaddition, as shown by selected examples.

Extending the FLP library: Electrophilic fluorophosphonium triflates bearing pyridyl or imidazolyl substituents act as intramolecular N/P frustrated Lewis pairs that are capable of small molecule activation, that is, reversible CO₂ sequestration and binding of carbonyls, nitriles and acetylenes.

The selective, reversible binding of carbon dioxide (CO₂) and its transformation into value‐added chemicals and materials is a challenge of today and success would significantly contribute to decline CO₂ accumulation in the atmosphere. ^[1] The chemistry of CO₂ binding and transformation receives tremendous attention as it is regarded as an important heat‐trapping (greenhouse) gas ^[2] but at the same time considered as readily available, abundant, nontoxic, and renewable C1 synthon in synthesis. ^[3] In this context, generous efforts continue to be devoted to the development of efficient physical or chemical CO₂ activation approaches.[ ^1a , ⁴ ] A key‐step in the capture‐and‐release strategies and catalytic conversion, however, is a reversible binding of the CO₂ molecule which needs to be enabled by low‐energy and readily‐cleavable bonds. ^[5] Thus, a variety of organic carbon, nitrogen and phosphorus based Lewis bases were developed that are capable of forming adducts with the CO₂ molecule. ^[6] Notably, a variety of frustrated Lewis pairs (FLPs) have been investigated towards CO₂ sequestration including inter‐ (I, II) ^[7] and intramolecular (III,IV) ^[8] FLPs derived from different combinations of electrophiles (e. g., borane‐, aluminum‐, or silylium‐based) and nucleophiles (e. g., phosphanes, carbenes, or amines; Scheme 1). However, the key step in a capture‐and‐release application for the conversion of CO₂ ^[1] is usually hampered by the fact that the majority of FLPs form stable adducts with CO₂.[ ^7a , ^7b , ⁸ ] Efforts towards binding of CO₂ by highly Lewis acidic amidophosphoranes were reported by the Stephan group (Scheme 1, V). ^[9] Although phosphonium cations have been extensively studied for their electrophilic properties, their application as Lewis acid in FLP chemistry is scarcely investigated so far. In thought of using fluorophosphonium cations in FLP chemistry, where in contrast to known systems the P atom is the Lewis acidic site, two different strategies are feasible: either the addition of a Lewis base to the fluorophosphonium salt or designing a fluorophosphonium derivative with an intramolecular Lewis basic site.

Scheme 1.

Selected examples of stable CO₂ adducts of inter‐ and intramolecular FLP systems (I–V) and CO₂ adducts 5 ⁺, 6 ⁺ and 7 ²⁺ derived from bifunctional fluorophosphonium compounds (this work).

Herein, we present the synthesis of pyridyl‐ and imidazolyl‐substituted fluorophosphonium triflate salts and their application as intramolecular N/P FLP systems. Both compounds readily form labile adducts with CO₂ and bind carbonyls, nitriles and acetylenes by 1,3‐dipolar cycloaddition shown by selected examples.

To illustrate our approach, we first reacted fluorophosphonium salt 1[OTf] ^[10] as a very strong Lewis acid in combination with pyridine as a Lewis base and p‐tolylaldehyde in CH₂Cl₂ (Scheme 2). ^[10] The formation of adduct 2 ⁺ is indicated by multinuclear NMR spectroscopy and the ³¹P NMR spectrum displays a doublet resonance at δ(P)=−53.2 ppm (¹ J _PF=644 Hz) being well in the range for penta‐coordinate P atoms. X‐ray analysis of 2[OTf] confirms that the Lewis acidic P atom of the fluorophosphonium cation and Lewis basic N atom of the pyridine act cooperatively acts as an intermolecular FLP system activating the carbonyl group of the aldehyde (Figure 1). The corresponding P1−O1 and C1−N1 distances are 1.7038(10) Å and 1.5206(17) Å, respectively. The C1−O1 distance of 1.3974(16) Å is about 0.2 Å longer compared to the C=O double bond in free aldehydes. ^[11] However, the application of this intermolecular FLP system is limited as we observed the decomposition of 1[OTf] in the presence of pyridine without a suitable substrate.

Scheme 2.

Adduct formation of 2[OTf] from the reaction of 1[OTf] with pyridine and p‐tolylaldehyde; i) pyridine (1 equiv.), p‐tolCHO (1 equiv.), CH₂Cl₂, RT, 4 h.

Figure 1.

Molecular structure of 2[OTf] ⋅ CH₂Cl₂ (hydrogen atoms, CH₂Cl₂, and non‐coordinating anions are omitted for clarity, ellipsoids are set at 50 % probability); Selected bond lengths (Å) and angles (°): P1−O1 1.7038(10), O1−C1 1.3974(16), C1−N1 1.5206(17).

We therefore targeted the synthesis of bifunctional fluorophosphonium salts such as 3[OTf] and 4[OTf] which contain a pyridyl‐ or imidazolyl‐substituent, respectively. The synthesis follows our recently reported one‐pot procedure, where N‐fluorobenzenesulfonimide (NFSI) is added to the corresponding phosphane (C₆F₅)₂PR (R=imidazolyl, pyridyl), followed by the addition of MeOTf (Scheme 3). ^[10] Triflate salts 3, 4[OTf] are isolated as colorless, air‐ and moisture‐sensitive salts in very good yields of >88 %. ^[12] We started to investigate the cooperative properties of these salts by pressurizing degassed CD₂Cl₂ solutions with CO₂ (2 bar). The corresponding ³¹P and ¹⁹F NMR spectra of the solution containing 3[OTf] reveal no interaction or reaction at ambient temperature indicated by the resonances for cation 3 ⁺ (δ(³¹P)=54.8 ppm, δ(¹⁹F)=−114.0 ppm, ¹ J _PF=996 Hz) in the spectra. However, a VT NMR investigation disclosed the reversible and temperature‐dependent binding of CO₂ by 3[OTf] (Figure 2, left).

Scheme 3.

Synthesis of fluorophosphonium triflate salts 3[OTf] and 4[OTf]; i) +NFSI (1 equiv.), PhF, RT, 16 h; ii) +MeOTf (1 equiv.), RT, 4 h, −MeNSI; NSI⁻=[N(SO₂Ph)₂]⁻.

Figure 2.

Equilibrium reaction of 3[OTf] and 4[OTf] with CO₂ (top); variable‐temperature ³¹P NMR spectra of CO₂ with 3[OTf] (bottom left, 2 bar in CD₂Cl₂) and 4[OTf] (bottom right, 1.5 bar in CD₃NO₂). Asterisks indicate small amounts of unidentified compounds.

The ³¹P NMR spectrum at 263 K shows, next to the dominant signal of the free cation 3 ⁺ (98 %), a small high field shifted doublet resonance at δ(³¹P)=−55.5 ppm (2 %, ¹ J _PF=769 Hz), which is well in the region for a penta‐coordinate phosphorus atom, thus, indicating the formation of the CO₂ adduct 5[OTf]. The corresponding resonance in the ¹⁹F NMR spectrum is detected as a downfield shifted doublet resonance at δ(¹⁹F)=−30.0 ppm. When the temperature is further decreased to 213 K the integral ratio of 5[OTf] increases to 41 : 59. The carbon atom of the bound CO₂ is observed as a doublet resonance at δ(¹³C)=141.8 ppm (² J _PC=3 Hz). Further decrease of the temperature to 193 K leads to the separation of a colorless precipitate, suggesting that 5[OTf] exhibits a reduced solubility in CD₂Cl₂ at low temperature. Accordingly, gradually increase to RT leads to the dissolution of 5[OTf] and its disappearance in the NMR spectrum. This process is reversible without decomposition of 3[OTf]. We investigate this interaction of 3[OTf] with CO₂ theoretically with two different models (Figure 3). In 3 ⁺‐CO₂ _A, CO₂ is activated by the incorporation of both the Lewis acidic and the Lewis basic site, whereas in model 3 ⁺‐CO₂ _B the activation exclusively proceeds through the Lewis acidic P atom. The CO₂ binding energy is slightly stronger for model 3 ⁺ ‐CO₂ _A (−20.4 kJ/mol) than model 3 ⁺‐CO₂ _B (−18.4 kJ/mol). Moreover, the calculated ³¹P NMR chemical shift change (Δδ=δ(3 ⁺‐CO₂ _A)−δ(3 ⁺)) for model 3 ⁺‐CO₂ _A (Δδ(³¹P)=−124 ppm) is in the range of the experimental result (Δδ(³¹P)=−110.7 ppm), while the calculated Δδ(³¹P) for model 3 ⁺‐CO₂ _B is only shifted by −17 ppm which is not observed in the respective ³¹P NMR spectrum. Thus, the CO₂ activation is proposed to proceed via the P and pyridyl N atoms, affording 5[OTf] in the configuration of model 3 ⁺‐CO₂ _A. As the DFT calculation reveals a slightly improved CO₂ binding energy (−23.0 kJ/mol) for 4[OTf], we also investigated the reactivity of 4[OTf] towards CO₂ (Figure 3). Pressurizing a degassed CH₂Cl₂ solution of 4[OTf] with CO₂ (0.5 bar) at ambient temperature results in the immediate formation of a colorless solid. However, the solid re‐dissolves immediately after releasing the pressure and all our separation attempts ended up with the recovery of 4[OTf]. We therefore monitored the reaction of 4[OTf] with CO₂ (1.5 bar) by NMR spectroscopy in degassed CD₃NO₂. The ³¹P NMR spectrum at ambient temperature shows signals of two different adducts at δ(³¹P)=−60.0 ppm (¹ J _PF=737 Hz, 5 %) and δ(³¹P)=−61.5 ppm (¹ J _PF=740 Hz, 14 %) besides the signal of 4[OTf] (δ(³¹P)=55.2 ppm, δ(¹⁹F)=−111.5 ppm, ¹ J _PF=1045 Hz, Figure 2, right). The corresponding signals in the ¹⁹F spectrum are detected at δ(¹⁹F)=−8.1 ppm (¹ J _FP=737 Hz) and δ(¹⁹F)=−16.9 ppm (¹ J _FP=740 Hz), respectively. Interestingly, the ¹³C NMR spectrum displays two signals for the bound CO₂ moiety at δ(¹³C)=139.9 ppm (² J _PC=7 Hz) as a doublet resonance and at δ(¹³C)=105.5 ppm (² J _PC=8 Hz) as a triplet resonance suggesting a coupling to two phosphorus atoms.¹³ This strongly indicates the formation of a 1 : 1 FLP‐CO₂ adduct 6[OTf] (δ(³¹P)=−61.5 ppm, δ(¹⁹F)=−16.9 ppm, ¹ J _PF=740 Hz) and a 2 : 1 FLP‐CO₂ adduct 7[OTf]₂ (δ(³¹P)=−60.0 ppm, δ(¹⁹F)=−8.1 ppm, ¹ J _PF=737 Hz). The ³¹P NMR chemical shift change of 6[OTf] (Δδ=δ(6 ⁺)−δ(4 ⁺), −116.7 ppm) is well in the range of calculated model 4 ⁺‐CO₂ _A (Δδ(³¹P)=−108 ppm). Decreasing the temperature stepwise to 243 K, the integral ratio of 7[OTf]₂ gradually increases to 66 %, while the integral ratio of 6[OTf] increases to 28 %. Variable‐temperature NMR studies at 0.5 or 2.0 bar CO₂ pressure did not show significant difference on the integral ratio of 6[OTf] and 7[OTf]₂. Although the calculated electrophilicities are similar for 3[OTf] (GEI=3.164 eV, FIA=748.0 kJ/mol) and 4[OTf] (GEI=3.121 eV, FIA=743.8 kJ/mol), ^[10] in no case do we observe a double activation of CO₂ with 3[OTf]. This suggests that a certain nucleophilicity of the Lewis basic site is crucial for the formation of the 2 : 1 FLP‐CO₂ adduct 7[OTf]₂. ^[14]

Figure 3.

Optimized geometries and free energies of 3 ⁺‐ and 4 ⁺‐CO₂ complexes (distances in Å, MP2/def2‐TZVP level of theory).

To our delight, we were able to obtain extremely sensitive but suitable co‐crystals containing both cations 6 ⁺ and 7 ²⁺ by vapor diffusion of CH₂Cl₂ into a CH₃NO₂ solution of 4[OTf] under CO₂ atmosphere (0.5 bar) at −30 °C (Figure 4). Although the double activation of CO₂ has been reported with FLPs based on hafnium (VI) and aluminum (VII) complexes (Scheme 4),[ ¹³ , ¹⁵ ] the tetrahedral cation 7 ²⁺ represents the only crystallographically characterized example of a metal‐free 2 : 1 FLP‐CO₂ adduct.

Figure 4.

Molecular structure of co‐crystal 6[OTf]⋅7[OTf]₂⋅CH₃NO₂ (hydrogen atoms, CH₃NO₂, and non‐coordinating anions are omitted for clarity, ellipsoids are set at 50 % probability). Selected bond lengths (Å) and angles (°); for 6 ⁺: P1−O1 1.773(5), O1−C1 1.307(8), C1−N1 1.442(10), C1−O2 1.206(10), O1−C1−O2 128.1(7); for 7 ²⁺: P1−O1 1.771(4), P2−O2 1.744(4), O1−C3 1.352(7), O2−C3 1.366(6), O1−C3−O2 114.4(5).

Scheme 4.

FLP‐CO₂ adducts VI and VII containing a doubly activated CO₂ molecule.

The molecular structure of 6 ⁺ which is consistent with the optimized structure 4⁺‐CO₂ _A, shows that a CO fragment of the CO₂ is integrated into a five‐membered PNC₂O heterocycle while the other O atom is exocyclic. The CO₂ moiety is bent, giving an O1−C1−O2 angle of 128.1(7)°. One oxygen is bound by the Lewis acidic P atom with a P1−O1 bond length of 1.773(5) Å and the carbon atom is stabilized by the Lewis basic N atom to give a N1−C1 bond length of 1.442(10) Å. The C1−O1 (1.307(8) Å) and C1=O2 (1.206(10) Å) bonds in the CO₂ fragment are comparable to other FLP‐CO₂ adducts (e. g., C−O 1.33 Å, C=O 1.21 Å).[ ^9a , ¹⁶ ] In the case of the 2 : 1 FLP‐CO₂ adduct 7 ²⁺, the CO₂ molecule is captured by two cations 4 ⁺ resulting in a distorted tetrahedral geometry of the central carbon atom. The P−O bond lengths (P1−O1 1.771(4) Å, P2−O2 1.744(4) Å) are similar to those of 6 ⁺. The C−O bonds (O1−C3 1.352(7) Å, O2−C3 1.366(6) Å) are both significantly longer than those in 6 ⁺, but comparable to those reported for the hafnium based FLP‐CO₂ adduct VI (1.383 and 1.369 Å, Figure 4). ^[15]

Inspired by these results, further studies concerned the 1,3‐dipolar cycloaddition of salts 3,4[OTf] with carbonyls, nitriles, and acetylenes to the respective heterocycles (Scheme 5). At room temperature, the formation of acetone adducts 8 a, b[OTf] (8 a ⁺/8 b ⁺: δ(³¹P)=−52.0/−60.0 ppm, δ(¹⁹F)=−6.5/9.6 ppm, ¹ J _PF=734/696 Hz, 97 and 84 % isolated yield) and acetonitrile adducts 9 a, b[OTf] (9 a⁺ /9 b⁺ : δ(³¹P)=−49.3/−65.0 ppm, δ(¹⁹F)=3.0/19.9 ppm, ¹ J _PF=738/702 Hz, 71 and 95 % isolated yield) are indicated by multinuclear NMR spectroscopy (Table 1) and are isolated in good to excellent yields after workup. The carbonyl carbon atoms in 8 a, b[OTf] (8 a ⁺/8 b ⁺: δ(¹³C)=101.8/96.7 ppm, ² J _CP=9/11 Hz) are significantly shifted to higher field compared to the free acetone (δ(¹³C)=207 ppm). ^[17] The nitrile carbon atoms in 9 a,b[OTf] are shifted to lower field (9 a ⁺/9 b ⁺: δ(¹³C)=151.0/150.2 ppm, ² J _CP=18/19 Hz). Interestingly, when compounds 9 a, b[OTf] are reacted with acetone at room temperature, the formation of 8 a, b[OTf] under release of CH₃CN is observed after a reaction time of 1 h (Scheme 5, ii). The quantitative exchange is indicated by the clean shift of the corresponding doublet resonance in the ³¹P NMR spectrum of the reaction mixture.

Scheme 5.

Reaction of 3/4[OTf] with unsaturated compounds; i) +Me₂CO (1 equiv.), CH₂Cl₂, RT, 1 h; ii) +MeCN (1 equiv.)_, CH₂Cl₂, RT, 1 h; iii) +acetylenes (0.5 equiv.), DCE, RT, 4 h; iv) +acetylenes (1 equiv.), DCE, 80 °C, 16 h; v) RT, 2 weeks, or 80 °C, 16 h, −3/4[OTf]; vi) +Me₂CO (1 equiv.), −MeCN, CH₂Cl₂, RT, 1 h.

Table 1.

Relevant ¹⁹F and ³¹P NMR data of the heterocycles 8–12[OTf].

Compound[a]	Solvent	δ(19F) (ppm)	δ(31P) (ppm)	1 J PF (Hz)
8 a[OTf]	CD2Cl2	−6.5	−52.0	734
8 b[OTf]	CD2Cl2	9.6	−61.0	696
9 a[OTf]	CD3CN	3.0	−49.3	738
9 b[OTf]	CD3NO2	19.9	−65.0	702
11 a[OTf]2	CD3CN	–	6.9	–
11 a[OTf]2	DCE[a]	–	−15.0	–
12 a[OTf]	DCE[a]	34.0	−88.8	707
12 b[OTf]	CD2Cl2	25.2	−98.8	718

[a] With C₆D₆ capillary.

When 3, 4[OTf] are reacted with acetylenes in dichloroethane (DCE) at room temperature for 4 h, the formation of 10 a, b and 11 a, b[OTf]₂ is observed, while 12 a, b[OTf] are cleanly formed by fluoride abstraction from 10 a, b to 11 a, b ²⁺ after reacting at 80 °C overnight or at ambient temperature for 2 weeks (Scheme 5, iii–v). Cation 11a ²⁺(GEI=7.408 eV, FIA=1066.9 kJ/mol) is more Lewis acidic than 3 ⁺ (GEI=3.164 eV, FIA=748.0 kJ/mol), thus, low reaction tendency from 11 a, b ⁺ to 12 a, b ⁺ seems to be influenced by solvent effects, which significantly decrease the Lewis acidity compared to the gas phase calculations.[ ¹⁰ , ¹⁸ ] 11 a, b[OTf]₂ are found as singlet resonances in the ¹P NMR spectra at δ(³¹P)=6.9 and −15.0 ppm, while 12 a, b[OTf] are observed as doublet resonances in the ³¹P and ¹⁹F NMR spectra (12 a ⁺/12 b ⁺: δ(³¹P)=−88.8/−98.8 ppm, δ(¹⁹F)=34.0/25.2 ppm, ¹ J _PF=707/718 Hz, Table 1). We were able to isolate 11 a[OTf]₂ ^[19] (78 %) and 12 b[OTf] (66 %) as pure products for full characterization.

The molecular structures of compounds 8 a, b[OTf], 9 a, b[OTf] and 12 b[OTf] are confirmed by X‐ray analysis of suitable single crystals which were obtained by slow vapor diffusion of n‐pentane into saturated CH₂Cl₂ solutions at −30 °C (Figure 5). All obtained structures show a distorted trigonal‐bipyramidal bonding environment at the P atom and display the expected five‐membered heterocycles with angle sum ranging from 536.8° to 540.0° (Table 2). In the structures of 8 a, b ⁺ and 9 a, b ⁺, the heteroatoms (O, N) and the fluorine atoms occupy the axial position. The P−F bonds (1.637(2) −1.673(13) Å) are elongated compared to those of the starting materials 3 ⁺ and 4 ⁺, but comparable to those in difluorophosphoranes (e. g., (C₆F₅)₃PF₂: 1.638(2) Å). ^[20] The resulting P‐heteroatom distances in the acetone adducts (8 a ⁺/8 b ⁺: 1.695(2)/1.695(3) Å) are significantly shorter compared to the acetonitrile adducts (9 a ⁺/9 b ⁺: 1.777(2)/1.788(2) Å) while the C−N bonds are around 0.15 Å longer (8 a ⁺/8 b ⁺: 1.495(4) Å/1.411(5) Å, 9 a ⁺/9 b ⁺: 1.250(3)/1.272(3) Å, Table 2).

Figure 5.

Molecular structures of selected FLP adducts (hydrogen atoms, solvent molecules, and non‐coordinating anions are omitted for clarity, ellipsoids are set at 50 % probability); selected structural parameters are included in Table 2.

Table 2.

Selected geometrical parameters of the crystallographically characterized FLP adducts.

Compound	P−F (Å)	P−X[a] (Å)	C−X[a] (Å)	N−C (Å)	Angle sum[b] (°)
6[OTf]	1.623(5)	1.773(5)	1.307(8)	1.442(10)	539.7
8 a[OTf]	1.673(13)	1.6989(15)	1.398(2)	1.519(3)	536.8
8 b[OTf]	1.637(2)	1.695(2)	1.411(5)	1.495(4)	537.2
9 a[OTf]	1.6427(16)	1.777(2)	1.250(3)	1.491(3)	540.0
9 b[OTf]	1.6426(16)	1.788(2)	1.272(3)	1.458(4)	539.9
12 b[OTf]	1.651(3)	1.773(5)	1.330(8)	1.415(6)	539.2

[a] X=O (6 ⁺, 8 a, b ⁺), N (9 a, b ⁺), C (12 b ⁺); [b] Angle sum of the newly formed five‐membered ring.

The C−O bond lengths of the carbonyl moiety 8 a ⁺/8 b ⁺: 1.495(4)/1.411(5) Å and the C−N distances of the nitrile moiety (9 a ⁺/9 b ⁺: 1.250(3)/1.272(3) Å) are typical for C−O single bonds and C=N double bonds, respectively. ^[11] The structure of 12 b ⁺ shows that the pyridyl moiety is in the axial position, opposite to the fluorine atom (Figure 5). Thus, the terminal carbon atom of the alkyne occupies one of the equatorial positions adopting the minimum energy configuration due to steric restraints. The newly formed P1‐C1 bond distance (1.790(6) Å) and the N1−C2 bond distance (1.414(6) Å) are comparable to those in the acetonitrile adduct 9 b ⁺ (Table 2). The C1−C2 bond length (1.330(8) Å) is well in the range of a typical C=C double bond. ^[11]

Deposition Numbers 2061970 (for 4[OTf] ⋅ CH₂Cl₂), 2061971 (for 2[OTf]), 2061972 (for 3[OTf] ⋅ C₆H₅F), 2061973 (for 6[OTf] ⋅ 7[OTf]₂ ⋅ CH₃NO₂), 2061974 (for 8 a[OTf] ⋅ CH₂Cl₂), 2061975 (for 8 b[OTf] ⋅ CH₂Cl₂), 2061976 (for 9 a[OTf] ⋅ (CH₂Cl₂)₂ ⋅ CH₃CN), 2061977 (for 9 b[OTf]), 2061978 (for 13), 2061979 (for 12b[OTf]), and 2061980 (for 14) contain the supplementary crystallographic data for this paper. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service.

In summary, electrophilic fluorophosphonium compounds bearing pyridyl (3[OTf]) or imidazolyl (4[OTf]) substituents, thus an additional Lewis basic site, represent a new type of intramolecular FLP that reacts with small molecules in a cooperative manner. The cyclizations of 3[OTf] and 4[OTf] with the 1,2‐dipolar compounds acetone, acetonitrile and acetylenes gave rise to the formation of the corresponding heterocyclic compounds, thus illustrating the cooperative reactivity of the new FLP derivatives. This is additionally demonstrated by the reversible formation of adducts with CO₂ (5 ⁺ and 6 ⁺) at low temperature, which was investigated by variable‐temperature NMR studies and X‐ray analysis. Surprisingly, the bifunctional phosphonium cation 4 ⁺ forms an adduct with CO₂ (7 ²⁺) comprising one molecule of CO₂ and two molecules of 4 ⁺, resulting in a spirocyclic geometry at the central carbon atom, which is hitherto unreported for an N/P FLP system. These novel bifunctional phosphonium cations extend the diverse library of FLP systems, and the reversible CO₂ adduct formation might provide new applications for in FLP chemistry, which we are currently investigating.

Conflict of interest

The authors declare no conflict of interest.

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Supporting Information

C.-X. Guo, K. Schwedtmann, J. Fidelius, F. Hennersdorf, A. Dickschat, A. Bauzá, A. Frontera, J. J. Weigand, Chem. Eur. J. 2021, 27, 13709.