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.

fluorophosphonium salt or designing a fluorophosphonium derivative with an intramolecular Lewis basic site.
Herein, we present the synthesis of pyridyl-and imidazolylsubstituted fluorophosphonium triflate salts and their application as intramolecular N/P FLP systems. Both compounds readily form labile adducts with CO 2 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 CH 2 Cl 2 (Scheme 2). [10] The formation of adduct 2 + is indicated by multinuclear NMR spectroscopy and the 31 P NMR spectrum displays a doublet resonance at δ(P) = À 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 P1À O1 and C1À N1 distances are 1.7038(10) Å and 1.5206 (17) Å, respectively. The C1À 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.
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 (C 6 F 5 ) 2 PR (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 CD 2 Cl 2 solutions with CO 2 (2 bar). The corresponding 31 P and 19 F NMR spectra of the solution containing 3[OTf] reveal no interaction or reaction at ambient temperature indicated by the resonances for cation 3 + (δ( 31 P) = 54.8 ppm, δ( 19 F) = À 114.0 ppm, 1 J PF = 996 Hz) in the spectra. However, a VT NMR investigation disclosed the reversible and temperature-dependent binding of CO 2 by 3[OTf] (Figure 2, left).
The 31 P 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( 31 P) = À 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 CO 2 adduct 5[OTf]. The corresponding resonance in the 19 F NMR spectrum is detected as a downfield shifted doublet resonance at δ( 19 F) = À 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 CO 2 is observed as a doublet resonance at δ( 13 C) = 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 CD 2 Cl 2 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 CO 2 theoretically with two different models ( Figure 3). In 3 + -CO 2 _A, CO 2 is activated by the incorporation of both the Lewis acidic and the Lewis basic site, whereas in model 3 + -CO 2 _B the activation exclusively proceeds through the Lewis acidic P atom. The CO 2 binding energy is slightly stronger for model 3 + -CO 2 _A (À 20.4 kJ/mol) than model 3 + -CO 2 _B (À 18.4 kJ/mol). Moreover, the calculated 31 P NMR chemical shift change (Δδ = δ(3 + -CO 2 _A)À δ(3 + )) for model 3 + -CO 2 _A (Δδ( 31 P) = À 124 ppm) is in the range of the experimental result (Δδ( 31 P) = À 110.7 ppm), while the calculated Δδ( 31 P) for model 3 + -CO 2 _B is only shifted by À 17 ppm which is not observed in the respective 31 P NMR spectrum. Thus, the CO 2 activation is proposed to proceed via the P and pyridyl N atoms, affording 5 [OTf] in the configuration of model 3 + -CO 2 _A. As the DFT 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.), CH 2 Cl 2 , RT, 4 h.
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 Scheme 4. FLP-CO 2 adducts VI and VII containing a doubly activated CO 2 molecule.
The CÀ O bond lengths of the carbonyl moiety 8 a + /8 b + : 1.495(4)/1.411(5) Å and the CÀ N distances of the nitrile moiety (9 a + /9 b + : 1.250(3)/1.272(3) Å) are typical for CÀ 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 N1À C2 bond distance (1.414(6) Å) are comparable to those in the acetonitrile adduct 9 b + ( Table 2). The C1À C2 bond length (1.330(8) Å) is well in the range of a typical C=C double bond. [11] Deposition 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 CO 2 (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 CO 2 (7 2 + ) comprising one molecule of CO 2 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 CO 2 adduct formation might provide new applications for in FLP chemistry, which we are currently investigating. 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.  , 8 a, b + ), N (9 a, b + ), C (12 b + ); [b] Angle sum of the newly formed five-membered ring.