Tris(pentafluoroethyl)difluorophosphorane and N-Heterocyclic Carbenes: Adduct Formation and Frustrated Lewis Pair Reactivity

The synthesis and characterization of Lewis acid/base adducts between tris(pentafluoroethyl)difluorophosphorane PF2(C2F5)(3) and selected N-heterocyclic carbenes (NHCs) R(2)Im (1,3-di-organyl-imidazolin-2-ylidene) and phosphines are reported. For NHCs with small alkyl substituents at nitrogen (R=Me, nPr, iPr) the adducts NHC . PF2(C2F5)(3) (2 a-h) were isolated. The reaction with the sterically more demanding NHCs Dipp(2)Im (1,3-bis-(2,6-di-iso-propylphenyl)-imidazolin-2-ylidene) (1 i) and tBu(2)Im (1,3-di-tert-butyl-imidazolin-2-ylidene) (1 j) afforded the aNHC adducts 3 i and 3 j (a denotes abnormal NHC coordination via a backbone carbon atom). The use of tBuMeIm (1-tert-butyl-3-methyl-imidazolin-2-ylidene) (1 m) led to partial decomposition of the NHC and formation of the salt [tBuMeIm-H][MeIm . PF2(C2F5)(3)] (4 m). The phosphorane PF2(C2F5)(3) forms adducts with PMe3 but does not react with PPh3 or PCy3. The mer-cis isomer of literature-known Me3P . PF2(C2F5)(3) (5 a) was structurally characterized. Mixtures of the phosphorane PF2(C2F5)(3) and the sterically encumbered NHCs tBu(2)Im, Dipp(2)Im, and Dipp(2)Im(H2) (1,3-bis-(2,6-di-iso-propylphenyl)-imidazolidin-2-ylidene) (1 k) showed properties of FLPs (Frustrated Lewis Pairs) as these mixtures were able to open the ring of THF (tetrahydrofuran) to yield NHC-(CH2)(4)O-PF2(C2F5)(3) 6 i-k. Furthermore, the deprotonation of the weak C-H acids CH3CN, acetone, and ethyl acetate was achieved, which led to the formation of the corresponding imidazolium salts and the phosphates [PF2(C2F5)(3)(CH2CN)](-) (7), [PF2(C2F5)(3)(OC(=CH2)CH3)](-) (8) and [PF2(C2F5)(3)(CH2CO2Et)](-) (9).

Automated summary

The synthesis and characterization of Lewis acid/base adducts between PF2(C2F5)(3) and selected N-heterocyclic carbenes (NHCs) and phosphines were reported. The steric effects of different alkyl substituents on NHCs influenced the stability of the adducts, with larger substituents leading to partial decomposition.