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

Abstract

AbstractThe synthesis and characterization of Lewis acid/base adducts between tris(pentafluoroethyl)difluorophosphorane PF2(C2F5)3 and selected N‐heterocyclic carbenes (NHCs) R2Im (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 Dipp2Im (1,3‐bis‐(2,6‐di‐iso‐propylphenyl)‐imidazolin‐2‐ylidene) (1 i) and tBu2Im (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 tBu2Im, Dipp2Im, and Dipp2ImH2 (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)4O−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).