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 PF 2 (C 2 F 5 ) 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, n Pr, i Pr) the adducts NHC ⋅ PF 2 (C 2 F 5 ) 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 t Bu 2 Im (1,3‐di‐ tert ‐butyl‐imidazolin‐2‐ylidene) ( 1 j ) afforded the a NHC adducts 3 i and 3 j ( a denotes “abnormal” NHC coordination via a backbone carbon atom). The use of t BuMeIm (1‐ tert ‐butyl‐3‐methyl‐imidazolin‐2‐ylidene) ( 1 m ) led to partial decomposition of the NHC and formation of the salt [ t BuMeIm−H][MeIm ⋅ PF 2 (C 2 F 5 ) 3 ] ( 4 m ). The phosphorane PF 2 (C 2 F 5 ) 3 forms adducts with PMe 3 but does not react with PPh 3 or PCy 3 . The mer ‐ cis isomer of literature‐known Me 3 P ⋅ PF 2 (C 2 F 5 ) 3 ( 5 a ) was structurally characterized. Mixtures of the phosphorane PF 2 (C 2 F 5 ) 3 and the sterically encumbered NHCs t Bu 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−(CH 2 ) 4 O−PF 2 (C 2 F 5 ) 3 6 i – k . Furthermore, the deprotonation of the weak C−H acids CH 3 CN, acetone, and ethyl acetate was achieved, which led to the formation of the corresponding imidazolium salts and the phosphates [PF 2 (C 2 F 5 ) 3 (CH 2 CN)] − ( 7 ), [PF 2 (C 2 F 5 ) 3 (OC(=CH 2 )CH 3 )] − ( 8 ) and [PF 2 (C 2 F 5 ) 3 (CH 2 CO 2 Et)] − ( 9 ).