The Viability of C5-Protonated- and C4,C5-Ditopic Carbanionic Abnormal NHCs: A New Dimension in NHC Chemistry

The first C5-protonated abnormal N-heterocyclic carbene (aNHC), PhC{N(2,6-iPr₂ C₆ H₃ )}₂ CHC: (4) is readily accessible by C4-deprotonation of [ArC{N(2,6-iPr₂ C₆ H₃ )}₂ CHCH]X (3 a-X) (Ar=Ph, X=Br or I) with a base. The aNHC 4 is stable at 298 K in [D₈ ]THF solution and has been spectroscopically characterized. The facile availability of 4 enables the synthesis of a series of main-group compounds as well as transition-metal complexes featuring a new phosphorus-aNHC hybrid ligand. Double deprotonation of [ArC{N(2,6-iPr₂ C₆ H₃ )}₂ CHCH]X (Ar=Ph, 3 a-X (X=Br or I); 4-Tol, 3 b-Br; 4-DMP, 3 c-Br; Tol=MeC₆ H₄ , DMP=Me₂ NC₆ H₄ ) with nBuLi yields the C4,C5-ditopic carbanionic aNHCs, [ArC{N(2,6-iPr₂ C₆ H₃ )}₂ (C:)₂ ]Li(THF)_n (Ar=Ph, 13 a; 4-Tol, 13 b; 4-DMP, 13 c), which on treatment with Ph₂ PCl affords cationic vicinal bisphosphine derivatives [ArC{N(2,6-iPr₂ C₆ H₃ )}₂ {C(PPh₂ }₂ ]X (Ar=Ph, 14 a-X, X=Br or I; 4-Tol, 14 b-Br; 4-DMP, 14 c-Br).