Lithium Complexes with Bridging and Terminal NHC ligands: The Decisive Influence of an Anionic Tether

Deprotonation of the fluorenyl-tethered imidazolinium salt [9-(C13H9)C2H4N(CH)C2H4N(2,4,6-Me3C6H2)][BF4] gave a spirocyclic compound that reacted with a synergic mixture of LiPh/LiN(SiMe3)2 or LinBu/LiN(SiMe3)2 to give a dilithium complex incorporating a bridging N(SiMe3)2 ligand. In contrast, deprotonation of the imidazolium salt [9-(C13H9)C2H4N(CH)C2H2N(Me)][Br] instead yielded the free NHC, which reacted with n-BuLi to form a dimeric, NHC-bridged dilithium complex. Addition of LiN(SiMe3)2 led to coordination and the formation of a dilithium complex with a bridging N(SiMe3)2 ligand, which was characterised in the solid state as a 1D coordination polymer. The reaction of 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene (SIPr) with lithium indenide and lithium fluorenide gave soluble species with terminal binding of the NHC to the lithium cation and η5 coordination of indenyl or fluorenyl. A symmetrical bridging mode for an NHC donor was therefore observed only if a tethered fluorenyl anion was present with no additional amide ligand.