A stable anionic N-heterocyclic carbene and its zwitterionic complexes

Pyrimidinium betaines (1), readily accessible via a straightforward modular synthesis from a formamidine and a monosubstituted malonic acid, are readily deprotonated by nBuLi (or KHMDS) to give the stable carbene species [2]Li+ (abbreviated as maloNHC). The latter represents the archetype of a subgroup of N-heterocyclic carbenes incorporating a malonate as remote anionic functional group within their heterocyclic backbone. While playing the dual role of monodentate 2 e(-) L type donor and noncoordinating charge carrier X, such ligands are seen to provide a rational route to zwitterionic complexes, as illustrated here by three examples (Rh, Fe, Ag). In particular, the reaction of [2]Li+ with [RhCl(1,5-COD)](2) produces the neutral 14 e(-) complex Rh (maloNHC)(COD) (3) in which coordinative unsaturation at the metal is relieved in the solid state by an uncommon labile bonding interaction between the C-ipso of one of the mesityl arms and the Rh center.