N-Heterocyclic Carbene (NHC) Ligand Exchange of Bis(NHC)-Pd(II) Complexes.

N-heterocyclic carbene (NHC)-Pd(II) bonds are recognized as rather robust bonds to equip palladium(II) centers with decent stability and stereoelectronic tunability. In this report, the authentic carbene-Pd(II) bonds in -ammonium- and sulfonate-functionalized NHC complexes were activated, and NHC ligand exchange was triggered to afford four well-defined, sulfobetaine-type, zwitterionic heterobiscarbene complexes. Combined experimental studies and density functional theory calculations indicated that electrostatic interactions play a minimal role in the energy of this reaction; instead, the exchange appears to be driven primarily by the precipitation of the resulting zwitterionic complexes.