N-Heterocyclic Carbene (NHC) Ligand Exchange of Bis(NHC)–Pd(II) Complexes
Résumé
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 N-ammonium-and sulfonate-functionalized NHC complexes were activated, and NHC ligand exchange was triggered to afford four well-defined, sulfobetainetype, 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.N-heterocyclic carbene (NHC) is a prestigious ligand for transition metals due to its strong σ donation to form robust carbene-M bonds. 1 In earlier applications, NHC-M bonds were considered essentially inert, providing metal complexes with enhanced stability and stereoelectronic tunability. However, recent reports have shown that the carbene ligand may be labile under certain conditions. Öfele and co-workers first reported that irradiation of a [Cr(NHC)(CO)5] complex led to the formation of a biscarbene chromium complex [Cr(NHC)2(CO)4] in boiling THF. 2 Later, analogous Mo and W complexes were prepared by similar reactions (Figure 1, eq. 1). 3 More recently, group.