Biscarbene palladium(II) complexes. Reactivity of saturated versus unsaturated N-heterocyclic carbenes

A series of designed palladium biscarbene complexes including saturated and unsaturated N-heterocyclic carbene (NHC) moieties have been prepared by the carbene transfer methods. All of these complexes have been characterized by 1H and 13C NMR spectroscopy as well as X-ray diffraction analysis. The reactivity of Pd−C(saturated NHC) is distinct from that of Pd−C(unsaturated NHC). The Pd−C(saturated NHC) bonds are fairly stable toward reagents such as CF3COOH, AgBF4 and I2, whereas Pd−C(unsaturated NHC) bonds are readily cleaved under the similar conditions. Notably, the catalytically activity of these palladium complexes on Suzuki−Miyaura coupling follows the order: (sat-NHC)2PdCl2 > (sat-NHC)(unsat-NHC)PdCl2 > (unsat-NHC)2PdCl2.