Synthesis of palladium(0) and -(II) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation

A transmetallation route, using silver(I) precursors, to several zero- and di-valent palladium complexes with chelating bis(N-heterocyclic carbene) ligands bearing various N-substituents has been established. The resulting complexes have been characterized by NMR and mass spectroscopy. In addition, the structure of a representative compound, [Pd0(bis-(Mes)NHC)(η2-ma)] (3a), was confirmed by X-ray crystal structure determination. In contrast to the transfer semihydrogenation, in which only low activity was observed, complex 3a showed activity (TOF = 49 molsub molcat −1 h−1) and selectivity comparable to its monodentate counterparts in the semihydrogenation of 1-phenyl-1-propyne with molecular hydrogen.