Rigid pyridyl substituted NHC ligands, their Pd(0) complexes and their application in selective transfer semihydrogenation of alkynes

The synthesis of an air-stable series of Pd-0 complexes with unsymmetric bidentate N-pyridine N-heterocyclic carbene ligands has been described. The carbenes were generated by synthesis of the silver( I) complexes from the imidazolium salts, followed by transmetallation of the C-N ligands to obtain the target electron-rich zerovalent palladium compounds. The bidentate coordination behaviour of the ligands was confirmed by H-1, 13-C NMR and X-ray spectroscopy. The complexes are active precatalysts for the selective transfer semihydrogenation of alkynes to Z-alkenes, with selectivities up to 99%.