Palladium Complexes Containing a Hemilabile Pyridylcarbene Ligand

A series of palladium complexes with a hemilabile terdentate carbene ligand, 1,3-bis(2-pyridyl)imidazol-2-ylidene (pyimypy), has been synthesized. Molecular structures of [pyimypyH]Cl · H 2 O ( 1 ), [Pd(pyimypy) 2 ][PF 6 ] 2 ( 4 ), [Pd(pyimypy) 2 Br]Br · 2H 2 O ( 5 ), [Pd(pyimypy) 2 I][I] 1/2 [PF 6 ] 1/2 ( 6 ), and [Pd(pyimypy)(PPh 3 ) 2 Br]Br · CH 3 CN · H 2 O ( 8 ) have been determined by single-crystal X-ray diffraction. In compound 4, the two bidentate pyimypy ligands are in a cis configuration and the palladium center has a highly distorted square-planar geometry. Compounds 5 and 6 have a bidentate and a monodentate pyimypy ligand. Compound 8 has two trans phosphine ligands, and the pyimypy behaves as a monodentate ligand. Compounds 4, 5, and 6 are fluxional in solution at room temperature. Halide- and solvent-assisted exchange processes involving coordinated and free pyridines are thought to be responsible for the fluxional behavior. Complex 4 is active toward the catalytic polymerization of CO/norbornylene.