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·H2O (1), [Pd(pyimypy)2][PF6]2 (4), [Pd(pyimypy)2Br]Br·2H2O (5), [Pd(pyimypy)2I][I]1/2[PF6]1/2 (6), and [Pd(pyimypy)(PPh3)2Br]Br·CH3CN·H2O (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.