Palladium Complexes Containing Potentially Chelating Pyridylidene-Type Carbene Ligands

Oxidative addition of 2-bromopyridine derivatives containing a potentially chelating donor group E (E = NMe2, SMe, SPh) to palladium(0) gives C,E-bound pyridylpalladium(II) complexes. Mono-, di-, and polymeric palladium complexes are obtained depending on the type of functionalization at the pyridyl nitrogen. With a lone pair at nitrogen, dimetallic products are isolated, while protonation gives monometallic pyridylidene-type complexes. Remarkably, N-methylation inhibits chelating ligand coordination and a one-dimensional polymer is formed instead. Heck-type arylation of styrene is used as a probe for the catalytic activity of the palladium pyridylidene complexes and reveals moderate activities. Mechanistic studies support a heterogeneous mode of action, including loss of the pyridylidene-type ligand from the metal coordination sphere. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)