Synthesis and characterization of palladacyclopentadiene complexes with N-heterocyclic carbene ligands

New palladacyclopentadiene compounds containing different chelate NHC-thioether and NHC-pyridine ligands have been prepared by transfer of the functionalized carbenes from the respective silver complexes to the polymeric precursors [PdC–COOR)4]n (R = Me, t-Bu). Their dynamic behaviour in solution was discussed and the solid-structure of 2c was determined by X-ray crystallography. The treatment of [Pd(C–COOCH3)4]n with two equivalents of the carbene silver complexes led to the (NHC)2Pd(C4–COOCH3)4 derivatives (3c–i), a new class of compounds with only Pd–C bonds. A serious limitation to this synthetic procedure is an excessive steric crowding around the metal centre. The complexes 3 are present in solution as a mixture of two atropoisomers, due to restricted rotation around the Carbene–Pd bond. The kinetics of equilibration between the two configurational isomers was studied for complex 3c, which was also structurally defined by X-ray crystallography (anti isomer). Finally a synthetic protocol was set up for the synthesis of mixed NHC-Phosphine and NHC-Isocyanide palladacyclopentadiene complexes. In this procedure the order of addition of the reactants is of great importance.