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.