Non-innocence of N-heterocyclic carbene ligands:: Intermolecular C-H activation in allyl palladium NHC complexes

An unprecedented reactivity of (allyl)Pd N-Heterocyclic carbenes(NHC) X complexes was reported, where the NHC shows a truly non-innocent behavior and suffers C-H activation at the ligand backbone. Reaction of [{(allyl)PdCl}2] with the in situ generated NHC ligand derived yielded the expected mononuclear palladium complex, as was confirmed by X-ray crystallographic analysis as well as by the characteristic 13C NMR signal for the carbene C2 atom and other spectroscopic and analytical data. Using sealed NMR tubes, the evolution of propene could be clearly detected after prolonged standing or heating, which obviously results from activation of that C-H bond by the pre-coordinated (allyl)PdCl fragment. The results broaden the scope of NHC chemistry and suggest to use imidazole-based NHC units as bridging ligands for the construction of oligo- and polynuclear complexes.