Accessing Chelating Extended Linker Bis(NHC) Palladium(II) Complexes: Sterically Triggered Divergent Reaction Pathways

We have demonstrated the profound effect that the N-substituent steric bulk of ethylene-linked bis­(N-heterocyclic carbene) (bis­(NHC)) species has on the mechanism of their formation via Pd­(OAc)2-assisted deprotometalations. The binding mode of an ethylene-linked bis­(NHC) ligand is chelating when it bears N-2,4-Me2C6H3 substituents but forms a mono­(NHC) with a pendant imidazolium group for N-2,6-iPr2C6H3. An N-mesityl-substituted bis­(NHC), with steric bulk intermediate between the two aforementioned substituents, generates a mixture of two chelating homoleptic normal/normal and heteroleptic normal/abnormal bis­(NHC) complexes, as predicted by DFT calculations. Interestingly, the latter was seen to undergo facile reductive elimination, affording a novel tricyclic monodentate NHC ligand.