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.