Examination of reaction intermediates in the synthesis of sterically demanding bis(NHC) palladium complexes

Palladium N-heterocyclic carbene (NHC) complexes are used in a wide range of catalytic applications, often affording similar reactivity to phosphine ligands, with added stability and activity. The synthesis of numerous linked bis(NHC) palladium complexes have been reported by various authors, however there have been relatively limited studies of the mechanism of formation of these linked bis(NHC) complexes. These complexes have proven difficult to produce by a single synthetic method, where minor changes to the ligand including varying N-substituents and increasing the linker length can affect the synthetic requirements of forming these complexes. The synthesis of the t-butyl methylene linked di-imidazolium salt, and the associated pendant and chelating complexes has been previously reported. In a recent continuation of this area we have observed that alterations to the linker length have resulted in unusual reactivity, with one analogue forming a mixed normal/abnormal dicarbene system. Computational studies of this analogue have provided insight into the reaction mechanism. We are examining the hydrogen bonding of the pendant imidazolium intermediate previously discussed in comparison to other analogues in order to see if this intermediate plays a role in directing the synthetic pathway. In order to probe the nature of this bonding we undertook single crystal neutron diffraction studies to accurately locate the hydrogen atom position on a series of analogues.