Synthesis of bis(N-heterocyclic carbene) palladium complexes derived from (S,S)-1,2-bis(1-hydroxypropyl)benzene

New o-xylylene-linked bis(benzimidazolium) salts were synthesized in six-steps from C2-symmetric chiral 1,4-diol, 1,2-bis(1-hydroxypropyl)benzene, as a starting material. The silver complex of bis(benzimidazol-2-ylidene) was obtained on treatment of bis(benzimidazolium) salt with silver oxide. The reaction of the silver bis-NHC with [PdCl2(PhCN)2] afforded the bis-NHC complex of palladium. The X-ray diffraction studies on Pd complexes revealed that these complexes have distorted square planar geometry around the Pd center coordinating the NHC ligand in mutually cis-position. The arene ring of o-xylylene unit hanged over the Pd center and thus these complexes showed C1-symmetric structures. The variable temperature NMR spectroscopy revealed that these Pd complexes showed fluxional behavior between C1- and C2-symmetric structures in solution state.

Graphical abstractNew chiral o-xylylene-linked bis(benzimidazolium) pro-ligands and their palladium complexes were synthesized from C2-symmetric chiral 1,4-diol, (S,S)-1,2-bis(1-hydroxypropyl)benezene, as a starting material. Pd complexes showed C1-symmetric structure by the interaction between π-electrons on the o-xylylene unit and the Pd center in solid state. In solution, these Pd complexes showed the fluxional behavior between C2- and C1-symmetric structures by the flipping motion of the o-xylylene unit.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Synthesis of new chiral o-xylylene-linked bis-NHC palladium complexes. ► Pd complexes show C1-symmetric structure in solid state. ► Showing the fluxional behavior in solution between C1- and C2-symmetric structure.