Title : Synthesis and characterization of Tetranuclear Palladium complexes of abnormal N-heterocyclic Carbene ligands and their catalytic application in C-C Coupling reaction
Based on the ligand scaffold of an imidazolyl/benziimidazolyl moiety and an N-CH2 C(=O)NHPh amide substituent, we prepared two novel series of ligand precursors with an ortho hydroxy group incorporated on the N-phenyl ring. We demonstrated that the structural fine tuning of the ligand scaffold allowed the facile synthesis of tetranuclear palladium complexes including the first example of such multinuclear complexes with abnormal NHC (aNHC) ligands. For those precursors with C2-phenyl group, tetranuclear complexes featuring a tridentate ligand of aNHC, amidate, and bridging phenoxide donors was produced. For those with C2-methyl group, an extra addition of pyridine was essential leading to the production of mononuclear tridentate palladium aNHC complexes or tetranuclear complexes with tridentate ligands of methylene, amidate, and bridging phenoxide donors. Representative mononuclear and tetranuclear palladium aNHC complexes were structurally characterized by X-ray diffraction studies, revealing extremely short Pd-C bond distances. The tetranuclear palladium aNHC complexes were used Mizoroki-Heck reaction in ionic salt, delivering higher activities than the mononuclear complexes. The structures of three of the new complexes were further established by single-crystal X-ray diffraction studies.
Audience Take Away:
- We demonstrated that the structural fine tuning of the ligand scaffold allowed the facile synthesis of tetranuclear palladium complexes.