Two series of electron-rich metal (Ni and Pd) complexes, namely a pyridine-coordinated tridentate NHC/amidate/ phenoxy metal complex and a tetradentate complex bearing an extra pyridyl group, were synthesized. Two carbene moieties, imidazole-based and benzimidazole-based NHCs, were also incorporated into the ligand backbones. Seven of the new complexes were structurally characterized and comparative structural analysis revealed that the tetradentate complex with a flanking pyridyl side-arm was less distorted, leading to chelation-assisted stronger NHC coordination and hence a shorter metal-carbene bond. The catalytic activities of the two sets of [a] L.

Mononuclear and tetranuclear palladium complexes with functionalized “abnormal” N-heterocyclic carbene (aNHC) ligands based on 3-phenylimidazo­[1,5-a]­pyridine were synthesized. All of the new complexes were structurally characterized by single-crystal X-ray diffraction studies. The new complexes were applied in the Mizoroki–Heck coupling reaction of aryl chlorides with alkenes in neat n-tetrabutylammonium bromide (TBAB). The mononuclear palladium complex with a tridentate phenoxy- and amidate-functionalized aNHC ligand displayed activity superior to that of the palladium complex with a bidentate amidate-functionalized aNHC ligand. The new tetranuclear complex with the tridentate ligand displayed the best activities, capable of the activation of deactivated aryl chlorides as substrates with a low Pd atom loading. Even challenging sterically demanding ortho-substituted aryl chlorides were successfully utilized as substrates. The studies revealed that the robustness of the catalyst precursor is crucial in delivering high catalytic activities. Also, the promising use of tetranuclear palladium complexes with functionalized aNHC ligands as the catalyst precursors in the Mizoroki–Heck coupling reaction in neat TBAB was demonstrated.