Journal
EUROPEAN JOURNAL OF INORGANIC CHEMISTRY
Volume 2020, Issue 37, Pages 3601-3611Publisher
WILEY-V C H VERLAG GMBH
DOI: 10.1002/ejic.202000530
Keywords
Carbenes; Heck reaction; Homogeneous catalysis; Tridentate ligands
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Funding
- Ministry of Science and Technology of Taiwan [MOST 108-2113-M-018-001]
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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 palladium complexes for the Mizoroki-Heck coupling reaction were investigated. The comparative data showed that despite the higher thermal stability and the presence of the flanking hemilabile pyridyl side-arm which provided unsaturation to the metal center, the catalytic performance was slightly inferior to that of the tridentate palladium complex with a throw-away pyridine ligand. The tridentate palladium complex with the benzimidazole-based NHC moiety was the most effective catalyst, allowing the utilization of a range of aryl chlorides including deactivated substrates with a low 0.2 mol-% Pd loading in moltenn-tetrabutylammonium bromide.
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