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Lee HM, Zeng JY, Hu CH, Lee MT: A new tridentate pincer phosphine/n-heterocyclic carbene ligand: palladium complexes, their structures, and catalytic activities. Inorg Chem. 2004 Oct 18;43(21):6822-9.
A new imidazolium salt, 1,3-bis (2-diphenylphosphanylethyl)-3H-imidazol-1-ium chloride (2), for the phosphine/N-heterocyclic carbene-based pincer ligand, PC (NHC) P, and its palladium complexes were reported. The complex, [Pd (PC (NHC) P) Cl] Cl (4), was prepared by the common route of silver carbene transfer reaction and a novel direct reaction between the ligand precursor, PC (NHC) P.HCl and PdCl (2) without the need of a base. Metathesis reactions of 4 with AgBF (4) in acetonitrile produced [Pd (PC (NHC) P)(CH (3) CN)](BF (4))(2) (5). The same reaction in the presence of excess pyridine gave [Pd (PC (NHC) P)(py)](BF (4))(2) (6). The X-ray structure determination on 4-6 revealed the chiral twisting of the central imidazole rings from the metal coordination plane. In solution, fast interconversion between left- and right-twisted forms occurs. The twisting reflects the weak pi-accepting property of the central NHC in PC (NHC) P. The uneven extent of twisting among the three complexes further implies the low rotational barrier about the Pd-NHC bond. Related theoretical computations confirm the small rotational energy barrier about the Pd-NHC bond (ca. 4 kcal/mol). Catalytic applications of 4 and 5 have shown that the complexes are modest catalysts in Suzuki coupling. The complexes were active catalysts in Heck coupling reactions with the dicationic complex 5 being more effective than the monocationic complex 4. |
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