Inorganic Chemistry, Vol.43, No.21, 6822-6829, 2004
A new tridentate pincer phosphine/N-heterocyclic carbene ligand: Palladium complexes, their structures, and catalytic activities
A new imidazolium salt, 1,3-bis(2-diphenylphosphanylethyl)-3H-imidazol-1-ium chloride (2), for the phosphine/N/heterocyclic carbene-based pincer ligand, (PCP)-P-NHC, and its palladium complexes were reported. The complex, [Pd(pd(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 PdCl2 without the need of a base. Metathesis reactions of 4 with AgBF4 in acetonitrile produced [Pd(pC(NHC)p) (CH3CN)](BF4)2 (5). The same reaction in the presence of excess pyridine gave [Pd(pC(NHC)p)(py)](BF4)(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 T-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.