Palladium Complexes with Carbene and Phosphine Ligands: Synthesis, Structural Characterization, and Direct Arylation Reactions between Aryl Halides and Alkynes

The new palladium complexes with NHC and phosphine ligands, cis-PdCl2(L-2)(pPh(3)) (2), cis-PdBr2(L-2)(PPh3) (3), and cis-PdCl2(L-3)(PPh3) (4) were prepared following a general protocol of a one pot reaction between PdCl2(COD), PPh3, and the ligand precursors LH .Y(Y = Cl, Br, BF4) (L 2 = 1,3-dibenzylimidazolin-2-ylidene; L-3 = 1,3-dibeiizylimidizol-2-ylidene). The cis-PdCl2(L-3)(PCY3) Complex (5) was prepared by the ligand substitution react ion between 4 and PCY3. The palladium complexes with NHC and pyridine complexes, trans-PdCl2(L)(py) (6: L = L-2; 7: L = L-3) were obtained by heating a mixture of PdCl2(COD) and LH.BF4 in pyridine. A similar reaction condition using CH3CN as solvent with KO'Bu as base afforded cis-PdCl2(L-3)(2) (8). Complexes 2-8 were successfully characterized by X-ray crystallographic studies, among which, intriguingly, two polymorphs of 8 were obtained. Thermogravimetric analysis showed that the cis-PdX2(NHC)(PR3) Complexes are more thermally stable than the trans-PdCl2(NHC)(py) complexes. Together with the known cis-PdCl2(L-1)(PCy3) (1) (L-1 = 1-benzyl-3-(N-phenylatectamido)imidazol-2-ylidene), they were screened for the direct arylation reaction between aryl halides and alkynes. The result indicate that the carbene/phosphine complexes 1-5 are superior precatalysts than 6-8 with higher activities than the commonly-used system of Pd(OAc)(2)/2PPh(3).