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

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Abstract

The new palladium complexes with NHC and phosphine ligands, cis-PdCl2(L2)(PPh3) (2), cis-PdBr2(L2)(PPh3) (3), and cis-PdCl2(L3)(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) (L2 = 1,3-dibenzylimidazolin-2-ylidene; L3 = 1,3-dibenzylimidazol-2-ylidene). The cis-PdCl2(L3)(PCy3) complex (5) was prepared by the ligand substitution reaction between 4 and PCy3. The palladium complexes with NHC and pyridine complexes, trans-PdCl2(L)(py) (6: L = L2; 7: L = L3) were obtained by heating a mixture of PdCl2(COD) and LH·BF4 in pyridine. A similar reaction condition using CH3CN as solvent with KOtBu as base afforded cis-PdCl2(L3)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(L1)(PCy3) (1) (L1 = 1-benzyl-3-(N-phenylacetamido)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)2/2PPh3.

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