Robust and Electron-Rich cis-Palladium(II) Complexes with Phosphine and Carbene Ligands as Catalytic Precursors in Suzuki Coupling Reactions

A new imidazolinium ligand precursor [(LH)-H-2]Cl (2) was prepared in 86% yield. Compared with its imidazolium counterpart. [(LH)-H-1]Cl (1), 2 is very sensitive to moisture and can undergo ring-opening reactions very readily. Palladium complexes with the ring-opened products from imidazolinium salts were isolated and characterized by X-ray crystallography. Theoretical studies confirmed that the imidazolinium salt has a higher propensity for the ring-opening reaction than the imidazolium counterpart. New mixed phosphine/carbene palladium complexes, cis-[PdCl2(L)(PR3)] (L=L-1 and L-2 R= Ph, Cy), were successfully prepared. These complexes are highly robust as revealed by variable-temperature NMR spectroscopic studies and thermal gravimetric analysis. The structural and electronic properties of the new complexes oil varying the carbene group (imidazol-2-ylidene group (unsaturated carbene) vs. imidazolin-2-yli-dene (saturated carbene)) and the phosphine group (PPh3 vs. PCy3) were studied in detail by X-ray crystallography X-ray photoelectron spectroscopy, and theoretical calculations. The catalytic study reveals that cis-[PdCl3(L-2)-(PCy3)] is a competent Pd-II precatalyst for Suzuki coupling reactions, in Which unreactive aryl chlorides can be applied as substrates.