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 [L(2)H]Cl (2) was prepared in 86 % yield. Compared with its imidazolium counterpart, [L(1)H]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-[PdCl(2)(L)(PR(3))] (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 on varying the carbene group (imidazol-2-ylidene group (unsaturated carbene) vs. imidazolin-2-ylidene (saturated carbene)) and the phosphine group (PPh(3) vs. PCy(3)) were studied in detail by X-ray crystallography, X-ray photoelectron spectroscopy, and theoretical calculations. The catalytic study reveals that cis-[PdCl(2)(L(2))(PCy(3))] is a competent Pd(II) precatalyst for Suzuki coupling reactions, in which unreactive aryl chlorides can be applied as substrates.