Phosphane-ligated ionic palladium complexes: Synthesis, characterization and application as efficient and reusable precatalysts for the homogeneous carbonylative sonogashira reaction under CuI-free conditions

The stable ionic Pd^II complexes 1A-4A were synthesized through the complexation of PdCl₂(MeCN)₂ with the phosphane-functionalized ionic liquids (FILs) 1-4 with π-acceptor character. Single-crystal X-ray diffraction analyses showed that 1A-4A were all composed of Pd^II-centred square-planar cations and triflate (OTf^-, CF₃SO₃^-) counteranions. The complex cations in 1A-3A possessed structural similarity to trans-PdCl₂(PPh ₃)₂. The cation in 4A was a new Pd^II-centered planar complex ligated by the phosphane-carbon anion-carbene (PCC) pincer in a tripodal manner. The stabilities of 1A-4A were improved due to the intensive π-backbonding interaction between the Pd and P atoms. When complexes 1A-4A were used as precatalysts for homogeneous carbonylative Sonogashira coupling of PhI with phenylacetylene free of CuI, 1A-3A exhibited excellent catalytic behaviour with a TON of up to 1700 at a CO pressure of 0.5 MPa and moderate temperature of 100 °C, whereas 4A exhibited poor activity towards the transformation of PhI due to the high stability of 4A. The recycling experiments of 3A in [Bpy]BF₄ (as a solvent) indicated that 3A could be recycled for 5 runs with neither activity loss nor metal leaching into the organic phase. Complex 3A also proved to be a general precatalyst for the carbonylative Sonogashira couplings of a wide range of aryl iodides with phenylacetylene. The selectivity of the desired carbonylation product depended more on electronic effects than the steric effects of the substituents of the aryl iodides. The ionic palladium complexes 1A-4A ligated by imidazolium-based-phosphanes were prepared and shown, by single-crystal X-ray diffraction analyses, to be composed of Pd^II-centered square-planar cations and OTf^- counteranions. The complexes were tested as precatalysts for the homogeneous carbonylative Sonogashira coupling of aryl iodides with phenylacetylene.