Phosphane-Ligated Ionic Palladium Complexes: Synthesis, Characterization and Application as Efficient and Reusable Precatalysts for the Homogeneous Carbonylative Sonogashira Reaction under CuI-Free Conditions

Graphical 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. The stable ionic Pd II complexes 1A – 4A were synthesized through the complexation of PdCl 2 (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 – , CF 3 SO 3 – ) counteranions. The complex cations in 1A – 3A possessed structural similarity to trans -PdCl 2 (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]BF 4 (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.