Oxine based unsymmetrical (O−, N, S/Se) pincer ligands and their palladium(II) complexes: synthesis, structural aspects and applications as a catalyst in amine and copper-free Sonogashira coupling
Unsymmetrical pincer ligands having an 8-hydroxyquinoline (oxine) core viz. 2-(phenylthio/selenomethyl) quinolin-8-ol (L1/L2), 2-(N,N-dimethylthiocarbamoyl) quinolin-8-ol (L3) and 2-(pyrrolidin-1-ylthiocarbamoyl) quinolin-8-ol (L4) were synthesized. 2-Methylquinolin-8-ol was converted to 2-bromomethylquninolin-8-ol, which reacted with PhENa (E = S or Se) to give L1 and L2, and the Willgerodt–Kindler reaction on an appropriate aldehyde derivative of quinoline gave L3 and L4. Upon reaction with Na2PdCl4/[Pd(CH3CN)2Cl2], L1–L4 coordinated as a (O−, N, E) donor (E = S/Se) resulting in complexes [Pd(L–H)Cl] (1–4; L = L1–L4). The molecular structures of L1, 1 and 2 were established by single crystal X-ray diffraction. The palladium in 1 and 2 has a nearly square planar geometry. The Pd–S bond distance in 1 is 2.2648(14) Å and in 2, the Pd–Se bond distance is 2.3641(7) Å. Somewhat rare weak interactions (viz. C–H⋯Pd and Se⋯Cl) were noticed in the crystals of 1 and 2, respectively. Complexes 1 and 2 were found to be efficient in catalyzing Sonogashira coupling under amine and copper free conditions. The catalyst loading of 0.5–1.0 mol% was found to be promising for the conversion of several aryl halides to their coupled products. The yields were lower for ArCl in comparison to ArBr/ArI. The catalytic activity of 1 was marginally lower than that of 2. DFT calculations support the catalytic activity order and bond lengths and angles of 1 and 2.