Bis(imidate)palladium(ii) complexes with labile ligands. Mimics of classical precursors?

Abstract

A novel synthetic route to prepare palladium(II) precursor analogous of classical [Pd(Cl)₂(solvent)₂] has been developed. Just stirring Pd₃(AcO)₆ in dimethyl sulfide at room temperature, in the stoichiometric presence of protic imidate ligands, resulted in the precipitation of the desired complexes [Pd(imidate)₂(SMe₂)₂] (imidate = succinimidate (succ) 1, phthalimidate (phthal) 2, maleimidate (mal) 3, saccharinate (sac) 4 or glutarimidate (glut) 5). The new complexes are very soluble in common solvents and have been fully characterized, including an X-ray diffraction analysis of 2. Analogous reactions with succinimide in acetonitrile or dimethylsulfoxide produced [Pd(succinimidate)₂(solvent)₂] (6 and 7, respectively) as off-white powders. Thermal decomposition of 6 produces a new species 6* with bridging imidate ligands that can be formulated as a trimer similar to Pd₃(AcO)₆. The usefulness of 1–5 as precursors has been tested by reactions against monodentated neutral donor ligands, PPh₃ (a compounds), or pyridine (py, b compounds), to produce ten new derivatives of the general formula trans-[Pd(imidate)₂(L)₂]. The single-crystal structures of compounds 2a, 3a, 4a, 4a′, 5a and 4b have also been established, allowing an interesting molecular and supramolecular structural discussion. A cis-conformation was induced when the bidentate chelate ligand 1,2-bis(diphenylphosphino)benzene (dppb, c compounds) was made to react with 1–5. Structural characterization by X-ray diffraction of complex 2c confirmed the proposed formula. Catalytic activity in Suzuki–Miyaura cross-coupling of aryl bromides and benzyl bromides with aryl boronic acids has been tested.