Structurally characterized dipalladium(ii)-oxamate metallacyclophanes as efficient catalysts for sustainable Heck and Suzuki reactions in ionic liquids

Abstract

A new generation of dipalladium-oxamate metallacyclophanes of formulas (n-NBu₄)₄ [Pd₂(ppba)₂] (1), (n-NBu₄)₄[Pd₂(dpvba)₂] (2), (n-NBu₄)₄[Pd₂(dpazba)₂] (3), (n-NBu₄)₄[Pd₂(dpeba)₂] (4) and (n-NBu₄)₄[Pd₂(tpeba)₂] (5) [n-NBu₄⁺ = tetra-n-butylammonium cation, H₄ppba = N,N′-1,4-phenylenebis(oxamic acid), H₄dpvba = N,N′-4,4′-diphenylethenebis(oxamic acid), H₄dpazba = N,N′-4,4′-diphenylazobis(oxamic acid), H₄dpeba = N,N′-4,4′-diphenylethynebis(oxamic acid) and H₄tpeba = N,N′-1,4-di(4-phenylethynyl)phenylenebis(oxamic acid)] was prepared. The crystal structure of the solvated species of 2–4, namely (n-NBu₄)₄[Pd₂(dpvba)₂]·6MeOH·2Et₂O (2a), (n-NBu₄)₄[Pd₂(dpazba)₂]·8MeOH (3a), and (n-NBu₄)₂[Pd₂(dpeba)₂]·8MeOH (4a), was determined by single crystal X-ray diffraction. These compounds crystallize in the triclinic space group P() and they have in common the presence of tetraanionic dipalladium(II) units corresponding to [3,3] metallacyclophane-type motifs connected by two N–Pd–N bonds. Each palladium(II) ion in 2a–4a is a four-coordinate ion with two carboxylate-oxygen and two amidate-nitrogen atoms of two fully deprotonated oxamate fragments from two bis(oxamate) ligands building a square planar surrounding, which forms a loop oligomer. The bis(oxamate) ligands exhibit a syn (trans, trans) arrangement. Because of their ionic character, the catalytic potential of dipalladium(II) compounds 1–5 was investigated for both Heck and Suzuki coupling reactions in several ionic liquids. Our results show that they are very efficient and robust catalysts, catalyzing the carbon–carbon bond forming reaction of iodoaryl with phenylboronic acid and ethyl acrylate in short times at the melting reaction temperature of n-Bu₄NBr used as a molten salt reaction medium. They remain active after at least six successive runs, without significant reductions in yields at 120 °C.