Synthesis of the First C-2 Cyclopalladated Derivatives of 1,3-Bis(2-pyridyl)benzene. Crystal Structures of [Hg(N-C-N)Cl], [Pd(N-C-N)Cl], and [Pd₂(N-C-N)₂(μ-OAc)]₂[Hg₂Cl₆]. Catalytic Activity in the Heck Reaction

Abstract

The synthesis of C-2 cyclopalladated derivatives of 1,3-bis(2-pyridyl)benzene (N-CH-N) has been achieved for the first time through a transmetalation reaction. C-2 mono- and dinuclear complexes have been obtained by reaction of Pd(OAc)2 and the organomercury(II) compound [Hg(N-C-N)Cl] (1) as precursors. The new species 2−11 were characterized in solution and, in the case of [Pd(N-C-N)Cl] (2) and [Pd2(N-C-N)2(OAc)]2[Hg2Cl6] (11) as well as of [Hg(N-C-N)Cl] (1), in the solid state. The X-ray crystal structures unambiguously show the pincer behavior of the N-C-N ligand in the palladium species and the linear coordination C−Hg−Cl in the mercury precursor. Displacement of the coordinated halide gives cationic species such as [Pd(N-C-N)(H2O)][BF4] (5) or, in the absence of donor solvents or anions, the dinuclear halide-bridged species [Pd2(N-C-N)2(μ-X)][BAr‘4] (X = Cl (6), Br (7), I (8); Ar‘ = 3,5-(CF3)2-C6H3). For comparison purposes [Pd(N-C-N)(OAc)] (9) and [Pd2(N-C-N)2(μ-OAc)][BAr‘4] (10) were also prepared. The mononuclear complexes 2, 4, 5, and 9 are high-performance catalysts in the Heck reaction of iodobenzene and methyl acrylate, even at moderate temperatures. Under microwave heating a turnover frequency as high as 280 000 h-1 is observed with complex 2 at the nominal temperature of 135 °C.