engleski

Cyclopalladation mechanism with azobenzene ligands

Three new complexes, A, B and C, each with two azobenzene ligands bonded to one or two palladium atoms, have been prepared. A and B were precipitated from CH_2Cl_ and C from CH_3OH. It has been found that in DMF solution the complexes A and B rearrange into C. The dynamics of the rearrangement, as well as of the interaction of azobenzene ligands with various Pd(II) chloro-compounds, has been studied by 1H-NMR and UV-VIS spectroscopies in order to explain the mechanism of the reactions. It has been found that there exists a complex equilibrium involving free Pd(II) compounds and their dimers, free ligand, and various complexes with Pd coordinated by one of the two azo-nitrogens. The rearrangement of A and B into C begins by substitution of one ligand with the solvent molecule (DMF), resulting in the complex with only one azobenzene ligand. Such an intermediate has been also found when starting with the free ligand and palladium(II) chloro-compound, so the rearrangement mechanism of A and B reduces to the interaction of free azobenzene ligands with the palladium(II) species. This interaction is largely affected by the kind of the palladium(II)-compound. In DMF, PdCl_2X_2, X=DMSO or CH_3CN, forms dimer complexes bridged by two chlorine atoms, while e.g. /PdCl_4/^2- stays in the monomer form. The dimer Pd(II)-species reacts with the azobenzene ligands much more rapidly than the monomer and produces the complex which subsequently releases one Pd(II) subunit, probably by replacement with the solvent molecule. The resulting complex enters the cyclopalladation reaction in which the organometallic bond between the Pd and aromatic carbon is formed with simultaneous release of HCl. The mechanistic details of this reaction and of the dimerization into the complex of type C are still being studied.

cyclopalladation; mechanism; azobenzene

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