A new synthesis for thermolabile low-valent palladium complexes by electron transfer reactions from nickel(0) to palladium(II) compounds

The nickel(0) complex [Ni(bpy)(cod)] (bpy: 2,2′-bipyridine, cod: cycloocta-1,5-diene) was used as a mild reducing reagent for the synthesis of the extremely reactive low-valent palladium complexes [Pd2X2(cod)2] (1: X = Cl, 2: X = Br), Pd(cod)2 (3) and Pd(norbornene)3 (4). The X-ray analysis of 1 showed that the two [Pd(cod)(Cl)] moieties are only connected by a short Pd(I)–Pd(I) bond (bond length: 2.5379(4) Å) with the chloride ions as monodentate ligands. The X-ray structure of 3 which is also known to be an extremely reactive compound could be determined by X-ray diffraction. As expected, the Pd(0) centre is surrounded by the two cod ligands to form a PdC4 tetrahedron with typical Pd–C bond lengths. The crystal structure of 3 shows it to be very similar to the closely related complexes M(cod)2 (M: Ni, Pt). The X-ray structure of 4 displays that the Pd(0) centre is in a trigonal planar environment of the three olefin groups. According to 1H NMR measurements the complexes have the same structure in solution as found in the solid state.

Graphical abstractThe nickel(0) complex [Ni(bpy)(cod)] (bpy: 2.2′-bipyridine, cod: cycloocta-1.5-diene) was used as a mild reducing reagent for the synthesis of the extremely reactive low-valent palladium complexes [Pd2X2(cod)2] (1: X = Cl, 2: X = Br), Pd(cod)2 (3) and Pd(norbornene)3 (4). Compounds 1–4 were structurally characterized by 1H NMR spectroscopy and X-ray analyses.Figure optionsDownload full-size imageDownload as PowerPoint slide