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Keywords:

  • Phosphane ligands;
  • Biphenyl;
  • Palladium;
  • Suzuki–Miyaura coupling;
  • VT NMR spectroscopy;
  • Cross-coupling

Abstract

New 6,6′-dibromo- and 6,6′-bis(dimethylamino)-substituted 2,2′-diphosphanylbiphenyl ligands 1114 were prepared starting from 2,2′-dibromo-4,4′-dimethyl-6,6′-dinitro-1,1′-biphenyl (4). Depending on the phosphane groups [diphenylphosphanyl (11, 13) or diisopropylphosphanyl (12, 14)] the palladium dichloride complexes show different coordination symmetry. Whereas the smaller diphenylphosphanyl groups lead to C2-symmetric complexes, the respective bis(diisopropyl)phosphanes 12 and 14 form C1-symmetric complexes that show fluxional behavior due to the restricted rotation of the isopropyl groups as well as the exchange of atom positions within the C1-symmetric conformer. All complexes have been tested as precatalysts in the Suzuki–Miyaura cross coupling reaction of 2-bromotoluene and phenylboronic acid. The activity of the catalytic system increases with the size of the diphosphanes and the donating ability of the ligand. In contrast to C2-symmetric palladium complex 15, platinum complex 19 was found to be C1-symmetric in the solid state despite the fact that both complexes have the small bis(diphenylphosphanyl)-substituted diphosphane ligand 11 in common. NiBr2 adduct 20 with a similar diphosphane 13 exists as a mixture of distorted square-planar and tetrahedral coordination sphere geometries in equilibrium with each other.