Synthesis of 6,6′-Bis(dimethylamino)- and 6,6′-Dibromo-Substituted 2,2′-Diphosphanylbiphenyls and Their Palladium Complexes

New 6,6-dibromo- and 6,6-bis(dimethylamino)-substituted 2,2-diphosphanylbiphenyl ligands 11-14 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 C-2-symmetric complexes, the respective bis(diisopropyl)phosphanes 12 and 14 form C-1-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 C-1-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 C-2-symmetric palladium complex 15, platinum complex 19 was found to be C-1-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.