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

Abstract

AbstractNew 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 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.