Catalytic activity and fluxional behavior of complexes based on RuHCl(CO)(PPh₃)₃ and xantphos-type ligands

With RuHCl(CO)(PPh₃)₃ as the starting material, the complexes RuHCl(CO)(PPh₃)(L) were prepared for L = Xantphos and closely related ligands. Their catalytic activity in the direct amination of cyclohexanol showed large differences depending on the different backbone structures. In those complexes the Xantphos-type ligand backbones are slightly bent and display fluxionality, studied by VT-NMR. This was assigned to the "flipping" of the backbone via the bridging atoms in the xanthene backbone. Via line shape analysis of the peaks, the Gibbs free energy of activation of the flipping movement was found to be around 56 kJ/mol in all cases. However, the activation enthalpy and entropy differed considerably. Employing RuCl₂(PPh₃)₃ as the precursor resulted in the trans-coordinated complexes RuCl₂(PPh₃)(L) for L = Xantphos, Sixantphos. Fluxionality was no longer observed, due to the fact that in these complexes the O atom in the backbone also coordinates to the Ru.