Flexible Diphosphine Ligands with Overall Charges of 0, +1, and +2: Critical Role of the Electrostatics in Favoring Trans over Cis Coordination

The influence of the formal electrostatic interaction on the cis/trans coordination mode at a PdCl2 center is investigated in a family of isostructural flexible diphosphine ligands Ph2P–X–C6H4–Y–PPh2, where X and Y stand for neutral or cationic N,C-imidazolylene linkers. While the neutral and monocationic diphosphine spontaneously behave as classical cis-chelating ligands, only the dicationic diphosphine, where the electrostatic repulsion between the formal positive charges specifically takes place, is observed to behave as a trans-chelating ligand. The crucial role of electrostatics is analyzed on the basis of 31P NMR data in solution and X-ray diffraction data in the crystal state. Comparative theoretical studies of the cis- and trans-chelated complexes, including EDA, static 31P NMR, MESP, and AIM analyses, have been undertaken on the basis of DFT calculations in the gas phase or in the acetonitrile continuum. Whereas the cis-coordination mode is shown to be thermodynamically favored for the neutral ligand, the trans-coordination mode is found to be preferred for the dicationic homologue. The stereochemical preference is thus shown to be parallel to the expected effect of the formal electrostatic interaction. The results open perspectives for control of the cis- and trans-chelating behavior of flexible bidentate ligands by more or less reversible charge transfer at the periphery of the coordination sphere of a metallic center.