INFLUENCE OF FLUORINE ON AROMATIC INTERACTIONS

Non-covalent interactions between aromatic ligands influence the confo rmations of metal complexes, and the system [M(OAr)(2)L(2)] has been u sed to investigate the difference between phenyl-phenyl, phenyl-pentaf luorophenyl and pentafluorophenyl-pentafluorophenyl interactions. X-Ra y crystal structures show that pentafluorophenyl groups adopt partiall y stacked orientations with the two aromatic rings close to parallel a nd with significant pi overlap. In contrast, phenyl groups are skewed away from each other with only edge-to-face contacts. Phenyl-pentafluo rophenyl interactions adopt a coplanar fully stacked geometry. These r esults have been rationalised on the basis of energy calculations (car ried out blind) using a variety of empirical models for treating weak non-covalent interactions. The major cause of the different behaviour of the three systems lies in the electrostatic interactions between th e pi systems.