π-Complexed polyfluoroarenes: a reactivity, bonding and spectroscopic study of (η 6 -C 6 F 6 )Cr(η 6 -C 6 H 6 ) and related molecules

The syntheses, structures and reactivity of fluorinated di-arene chromium sandwich complexes (chromarenes) are reviewed, and their bonding characteristics probed by an analysis of their spectroscopic properties. In particular, density functional theory, together with a detailed vibrational study of (C6F6)Cr(C6H6), has been used to study a system containing the very rare hexahapto-coordinated hexafluorobenzene ligand. The structural, reactivity and spectroscopic data yield a consistent picture whereby electron density is transferred from the chromium to both rings, but to a much greater extent to the fluorinated ligand. The result is a marked decrease in the stretching and bending vibrational modes of the fluorinated ring, and also dramatic changes in the 19F NMR chemical shifts and coupling constants. This contrasts with the situation in the organic mixed arene system C6F6/C6H6 where intermolecular interactions are rather minimal.