Chromium Sandwich Complexes of Polycyclic Aromatic Hydrocarbons: Triphenylene and Fluoranthene as η6 Ligands

Abstract

The sandwich complexes bis(η6-triphenylene)chromium (12) and bis(η6-fluoranthene)chromium (13) have been prepared by means of metal atom/ligand vapor cocondensation. Whereas for triphenylene exclusive coordination to the peripheral rings is observed, the situation is more complicated for fluoranthene. According to NMR evidence initial metal coordination to the benzene (B) as well as to the naphthalene (N) section of the fluoranthene ligand occurs, leading to the isomers 13(I) (η6-B, η6-B), 13(II) (η6-B, η6-N) and 13(III) (η6-N, η6-N). Since the substitutional lability of the chromium–naphthalene bond largely exceeds that of the chromium–benzene bond, the isomer distribution depends on the workup conditions; 13(I) is clearly the most stable isomer. Crystal structure determinations performed for the salts [12][BPh4] and [13][I] point to the preference for syn orientation of the polycyclic aromatic hydrocarbons and to a minute metal slippage in the peripheral direction. The triphenylene complex 12 features the electrochemically reversible redox couples 12 (+/0, metal-centered), 12 (0/–, ligand-centered) and 12 (–/2–, ligand-centered), the latter displaying a redox splitting of 300 mV. Conversely, for the fluoranthene complex 13, secondary reduction 13 (–/2–) is irreversible. This finding is consistent with the larger redox splitting of ca. 480 mV which indicates more extensive interligand interaction in the dianion 132–, thereby favoring metal–ligand cleavage. While the radical cations 12+· and 13+· are amenable to EPR study, the radical anions 12–· and 13–· are too unstable. Instead, the radical anions of the free ligands are observed by EPR upon electrochemical reduction. In the case of 12, the temporary existence of the radical anion 12–· is indicated, however.