METAL PI-COMPLEXES OF BENZENE DERIVATIVES, 52 - CHROMIUM SANDWICH COMPLEXES OF POLYCYCLIC AROMATIC-HYDROCARBONS - TRIPHENYLENE AND FLUORANTHENE AS ETA(6) LIGANDS
C. Elschenbroich et al., METAL PI-COMPLEXES OF BENZENE DERIVATIVES, 52 - CHROMIUM SANDWICH COMPLEXES OF POLYCYCLIC AROMATIC-HYDROCARBONS - TRIPHENYLENE AND FLUORANTHENE AS ETA(6) LIGANDS, EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, (10), 1998, pp. 1391-1401
The sandwich complexes bis(eta(6)-triphenylene)chromium (12) and bis(e
ta(6)-fluoranthene)chromium (13) have been prepared by means of metal
atom/ligand vapor cocondensation. Whereas for triphenylene exclusive c
oordination 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) sec
tion of the fluoranthene ligand occurs, leading to the isomers 13(I) (
eta(6)-B, eta(6)-B), 13(II) (eta(6)-B, eta(6)-N) and 13(III) (eta(6)-N
, eta(6)-N). Since the substitutional lability of the chromium-naphtha
lene bond largely exceeds that of the chromium-benzene bond, the isome
r 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 orientat
ion of the polycyclic aromatic hydrocarbons and to a minute metal slip
page in the peripheral direction. The triphenylene complex 12 features
the electrochemically reversible redox couples 12 (+/0, metal-centere
d), 12 (0/-, ligand-centered) and 12 (-/2-, ligand-centered), the latt
er displaying a redox splitting of 300 mV. Conversely, for the fluoran
thene complex 13, secondary reduction 13 (-/2-) is irreversible. This
finding is consistent with the larger redox splitting of ca. 480 mV wh
ich indicates more extensive interligand interaction in the dianion 13
(2-), thereby favoring metal-ligand cleavage. While the radical cation
s 12(+.) and 13(+.) are amenable to EPR study, the radical anions 12(-
.) and 13(-.) are too unstable. Instead, the radical anions of the fre
e ligands are observed by EPR upon electrochemical reduction. In the c
ase of 12, the temporary existence of the radical anion 12(-.) is indi
cated, however.