SYNTHESIS OF (ETA-6-ARENE)TRICARBONYLCHROMIUM COMPLEXES - REGIOSELECTIVE REACTION OF CHROMIUM HEXACARBONYL WITH POLYCYCLIC AROMATIC-HYDROCARBONS

The reaction of chromium hexacarbonyl with 21 substituted polycyclic aromatic hydrocarbons (PAHs) was studied. The PAHs employed for study contain two to four aromatic benzo rings and include parent PAHs, mono- and dimethylated PAHs, PAHs with one or two rings saturated, and gamma-keto-hexacyclic and phenyl-substituted derivatives. In general, a partially hydrogenated ring substituent, presumably due to its electron-donating character, directs the reaction regioselectivity to the substituted aromatic ring. This suggests that steric hindrance may not be as important a factor as electron-donating ability. Further evidence for this conclusion is that a methyl substituent, also having electron-donating character, exhibits a similar effect, whereas an electron-withdrawing keto functional group directs the reaction away from the substituted aromatic ring. On the basis of the electronic effects of the substituents observed in this study, we propose that total pi-electron density, in terms of total pi-bond order, of a benzenoid ring in a PAH is a decisive factor in determining the regioselectivity of the coordination reactions between PAHs and chromium hexacarbonyl. In contrast to previous reports that a [Cr(CO)3] group always bonds to the terminal aromatic ring of a PAH, we have found that reactions between chromium hexacarbonyl and two PAHs, 9-methylanthracene and 9,10-dimethylanthracene, occur on the central aromatic ring.