Fluxional Behavior and Substituent Site Preferences in Cationic Monosubstituted Tropyliumiron Tricarbonyl Complexes

The fluxional monosubstituted tropyliumiron tricarbonyl complexes, RC₇H ₆Fe(CO)₃⁺ (R = methyl, isopropyl, cyclopropyl, and phenyl), have been studied by variable temperature 1H and ^l3C NMR. Static systems can be observed at low temperatures (ca. — 100 °C) and reveal the isomer distributions in the various ions studied and thus the substituent site preferences. For R = methyl, the 2-methyl, 3-methyl, and 6-methyl (vinyl substituted) ions can be observed in a ratio of ca. 43: 7: 50 at —100 °C. For R = isopropyl, no 3 isomer is observed and the 2-isopropyl:6-isopropyl ratio is ca. 15:85. When R = phenyl, the vinyl-substituted 6 isomer is the only species observed, while in contrast for R = cyclopropyl the 2 isomer predominates (80%) with the 6-cyclopropyl isomer as the minor component (20%). The substituent site preferences indicate that there is relatively little charge on the carbons of the metal-complexed pentadienyl ring, but among the three different sites, most positive charge resides at C₂, C4. A fine balance of electronic vs. steric factors control the C₂:C₆ isomer ratio. The dynamic behavior of the complexes was studied at higher temperatures and, in general, three dynamic processes all occurring via 1, 2 iron shifts could be observed. In order of increasing activation energies, these processes were (1) degenerate isomerization of the 2 isomer with its enantiomer via the symmetrical 3 isomer, (2) degenerate interconversion of the 6 isomer with its enantiomer, and (3) the interconversion of the equilibrating 2 and 3 isomers with the 6 isomer via the unobserved 1 isomer. Detailed rates and free energies of activation for these processes are given.