Topological effects on intramolecular electron transfer via quantum interference

The three isomers of diferrocenylbenzenes (ortho, 1o; meta, 1m; para, 1p) as well as 5-substituted derivatives of m-diferroceylbenzene with R = NH2 (2), Cl (3), CH3 (4), CN (5), NO2 (6), and N(CH3)(3)(3+) (7) have been prepared. Crystal structures of 10, 3, and 5 have been solved. In 3 and 5, the cyclopentadienyl rings are nearly parallel to the benzene mean planes with angles ranging from 9.99(5)degrees to 14.74(5)degrees. One ferrocene group is above and the other below the mean molecular plane. For lo, there is an important twist between the benzene and cyclopentadiene rings (68.6(8)degrees and 32.5(8)degrees) for steric reasons. Controlled potential electrolysis yields the mixed-valence ferrocene/ ferrocenium species in comproportionation equilibrium with homovalent species. Intervalence transitions have been observed and corrected from comproportionation. From the intervalence band parameters, metal-metal couplings (V-ab) are calculated using Hush's equation. The values are much higher for 1o (0.025 eV) and 1p (0.043 eV) than for 1m (0.012 eV) and exhibit little or no variation for the substituted m-diferrocenylbenzenes 2-6. These results are rationalized by extended Huckel molecular orbital calculations. The weakness of the interaction in Im can be ultimately traced to a quantum Interference effect, i.e., a cancellation of the contributions of two electron transfer paths. This cancellation occurs because each path implies a mixing of metal orbitals with a different ligand orbital, and the resulting molecular orbitals exhibit different symmetries.