On the question of redox-induced haptotropic rearrangements in the electrochemical reduction of η ⁶-triolefin chromium complexes having nonaromatic ligands

The electrochemical reduction of two types of Cr(η ⁶- triolefin)(CO) ₃ complexes has been studied by voltammetry and by electrolysis. In contrast to Cr(η ⁶-arene)(CO) ₃ compounds, the bicyclic ligand in Cr(η ⁴:η ²-C ₉H ₈Ph ₂)(CO) ₃ (1a) and the fulvene-type ligand in Cr(η ⁶-C ₅H ₄CPh ₂)(CO) ₃ (2) are nonaromatic and display two separate one-electron reductions to the nominally 19e and 20e complexes: E _1/2(1) = -2.00 V and E _1/2(2) = -2.20 V for 1a, E _1/2(1) = -1.40 V and E _1/2(2) = -2.10 V for 2 in THF/0.1 M [NBu ₄][PF ₆] (potentials vs ferrocene/ferrocenium). On the basis of the IR and ESR data of 1a ^- and on the redox behavior when one or more phenyl groups in 1a are replaced by a SiMe ₃ group, 1a ^- is viewed as a delocalized metal-ligand radical that is likely to retain the η ⁴η ²:- hapticity of the bicyclic ligand. In contrast, an η ⁶/η ⁵ haptotropic rearrangement is very fast and perhaps concomitant with the 2/2 ^- electron-transfer process. The rearrangement lends thermodynamic stability to the monoanion 2 ^-, manifested as an increased separation of the two one-electron potentials (ΔE _1/2 = E _1/2(1) - E _1/2(2)) to +700 mV for 2, compared to +200 mV for 1a and the negative values previously reported for Cr(arene) complexes. The dianions 1a ^2- and 2 ^2- undergo facile protonation reactions. Whereas the protonation product 1aH ^- is reoxidized with loss of H ⁺ to regenerate la, the protonation product 2aH ^- is a stable 18-electron cyclopentadienyl-substituted compound which, when oxidized, retains its chromium-Cp character. The results are evaluated in light of the redox-induced haptotropic rearrangements previously identified for relevant metal-arene complexes.