On the Question of Redox-Induced Haptotropic Rearrangements in the Electrochemical Reduction of η6-Triolefin Chromium Complexes Having Nonaromatic Ligands

• Published in: Organometallics 2006-01, Vol.25 (1), p.275-282
• Main Authors: Garg, Anupam, Nemer, David M, Choi, Hojae, Sheridan, John B, Geiger, William E
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om058046u

The electrochemical reduction of two types of Cr(η6-triolefin)(CO)3 complexes has been studied by voltammetry and by electrolysis. In contrast to Cr(η6-arene)(CO)3 compounds, the bicyclic ligand in Cr(η4:η2-C9H8Ph2)(CO)3 (1a) and the fulvene-type ligand in Cr(η6-C5H4CPh2)(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 [NBu4][PF6] (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 SiMe3 group, 1a - is viewed as a delocalized metal−ligand radical that is likely to retain the η4:η2 hapticity of the bicyclic ligand. In contrast, an η6/η5 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 1a, 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.