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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry