A Mixed-Ring Sandwich Complex from Unexpected Ring Contraction in [Re(η6‑C6H5Br)(η6‑C6R6)](PF6)

The contraction of coordinated aromatic hydrocarbons is a rare reactivity pattern in organometallic chemistry. We describe the conversion of a bromobenzene coordinated to a ReI center into a cyclopentadienyl aldehyde. Under mildly alkaline conditions, the expected phenol complex is formed with Re and 99Tc but under strong basic conditions; ring contraction occurs in close to quantitative yields for Re only. A mechanism for this unprecedented reaction is proposed based on 1H and 2H NMR spectra and density functional theory calculations.