Strain-Induced Photochemical Opening of Ferrocene[6]cycloparaphenylene: Uncaging of Fe2+ with Visible Light

We present the synthesis, structure and remarkable reactivity of the first carbon nanohoop that incorporates ferrocene in the macrocyclic backbone. The high strain imposed on the ferrocene by the curved nanohoop structure enables unprecedent photochemical reactivity of this otherwise photochemically inert metallocene complex. Visible light activation triggers a ring-opening of the nanohoop structure dissociating the Fe–cyclopentadienyl bond in the presence of 1,10-phenanthroline. This process uncages Fe2+ ions captured in the form of [Fe(phen)3]2+ complex in high chemical yield and can operate efficiently in a water-rich solvent. The measured quantum yields of [Fe(phen)3]2+ formation show that embedding ferrocene into a strained nanohoop boosts its photoreactivity by three orders of magnitude compared to an unstrained ferrocene macrocycle or ferrocene itself. Our data suggest that the dissociation occurs by intercepting the photoexcited triplet state of the nanohoop by the nucleophilic ligand. The strategy portrayed in this work proposes that new, tunable reactivity of analogous metallamacrocycles can be achieved with spatial and temporal control, which will aid development of responsive materials for metal ions delivery and organometallic, supramolecular, or polymer chemistry.