Strain-Induced Photochemical Opening of Ferrocene[6]cycloparaphenylene: Uncaging of Fe$^{2+}$ with Green Light

We present the synthesis, structural analysis, and remarkable reactivity of the first carbon nanohoop that fully incorporates ferrocene in the macrocyclic backbone. The high strain imposed on the ferrocene by the curved nanohoop structure enables unprecedented photochemical reactivity of this otherwise photochemically inert metallocene complex. Visible light activation triggers a ring-opening of the nanohoop structure, fully dissociating the Fe–cyclopentadienyl bonds 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 with green light excitation. The measured quantum yields of [Fe(phen)3]2+ formation show that embedding ferrocene into a strained nanohoop boosts its photoreactivity by 3 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 a nucleophilic solvent or external 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 and abet the development of responsive materials for metal ion delivery and supramolecular, organometallic, or polymer chemistry.