An acetylene-bridged ferrocene macrocycle: efficient synthesis and electron transfer mechanism in mixed-valence systems.

Cyclic oligomers with multiple redox centers are ideal models for intramolecular electron transfer processes, as they feature well-defined spatial geometries and degenerate energy states. The design and synthesis of such structures with strongly interacting monomers, however, remains a significant challenge. Here, we report a one-pot synthesis of an acetylene-bridged ferrocene macrocycle (9) using alkyne metathesis, with a remarkable 43% isolated yield. The macrocycle adopts a chiral / conformation in the crystal, reminiscent of the iconic Penrose triangle. Electrochemical studies suggested that redox processes of all three ferrocene units are reversible and highly correlated, despite relatively long Fe-Fe distances. Hydrogenation of acetylene bridges yielded an analogous trimeric ferrocene macrocycle (14), whose redox waves showed less separation due to the lack of conjugation and through-bond charge transfer. Assuming that the through-space interaction energy is the same for both macrocycles, we estimated that conjugation through acetylene bridges accounts for 25-36% of overall interaction. Trication 9 was obtained by chemical oxidation, and it showed EPR signals with weak anisotropy, indicative of fast intramolecular electron transfer. Varied-temperature (VT) EPR studies suggested intramolecular antiferromagnetic interaction and a doublet ground state (Δ = -0.06 kcal mol) for 9.