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

Abstract

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 PPM/MMP 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 9³⁺ 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 (ΔE_D–Q = −0.06 kcal mol⁻¹) for 9³⁺.