Structural, electrochemical and photophysical properties of an exocyclic di-ruthenium complex and its application as a photosensitizer

The reaction of cis-bis(2,2′-bipyridine)dichlororuthenium(ii) hydrate with a conformationally mobile bipyridyl macrocycle afforded [(bpy)₂Ru(μ-L)Ru(bpy)₂]Cl₄·6H₂O, a bridged di-Ru complex. Single crystal X-ray diffraction showed the macrocyclic ligand adopting a bowl-like structure with the exo-coordinated Ru(ii) centers separated by 7.29 Å. Photophysical characterization showed that the complex absorbs in the visible region (λ_max = 451 nm) with an emission maximum at 610 nm (τ = 706 ns, Φ_PL = 0.021). Electrochemical studies indicate the di-Ru complex undergoes three one-electron reversible reductions and a reversible one-electron oxidation process. This electrochemical reversibility is a key characteristic for its use as an electron transfer agents. The complex was evaluated as a photocatalyst for the electronically mismatched Diels-Alder reaction of isoprene and trans-anethole using visible light. It afforded the expected product in good conversion (69%) and selectivity (dr > 10:1) at low loadings (0.5-5.0 mol%) and the sensitizer/catalyst was readily recycled. These results suggest that the bipyridyl macrocycle could be widely applied as a bridging ligand for the generation of chromophore-catalyst assemblies.