Abstract
The reaction between indigo, H2Ind, and {Ru(bpy)2(EtOH)2}2+ in EtOH/NaOH produced the compounds [Ru(bpy)2(HInd)]ClO4 [1]ClO4, rac-{[Ru(bpy)2]2(μ-Ind)}(ClO4)2 [2](ClO4)2, and meso-{[Ru(bpy)2]2(μ-Ind)}(ClO4)3 [2](ClO4)3, which were structurally characterised, the latter as the first stable, isolable radical complex of indigo. The redox pair 22+/23+ showed little structural difference, as confirmed using DFT calculations. The redox series 1n and 2n were investigated using voltammetry and spectroelectrochemistry (EPR, UV-vis-NIR). Remarkably, the EPR results for 1, 12+, 2+ and 23+ revealed mostly ligand based spin in ruthenium(ii) complexes of the indigo-derived radical ligands HInd˙2-, HInd˙, Ind˙3- and Ind˙-, in agreement with the DFT calculated spin densities. The dominance of the frontier orbitals by the metal-stabilised indigo chromophore was also confirmed via the TD-DFT based assignment of near-infrared absorptions as intra-indigo or ligand-to-ligand charge transfer transitions.
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