A structurally characterised redox pair involving an indigo radical: indigo based redox activity in complexes with one or two [Ru(bpy)2] fragments

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.