Synthesis and spectroelectrochemistry of Ir(bpy)(phen)(phi)³⁺, a tris(heteroleptic) metallointercalator

A tris(heteroleptic) phenanthrenequinone diimine (phi) complex of Ir(III), Ir(bpy)(phen)(phi)³⁺, was synthesized through the stepwise introduction of three different bidentate ligands, and the Λ- and Δ-enantiomers were resolved and characterized by CD spectroscopy. Like other phi complexes, this tris(heteroleptic) iridium complex binds avidly to DNA by intercalation. Electrochemical studies show that Ir(bpy)(phen)(phi)³⁺ undergoes a reversible one-electron reduction at E₀ = -0.025 V in 0.1 M TBAH/DMF (versus Ag/AgCl), and spectroelectrochemical studies indicate that this reduction is centered on the phi ligand. The EPR spectrum of electrochemically generated Ir(bpy)(phen)(phi)²⁺ is consistent with a phi-based radical. The electrochemistry of Ir(bpy)(phen)(phi)³⁺ was also probed at a DNA-modified electrode, where a DNA binding affinity of K = 1.1 × 10⁶ M^-1 was measured. In contrast to Ir(bpy)(phen)(phi)³⁺ free in solution, the complex bound to DNA undergoes a concerted two-electron reduction, to form a diradical species. On the basis of UV-visible and EPR spectroscopies, it is found that disproportionation of electrochemically generated Ir(bpy)(phen)(phi)²⁺ occurs upon DNA binding. These results underscore the rich redox chemistry associated with metallointercalators bound to DNA.