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 o f three different bidentate ligands, and the Lambda- and Delta -enantiomers were resolved and characterized by CD spectroscopy. Like other phi complex es, this tris(heteroleptic) iridium complex binds avidly to DNA by intercal ation. Electrochemical studies show that Ir(bpy)(phen)(phi)(3+) undergoes a reversible one-electron reduction at E-0 = -0.025 V in 0.1 M TBAH/DMF (ver sus Ag/AgCl), and spectroelectrochemical studies indicate that this reducti on is centered on the phi ligand. The EPR spectrum of electrochemically gen erated lr(bpy)(phen)(phi)(2+) is consistent with a phi-based radical. The e lectrochemistry of Ir(bpy)(phen)(phi)(3+) was also probed at a DNA-modified electrode, where a DNA binding affinity of K = 1.1 x 10(6) M-1 was measure d. In contrast to Ir(bpy)(phen)(phi)(3+) free in solution, the complex boun d 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 tha t disproportionation of electrochemically generated lr(bpy)(phen)(phi)(2+) occurs upon DNA binding. These results underscore the rich redox chemistry associated with metallointercalators bound to DNA.