A triangular prismatic hexanuclear iridium(iii) complex bridged by flavin analogues showing reversible redox processes

Abstract

[Ir6(μ-alloCl2 2-)3(Cp* )6(OH)3](PF6)3 (1) having 7,8-dichloroalloxazine dianion (alloCl2 2-) as bridging ligands was synthesized and characterized by X-ray crystallography, spectroscopic and electrochemical measurements. The alloxazine ligands showed unprecedented coordination modes to link the six Ir(iii) centres. The complex exhibited remarkable stability and reversible six-electron redox processes at the bridging alloxazine ligands in organic solvents. The first reversible reduction process occurred on each of three alloxazine ligands in 1 to produce a three-electron-reduced species, [IrIII 6Cp* 6(μ-alloCl2 3-)3(OH)3], and was observed as an apparent one-step reduction process at -0.65 V (vs. Fc0/+). The second reversible reduction process on each of the three alloxazine ligands in 1 was recorded at almost the same potential, -0.78 V (vs. Fc0/+), to afford the six-electron-reduced form, [Ir III 6Cp*6(μ-alloCl2 4-)3(OH)3]3-. The radical anion of the alloxazine derivative was detected by EPR measurements at room temperature. After the six-electron reduction of 1 with cobaltocene, the backward oxidation processes of reduced forms with p-chloranil were traced by UV-Vis spectroscopy to confirm the recovery of the original spectrum of 1. © 2013 The Royal Society of Chemistry.