Nitrile Substituents at the Conjugated Dipyridophenazine Moiety as Infrared Redox Markers in Electrochemically Reduced Heteroleptic Ru(II) Polypyridyl Complexes

Ruthenium(II) complexes [Ru(tap)2(NN)]2+ (tap = 1,4,5,8-tetraazaphenanthrene, NN = 11-cyano-dipyrido[3,2-a:2′,3′-c]phenazine (11-CN-dppz) and 11,12-dicyano-dipyrido[3,2-a:2′,3′-c]phenazine (11,12-CN-dppz)) feature the CN groups as infrared (IR)-active redox markers. They were studied by cyclic voltammetry, UV–vis, and IR spectroelectrochemistry (SEC), and density functional theory calculations to assign the four 1e– reduction waves R1–R4 observed in dichloromethane. Generally, the NN ligands are reduced first (R1). For [Ru(tap)2(11,12-CN-dppz)]2+, R1 is sufficiently separated from R2 and delocalized over both tap ligands. Accordingly, IR SEC conducted at R1 shows a large red shift of the νs,as(CN) modes by −18/–28 cm–1, accompanied by a 4-fold enhancement of the νs(CN) intensity, comparably with reference data for free 11,12-CN-dppz. The first tap-based reduction of spin-doublet [Ru(tap)2(11,12-CN-dppz)]+ to spin-triplet [Ru(tap)2(11,12-CN-dppz)] at R2 decreased ν(CN) by merely −2 cm–1, while the intensity enhancement reached an overall factor of 8. Comparably, a red shift of ν(CN) by −27 cm–1 resulted from the 1e– reduction of [Ru(tap)2(11-CN-dppz)]2+ at R1 (poorly resolved from R2), and the intensity enhancement was roughly 3-fold. Concomitant 1e– reductions of the tap ligands (R2 and R3) caused only minor ν(CN) shifts of −3 cm–1 and increased the absorbance by overall factors of 6.5 and 8, respectively