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 C≡N 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 ν s,as (C≡N) modes by -18/-28 cm -1 , accompanied by a 4-fold enhancement of the ν s (C≡N) 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(C≡N) by merely -2 cm -1 , while the intensity enhancement reached an overall factor of 8. Comparably, a red shift of ν(C≡N) 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(C≡N) shifts of -3 cm -1 and increased the absorbance by overall factors of 6.5 and 8, respectively.