Nitrile substituents at the conjugated dipyridophenazine moiety as infra-red redox markers in electrochemically reduced heteroleptic Ru(II) polypyridyl complexes

Sumner, E., Pižl, M., McQuaid, K. T. and Hartl, F. ORCID: https://orcid.org/0000-0002-7013-5360 (2024) Nitrile substituents at the conjugated dipyridophenazine moiety as infra-red redox markers in electrochemically reduced heteroleptic Ru(II) polypyridyl complexes. Inorganic Chemistry, 63 (5). pp. 2460-2469. ISSN 0020-1669 doi: 10.1021/acs.inorgchem.3c03484

Abstract/Summary

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 IR-active redox markers. They were studied by cyclic voltammetry, UV-Vis and IR spectroelectrochemistry (SEC), and DFT 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 delocalized over both tap ligands. Accordingly, IR SEC conducted at R1 shows a large red shift of the s,as(CN) modes by –18/–28 cm–1, accompanied by 4-fold enhancement of the 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 (CN) by mere –2 cm–1 whilst the intensity enhancement reached an overall factor of 8. Comparably, a red shift of (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 (CN) shifts of –3 cm–1 and increased the absorbance by an overall factor of 6.5 and 8, respectively.