Optical and Infrared Spectroelectrochemical Studies of CN-Substituted Bipyridyl Complexes of Ruthenium(II)

Ruthenium(II) polypyridyl complexes [Ru(CN-Me-bpy)x(bpy)3–x]2+ (CN-Me-bpy = 4,4′-dicyano-5,5′-dimethyl-2,2′-bipyridine, bpy = 2,2′-bipyridine, and x = 1–3, abbreviated as 12+, 22+, and 32+) undergo four (12+) or five (22+ and 32+) successive one-electron reduction steps between −1.3 and −2.75 V versus ferrocenium/ferrocene (Fc+/Fc) in tetrahydrofuran. The CN-Me-bpy ligands are reduced first, with successive one-electron reductions in 22+ and 32+ being separated by 150–210 mV; reduction of the unsubstituted bpy ligand in 12+ and 22+ occurs only when all CN-Me-bpy ligands have been converted to their radical anions. Absorption spectra of the first three reduction products of each complex were measured across the UV, visible, near-IR (NIR), and mid-IR regions and interpreted with the help of density functional theory calculations. Reduction of the CN-Me-bpy ligand shifts the ν(C≡N) IR band by ca. −45 cm–1, enhances its intensity ∼35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2+, 3+, and 30 show distinct ν(C≡N) features due to the presence of both CN-Me-bpy and CN-Me-bpy•–, confirming that each reduction is localized on a single ligand. NIR spectra of 10, 1–, and 2– exhibit a prominent band attributable to the CN-Me-bpy•– moiety between 6000 and 7500 cm–1, whereas bpy•–-based absorption occurs between 4500 and 6000 cm–1; complexes 2+, 3+, and 30 also exhibit a band at ca. 3300 cm–1 due to a CN-Me-bpy•– → CN-Me-bpy interligand charge-transfer transition. In the UV–vis region, the decrease of π → π* intraligand bands of the neutral ligands and the emergence of the corresponding bands of the radical anions are most diagnostic. The first reduction product of 12+ is spectroscopically similar to the lowest triplet metal-to-ligand charge-transfer excited state, which shows pronounced NIR absorption, and its ν(C≡N) IR band is shifted by −38 cm–1 and 5–7-fold-enhanced relative to the ground state.