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

• Published in: Inorganic chemistry Vol. 60; no. 6; pp. 3514 - 3523
• Main Authors: Taylor, James O, Pižl, Martin, Kloz, Miroslav, Rebarz, Mateusz, McCusker, Catherine E, McCusker, James K, Záliš, Stanislav, Hartl, František, Vlček, Antonín
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 15.03.2021; American Chemical Society (ACS)
• Subjects: Infrared light; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; Ligands; Light absorption; Metal to ligand charge transfer; Redox reactions
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/acs.inorgchem.0c03579

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 1 2+ , 2 2+ , and 3 2+ ) undergo four (1 2+ ) or five (2 2+ and 3 2+ ) 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 2 2+ and 3 2+ being separated by 150–210 mV; reduction of the unsubstituted bpy ligand in 1 2+ and 2 2+ 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 1 2+ 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.