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

Abstract

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(CN) 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(CN) 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(CN) IR band is shifted by -38 cm-1 and 5-7-fold-enhanced relative to the ground state.