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 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 nu(C N) IR band by ca. -45 cm(-1), enhances its intensity similar to 35 times, and splits the symmetrical and antisymmetrical modes. Semireduced complexes containing two and three CN-derivatized ligands 2(+), 3(+), and 3(0) show distinct nu(C N) features due to the presence of both CN-Me-bpy and CN-Me-bpy(center dot-), confirming that each reduction is localized on a single ligand. NIR spectra of 1(0), 1(-), and 2(-) exhibit a prominent band attributable to the CN-Me-bpy(center dot-) moiety between 6000 and 7500 cm(-1), whereas bpy(center dot-)-based absorption occurs between 4500 and 6000 cm(-1); complexes 2(+), 3(+), and 3(0) also exhibit a band at ca. 3300 cm(-1) due to a CN-Me-bpy(center dot-) -> CN-Me-bpy interligand charge-transfer transition. In the UV-vis region, the decrease of pi -> pi* 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 nu(C N) IR band is shifted by -38 cm(-1) and 5-7-fold-enhanced relative to the ground state.

Automated summary

Ruthenium(II) polypyridyl complexes undergo four to five successive one-electron reduction steps in tetrahydrofuran, starting with the reduction of CN-Me-bpy ligands followed by the unsubstituted bpy ligands. Absorption spectra reveal distinct features of each reduction localized on a single ligand, with significant changes in IR and UV-vis regions indicating the transformation of the complexes.