Chemistry of Azoimidazoles: Synthesis, Spectral Characterization, Electrochemical Studies, and X-ray Crystal Structures of Isomeric Dichloro Bis[1-alkyl-2-(arylazo)imidazole] Complexes of Ruthenium(II)

Several new ligands, azoimidazoles belonging to the class 1-methyl-2-(arylazo)imidazoles (L1 (3)) and 1-benzyl-2-(arylazo)imidazoles (L2 (4)) (R = H (a), Me (b), OMe (c), Cl (d), NO2 (e)) have been synthesized and reacted with RuCl3 in ethanol under refluxing conditions. Two isomers of the composition RuL2Cl2, green (i) and blue (iii), are chromatographically separated. The green isomer is quantitatively transformed to the blue isomer on refluxing in a high boiling solvent. The isomeric structures have been confirmed by X-ray crystallography. Crystal data are as follows. Green complex C38H34Cl2N8Ru (6a): crystal system monoclinic; space group C2/c; a = 15.680(8) Å; b = 22.766(14) Å; c = 11.473(5) Å; β = 119.27(4)°; V = 3573(3) Å3 Z = 4; R = 3.59%; Rw = 4.38%. Blue complex C22H24Cl2N8Ru (7b): crystal system monoclinic; space group P21/n, a = 9.547(6) Å; b = 22.554(14) Å; c = 11.748(8) Å; β = 99.07(5)°; V = 2498(3) Å3; Z = 4; R = 3.15%; Rw = 4.51%. With reference to the pairs of Cl, N(imidazole), and N(azo) bound to Ru, the green isomer (6a) has a trans−cis−cis configuration and the blue isomer (7b) is cis−trans−cis. In both structures the Ru−N(azo) distances are relatively shorter than Ru−N(imidazole), indicating stronger bonding in the former and the presence of a Ru−L π-interaction that is localized in the Ru−azo fragment. The isomer configuration is supported by IR and 1H NMR data. The compounds exhibit t2(Ru) → π*(L) MLCT transitions in the visible region. Redox studies show the Ru(III)/Ru(II) couple in the green complexes (5, 6) at 0.6−0.7 V and in the blue complexes at 0.7−0.8 V versus SCE and two successive azo reductions. The difference in the first metal and ligand redox potentials is linearly correlated with νCT (t2(Ru) → π*(L).