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 (L-1 (3)) and 1-benzyl 2-(arylazo)imidazoles (L-2 (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 15.650(8) Angstrom; b = 22.766(14) Angstrom: c = 11.473(5) Angstrom: beta = 119.27(4)degrees; V = 3573(3) Angstrom(3) Z = 4; R = 3.59%; R-w = 4.38%. Blue complex C22H24Cl2N8Ru(7b): crystal system monoclinic; space group P2(1)/n, a = 9.547(6) Angstrom; b = 22.554(14) Angstrom; c = 11.745(8) Angstrom: beta = 99.07(5)degrees; V = 2498(3) Angstrom(3); Z = 4; R = 3.15%; R-w = 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 pi-interaction that is localized in the Ru-azo fragment. The isomer configuration is supported by IR and H-1 NMR data. The compounds exhibit t(2)(Ru) --> pi*(L) MLCT transitions in the visible region. Redox studies show the Ru(TLI)/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 e azo reductions. The difference in the first metal and ligand redox potentials is linearly correlated with nu(CT) (t(2)(Ru) --> pi*(L).