Articles 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) Tarun Kumar Misra, ² Debasis Das, ² Chittaranjan Sinha,* ,² Prasanta Ghosh, ‡ and Chandan Kumar Pal ‡ Department of Chemistry, The University of Burdwan, Burdwan 713 104, India, and Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India ReceiVed April 16, 1997 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), NO 2 (e)) have been synthesized and reacted with RuCl 3 in ethanol under refluxing conditions. Two isomers of the composition RuL 2 Cl 2 , 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 C 38 H 34 Cl 2 N 8 Ru (6a): crystal system monoclinic; space group C2/c; a ) 15.680(8) Å; b ) 22.766(14) Å; c ) 11.473(5) Å;  ) 119.27(4)°; V ) 3573(3) Å 3 Z ) 4; R ) 3.59%; R w ) 4.38%. Blue complex C 22 H 24 Cl 2 N 8 Ru (7b): crystal system monoclinic; space group P2 1 /n, a ) 9.547(6) Å; b ) 22.554(14) Å; c ) 11.748(8) Å;  ) 99.07(5)°; V ) 2498(3) Å 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 π-interaction that is localized in the Ru-azo fragment. The isomer configuration is supported by IR and 1 H NMR data. The compounds exhibit t 2 (Ru) f 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 ν CT (t 2 (Ru) f π*(L). Introduction The ruthenium chemistry of unsaturated nitrogenous ligands has developed 1-10 in recent times, primarily due to the wide range of oxidation states, varieties of reactivities of the complexes, directional electron and energy transfer, light-to- electrical energy conversion, photophysical and photochemical ² The University of Burdwan. ‡ Indian Association for the Cultivation of Science. (1) (a) Reedijk, J. In ComprehensiVe Coordination Chemistry; Wilkinson, G., McCleverty, J. A., Eds.; Pergamon: Oxford, U.K., 1987; Vol. 2, p 43. (b) Wong, W. T. Coord. 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