Heteroleptic tris-chelates of ruthenium(II)-bipyridine-arylazo-imidazoles:Synthesis, spectral characterisation and electrochemical properties. Single crystal X-ray structure of [(2,2 '-bipyridine)-bis{1-methyl-2-(p-tolylazo)imidazole}ruthenium(II)] perchlorate

The reaction of cis-trans-cis-RuCl2(RaaiR')(2) [RaaiR' = 1-alkyl-2-(arylazo )imidazole, p-R-C6H4-N=N-C3H2-NN-1-R', where R = H (a), Me (b), Cl (c) and R'= Me (1), Et (2) and CH2Ph (3)] either with 2,2'-bipyridine (bpy) and AgN O3 followed by NaClO4 or [Ag(bpy)(2)](ClO4) in boiling acetone led to red-b rown [Ru(bpy)(RaaiR')(2)](ClO4)(2). The complex may exist in three isomeric structures. The structure of [Ru(bpy)(MeaaiMe)(2)]ClO4)(2) was confirmed b y X-ray diffraction study and suggests that only one isomer is crystallised . Electronic spectra exhibit a strong MLCT band at 520 +/- 5 nm along with two weak transitions at longer wavelength. The complexes emit at 77 K and q uantum yields vary from 0.011-0.025. The H-1 NMR spectral measurements of t he complexes suggest the existence of two isomers in 1 :0.3 concentration r atio. The methylene, -CH2-, in RaaiEt of [Ru(bpy)(RaaiEt)(2)](2+) gives a c omplex AB type multiplet while the RaaiCH(2)Ph moiety of [Ru(bpy)(RaaiCH(2) Ph)(2)](2+) shows AB type quartets. Cyclic voltammograms show a Ru(III)/Ru( II) redox couple along with three successive ligand reductions. The plot of difference in potential of first oxidation and reduction versus energy of main MLCT band (<(<nu>)over tilde>(CT)) is linear. Electrochemical parametr ization of the Ru(III)/Ru(II) redox couple determines ligand potential E-L( L). Using E-L(L) the potential of the present complexes has been predicted and is linearly related to the observed potential.