Heteroleptic tris-chelates of ruthenium(ii)-bipyridine-arylazoimidazoles: 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

Abstract

The reaction of cis-trans-cis-RuCl₂(RaaiR′)₂ [RaaiR′ = 1-alkyl-2-(arylazo)imidazole, p-R–C₆H₄–NN–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 AgNO₃ followed by NaClO₄ or [Ag(bpy)₂](ClO₄) in boiling acetone led to red-brown [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 by 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 quantum yields vary from 0.011–0.025. The ¹H NMR spectral measurements of the complexes suggest the existence of two isomers in 1 ∶ 0.3 concentration ratio. The methylene, –CH₂–, in RaaiEt of [Ru(bpy)(RaaiEt)₂]²⁺ gives a complex AB type multiplet while the RaaiCH₂Ph moiety of [Ru(bpy)(RaaiCH₂Ph)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 (_CT) is linear. Electrochemical parametrization 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.