Heteroleptic tris-chelates of ruthenium(II): Synthesis, spectral characterisation and electrochemical properties

A facile reaction of cis–trans–cis-RuCl2(RaaiR′)2 [RaaiR′ = 1-alkyl-2-(arylazo)imidazole, m-R-C6H4-NN-C3H2-NN-1-R′, where R = H (a), OMe (b), NO2 (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 has isolated red-brown [Ru(bpy)(RaaiR′)2](ClO4)2 (1a–c, 2a–c, 3a–c). The maximum molecular peak of [Ru(bpy)(OMeaaiMe)2](ClO4)2 (1b) is observed at m/z 888.01 (100%) in the FAB mass spectrum. IR spectra of the complexes show CN and NN stretching at 1590 and 1370 cm−1 which is red shifted by 40 and 90 cm−1 from the free ligand value supports Ru-azo nitrogen pi bonding interaction. The emission spectra in frozen glass (77 K) are sharper and considerably more intense than the room temperature spectra. The 1H NMR spectral measurements suggest methylene, –CH2–, in RaaiEt gives a complex AB type multiplet while in RaaiCH2Ph it shows AB type quartets. Considering two arylazoimidazole moities there are 20 different carbon atoms in the molecule which gives a total of 20 different peaks in the 13C NMR spectrum. In the 1H–1H COSY spectrum of the present complexes, absence of any off-diagonal peaks extending from δ = 14.12 and 9.55 ppm confirm their assignment of no proton on N(1) and N(3), respectively. Contour peaks in the 1H–13C HMQC spectrum in the present complexes, the absence of any contours at δ = 147.12, 160.76, 155.67 and 157.68 ppm assign them to the C(2), C(6), C(8) and C(e and e″) carbon atoms, respectively. Cyclic voltammogram shows 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 (νCT) is linear. Electrochemical parametrisation of Ru(III)/Ru(II) redox couple determines ligand potential EL(L).