Synthesis, spectral and electrochemical studies of isomeric dichloro-bis[N(1)-ethyl-2-(arylazo)imidazole]ruthenium(II) and the reaction of the green isomer with tertiary phosphines

Abstract

N(1)-Ethyl-2-(arylazo)imidazoles (L) react with RuCl3 giving green (5) and blue RuL2Cl2 (6) isomers. The green isomer is quantitatively transformed into the blue isomer on boiling under reflux in 1,2-dichlorobenzene. The ligand is of the N,N′-chelating type and in principle, provides five geometrical isomers. Considering the coordination pairs in the order: Cl, N(imidazole), N and N(azo),N′ the green isomer is␣spectroscopically characterised as trans-cis-cis-tcc-RuL2Cl2 (5) and the blue isomer is cis-trans-cis-ctc-RuL2Cl2 (6). The green isomer reacts smoothly with tertiary phosphines giving species of type [RuCl(P)L2]+ (7,8) and [Ru(P-P)L2]+2 (9,10) in which Cl, P and P-P respectively occupy cis-positions [P = PPh3, PPh2Me, P-P = Ph2P(CH2)2PPh2 (dppe) and Ph2P(CH2)3PPh2 (dppp)]. All the complexes display allowed t2(Ru) → π* (L) transitions in the visible region. A systematic shift of this MLCT band to higher energy occurs in the order: tcc-RuL2Cl2 (5) 1.2V versus s.c.e.) in phosphine derivatives (7–10). The azo reduction (E0 L) appears at negative values with respect to s.c.e. and is found to be sensitive to substitution in the phenyl ring of the ligand frame. The potential difference (E0 M−E0 L) is linearly related to the MLCT band energies (νCT) of green and blue RuL2Cl2 complexes. The phosphine derivatives behave differently, a linear correlation being observed between νCT and E0 M. The isomer configuration of RuL2Cl2 and the stereochemistries of the phosphine derivatives are established by 1H n.m.r. spectral data. The green (5) and blue (6) isomers exhibit single -Me or -OMe signals indicatives of C2-symmetry. The phosphine complexes [Ru(P)Cl(L2)2]+/[Ru(P-P)(L2)2]+2 show two equally intense methyl signals suggesting C1-symmetry in the derivatives.