Synthesis, structure, redox activity and spectroscopic properties of ruthenium(II) complexes with 3,5-bis(benzothiazol-2-yl)pyrazole, 3,5-bis(benzimidazol-2-yl)pyrazole and 2,2′-bipyridine as co-ligands

Ruthenium(II) complexes of composition [(bipy)(2)Ru(Hpzbzth)][ClO4](2). 3H(2)O 1, [(bipy)(2)Ru(pzbzth)][ClO4]. 2H(2)O 2, [(bipy)(2)Ru(pzbzth)Ru(bipy)(2)][ClO4](3). H2O 3, [(bipy)(2)Ru(H(3)pzbzim)][ClO4](2). 2H(2)O 4 and [(bipy)(2)Ru(Hpzbzim)]. 2H(2)O 5, where Hpzbzth = 3,5-bis(benzthiazol-2-yl)pyrazole, H(3)pzbzim = 3.5-bis(benzimidazol-2-yl)pyrazole and bipy = 2,2'-bipyridine, have been synthesized and characterized. The crystal structure of 3, in which the two metal centres are bridged by the pyrazolate moiety of the pzbzth anion, has been determined. The two RuN6 chromophores in this complex are separated by 4.723(3) Angstrom. From the significant down-field shift of the pyrazolate CH proton in 3 (8.92 ppm) with respect to 1 (7.78 ppm) and 4 (7.82 ppm), the involvement of S ... H(C)... S type interaction in 3 has been proposed. The equilibrium constants of the species involving dissociation of the NH protons of the bridging ligand and the change in the oxidation state of ruthenium from +2 to +3 have been determined in acetonitrile-water (3:2) by cyclic voltammetric and spectrophotometric methods. Redox titrations of complexes 1, 3 and 4 by cerium(lv) have revealed that the disappearence of the metal-to-ligand charge transfer band is accompanied by the appearence of the ligand-to-metal charge transfer band at higher wavelengths. In the case of 3, when 1 equivalent of cerium(IV) is added, the mixed-valence (RuRuIII)-Ru-II species is generated which exhibits an absorption maximum at 950 nm due to the intervalence-transfer transition. The luminescence spectral behaviour of complexes 1-4 has been examined in methanol-ethanol (1:4) solution (at 300 K) as well as in glassy state (at 77 K).