Synthesis, spectroscopic properties and proton-coupled redox activities of mononuclear osmium(II) mixed-chelates derived from pyrazole-3,5-bis(benzimidazole) and 2,2'-bipyridine

The mixed-ligand osmium(II) complexes [(bpy)<SUB>2</SUB>Os(H<SUB>3</SUB>pzbzim)](ClO<SUB>4</SUB>)<SUB>2</SUB>·2H<SUB>2</SUB>O (1) and [(bpy)<SUB>2</SUB>Os(Hpzbzim)]·2H<SUB>2</SUB>O (2), where H<SUB>3</SUB>pzbzim=pyrazole-3,5-bis(benzimidazole) and bpy=2,2'-bipyridine, have been synthesized and spectroscopically characterized. In complex (2), the NH deprotonation occurs at the pyrazole and at one of the metal-bound benzimidazole sites. As compared to complex (1), which undergoes reversible oxidation at 0.48 V (vs. Ag/AgCl) in acetonitrile, the redox potential of (2) is shifted to a much lower value, 0.14 V. The proton-coupled redox activities of the complexes have been studied over the pH range 1-12 in acetonitrile-water (3:2). From the pH versus E<SUB>1/2</SUB> profile, the equilibrium constants of the complex species in the protonated/deprotonated forms and the metal ion in +2/+3 oxidation states have been determined. The pK<SUB>a</SUB> values of the complexes in the +2 state have also been determined spectrophotometrically. The spectrophotometric titration of complex (1) with cerium(IV) has revealed the disappearance of the more intense and higher energy Os<SUP>II</SUP>dΠ→Π<SUP>∗</SUP>(bpy) MLCT band and the appearance of the less intense and lower energy Π(H<SUB>3</SUB>pzbzim)→Os<SUP>III</SUP>dΠ LMCT band. The luminescence spectrum of (1) in methanol-ethanol at 77 K has shown it to be a weak emitter, while no emission is observed for (2)