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

Abstract

The mixed-ligand osmium(II) complexes [(bpy) 2Os(H 3pzbzim)](ClO 4) 2·2H 2O ( 1) and [(bpy) 2Os(Hpzbzim)]·2H 2O ( 2), where H 3pzbzim=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 1/2 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 p K a 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 IIdπ→π*(bpy) MLCT band and the appearance of the less intense and lower energy π(H 3pzbzim)→Os IIIdπ 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).