Mononuclear and binuclear ruthenium(II) complexes containing 2,2'-bipyridine or 1,10-phenanthroline and pyrazole-3,5-bis(benzimidazole). Synthesis, structure, isomerism, spectroscopy, and proton-coupled redox activity

Abstract

A number of mixed-ligand mononuclear and binuclear ruthenium(II) complexes of composition [(bpy)₂Ru(H₃pzbzim)](ClO₄)₂·2H₂O (1), [(phen)₂Ru(H₃pzbzim)](ClO₄)₂·3H₂O (2), [(bpy)₂Ru(H₂pzbzim)Ru(bpy)₂](ClO₄)₃·5H₂O (3), [(phen)₂Ru(H₂pzbzim)Ru(phen)₂](ClO₄)₃·4H₂O (4), [(bpy)₂Ru(H₂pzbzim)Ru(phen)₂](ClO₄)₃·4H₂O (5), [(bpy)₂Ru(pzbzim)Ru(bpy)₂](ClO₄)·3H₂O (6), and [(phen)₂Ru(pzbzim)Ru(phen)₂](ClO₄)·2H₂O (7), where H₃pzbzim = pyrazole-3,5-bis(benzimidazole), bpy = 2,2'-bipyridine, and phen = 1,10-phenanthroline, have been prepared and characterized. Complexes 3-5 isolated as mixtures of diastereoisomers have been separated by fractional recrystallization. In the cases of 3 and 4, the meso (ΛΔ) and racemate (rac) (ΛΛ, ΔΔ) forms, and for 5, two enantiomeric pairs [(ΛΔ, ΔΛ) and (ΛΛ, ΔΔ)] have been obtained. These, as well as the meso and rac diastereoisomers of 6, have been characterized by ¹H and ¹³C NMR spectroscopy. The crystal structure of the meso (ΛΔ) form of 3 (C₅₇H₅₃N₁₄C_l3O₁₇Ru₂) has been determined, which crystallizes in the monoclinic space group P2₁/c with a = 11.672(2) Å, b = 41.696(9) Å, c = 12.871(2) Å, β = 90.03(2)°, and Z = 4. The acid-base and redox chemistry of the binuclear complexes has been studied over the pH range 1-12 in acetonitrile-water (3:2) medium. The equilibrium constants of the species involving protonation and deprotonation of the benzimidazole NH protons and the metal oxidation states covering +2 and +3 have been evaluated by spectrophotometric and cyclic voltammetric measurements. During spectrophotometric titrations of the complexes with cerium(IV), the metal-to-ligand charge transfer transitions are replaced by the newly generated ligand-to-metal charge transfer transition. The luminescence spectra of the complexes in solution (at 298 K) and in frozen glass (at 77 K) and their quantum yields have been reported. In contrast to the luminescent behavior of 1-5, corresponding deprotonated compounds do not fluoresce even at 77 K.