Structural, spectroscopic, and proton-coupled electron-transfer behavior of pyrazolyl-3,5-bis(benzimidazole)-bridged homo- and heterochiral Ru^IIRu^II, Os^IIOs^II, and Os^IIRu^II 2,2'-bipyridine complexes

Abstract

The homo- and heterobimetallic complexes [(bpy)₂M^II(H₂pzbzim)M'^II(bpy)₂](ClO₄)₃·nH₂O (1, 3, 5) and their corresponding deprotonated complexes [(bpy)₂M^II(pzbzim)M'^II(bpy)₂](ClO₄)·nH₂O (2, 4, 6) [where M^II, M'^II = Ru (1, 2) = Os (3, 4); M^II = Os and M'^II = Ru (3, 5); bpy = 2,2'-bipyridine; H₃pzbzim = pyrazole-3,5-bis(benzimidazole)] were synthesized, separated to their heterochiral (a, ΛΔ/ΔΛ) and homochiral (b, ΛΛ/ΔΔ) diastereoisomers, and characterized by elemental analyses, ESI-MS, and ¹H NMR spectroscopy. The X-ray structures of 1a, 3a, and 5a show the involvement of two pyridine rings of two bpy ligands in strong intramolecular nonbonded π-π interaction. The occurrence of a C-H···π interaction between an aromatic C-H and the π-cloud of a pyridine ring leads to strong electronic shielding of this proton (¹H NMR). In all cases, the two diastereoisomers show practically no differences in their absorption spectra, redox potentials, and pK values. The large shifts in the E_½ values to less positive potentials and substantial redshifts in the MLCT bands that occur on deprotonation of 1, 3, and 5 are energetically correlated. From the profiles of E_½(1), (2) vs. pH over the pH range 1-12, the equilibrium constants and standard redox potentials for all the complex species in the metal oxidation states II·II, II·III, and III·III and the bridged ligand in the protonation states H₂pzbzim^-, Hpzbzim^2-, and pzbzim^3- have been evaluated. Using these values the bond dissociation free energies for the benzimidazole N-H bonds have been estimated. Spectroelectrochemical studies have been carried out for 1a, 3a, and 5a in the range 400-1100 nm. With stepwise oxidation of the metal centers replacement of MLCT bands by LMCT bands occur gradually with the observation of sharp isosbestic points. In the case of 1a, a band observed at λ_max = 910 nm for the Ru^IIRu^III species has been ascribed to intervalence charge transfer (IVCT) transition.