Structural, photophysical and electrochemical studies of [RuN6]2+ complexes having polypyridine and azole mixed-donor sites

The mixed-ligand ruthenium(II) complexes [(phen)2Ru(pzbzimH3)](ClO4)2·3H2O (1), [(phen)2Ru(bzimH)2](ClO4)2·3H2O (2) and [(bpy)2Ru(bpybzimH2)](ClO4)2 (3), where phen = 1,10-phenanthroline, bpy = 2,2′-bipyridine, pzbzimH3 = pyrazole-3,5-bis(benzimidazole), bzimH = benzimidazole and bpybzimH2 = 6,6′-bis(benzimidazole-2-yl)-2,2′-bipyridine have been synthesized and spectroscopically characterized. The X-ray structures of the three compounds have been determined which show that relative to the polypyridine ligands (phen or bpy) two donor nitrogens of the second ligand occupy cis position. In case of 3, bpybzimH2 ligand is coordinated in puckered form where its bpy unit acts in a monodentate fashion. The electrochemical properties, absorption and emission spectral characteristics and lifetimes of luminescence decay of the complexes have been compared. Deprotonation of the azole NH moieties of the complexes lead to substantial lowering of redox potentials of the RuII/RuIII couple as well as the MLCT and emission band energies. Spectrophotometric and spectrofluorometric titrations of complexes 1 and 3 have been carried out in 3:2 acetonitrile–water as a function of pH over the range 3.5–12.0 and the pK values have been determined. The kinetic parameters for the decay of the 3MLCT excited states of 1 at different pH at 298 K have been evaluated.

Graphical abstractThe redox potentials, absorption and emission band energies and lifetimes of luminescent decay undergo significant changes as the azole protons of the complexes get deprotonated with the increase of pH.Figure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights► Mixed-ligand Ru(II) complexes 1, 2 and 3 have been synthesized and structurally characterized. ► Strong intramolecular C–H⋯π (1 and 2) and π⋯π (3) interactions have been noted. ► E1/2 of RuII/RuIII, MLCT absorption and emission band energies get substantially lowered. ► The pK values of 1 and 3 have been determined. ► Lifetime of 3MLCT state of 1 has been determined as a function of pH.