Derivatives of the [Ru(bipy)(CN)4]2−chromophore with pendant pyridyl-based binding sites: synthesis, pH dependent-luminescence, and time-resolved infrared spectroscopic studies

Abstract

Reaction of K₄[Ru(CN)₆] with 2,2′:4′,4″-terpyridine (L¹) or 2,2′:3′,2″:6″,2‴-quaterpyridine (L²) in acidic aqueous methanol affords the complexes K₂[Ru(L¹)(CN)₄] and K₂[Ru(L²)(CN)₄] respectively, both containing the {Ru(bipy)(CN)₄}²⁻ chromophore but with pendant pyridyl or bipyridyl units, respectively. Time-resolved IR analysis of K₂[Ru(CN)₄(L²)] in MeCN–D₂O showed that the most intense CN stretching vibration shifted to higher energy by ca. 50 cm⁻¹ after laser excitation, consistent with formation of a Ru(III)/(L²)˙⁻ MLCT excited state for which the lifetime measurement (38 ± 5 ns, measured by TRIR) agrees reasonably well with the value measured by luminescence methods (30 ± 2 ns). Study of the pH dependence of the absorption and emission spectra of the two complexes revealed the presence of two different effects arising from protonation of the pendant pyridyl/bipyridyl site (which occurs with pK_a ≊ 3.1 in each case) and protonation of the cyanide ligands (which occurs with pK_a ≊ 2 in each case). For K₂[Ru(L¹)(CN)₄], protonation of the pendant pyridyl unit results in the ¹MLCT excited state being lowered in energy by ca. 1000 cm⁻¹, whereas at lower pH values (2.5–1), protonation of the cyanide ligands raises the ¹MLCT excited state energy by over 2000 cm⁻¹. For K₂[Ru(L²)(CN)₄] in contrast, protonation of the pendant bipyridyl unit has no detectable effect on the ¹MLCT energy, as the pendant site is electronically decoupled from the complex core by a substantial twist between the free and coordinated bipy components of L²; protonation of the cyanides at lower pH values however destabilises the ¹MLCT excited state. Protonation of the pendant pyridyl sites results in complete (for [Ru(L¹)(CN)₄]²⁻) or near-complete (for [Ru(L²)(CN)₄]²⁻) quenching of the luminescence; possible reasons for this behaviour are discussed. The crystal structures of the two related complexes [Ru(^tBu₂bipy)₂(L¹)][PF₆]₂ and [Cl₂Pt(μ-L²)Ru(bipy)₂][PF₆]₂ are also described to illustrate arguments about the conformations of L¹ and L².