Balancing the acidity of the pendant urea arm of bis-heteroleptic ruthenium(ii) complex containing pyridyl triazole for improved oxyanion recognition

Abstract

Two new Ru^II-based bis-heteroleptic ditopic receptors 1[PF₆]₂ (C₄₄H₃₄F₁₂N₁₀OP₂Ru) and 2[PF₆]₂ (C₄₀H₂₇F₁₇N₁₀OP₂Ru), decorated with a 1-naphthyl and pentafluorophenyl urea pendant arm, respectively, along with the previously reported 3[PF₆]₂ (C₄₀H₃₁F₁₃N₁₀OP₂Ru), containing a pendant 4-fluorophenyl urea unit and 4[PF₆]₂, devoid of a pendant urea arm, have been studied to establish the role of urea proton acidity on the sensing and extraction of oxyanions in the presence of triazole C–H as an additional hydrogen bonding motif. ¹H-NMR, isothermal titration calorimetry (ITC) and photophysical experiments show selective binding of 1[PF₆]₂ and 2[PF₆]₂ toward oxyanions such as phosphates (e.g., H₂PO₄⁻ and HP₂O₇³⁻) and carboxylates (e.g., CH₃CO₂⁻ and PhCO₂⁻) like 3[PF₆]₂. This generalizes the role of triazole C–H and urea in the Ru^II-based bis-heteroleptic ditopic receptors towards recognition of such anions. Interestingly, complex 1[PF₆]₂ having intermediate acidic urea –NH protons showed the highest binding affinity with phosphates as compared to the other urea analogues 2[PF₆]₂ and 3[PF₆]₂ as well as the non-urea analogue, 4[PF₆]₂ (C₃₂H₂₄F₁₂N₈P₂Ru). Moreover, 2[PF₆]₂ having the most acidic –NH protons showed higher binding affinity towards carboxylates as compared to that of 1[PF₆]₂/3[PF₆]₂. Detailed photo-physical studies revealed that 1[PF₆]₂ is a farsuperior and more selective H₂PO₄⁻ sensor compared to 2[PF₆]₂/3[PF₆]₂/4[PF₆]₂, as evidenced by the higher degree of amplification of Ru^II center-based MLCT emission, greater change in excited state lifetime, lower detection limit and higher degree of selectivity. Furthermore, 1[PF₆]₂ also acts as a moderate liquid–liquid extraction agent of H₂PO₄⁻, CH₃CO₂⁻ and PhCO₂⁻ anions, which was comparable to 3[PF₆]₂ and much higher as compared to 2[PF₆]₂/4[PF₆]₂.