Bis-heteroleptic ruthenium(II) complex of a triazole ligand as a selective probe for phosphates

Abstract

A new bis-heteroleptic ruthenium(II) complex (1) of 2-(1-methyl-1H-1,2,3-triazol-4-yl) pyridine (L) ligand was extensively explored for anion sensing studies. 1[PF₆]₂ shows selective sensing of dihydrogen phosphate (H₂PO₄^–)/hydrogen pyrophosphate (HP₂O₇^3–) among halides, HCO₃^–, AcO^–, NO₃^–, ClO₄^–, HSO₄^–, OH^–, BzO^–, H₂PO₄^–, and HP₂O₇^3– in acetonitrile. Enhancement of emission intensity of 1[PF₆]₂ along with a 10 nm red shift of the emission maximum is observed in the presence of H₂PO₄^–/HP₂O₇^3– selectively. The photoluminescence (PL) titration experiment of 1[PF₆]₂ results in binding constants (K_a) of 5.28 × 10⁴ M^–1 and 4.67 × 10⁴ M^–1 for H₂PO₄^– and HP₂O₇^3–, respectively, which is in good agreement with the K_a values obtained from UV–vis titration experiments (2.97 × 10⁴ M^–1 and 2.45 × 10⁴ M^–1 for H₂PO₄^– and HP₂O₇^3–, respectively). High selectivity of 1[PF₆]₂ toward these two anions in acetonitrile is further confirmed by PL intensity measurement of 1[PF₆]₂ upon addition of these two anions in the presence of a large excess of other competitive anions. Further, considerable changes in the lifetime (τ) as well as in the decay pattern of 1[PF₆]₂ in the presence of H₂PO₄^–/HP₂O₇^3– among all tested anions support the selective binding property of 1[PF₆]₂ toward these two anions. Significant downfield shift of the triazole −CH proton of 1[PF₆]₂ with 1 equiv of H₂PO₄^– (Δδ = 0.26 ppm) and HP₂O₇^3– (Δδ = 0.23 ppm) in deuterated dimethyl sulfoxide proclaim binding mechanism via C–H···anion interaction in solution state. Finally, single-crystal X-ray structural analysis confirms the first example of dihydrogen pyrophosphate (H₂P₂O₇^2–) recognition via solitary C–H···anion interactions.