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

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[PF ₆]₂ along with a 10 nm red shift of the emission maximum is observed in the presence of H₂PO₄^-/HP ₂O₇^3- 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[PF ₆]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^-/HP ₂O₇^3- 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[PF ₆]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 (H₂P₂O ₇^2-) recognition via solitary C- H···anion interactions.