A cyanuric acid platform based tripodal bis-heteroleptic Ru(II) complex of click generated ligand for selective sensing of phosphates via C–H•••anion interaction

Abstract

A new bis-heteroleptic trinuclear Ru(II) complex (1[PF₆]₆) has been synthesized from electron deficient cyanuric acid platform based copper-catalyzed azide–alkyne cycloaddition, i.e., CuAAC click generated ligand, 1,3,5-tris [(2-aminoethyl-1H-1,2,3-triazol-4-yl)-pyridine]-1,3,5-triazinane-2,4,6-trione (L1). Complex 1[PF₆]₆ displays weak luminescence (ϕ_f = 0.002) at room temperature with a short lifetime of ∼5 ns in acetonitrile. It shows selective sensing of hydrogen pyrophosphate (HP₂O₇^3–) through 20-fold enhanced emission intensity (ϕ_f = 0.039) with a 15 nm red shift in emission maxima even in the presence of a large excess of various competitive anions like F^–, Cl^–, AcO^–, BzO^–, NO₃^–, HCO₃^–, HSO₄^–, HO^–, and H₂PO₄^– in acetonitrile. Selective change in the decay profile as well as in the lifetime of 1[PF₆]₆ in the presence of HP₂O₇^3– (108 ns) further supports its selectivity toward HP₂O₇^3–. UV–vis and photoluminescence titration profiles and corresponding Job’s plot analyses suggest 1:3 host–guest stoichiometric binding between 1[PF₆]₆ and HP₂O₇^3–. High emission enhancement of 1[PF₆]₆ in the presence of HP₂O₇^3– has resulted in the detection limit of the anion being as low as 0.02 μM. However, 1[PF₆]₆ shows selectivity toward higher analogues of phosphates (e.g., ATP, ADP, and AMP) over HP₂O₇^3–/H₂PO₄^– in 10% Tris HCl buffer (10 mM)/acetonitrile medium. Downfield shifting of the triazole C–H in a ¹H NMR titration study confirms that the binding of HP₂O₇^3–/H₂PO₄^– is occurring via C–H···anion interaction. The single crystal X-ray structure of complex 1 having NO₃^– counteranion, 1[NO₃]₆ shows binding of NO₃^– with complex 1 via C–H···NO₃^– interactions.