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A new benzene platform based tripodal halogen bond (XB) donor receptor, 1a, has shown selectivity towards bromide over chloride and other interfering anions. Importantly, bromide selectivity of 1a has been utilized towards the selective removal of bromide (complex 1) from a mixture of interfering anions via a competitive crystallization process.

Tris(2‐aminoethyl)amine (tren) based 4‐cyanophenyl‐substituted tripodal L, tris{[(4‐cyanophenyl)amino]ethyl}thiourea receptor, was synthesized and explored thoroughly for anion recognition in solution by NMR spectroscopy and isothermal titration calorimetry (ITC) as well as in the solid state by single‐crystal X‐ray diffraction studies. Anion recognition properties of L were further exploited toward the extraction of sulfate as well as fluoride from aqueous media using a liquid–liquid extraction technique. A solution‐state anion binding study using NMR spectroscopy in [D6]DMSO and ITC measurements in dry acetonitrile show a relatively higher association constant of L with halides (F– and Cl–) over oxyanions (H2PO4– and HSO4–). The single‐crystal X‐ray structural analysis of complex 1 reveals a monotopic encapsulation of fluoride in L through six N–H···F– interactions with a distorted trigonal‐prismatic geometry, whereas sulfate and carbonate induce dimeric assemblies of L in complexes 2 and 3, respectively. In the case of sulfate, a tight dimeric capsular assembly of ca. 9.5 Å is observed through 15 N–H···O interactions, whereas carbonate forms a sandwich‐like dimeric molecular aggregation through 14 N–H···O interactions. In the presence of tetrabutylammonium iodide as the phase transfer agent, L has shown ca. 70 % extraction of fluoride (based on L) and ca. 40 % extraction of sulfate (based on L) from aqueous solutions using an anion‐exchange‐based liquid–liquid extraction strategy. Extraction of these anions is unambiguously demonstrated by 1H NMR, 19F NMR and FTIR spectroscopy, PXRD and single‐crystal X‐ray diffraction studies.

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 × 10(4) M(-1) and 4.67 × 10(4) M(-1) for H2PO4(-) and HP2O7(3-), respectively, which is in good agreement with the Ka values obtained from UV-vis titration experiments (2.97 × 10(4) M(-1) and 2.45 × 10(4) 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.