Anion Binding by Protonated Forms of the Tripodal Ligand Tren

The interaction of the protonated forms of tris(2-aminoethyl)amine (tren) with NO3−, SO42−, TsO−, PO43−, P2O74−, and P3O105− was studied by means of potentiometric and microcalorimetric measurements in a 0.10 M NMe4Cl aqueous solution at 298.1 ± 0.1 K, affording stability constants and the relevant energetic terms ΔH° and TΔS° of complexation. Thermodynamic data show that these anion complexation processes are mainly controlled by electrostatic forces, although hydrogen-bond interactions and solvation effects also contribute to complex stability, leading, in some cases, to special ΔH° and TΔS° contributions. The crystal structures of [H3L][NO3]3 and [H3L][TsO]3 evidence a preferred tridentate coordination mode of the triprotonated ligands in the solid state. Accordingly, the H3L3+ receptor binds a single oxygen atom of both NO3− and TsO− by means of its three protonated fingers, although in the crystal structure of [H3L][TsO]3, one conformer displaying bidentate coordination was also found. Modeling studies performed on the [H3L(NO3)]2+ complex suggested that the tridentate binding mode is the preferred one in aqueous solution, while in the gas phase, a different complex conformation in which the receptor interacts with all three oxygen atoms of NO3− is more stable.