Lanthanide(III) Complexation with an Amide Derived Pyridinophane

Abstract

Herein we report a detailed investigation of the solid state and solution structures of lanthanide­(III) complexes with the 18-membered pyridinophane ligand containing acetamide pendant arms TPPTAM (TPPTAM = 2,2′,2″-(3,7,11-triaza-1,5,9­(2,6)-tripyridinacyclododecaphane-3,7,11-triyl)­triacetamide). The ligand crystallizes in the form of a clathrated hydrate, where the clathrated water molecule establishes hydrogen-bonding interactions with the amide NH groups and two N atoms of the macrocycle. The X-ray structures of 13 different Ln³⁺ complexes obtained as the nitrate salts (Ln³⁺ = La³⁺–Yb³⁺, except Pm³⁺) have been determined. Additionally, the X-ray structure of the La³⁺ complex obtained as the triflate salt was also obtained. In all cases the ligand provides 9-fold coordination to the Ln³⁺ ion, ten coordination being completed by an oxygen atom of a coordinated water molecule or a nitrate or triflate anion. The bond distances of the metal coordination environment show a quadratic change along the lanthanide series, as expected for isostructural series of Ln³⁺ complexes. Luminescence lifetime measurements obtained from solutions of the Eu³⁺ and Tb³⁺ complexes in H₂O and D₂O point to the presence of a water molecule coordinated to the metal ion in aqueous solutions. The analysis of the Ln³⁺-induced paramagnetic shifts indicates that the complexes are ten-coordinated throughout the lanthanide series from Ce³⁺ to Yb³⁺, and that the solution structure is very similar to the structures observed in the solid state. The complexes of the light Ln³⁺ ions are fluxional due to a fast Δ­(λλλλλλ) ↔ Λ­(δδδδδδ) interconversion that involves the inversion of the macrocyclic ligand and the rotation of the acetamide pendant arms. The complexes of the small Ln³⁺ ions are considerably more rigid, the activation free energy determined from VT ¹H NMR for the Lu³⁺ complex being Δ<i>G</i>^⧧₂₉₈ = 72.4 ± 5.1 kJ mol^–1