Lanthanide(III) Complexation with an Amide Derived Pyridinophane

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 ΔG⧧298 = 72.4 ± 5.1 kJ
mol–1.