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 Ln(3+) complexes obtained as the nitrate salts (Ln(3+) = La(3+)-Yb(3+), except Pm(3+)) have been determined. Additionally, the X-ray structure of the La(3+) complex obtained as the triflate salt was also obtained. In all cases the ligand provides 9-fold coordination to the Ln(3+) 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 Ln(3+) complexes. Luminescence lifetime measurements obtained from solutions of the Eu(3+) and Tb(3+) 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 Ln(3+)-induced paramagnetic shifts indicates that the complexes are ten-coordinated throughout the lanthanide series from Ce(3+) to Yb(3+), and that the solution structure is very similar to the structures observed in the solid state. The complexes of the light Ln(3+) 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 Ln(3+) ions are considerably more rigid, the activation free energy determined from VT (1)H NMR for the Lu(3+) complex being ΔG(⧧)298 = 72.4 ± 5.1 kJ mol(-1).

Exceptionally Inert Lanthanide(III) PARACEST MRI Contrast Agents Based on an 18‐Membered Macrocyclic Platform

“…The absorption spectra of the [EuL 1 ] 3+ complex presents an absorption band with a maximum at λ =269 nm attributed to the pyridyl chromophore 24. The emission spectrum recorded under excitation through this band presents the expected 5 D o → 7 F J ( J =0–4) transitions characteristic of the Eu 3+ ion (see Figure S2 in the Supporting Information).…”

Inert macrocyclic Eu3+ complex with affirmative paraCEST features

“…3). The absorption spectrum presents a maximum at 268 nm (ε = 11 000 M −1 cm −1 ) characteristic of the pyridyl group, 58 and this band can be used to sensitized Eu 3+ luminescence emission. The absorption and excitation spectra are almost identical, which indicates that Eu 3+ sensitization occurs via energy transfer from excited states of the ligand to Eu 3+ excited states.…”

The crystal structure of tris(μ2-1,3-bis(4,4,4-trifluoro-3-oxido-1-(oxo)but-2-en-1-yl)phenyl-κ4 O,O′:O′′,O′′′)-bis(1,2-dimethoxyethane-κ2 O,O′)dilutetium(III), C50H38F18Lu2O16

“…Lutetium(III) complexes have wide range applications in catalysis in the past few years [4][5][6]. Some directly related structures of the title structure have been reported [3,[7][8].…”