Macrocyclic Pyclen-Based Gd³⁺ Complex with High Relaxivity and pH Response.

Area of Science:

• Coordination Chemistry
• Supramolecular Chemistry
• Materials Science

Background:

• Macrocyclic ligands are crucial in coordination chemistry for their ability to encapsulate metal ions.
• Lanthanide complexes are vital in various applications, including medical imaging and luminescence.

Purpose of the Study:

• To synthesize and characterize a novel macrocyclic ligand (H4L) and its lanthanide complexes.
• To investigate the structural, relaxivity, and luminescence properties of these complexes.
• To explore the influence of pH on the solution behavior and aggregation of the complexes.

Main Methods:

• X-ray diffraction for solid-state structure determination.
• Relaxivity measurements for Gadolinium (Gd3+) complex.
• Luminescence lifetime measurements for Europium (Eu3+) complex in H2O and D2O.
• Density Functional Theory (DFT) calculations for coordination environment.
• Proton Nuclear Magnetic Resonance (1H NMR) and Diffusion Ordered Spectroscopy (DOSY) for solution studies.

Main Results:

• The crystal structure of the free ligand revealed specific protonation sites.
• Gd3+ complex relaxivity is pH-dependent, increasing at lower pH.
• Eu3+ complex in solution at low pH features two inner-sphere water molecules.
• DFT calculations elucidated the coordination number and environment of lanthanide ions.
• At high pH, Eu3+ complexes exhibit biexponential decay and form small aggregates, indicating pH-induced structural changes.

Conclusions:

• The synthesized macrocyclic ligand forms stable lanthanide complexes with tunable properties.
• pH significantly influences the hydration state and solution behavior of the lanthanide complexes.
• The findings provide insights into the design of lanthanide-based functional materials.