TRASUTA: The Effect of the Structural Rigidity of a Mesocyclic AAZTA-like Chelating Agent on the Thermodynamic, Kinetic, and Structural Properties of Some Divalent Metal and Ga3+ Complexes

Mesocyclic chelating agents such as AAZTA and its derivatives have been recently reported to overcome the relatively low thermodynamic stability of metal complexes of acyclic chelating agents and the slow complexation kinetics of macrocyclic chelating agents. This work reports the preparation of a spirobicyclic hexadentate AAZTA-like chelating agent (TRASUTA) and the investigation of the thermodynamic, kinetic, and structural properties of the corresponding chelates with the PET-relevant Ga3+ and selected metal ions. A combination of analytical techniques allowed identification of a coordination isomerization process, involving the coordinating side arms and the inversion of a nitrogen atom and leading to lower thermodynamic and kinetic inertness with respect to mononuclear mesocyclic analogues. The bicyclic system of TRASUTA retains significant dynamics despite the conformational constraint imposed by the spiro-fusion, resulting in a lower stability of the corresponding metal chelates.