Ligand Preorganization in Metal Ion Complexation: Molecular Mechanics/Dynamics, Kinetics, and Laser-Excited Luminescence Studies of Trivalent Lanthanide Complex Formation with Macrocyclic Ligands TETA and DOTA

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Abstract
The molecular mechanics and dynamics calculations, kinetics, and laser-excited luminescence studies were carried out for trivalent lanthanide (Ln3+) complexes of macrocyclic polyaminopolycarboxylate ligands TETA and DOTA (where TETA is 1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid and DOTA is 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) to further understand the observed thermodynamic, kinetic, and structural properties and to examine how ligand preorganization affects metal ion complexation. Excitation spectroscopy (emission monitored at 614.0 nm) of the 7F0 → 5D0 transition of Eu3+ was used to study the aqueous properties of the Eu3+−TETA system. A stopped-flow spectrophotometric method was used to study the formation kinetics of the aqueous Ce3+−TETA/DOTA systems in the pH range 6.1−6.7. Molecular mechanics calculation results are consistent with the proposed mechanism of Ln(DOTA)- formation, i.e., formation of a carboxylate O-bonded precursor, followed by metal ion moving into the preformed macrocyclic cavity. For Ln(TETA)- formation, at least two carboxylate O-bonded intermediates have been predicted and Ln3+ ion assisted reorganization of the TETA ligand is present. The calculated bond distances and overall structures of Ln(DOTA)- and Ln(TETA)- were in agreement with the single-crystal and solution NMR structural data. The origin of the difference in thermodynamic stability of Ln(DOTA)- and Ln(TETA)- complexes and the corresponding formation intermediates is mainly due to the differences in water-occupancy energy (i.e., whether there is an apical coordinated water molecule), the ligand strain energy, and the cation−ligand interaction energy. Kinetic studies revealed that the formation rates of the Ce(TETA)- complex are smaller at lower pH and temperature but become greater at higher pH and temperature, as compared to those of the Ce(DOTA)- complex. This is attributed to the lanthanide ion and both mono- and di-hydroxide ion assisted TETA conformational reorganization and higher kinetic activation parameters. The presence of a di-hydroxide ion assisted intermediate rearrangement pathway could make the Ce(TETA)- complex formation rate faster at higher pH, and the higher activation barrier makes Ce(TETA)- complex formation rate slower at lower pH, as compared to those of the Ce(DOTA)- complex.

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