The spectroscopic study of Kinetic properties

of Pb(II) Complexes with Phosphonic Acid Derivative Ligands


                                Viktorie ŠIRŮČKOVÁ, Přemysl LUBAL^1


^1Department of Chemistry, Faculty of Science, Masaryk University, Kotlářská 2,
611 37 Brno, Czech Republic

E-mail: siruckova.viki@gmail.com


Pb(II) has been gaining popularity in various scientific fields – for example, in biomedicine for
its potential to be a cancer-treating theranostic agent (in which ^203Pb can be used for tumor
visualization and ^212Pb for tumor treatment) in the form of  Pb(II) complexes of macrocyclic
ligands. Macrocyclic ligands containing pendant arms with phosphonic/phosphinic acid-based groups
enable greater structural variability of the macrocyclic complex as well as variability in the
physico-chemical properties of the Pb(II) complex. In order to develop macrocyclic complexes with
the desired properties for medicinal purposes, the kinetics and thermodynamics of the complexes
must be well-understood. This poster presents a detailed kinetic and thermodynamic study of Pb(II)
complexes with cyclen-based macrocycles containing one (DO3AP^ABn, DO3AP^PrA, DO3AP) or four (DOTP,
DOT^Ph, DOTP^Ph, DOTP^OEt) phosphonic acid derivative pendant arms. The ligand DOTA, which contains
four acetic acid pendant arms, is used as a comparison.

The kinetics of formation and dissociation of Pb(II) complexes in various pH conditions were
monitored using conventional molecular absorption spectroscopy at λ = 260nm, while for very fast
reactions (on the order of seconds), stopped-flow instrumentation was employed. The results for the
formation kinetics indicated that complexes with ligands with one phosphonic-acid pendant arm had
similar rate constants, while the formation of complexes with all four phosphonic-acid based
ligands was slightly faster. The mechanism of formation was determined by doing studies with excess
Pb(II) and is presented – in general, the complex is formed first as an out-of-cage complex with
the pendant arms coordinating to the Pb(II) ion, which is then incorporated into the cavity through
a second, slower step. The kinetic inertness of the complexes follows the order DOTA > DO3AP^ABn ≈
DO3AP^PrA > DO3AP > complexes with all four phosphonic-acid derivative pendant arms. Preliminary
computational studies were done to determine any relationship between structural parameters (such
as bond length) and kinetic parameters. Overall, complexes with a longer Pb-N bond length had a
faster rate of formation; those with a longer Pb-O bond length had a slower rate of dissociation.


Acknowledgements

This work was supported by Masaryk University (MUNI/A/1539/2021), Ministry of Education of the
Czech Republic (LTC20044) and EU (COST CA18202 NECTAR Action).