Assessing the formation of weak sodium complexes with negatively charged ligands

Berto, S.; Chiavazza, E.; Canepa, P.; Prenesti, E.; Daniele, P. G.

doi:10.1039/C6CP00192K

Assessing the formation of weak sodium complexes with negatively charged ligands

S. Berto, E. Chiavazza, P. Canepa, E. Prenesti, and P. G. Daniele; Phys. Chem. Chem. Phys. 18, 13118–13125 (2016).

BibTeX

@article{Berto2016assessing,
  author  = {Berto, S. and Chiavazza, E. and Canepa, P. and Prenesti, E. and Daniele, P. G.},
  title   = {{Assessing the formation of weak sodium complexes with negatively charged ligands}},
  journal = {Physical Chemistry Chemical Physics},
  volume  = {18},
  pages   = {13118--13125},
  year    = {2016},
  doi     = {10.1039/C6CP00192K}
}

Brief summary

Sodium ions form only weak complexes with dissolved molecules, but these still matter in biology and chemistry. Measuring them precisely with sodium-selective electrodes, and interpreting them with quantum-mechanical calculations, we show that complexes grow stronger as the ligand carries more charge, and that nitrogen atoms in flexible ligands such as EDTA help hold the sodium.

Full abstract

The stability of sodium complexes with poly-carboxylic and polyamino-carboxylic acids is investigated with ion-selective electrode-Na⁺ potentiometry, working at strictly constant ionic strength. It is observed that the formation constants of the Na⁺ complexes with monoligand stoichiometry (ML) increase with the number of charges on the ligand. For example, in poly-carboxylic acids this dependency is linear and is well captured by an experimental equation. A different behaviour is observed for the poly-amino carboxylic acids, which show higher complexation capabilities reaching a plateau of the binding energy past a specific ligand size. The experimental results are discussed qualitatively using ab initio calculations based on DFT B3LYP, and the principal electronic characteristics of the ligands under investigation are identified. As a result of the flexibility imparted by the long chains of polyamino-carboxylic ligands, both experimental and theoretical models demonstrate that nitrogen atoms in proximity of Na⁺ ions can participate in the metal coordination, thus providing further stabilization for the complexes. Moreover, by increasing the ligand size the stabilization gained in terms of ΔG reached a plateau for EDTA, in agreement with experimental observations.