Complexation of the lanthanides by cryptand 2.2.2 in propylene carbonate: an electrochemical determination of the thermodynamic parameters

The complexation of the lanthanide ions by cryptand 2.2.2 in anhydrous propylene carbonate has been investigated by a competitive potentiometric method between 25 and 90 °C. The stability of the lanthanide cryptates increases from LaIII(log K= 12.9) to YbIII(log K= 17.6). All the complexes are enthalpy stabilized. The entropic term favours complexation at the beginning of the lanthanide series but rapidly becomes unfavourable, presumably because of a progressive rearrangement of the ligand. These complexation trends contrast significantly with results published previously and reflect the importance of the solvent in the complexation process.