On the determination of the number of water molecules, q, coordinated to europium(III) ions in solution from luminescence decay lifetimes

A refined equation (in its simplest form: q=1.11[τ−1H2O−τ−1D2O−0.31]) which allows the prediction of the number of water molecules in the first coordination sphere of a europium(III) (Eu3+) complex, q, in aqueous solution is presented. It is recognized that in the long history of the determination of the q-values of Eu3+ complexes from luminescent data of the excited metal ions in H2O and D2O solutions, certain inconsistencies are present. In some cases the q-values determined have either been non-integral when they should be integral, or have been in conflict with q-values predicted by other means. The original q-value equation put forth by this laboratory correlated the luminescence lifetime data of crystalline Eu3+ complexes to the known q-values based on XRD data from single crystals. In the current report, the difference in the decay rate of the Eu3+ luminescence of metal complexes in H2O and D2O solution is linearly correlated to q-values predicted using the original equation as a guide. Our current interpretation of the luminescence data of 25 Eu3+ complexes taken from the literature, along with the extensive research of many labs involving the effect of oscillators other than water molecules in the first coordination sphere of the Eu3+ ion, has yielded the following equation: q=1.11[τ−1H2O−τ−1D2O−0.31+0.45nOH+0.99nNH+0.075nOCNH] where nOH is the number of alcoholic OH oscillators in the first coordination sphere of Eu3+, nNH is the number of amine NH oscillators in the first coordination sphere of Eu3+, and nOCNH is the number of amide NH oscillators in which the amide carboxylic oxygen is in the first coordination sphere of Eu3+. The coefficient of determination parameter of the linear least-squares fit to the data is 0.998 and the standard error of the fit is ±0.1 in q. The equation is used to account for the effect of water molecules in the second coordination sphere of the Eu3+ ion as well as to cast light on aqueous Eu3+ complexes that have known labile coordination spheres.