A theoretical study on molecular structure of Eu(III)-salicylate complexes in aqueous system

The coordination geometry of Eu(III)-salicylate complexes in aqueous phase was obtained using a density functional theory (DFT) calculation. Geometry optimization with hybrid functional B3LYP revealed that the 1:1 carboxylate-coordinated complex takes uni-dentate coordination between carboxylate and central Eu(III), and holds eight coordination water molecules in its first coordination sphere, resulting in the constitution [Eu(HSA)·8H2O]2+, where (HSA) stands for salicylate with protonated OH group. Adopting a determination procedure including a continuum solvent using the CPCM model made a significant contribution. Namely, we found that the bi-dentate model with the constitution [Eu(HSA)·7H2O]2+ was the most stable structure of all models in the gas-phase, while the uni-dentate model [Eu(HSA)·8H2O]2+ became the most stable when using the CPCM solvent model. This agreed well with our recent experimental study using time-resolved laser-induced fluorescence spectroscopy (TRLIFS), where 5-sulfosalicylate forms a complex with Eu(III) via carboxyl group with uni-dentate coordination. The most stable geometry of the corresponding chelate complexes, Eu(SA)+, was obtained by the same procedure, and the constitution was identified as [Eu(SA)·6H2O]+, where (SA) stands for salicylate with dissociated OH group. This suggests an important chemical finding that the following intra-molecular chelate ring closure reaction holds: [Eu(HSA)·8(H2O)]2+→[Eu(SA)·6(H2O)]+H3O+(H2O). This means that two water molecules are dissociated with the penetration of the OH group into the first coordination sphere, and the coordination number of the central Eu(III) is reduced from nine to eight with the chelate ring closing reaction.