Solution Structure, Electronic Energy Levels, and Photophysical Properties of [Eu(MeOH)(NO₃)]^3- Complexes

The structure of lanthanide(III) ions in solutions high in nitrate has been debated since the early days of lanthanide coordination chemistry. The structure and properties of lanthanides in these solutions are essential in industrial rare-earth separation, as well as in the fundamental solution chemistry of these elements. Pending decades of debate, it was established that nitrate is bidentate and coordinates in the inner sphere, and complexes have been observed with as many as four nitrates coordinated to a single lanthanide(III) center in nonaqueous solutions. We revisit the interactions between nitrate and europium(III) in methanol using optical spectroscopy, X-ray total scattering, and the current understanding of europium(III) photophysics. By a combination of direct and indirect methods to probe the structure, it was found that four distinct species from Eu(MeOH)₉³⁺ to [Eu(MeOH)₃(NO₃)₃] are present in solutions containing from 0 to 2 M NO₃^- ions. It was shown that the changes in transition probabilities together with high-resolution spectra can provide information on speciation and how the minute changes in ligand field affect the microstates. By a comparison to total X-ray scattering, it was concluded that the optical spectra alone allow the constitution and symmetry of the europium(III) species to be determined. Most notably, the minute changes in the all oxygen atom coordination imply significant changes in the optical properties of the europium(III) center.