Delicate, a study of structural change in ten-coordinated La(III), Ce(III), Pr(III), Nd(III), Sm(III) and Eu(III) sulfates

We recently presented a new method that allows for a direct structural comparison of coordination complexes. The main difference to other approaches is that our AlignIT approach uses a common scaling and orientation of the complexes, when computing the symmetry deviation values, σideal. Here, six apparently isostructural lanthanide(III) sulfates K6[(Ln)2(SO4)6]/K5Na[(Ln)2(SO4)6] with ten-coordinated lanthanide(III) sites (Ln(O)10) were prepared, and the single-crystal structures were determined and compared using the symmetry deviation values (σideal). The six structures were shown to fall in two groups, Pr(III) and Eu(III) are identical (σideal = 0.04) bar size. With a maximum σideal of 0.07, the same was found to be true for the La(III), Ce(III), Nd(III), Sm(III) structures. The two groups are shown to be significantly different (σideal > 2.7), yet the coordination geometry of all six are best described as bicapped square antiprisms (σideal = 1.15-1.68). The structures differ in more than symmetry as the smaller lanthanides are shown to crystallize with one sodium and five potassium ions, while the larger crystallize with six potassium ions. This does not change the structure and we postulate that the structural variation is due to a delicate size matching between alkali and lanthanide ions. For the first six lanthanides this structure is very robust, which is confirmed as seven doped systems readily crystallized. These doped systems were prepared in order to use europium(III) luminescence as a structural probe. Unsurprisingly, only the doped Eu-La and Eu-Ce systems were found to be luminescent. Between these two and the all-europium(III) system, the intricate structural differences were shown to be enough to change both crystal field splitting and luminescence lifetime. We conclude that even simple dilution experiments of luminescent lanthanide(III) ions in innocent hosts must consider that actual structure can act as a modulator for the observed properties.