Ionic Contraction across the Lanthanide Series Decreases the Temperature-Induced Disorder of the Water Coordination Sphere

Abstract

In liquid, temperature affects the structures of lanthanide complexes in multiple ways that depend upon complex interactions between ligands, anions, and solvent molecules. The relative simplicity of lanthanide aqua ions (Ln3+) make them well suited to determine how temperature induces structural changes in lanthanide complexes. We performed a combination of ab initio molecular dynamics (AIMD) simulations and extended X-ray absorption fine structure (EXAFS) measurements, both at 25 and 90 °C, to determine how temperature affects the first- and second-coordination spheres of three Ln3+ (Ce3+, Sm3+, and Lu3+) aqua ions. AIMD simulations show first lanthanide coordination spheres that are similar at 25 and 90 °C, more so for the Lu3+ ion that remains as eight-coordinate than for the Ce3+ and Sm3+ ions that change their preferred coordination number from nine (at 25 °C) to eight (at 90 °C). The measured EXAFS spectra are very similar at 25 and 90 °C, for the Ce3+, Sm3+, and Lu3+ ions, suggesting that the dynamical disorder of the Ln3+ ions in liquid water is sufficient such that temperature-induced changes do not clearly manifest changes in the structure of the three ions. Both AIMD simulations and EXAFS measurements show very similar structures of the first coordination sphere of the Lu3+ ion at 25 and 90 °C.

The solution structures and relative stability constants of lanthanide–EDTA complexes predicted from computation

“…For the Lu 3+ ion, both the aqua ion and complex with EDTA 4− remained 8-coordinate and retained their first coordination sphere geometry, as shown with RMSD values to ideal geometries between the two temperatures that are within their standard deviation: 0.39 ± 0.06 Å and 0.40 ± 0.05 Å for the bicapped trigonal prismatic geometry; 0.41 ± 0.08 Å and 0.39 ± 0.07 Å for the square antiprism; and 0.29 ± 0.04 Å and 0.33 ± 0.07 Å for the dodecahedral geometry; at 25 °C and 90 °C respectively. Although the Lu 3+ aqua ion favors the square antiprism geometry at 25 °C, 41 and the dodecahedral geometry at 90 °C, 33 the Lu 3+ ion favors the dodecahedral geometry at both temperatures in complex with EDTA 4− revealing ligand stabilization of the first Lu 3+ coordination sphere.…”

“…AIMD simulations in this work were performed in the NVT ensemble (constant volume and temperature), with a 1.0 fs time step, at 25 1C or 90 1C, to obtain at least 10 ps of equilibrated trajectory. The AIMD simulations at 90 1C were performed as in our previous work, 33 by taking an equilibrated frame at 25 1C and performing an NVT simulation at 90 1C until at least B10 ps of equilibrated trajectory were sampled. The analysis of radial distribution functions (RDFs), coordination numbers (CNs), and root mean square deviations (RMSDs, see SI) to ideal geometries of the studied systems was done for equilibrated parts of the trajectories that corresponded to at least B10 ps.…”

“…4), even for the [La 3+ -HEDTA 3À Á(H 2 O) n ] 0 complex where the small change is the result of the coordination number change between 90 1C and 25 1C, as was observed with the Ln aqua ions where the temperature-induced disorder decreased along the Ln series. 33 In the Ln 3+ aqua ions, increasing from 25 1C to 90 1C resulted in a change in coordination number of the earlier Ln elements (Ce 3+ , Sm 3+ ) but not in the Lu 3+ aqua ion; a similar behavior is observed in La 3+ with a weakly-bound ligand (HEDTA 3À ) but not in La 3+ with a stronger-binding ligand (EDTA 4À ) that stabilizes the entire complex keeping coordinated water molecules in the first shell of the Ln-ligand complex. For the Lu 3+ ion, both the aqua ion and complex with EDTA 4À remained 8-coordinate and retained their first coordination sphere geometry, as shown with RMSD values to ideal geometries between the two temperatures that are within their standard deviation: 0.39 AE 0.06 Å and 0.40 AE 0.05 Å for the bicapped trigonal prismatic geometry; 0.41 AE 0.08 Å and 0.39 AE 0.07 Å for the square antiprism; and 0.29 AE 0.04 Å and 0.33 AE 0.07 Å for the dodecahedral geometry; at 25 1C and 90 1C respectively.…”

Achieving stable photoluminescence by double thiacalix[4]arene-capping: the lanthanide-oxo cluster core matters

“…Looking at the polynuclear core structure, we postulate that the large number of water molecules and hydroxyl bridges on the nonanuclear [Tb 9 (μ 5 -OH) 2 (μ 3 -O) 8 ] core may likely undergo dehydration under heating, which leads to the collapsing of the nonanuclear cluster structure in 4 . 63–65…”

“…Looking at the polynuclear core structure, we postulate that the large number of water molecules and hydroxyl bridges on the nonanuclear [Tb 9 (m 5 -OH) 2 (m 3 -O) 8 ] core may likely undergo dehydration under heating, which leads to the collapsing of the nonanuclear cluster structure in 4. [63][64][65] Because of the superior structural stability, we further characterize the luminescent stability of 2 under higher temperature and extended UV exposure. First, we conduct the solid-state temperature-dependent photoluminescence measurements from 25 to 275 C under 350 nm excitation on 2.…”