Nuclear Magnetic Resonance Spectroscopy in Ln/An Research

Since signal separation by lanthanide shift reagents [1,2] has been replaced by elaborate pulse sequences and high-field spectrometers, lanthanides have advanced from auxiliaries to real objects of interest, also as inactive analogues for trivalent actinides in consequence of their similar chemistry.
Here we want to report on interactions and structures of the Ln(III) (La³⁺, Eu³⁺ and, where applicable, Y³⁺) with selected systems, i.e., L-lactate [3], inorganic (poly)borates [4] and organoborates [5]. Small organic molecules such as lactate are important as model molecules and potential complexing agents found throughout the biosphere. Borates are ubiquitous in nature. In the context of nuclear waste disposal they occur in remarkable amounts in salt formations being potential host rocks for nuclear waste repositories, but also in boron containing cooling water or borosilicate glass coquilles for spent nuclear fuel. Organoborates are considered due to possible reaction of the former compounds and, additionally, suggested as analogues to model the interaction between Ln/An and borates in general.
Among several possible structures, infrared (IR) and NMR measurements, supported by density functional theory (DFT) calculation, revealed that lactate forms Ln(III) (and Am³⁺) complexes with both the carboxyl and hydroxyl group involved. Polyborates, i.e., triborate and pentaborate form soluble weak aqueous Ln(III) complexes prior to precipitation as amorphous white solids, whereas condensation to higher polyborates can be excluded. Two signals in both the ⁸⁹Y and the ¹¹B NMR spectra probably arise from two coordination sites, which may reflect the polyborate species found in the supernatant solution. The organoborates formed by the reaction of boric acid and, e.g., lactate or salicylate also possess a tetra-coordinated boron atom [BO₄], considered as the responsible site for Ln(III) interaction in inorganic (poly)borates. Since the (poly)borate/boric acid equilibrium is strongly concentration and pH dependent, their replacement by organic analogues allows investigations at both lower total boron concentrations and pH values.

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[4] Schott, J.; Kretzschmar, J.; Acker, M.; Eidner, S.; Kumke, M. U.; Drobot, B.; Barkleit, A.; Taut, S.; Brendler, V.; Stumpf, T. Dalton Trans. 2014, 43, 11516–11528.
[5] Schott, J; Kretzschmar, J; Acker, M.; Tsushima, S.; Barkleit, A.; Taut, S.; Brendler, V.; Stumpf, T., Dalton Trans., in preparation.