Nuclear Magnetic Resonance Spectroscopy in Rare Earth and Actinide Research

Lanthanide complexes have become a useful tool in nuclear magnetic resonance (NMR) spectroscopy within the last 40 years as lanthanide shift reagents (LSR) [1,2]. Since signal separation by LSR has been overcome 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., O-phospho-L-serine, L-lactate [3] and (poly)borates [4]. Both organics are important as model molecules and potential complexing agents found throughout the biosphere and in vivo. Borates occur in remarkable amounts in geological (salt) formations for nuclear waste repositories, but also in boron containing cooling water or borosilicate glass coquilles for spent nuclear fuel.
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. The phosphorylated amino acid phosphoserine, able to act as a bifunctional ligand, shows Ln(III) complexation by both the phosphate and the carboxylate group as studied by solution and solid state NMR methods. Upon complexation, even at low pH, the respective protons are abstracted, followed by aggregation and precipitation, possibly forming coordination polyhedra rather than 1:1 (chelate) complexes. Polyborates, i.e., triborate and pentaborate form soluble weak aqueous Ln(III) complexes prior to precipitation as 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.

[1] C. C. Hinckley, J. Am. Chem. Soc. 1969, 91, 5160–5162.
[2] O. A. Gansow, M. R. Willcott, R. E. Lenkinski, J. Am. Chem. Soc. 1971, 93, 4295–4297.
[3] A. Barkleit, J. Kretzschmar, S. Tsushima, M. Acker, Dalton Trans. 2014, submitted.
[4] J. Schott, J. Kretzschmar, M. Acker, S. Eidner, M. U. Kumke, B. Drobot, A. Barkleit, S. Taut, V. Brendler, T. Stumpf, Dalton Trans. 2014, accepted.