NMR and TRLFS studies of Ln(iii) and An(iii) C5-BPP complexes

Abstract

C5-BPP is a highly efficient N-donor ligand for the separation of trivalent actinides, An(iii), from trivalent lanthanides, Ln(iii). The molecular origin of the selectivity of C5-BPP and many other N-donor ligands of the BTP-type is still not entirely understood. We present here the first NMR studies on C5-BPP Ln(iii) and An(iii) complexes. C5-BPP is synthesized with 10% ¹⁵N labeling and characterized by NMR and LIFDI-MS methods. ¹⁵N NMR spectroscopy gives a detailed insight into the bonding of C5-BPP with lanthanides and Am(iii) as a representative for trivalent actinide cations, revealing significant differences in ¹⁵N chemical shift for coordinating nitrogen atoms compared to Ln(iii) complexes. The temperature dependence of NMR chemical shifts observed for the Am(iii) complex indicates a weak paramagnetism. This as well as the observed large chemical shift for coordinating nitrogen atoms show that metal-ligand bonding in Am(C5-BPP)₃ has a larger share of covalence than in lanthanide complexes, confirming earlier studies. The Am(C5-BPP)₃ NMR sample is furthermore spiked with Cm(iii) and characterized by time-resolved laser fluorescence spectroscopy (TRLFS), yielding important information on the speciation of trace amounts of minor complex species.

Scheme 1. Synthesis and labeling of the pyrazole moiety in C5-BPP with 10% ¹⁵N; adapted from the synthesis protocol in ref. 20.

Fig. 1. Molecular structure and numbering scheme of the [²⁴³Am(C5-BPP)₃]³⁺ complex.

Fig. 2. ¹⁵N direct excitation spectra in MeOD-d₄ of all C5-BPP complexes in this study. N₉ signals are labeled with green circles, N₈ signals with red circles.

Fig. 3. Chemical shift differences between the NMR signals in the Sm(iii) (top) and Am(iii) (bottom) complexes compared to the La(iii) complex for all nuclei. All values are given in ppm.

Fig. 4. ¹H,¹⁵N-gHMQC spectrum of Am[({¹⁵N}C5-BPP)₃](OTf)₃ in MeOD-d₄, optimized for a coupling constant J_HN = 5 Hz. The expected value range is taken from similar experiments with the diamagnetic lanthanide complexes. The correlation signals in the gray circles originate from an additional minor complex species.

Fig. 5. Sections of the ¹⁵N direct excitation spectra of [Am({¹⁵N}C5-BPP)₃](OTf)₃ in MeOD-d₄ at increasing temperatures (N₉ left side, N₈ right side). All spectra are referenced to the internal standard TMS by the lock signal.

Fig. 6. Normalized fluorescence spectra of Cm(iii) in MeOD-d₄ with increasing amount of C5-BPP. [Cm(iii)]_ini = 6.6 × 10^–8 mol L^–1, [Am(iii)]_ini = 6.0 × 10^–6 mol L^–1.