H2O coordination in macropa complexes of f elements (Ac, La, Lu): feasibility of the 11th coordination site

Attila Kovács,Zoltán Varga

doi:10.1007/s11224-020-01717-3

Attila Kovács, Zoltán Varga

Open Access

https://doi.org/10.1007/s11224-020-01717-3

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Abstract

The feasibility of an additional ligand coordination at the 11th coordination site of actinium, lanthanum, and lutetium ions in 10-fold coordinated macropa complexes has been studied by means of density functional theory calculations. The study covered the two main macropa conformers, Δ(δλδ)(δλδ) and Δ(λδλ)(λδλ), favoured by larger (Ac3+, La3+) and smaller (Lu3+) ions, respectively. At the molecular level, the coordination of H2O is the most favourable to the largest Ac3+ while only slightly less to La3+. Protonation of the picoline arms enhances the coordination by shifting the metal ion closer to the open site of the ligand. The choice of macropa conformer has only a slight influence on the strength and bonding properties of the H2O coordination. Aqueous solution environment decreases considerably the energy gain of H2O coordination at the 11th coordination site.

Highlights

Radioactive isotopes are used in medicine since 1936, when Dr John Lawrence began treating a 28-year-old leukaemia patient with radioactive 32P [1]
We study the effect of an H2O ligand on the structural and bonding properties of macropa complexes with the f elements La, Lu, and Ac
We focus on the total interaction energy as well as on the ratio of the ΔVelst and ΔEoi components upon H2O coordination in the two model structures

Summary

Introduction

Radioactive isotopes are used in medicine since 1936, when Dr John Lawrence (brother of Nobel laureate Ernest Lawrence) began treating a 28-year-old leukaemia patient with radioactive 32P [1]. A single NMR study indicated that macropa complexes of 4f elements prefer different structures depending on the size of the Ln3+ ions: La3+ was shown to coordinate with the Δ(δλδ)(δλδ) while the small Yb3+ with the Δ(λδλ)(λδλ) conformation of macropa [25]. The geometrical parameter most characteristic on H2O coordination is the M–O5 bond distance (Table 1) They are shorter by 0.1–0.2 Å in the M(H2L)H2O3+ model structures as compared to the M(L)H2O+ ones, indicating a considerably stronger M...OH2 interaction in the protonated structures. (vi) In terms of the interaction energies, the H2O coordination is considerably weaker in the M(L)H2O+ complexes than in the M(H2L)H2O3+ ones (cf Figure 6a) This is in good agreement with the structural characteristics due to the stronger capture of the metal by the anionic L2− ligand and the resulting considerably longer M-O5 distances in these complexes (vide supra). H2O coordination in polar solvents is not probable

Conclusions

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