Abstract
Complexes of group III metals (rare earth and actinides) with 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (BTP) have been investigated by computational (DFT) and, in limited cases, by experimental (FT-IR, X-ray) techniques with the goal of determining the characteristics of metal–ligand interactions. The DFT calculations using the M062X exchange-correlation functional revealed that metal–ligand distances correlate with the ionic radii of the metals, in agreement with available X-ray diffraction results on the Sc, Y, La, U, and Pu complexes. A related blue-shift trend could be observed in seven characteristic bands in the IR spectra associated with metal–ligand vibrations. The computations uncovered considerable charge transfer interactions, particularly in the actinide complexes, as important covalent contributions to the metal–ligand bonding. The covalent character of the metal–ligand bonds decreases in the actinides, from U to Cm.
Highlights
After the first synthesis of 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (BTP) in 1999 [1,2], it has become one of the most powerful extractants, separating actinides (An) and lanthanides (Ln) from nuclear waste [3]
The relevant actinides in this process are the trivalent americium and curium ions, as uranium and plutonium can be removed from the waste beforehand by the PUREX process [4]
A characteristic feature is the interchange of the relative order of the M–N1 and M–N2 distances around r(III) = 1.1 Å
Summary
After the first synthesis of 2,6-bis(5,6-dipropyl-1,2,4-triazin-3-yl)pyridine (BTP) in 1999 [1,2], it has become one of the most powerful extractants, separating actinides (An) and lanthanides (Ln) from nuclear waste [3]. Complexing ligands with soft donor atoms are likely to bind more strongly to the actinides than to the lanthanides. The origin of this behavior lies in the somewhat stronger donor-acceptor abilities of the actinide 5f and 6d atomic orbitals with the molecular orbitals of the donor ligands, as compared to the 4f/5d orbitals of the lanthanides [5,6,7]. The role of steric effects was suggested between the [1,2,4]-triazin-3-yl versus the 2-pyridinyl rings for complexing with lanthanides [10]; due to the better steric properties of the former group, its nitrogen can approach metals with small ionic radii more closely
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