BTBPs versus BTPhens: Some Reasons for Their Differences in Properties Concerning the Partitioning of Minor Actinides and the Advantages of BTPhens

Two members of the
tetradentate N-donor ligand families 6,6′-bis­(1,2,4-triazin-3-yl)-2,2′-bipyridine
(BTBP) and 2,9-bis­(1,2,4-triazin-3-yl)-1,10-phenanthroline (BTPhen)
currently being developed for separating actinides from lanthanides
have been studied. It has been confirmed that CyMe4-BTPhen 2 has faster complexation kinetics than CyMe4-BTBP 1. The values for the HOMO–LUMO gap of 2 are comparable with those of CyMe4-BTBP 1 for which the HOMO–LUMO gap was previously calculated to
be 2.13 eV. The displacement of BTBP from its bis-lanthanum­(III) complex
by BTPhen was observed by NMR, and constitutes the only direct evidence
for the greater thermodynamic stability of the complexes of BTPhen.
NMR competition experiments suggest the following order of bis-complex
stability: 1:2 bis-BTPhen complex ≥ heteroleptic BTBP/BTPhen
1:2 bis-complex > 1:2 bis-BTBP complex. Kinetics studies on some
bis-triazine N-donor ligands using the stopped-flow
technique showed a clear relationship between the rates of metal ion
complexation and the degree to which the ligand is preorganized for
metal binding. The BTBPs must overcome a significant (ca. 12 kcal
mol–1) energy barrier to rotation about the central
biaryl C–C axis in order to achieve the cis–cis conformation that is required to form
a complex, whereas the cis–cis conformation is fixed in the BTPhens. Complexation thermodynamics
and kinetics studies in acetonitrile show subtle differences between
the thermodynamic stabilities of the complexes formed, with similar
stability constants being found for both ligands. The first crystal
structure of a 1:1 complex of CyMe4-BTPhen 2 with Y­(NO3)3 is also reported. The metal ion
is 10-coordinate being bonded to the tetradentate ligand 2 and three bidentate nitrate ions. The tetradentate ligand is nearly
planar with angles between consecutive rings of 16.4(2)°, 6.4(2)°,
9.7(2)°, respectively.