Trivalent Actinide and Lanthanide Complexation of 5,6-Dialkyl-2,6-bis(1,2,4-triazin-3-yl)pyridine (RBTP; R = H, Me, Et) Derivatives: A Combined Experimental and First-Principles Study

Complexations of lanthanide ions with 5,6-dialkyl-2,6-bis­(1,2,4-triazin-3-yl)­pyridine
[RBTP; R = H (HBTP), methyl (MeBTP), ethyl (EtBTP)] derivatives have
been studied in the acetonitrile medium by electrospray ionization
mass spectrometry, time-resolved laser-induced fluorescence spectroscopy,
and UV–vis spectrophotometric titration. These studies were
carried out in the absence and presence of a nitrate ion in order
to understand the effect of the nitrate ion on their complexation
behavior, particularly in the poor solvating acetonitrile medium where
strong nitrate complexation of hard lanthanide ions is expected. Consistent
results from all three techniques undoubtedly show the formation of
lower stoichiometric complexes in the presence of excess nitrate ion.
This kind of nitrate ion effect on the speciation of Ln3+ complexes of RBTP ligands has not so far been reported in the literature.
Different Am3+ and Ln3+ complexes were observed
with RBTP ligands in the presence of 0.01 M tetramethylammonium nitrate,
and their stability constant values are determined using UV–vis
spectrophotometric titrations. The formation of higher stoichiometric
complexes and higher stability constants for Am3+ compared
to Ln3+ ions indicates the selectivity of these classes
of ligands. A single-crystal X-ray diffraction (XRD) study of europium­(III)
complexes shows the formation of a dimeric complex with HBTP and a
monomeric complex with EtBTP, whereas MeBTP forms both the dimeric
and monomeric complexes. Density functional theory calculations confirm
the findings from single-crystal XRD and also predict the structures
of Eu3+ and Am3+ complexes observed experimentally.