Structural and Density Functional Studies of Uranium(III) and Lanthanum(III) Complexes with a Neutral Tripodal N-Donor Ligand Suggesting the Presence of a U−N Back-Bonding Interaction

The crystal structures of two trisiodide octacoordinated uranium(III) complexes of tris[(2-pyrazinyl)methyl]amine
(tpza), which differ only by the ligand occupying the eighth coordination site (thf or MeCN), and of their lanthanum(III) analogues have been determined. In the acetonitrile adducts the M−Npyrazine distances are very similar for U(III)
and La(III), while the U−Nacetonitrile distance is 0.05 Å shorter than the La−Nacetonitrile distance. In the [M(tpza)I3(thf)]
complexes in which the monodentate acetonitrile ligand, a weak π-acceptor ligand, is replaced by a thf molecule,
a σ-donor only, the mean value of the distance U−Npyrazine is 0.05 Å shorter than the mean value of the La−Npyrazine
distance. Since we are comparing isostructural compounds of ions with very similar ionic radii, these differences
indicate the presence of a stronger M−N interaction in the U(III) complexes and therefore suggest the presence of
a covalent contribution to the U−N bonding. The selectivity of the tpza ligand toward U(III) complexation (with
respect to that of La(III)) in the presence of σ-donor-only ligands has been quantified by the value of KU(tpza)/KLa(tpza)
measured to be 3.3 ± 0.5. The analysis of the metal−N-donor ligand bonding was carried out by a quasi-relativistic
density functional theory study on small model compounds, of formula I3M−L (M = La, Nd, U; L = acetonitrile,
pyrazine) and I3M−(pyrazine)3 (M = La, U). The structural data obtained from geometry optimizations on these
systems reproduce experimental trends, i.e., a decrease in the M−N distance from La to U, combined with an
increase of the C−N distance in the acetonitrile derivatives. A detailed orbital analysis carried out on the resulting
optimized complexes did not reveal any orbital interaction between the trivalent lanthanide cations (Ln3+) and the
N-donor ligands. In contrast, a back-donation electron transfer from 5f U3+ orbitals to the π* virtual orbital of the
ligand was observed for both acetonitrile and pyrazine. Evaluation of the total bonding energy between the MI3 and
L fragments shows that this orbital interaction leads to a stabilization of the uranium(III) system compared to the
lanthanide species.