A Theoretical Study of Unsupported Uranium–Ruthenium Bonds Based on Tripodal Ligands

The
investigation of metal–metal bonds has greatly promoted
the development of structural and applied chemistry, which is also
a special and important branch of organometallic chemistry. According
to the structurally authenticated [1, TrenTMSU–RuCp­(CO)2] complex containing an unsupported
U–Ru bond reported in an experiment, we have constructed a
series of similar structures, [2, TrenP–iPr2U–RuCp­(CO)2], [3, TrapenTMSU–RuCp­(CO)2] and
[4, (Ad,MeArO)3NU–RuCp­(CO)2], with different tripodal ligands and systematically analyzed
the nature of U–Ru bonding by using scalar-relativistic quantum
chemical calculations. On the basis of the analysis of the localized
molecular orbital, the U–Ru bonds are predominantly composed
of Ru 4d orbitals and partly U 6d and 5f orbitals, which indicates
that the U–Ru bonds are highly polarized. Noticeably, the contribution
of U 6d orbitals is smaller than that of U 5f orbitals for complex 4, which probably contributed to the peak U–Ru covalency
based on the values of bond order, delocalization index, and quantum
theory of atoms in molecules. The carbonyl stretching vibrational
frequencies, the molecular orbitals, and the results of energy decomposition
analysis indicate that the strength of the U–Ru bond in the
four complexes increases in the following order: 2 < 1 < 3 < 4. This reveals that
the U–Ru bond strength is sensitive to the tripodal ligands,
including the substituent and skeleton. Furthermore, the binding energies
indicate that complex 4 is more thermodynamically accessible.
This work improves our understanding of the unsupported U–Ru
complexes based on the tripodal ligand and is greatly significant
for the selection of precursors to construct more stable actinide–metal
bonds.