Synthesis, Characterization, and Density Functional Theory Analysis of Uranium and Thorium Complexes Containing Nitrogen-Rich 5‑Methyltetrazolate Ligands

Two
nitrogen-rich, isostructural complexes of uranium and thorium, (C5Me5)2U­[η2-(N,N′)-tetrazolate]2 (7) and
(C5Me5)2Th­[η2-(N,N′)-tetrazolate]2 (8),
containing 5-methyltetrazolate, have been synthesized and structurally
characterized by single-crystal X-ray diffraction, electrochemical
methods, UV–visible–near-IR spectroscopy, and variable-temperature 1H NMR spectroscopy. Density functional theory (DFT) calculations
yield favorable free energies of formation (approximately −375
kJ/mol) and optimized structures in good agreement with the experimental
crystal structures. Additionally, calculated NMR chemical shifts of 7 and 8 are in good agreement with the variable-temperature 1H NMR experiments. Time-dependent DFT calculations of both
complexes yield UV–visible spectroscopic features that are
consistent with experiment and provide assignments of the corresponding
electronic transitions. The electronic transitions in the UV–visible
spectroscopic region are attributed to C5Me5 ligand-to-metal charge transfer. The low-lying molecular orbitals
of the tetrazolate ligands (∼2 eV below the HOMO) do not contribute
appreciably to experimentally observed electronic transitions. The
combined experimental and theoretical analysis of these new nitrogen-rich
uranium and thorium complexes indicates the tetrazolate ligand behaves
primarily as a σ-donor.