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.