Matrix-Isolation Infrared Spectra and Electronic Structure Calculations for Dinitrogen Complexes with Uranium Trioxide Molecules UO3(η1‑NN)1–4

Investigations of the interactions of uranium trioxide (UO3) with other species are expected to provide a new perspective on its reaction and bonding behaviors. Herein, we present a combined matrix-isolation infrared spectroscopy and theoretical study of the geometries, vibrational frequencies, electronic structures, and bonding patterns for a series of dinitrogen (N2) complexes with UO3 moieties UO3(η1-NN)1–4. The complexes are prepared by reactions of laser-ablated uranium atoms with O2/N2 mixtures or laser-ablated UO3 molecules with N2 in solid argon. UO3(η1-NN)1–4 are classified as “nonclassical” metal–N2 complexes with increased Δν­(N2) values according to the experimental observations and the computed blue-shifts of N–N stretching frequencies and N–N bond length contractions. Electronic structure analysis suggests that UO3(η1-NN)1–4 are σ-only complexes with a total lack of π-back-donation. The energy decomposition analysis combined with natural orbitals for chemical valence calculations reveal that the bonding between the UO3 moiety and N2 ligands in UO3(η1-NN)1–4 arises from the roughly equal electrostatic attractions and orbital mixings. The inspection of orbital interactions from pairwise contributions indicates that the strongest orbital stabilization comes from the σ-donations of the 4σ*- and 5σ-based ligand molecular orbitals (MOs) into the hybrid 7s/6d x 2–y 2 MO of the U center. The electron polarization induced by electrostatic effects in the Ninner ← Nouter direction provides complementary contributions to the orbital stabilization in UO3(η1-NN)1–4. In addition, the reactions of UO3 with N2 ligands and the origination of the nonclassical behavior in UO3(η1-NN)1–4 are discussed.