Infrared Spectra and Electronic Structure Calculations for the NUN(NN)1–5 and NU(NN)1–6 Complexes in Solid Argon

Progress in the chemistry of molecular actinide-nitride compounds

The chemistry of actinide-nitrides has witnessed significant advances in the last ten years with a large focus on uranium and a few breakthroughs with thorium. Following the early discovery of the first terminal and bridging nitride complexes, various synthetic routes to uranium nitrides have since been identified, although the range of ligands capable of stabilizing uranium nitrides still remains scarce. In particular, both terminal- and bridging-nitrides possess attractive advantages for potential reactivity, especially in light of the recent development of uranium complexes for dinitrogen reduction and functionalization. The first molecular thorium bridged-nitride complexes have also been recently identified, anticipating the possibility of expanding nitride chemistry not only to low-valent thorium, but also to the transuranic elements.

Photodissociation and Infrared Spectroscopy of Uranium-Nitrogen Cation Complexes

Laser vaporization of uranium in a pulsed supersonic expansion of nitrogen is used to produce complexes of the form U⁺(N₂)_n (n = 1-8). These ions are mass selected in a reflectron time-of-flight spectrometer and studied with visible and UV laser fixed-frequency photodissociation and with tunable infrared laser photodissociation spectroscopy. The dissociation patterns and spectroscopy of U⁺(N₂)_n indicate that N₂ ligands are intact molecules and that there is no insertion chemistry resulting in UN⁺ or NUN⁺. Fixed frequency photodissociation at 532 and 355 nm indicate that the U⁺-N₂ bond dissociation energy varies little with changing coordination. The photon energy and the number of ligands eliminated allow an estimate of the average U⁺-N₂ dissociation energy of 12 kcal/mol. Infrared bands are observed for these complexes near the N-N stretch vibration via elimination of N₂ molecules. These resonances are observed to be shifted about 130 cm^-1 to the red from the free-N₂ frequency for complexes with n = 3-8. Density functional theory indicates that U⁺ is most stable in the sextet state in these complexes and that N₂ molecules bind in end-on configurations. The fully coordinated complex is predicted to be U⁺(N₂)₈, which has a cubic structure. The vibrational frequencies predicted by theory are consistently lower than those in the experiment, independent of the isomeric structure or spin state of the complexes. Despite its failure to reproduce the infrared spectra, theory provides an average U⁺-N₂ dissociation energy of 11.8 ± 0.5 kcal/mol, in good agreement with the value from the experiments.

Mapping the Electronic Structure of the Uranium(VI) Dinitride Molecule, UN2

A combined anion photoelectron spectroscopic and relativistic coupled-cluster computational study of the electronic structure of the UN₂ molecule is presented. Because the photoelectron spectrum of the uranium dinitride negative ion, UN₂^-, directly reflects the electronic structure of neutral UN₂, we have measured and relied upon the photoelectron spectrum of the UN₂^- anion as a means of mapping the electronic structure of neutral UN₂. In addition to the electron affinity of the UN₂ ground state, energy levels of the UN₂ excited states were well characterized by the close interplay between the experiment and high-level theory. We found that both electron attachment and electronic excitation significantly bend the UN₂ molecule and elongate its U≡N bond. Implications for the activation of UN₂ are discussed.

Theoretical Insights into Halogenated Uranium Cyanide/Isocyanide Compounds

Two kinds of halogenated uranium cyanide/isocyanide compounds, XUCN and XUNC (X = halogen) formed by the insertion of uranium atom into X-C(N) bonds of XCN (or XNC), were investigated by DFT and ab initio methods. Although XNC is less stable thermodynamically than XCN, XUNC is more stable than XUCN and is expected to be prepared and characterized in matrix isolation experiments. The C-N stretching vibration mode (ν_C-N) is the primary fingerprint for the identification of these isomers due to its red-shift character with respect to the relevant precursor. Atoms-in-molecule (AIM) analysis illustrates that both X-U and U-C(N) bonds in XUCN and XUNC show closed-shell interaction character, although partial covalent character contributes to them, and can be denoted as X^-U²⁺(CN)^- and X^-U²⁺(NC)^-, respectively. Charge decomposition analysis (CDA) further reveals that the isocyanide exhibits better donation performance than the cyanide, which should be the root cause of the difference between XUCN and XUNC.

Detection and Electronic Structure of Naked Actinide Complexes: Rhombic-Ring (AnN)2 Molecules Stabilized by Delocalized π-Bonding

The major products of the reaction of laser ablated and excited U atoms and N2 are the linear N≡U≡N dinitride molecule, isoelectronic with the uranyl dication, and the diatomic nitride U≡N. These molecules form novel cyclic dimers, (UN)2 and (NUN)2, with complex electronic structures, in matrix isolation experiments, which increase on UV photolysis. In addition, (NUN)2 increases at the expense of (UN)2 upon warming the codeposited matrix samples into the 20-40 K range as attested by additional nitrogen and argon matrix infrared spectra recorded after cooling the samples back to 4 or 7 K. These molecules are identified through matrix infrared spectra with nitrogen isotopic substitution and by comparing the observed matrix frequencies with those from electronic structure calculations. The dimerization is strong (theory predicts the dimer to be on the order of 100 kcal/mol more stable than the monomers), since the ground state involves 12 bonding electrons, 8 in the σ-system, and 4 in the delocalized π-system. This delocalized π bonding is present in the U, Th, La, and Hf analogues further demonstrating the interesting interplay between the 5f and 6d orbitals in actinide chemistry. The (UN)2(+) cation is also observed in solid argon, and calculations indicate that the bonding in the ring is preserved. On the other hand, the NUN dimer is of lower C2h symmetry, and the initial NUN molecules are recognizable in this more weakly bonded (ΔE = -64 kcal/mol) structure. The NThN molecules bind more strongly in the (NThN)2 dimer than the NUN molecules in (NUN)2 since NUN itself is more stable than NThN.

The Renaissance of Non-Aqueous Uranium Chemistry

Prior to the year 2000, non-aqueous uranium chemistry mainly involved metallocene and classical alkyl, amide, or alkoxide compounds as well as established carbene, imido, and oxo derivatives. Since then, there has been a resurgence of the area, and dramatic developments of supporting ligands and multiply bonded ligand types, small-molecule activation, and magnetism have been reported. This Review 1) introduces the reader to some of the specialist theories of the area, 2) covers all-important starting materials, 3) surveys contemporary ligand classes installed at uranium, including alkyl, aryl, arene, carbene, amide, imide, nitride, alkoxide, aryloxide, and oxo compounds, 4) describes advances in the area of single-molecule magnetism, and 5) summarizes the coordination and activation of small molecules, including carbon monoxide, carbon dioxide, nitric oxide, dinitrogen, white phosphorus, and alkanes.

Terminal U≡E (E = N, P, As, Sb, and Bi) bonds in uranium complexes: a theoretical perspective

The compound L-U-N [L = [N(CH2CH2NSiPr(i)3)3](3-), Pr(i) = CH(CH3)2] containing a terminal U-N triple bond has been synthesized and isolated successfully in experiments. To investigate the trend in the bonding nature of its pnictogen analogues, we have studied the L-U-E (E = N, P, As, Sb, and Bi) complexes using the scalar relativistic density functional theory. The terminal U-E multiple bond length increases in the order of U-N ≪ U-P < U-As < U-Sb < U-Bi, which can be supported by the hard and soft acids and bases (HSAB) theory. The U-E bond length, molecular orbital (MO), and natural bond orbital (NBO) reveal that the terminal U-E bonds should be genuine triple bonds containing one σ- and two π-bonding orbitals. Quantum theory of atoms in molecules (QTAIM) topological analysis and the electron localization function (ELF) suggest that the terminal U-E bond possesses covalent character and the covalency of U-E bonds decrease sharply when the terminal atom becomes heavier. This work presents a comparison about the bonding characteristic between the terminal U≡N bond and its heavier pnictogen (P, As, Sb, and Bi) analogues. It is expected that this work would shed light on the evaluation of the amount of 5f orbital participation in multiple bonds and further facilitate our deeper understanding of f-block elements.

Effects of density functionals and dispersion interactions on geometries, bond energies and harmonic frequencies of EUX3 (E=N, P, CH; X=H, F, Cl)

Quantum-chemical calculations have been performed to evaluate the geometries, bonding nature and harmonic frequencies of the compounds [EUX3] at DFT, DFT-D3, DFT-D3(BJ) and DFT-dDSc levels using different density functionals BP86, BLYP, PBE, revPBE, PW91, TPSS and M06-L. The stretching frequency of UN bond in [NUF3] calculated with DFT/BLYP closely resembles with the experimental value. The performance of different density functionals for accurate UN vibrational frequencies follows the order BLYP>revPBE>BP86>PW91>TPSS>PBE>M06-L. The BLYP functional gives accurate value of the UE bond distances. The uranium atom in the studied compounds [EUX3] is positively charged. Upon going from [EUF3] to [EUCl3], the partial Hirshfeld charge on uranium atom decreases because of the lower electronegativity of chlorine compared to flourine. The Gopinathan-Jug bond order for UE bonds ranges from 2.90 to 3.29. The UE bond dissociation energies vary with different density functionals as M06-L<TPSS<BLYP<revPBE<BP86<PBE≈PW91. The orbital interactions ΔEorb, in all studied compounds [EUX3] are larger than the electrostatic interaction ΔEelstat, which means the UN bonds in these compound have greater degree of covalent character (in the range 63.8-77.2%). The UE σ-bonding interaction is the dominant bonding interaction in the nitride and methylidyne complexes while it is weaker in [PUX3]. The dispersion energy contributions to the total bond dissociation energies are rather small. Compared to the Grimme's D3(BJ) corrections, the Corminboeuf's dispersion corrections are larger with metaGGA functionals (TPSS, M06-L) while smaller with GGA functionals.

Two-electron reductive carbonylation of terminal uranium(V) and uranium(VI) nitrides to cyanate by carbon monoxide

Two-electron reductive carbonylation of the uranium(VI) nitride [U(Tren(TIPS))(N)] (2, Tren(TIPS)=N(CH2CH2NSiiPr3)3) with CO gave the uranium(IV) cyanate [U(Tren(TIPS))(NCO)] (3). KC8 reduction of 3 resulted in cyanate dissociation to give [U(Tren(TIPS))] (4) and KNCO, or cyanate retention in [U(Tren(TIPS))(NCO)][K(B15C5)2] (5, B15C5=benzo-15-crown-5 ether) with B15C5. Complexes 5 and 4 and KNCO were also prepared from CO and the uranium(V) nitride [{U(Tren(TIPS))(N)K}2] (6), with or without B15C5, respectively. Complex 5 can be prepared directly from CO and [U(Tren(TIPS))(N)][K(B15C5)2] (7). Notably, 7 reacts with CO much faster than 2. This unprecedented f-block reactivity was modeled theoretically, revealing nucleophilic attack of the π* orbital of CO by the nitride with activation energy barriers of 24.7 and 11.3 kcal mol(-1) for uranium(VI) and uranium(V), respectively. A remarkably simple two-step, two-electron cycle for the conversion of azide to nitride to cyanate using 4, NaN3 and CO is presented.