Uranium–nitride chemistry: uranium–uranium electronic communication mediated by nitride bridges

Treatment of [UIV(N3)(TrenTIPS)] (1, TrenTIPS = {N(CH2CH2NSiPri3)3}3−) with excess Li resulted in the isolation of [{UIV(μ-NLi2)(TrenTIPS)}2] (2), which exhibits a diuranium(iv) ‘diamond-core’ dinitride motif. Over-reduction of 1 produces [UIII(TrenTIPS)] (3), and together with known [{UV(μ-NLi)(TrenTIPS)}2] (4) an overall reduction sequence 1 → 4 → 2 → 3 is proposed. Attempts to produce an odd-electron nitride from 2 resulted in the formation of [{UIV(TrenTIPS)}2(μ-NH)(μ-NLi2)Li] (5). Use of heavier alkali metals did not result in the formation of analogues of 2, emphasising the role of the high charge-to-radius-ratio of lithium stabilising the charge build up at the nitride. Variable-temperature magnetic data for 2 and 5 reveal large low-temperature magnetic moments, suggesting doubly degenerate ground states, where the effective symmetry of the strong crystal field of the nitride dominates over the spin–orbit coupled nature of the ground multiplet of uranium(iv). Spin Hamiltonian modelling of the magnetic data for 2 and 5 suggest U⋯U anti-ferromagnetic coupling of −4.1 and −3.4 cm−1, respectively. The nature of the U⋯U electronic communication was probed computationally, revealing a borderline case where the prospect of direct uranium–uranium bonding was raised, but in-depth computational analysis reveals that if any uranium–uranium bonding is present it is weak, and instead the nitride centres dominate the mediation of U⋯U electronic communication. This highlights the importance of obtaining high-level ab initio insight when probing potential actinide–actinide electronic communication and bonding in weakly coupled systems. The computational analysis highlights analogies between the ‘diamond-core’ dinitride of 2 and matrix-isolated binary U2N2.


Preparation of [{(Tren TIPS )UN}2Li4] (2)
Method A: A solution of 1 (3.57 g, 4.00 mmol) in toluene (10 ml) was added to a cold (−78 °C) slurry of Li metal (0.20 g, 28.57 mmol) in toluene (20 ml). The mixture was allowed to slowly warm to room temperature and was then stirred for 5 days. Each day the mixture was sonicated for 1 hr. After this time the mixture turned dark blue/red and a red precipitate had formed. The red precipitate was isolated by filtration (via cannula), then extracted into boiling toluene (60 ml) and filtered through a frit. The residue was washed with boiling toluene (2 ´ 10 ml) and filtered. The combined filtrate was concentrated to ~30 ml and stored at −30 °C to yield 2 as a red crystalline solid. The product was isolated by filtration, washed with pentane (2 x 10 ml) and dried in vacuo. Method B: A solution of 4 (3.48 g, 2.00 mmol) in toluene (10 ml) was added to a cold (−78 °C) slurry of Li metal (0.04 g, 5.8 mmol) in toluene (20 ml). The mixture was allowed to slowly to warm to room temperature and was then stirred for 5 days. Each day the mixture was sonicated for 1 hr. The resulting red precipitate was extracted into boiling toluene (60 ml) and filtered through a frit. The residue was washed with boiling toluene (2 x 10 ml) and filtered. The combined filtrate was stored at −30 °C to yield 2 as a red crystalline solid. The product was isolated by filtration, washed with pentane (2 ´ 10 ml) and dried in vacuo. (m), 564 (w), 513 (w). Once obtained in crystalline form, 2 is insoluble in aromatic solvents and it decomposes in polar solvents so 1 H and 29 Si NMR and UV/Vis/NIR spectra could not be obtained.

Preparation of [{(Tren TIPS )UN}2HLi3] (5)
Method A: Toluene (15 ml) was added to a mixture of 2 (0.44 g, 0.25 mmol) and benzo-9-crown-3 (0.18 g, 1 mmol). The resulting red mixture was gently heated to dissolve both reagents, then filtered and the volume was reduced to ca. 5 ml. Storage of the mixture at −30 °C afforded red crystals of 5.
Yield: 0.06 g, 13%. Method B: Toluene (10 ml) was added to a pre-cooled (−78 °C) mixture of 2 (0.20 S4 g, 0.11 mmol) and AgBPh4 (0.048 g, 0.11 mmol). The resulting red suspension was allowed to warm to room temperature, sonicated for 1 hr, then stirred for 72 hrs. Volatiles were removed in vacuo and the resulting red solid was recrystallised from hot toluene (2 ml) to afford red crystals of 5 on storing at room temperature. Yield: 0.

Representative attempted double reductions of 1 with excess Na-Cs or MC8 (M = K-Cs)
In a typical procedure, toluene (20 ml) was added to a precooled (-78 °C) mixture of 1 (0.45 g, 0.50 mmol) and the respective reductant (2.00 mmol). The resulting mixture was allowed to warm to ambient temperature and stirred for 24 hrs, during which time it was sonicated three times (1 hr each time) before being stirred for another 24 hrs. Hot filtration then cooling afforded crystalline 4M (typically in ~55% yield). Extended reaction times resulted in overall decomposition to Tren TIPS H3 and other unidentified species.

Cs)
In a typical procedure, toluene (20 ml) was added to a precooled (-78 °C) mixture of 1 (0.45 g, 0.50 mmol) and the respective reductant (1.00 mmol). The resulting mixture was allowed to warm to ambient temperature and stirred for 24 hrs, during which time it was sonicated three times (1 hr each time) before being stirred for another 24 hrs. Hot filtration then cooling afforded crystalline 4M (typically in 45% yield). Extended reaction times resulted in overall decomposition to Tren TIPS H3 and other unidentified species.

Cs)
In a typical procedure, toluene (20 ml) was added to a precooled (-78 °C) mixture of 4M (0.50 mmol) and the respective reductant (2.00 mmol). The resulting mixture was allowed to warm to ambient temperature and stirred for 24 hrs, during which time it was sonicated three times (1 hr each time) before being stirred for another 24 hrs. Hot filtration then cooling afforded crystalline 4M (typically in 40% yield). Extended reaction times resulted in overall decomposition to Tren TIPS H3 and other unidentified species.

Computational Details
DFT calculations on 4 were performed with Gaussian 16 revision A.03, 10 and with Turbomole 7.3 for 3. 11 Calculations were spin unrestricted and used the GGA functional PBE, 12 as well as the hybrid PBE0. 13 The 60erelativistic effective core potential (RECP) of the Stuttgart/Cologne group (ECP60MWB) was used alongside the associated segmented basis set, [14][15][16] and on other elements the cc-pVDZ basis set was used. [17][18][19] Grimme's D3 dampening function was used to account for dispersion interactions. 20 Integration grids and convergence criteria were left at their default in Gaussian 16,and in Turbomole the m4 integration grid was used, with convergence criteria being left at their default.
CASSCF and RASSCF calculations were performed on model systems using OpenMolcas 18.09 21 Calculations were performed in Ci symmetry, reflecting the symmetry of the XRD crystal structure.
The ANO-RCC basis set was used; on uranium, and the ring nitrogen atoms, the VTZP contraction  Tables   Table S1. Absolute and relative energies of 2 and 2A with the functional PBE, for several multiplicities at the quintet optimised geometry. * = failed to converge.