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.

Samples were carefully checked for purity and data reproducibility between independently prepared batches for each compound examined.Care was taken to ensure complete thermalisation of the sample before each data point was measured, and samples were immobilised in an eicosane matrix to prevent sample reorientation during measurements.Diamagnetic corrections were applied using tabulated Pascal constants and measurements were corrected for the effect of the blank sample holders (flame sealed Wilmad NMR tube and straw) and eicosane matrix.Elemental microanalyses were carried out by Mr Martin Jennings at the Micro Analytical Laboratory, School of Chemistry, University of Manchester.

and measurements were corrected for the ef
ect of the blank sample holders (flame sealed Wilmad NMR tube and straw) and eicosane matrix.Elemental microanalyses were carried out by Mr Martin Jennings at the Micro Analytical Laboratory, School of Chemistry, University of Manchester.

The compounds [(Tren TIPS )U(N3)] (1) and [{(Tren TIPS )UNLi}2] (4) were prepared as described previously. 8,9Alkali metals were washed with hexane to remove any protective mineral oil coatings and were stored under argon.The lithium powder was then used directly but other alkali metals were freshly cut and freed of any passivated oxide layer before use.


Preparation of [{(Tren TIPS )UN}2Li4] (2)

Method A: A solution of 1 (3.57g, 4.00 mmol) in toluene (10 ml) was added to a cold (−78 °C) slurry of Li metal (0.20 g, 28.57mmol) 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 te The compounds [(Tren TIPS )U(N3)] (1) and [{(Tren TIPS )UNLi}2] (4) were prepared as described previously. 8,9Alkali metals were washed with hexane to remove any protective mineral oil coatings and were stored under argon.The lithium powder was then used directly but other alkali metals were freshly cut and freed of any passivated oxide layer before use.

Preparation of [{(Tren TIPS )UN}2Li4] (2)
Method A: A solution of 1 (3.57g, 4.00 mmol) in toluene (10 ml) was added to a cold (−78 °C) slurry of Li metal (0.20 g, 28.57mmol) 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.Representative yield of either method: 2.05 g, 58%.Anal.calcd for C66H150N10Li4Si6U2: C, 45.14; H, 8.61; N, 7.97%.Found: C, 45.45; H, 8.57; N, 7.88%.FTIR ν/cm -1 (Nujol): 1631 (w), 1377 (w), 1300 (w), 1261 (w), 1052 (bs), 1025 (s), 990 (w), 933 (s), 917(m), 882 (s), 738 (s), 671 (m), 620 (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.

le 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 m

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.

ed
n crystalline form, 5 is insoluble in aromatic solvents and it decomposes in polar solve

s so
1 H and 29 Si NMR and UV/Vis/NIR spectra could not be obtained.


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

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.

forded crystalline 4M
typically in ~55% yield).Extended reaction times resulted in overall decomposition to Tren TIPS H3 and other unidentified species.


Representative attempted double reductions of 1 with stoichiometric 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 (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 Tre

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.
TIPS H3 and other unidentified species.

Representative attempted reductions of 4M (M = Na-Cs) using excess Na-Cs or MC8 (M = K-


Cs)

In a typical procedure, toluene (20 ml) was added Representative attempted reductions of 4M (M = Na-Cs) using excess Na-Cs or MC8 (M = K-

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.
o 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 2

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. 138][19] Grimme's D3 dampening function was used to account for dispersion interactions. 20Integration 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.

20Integration grids and con
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 was used and VDZ on all other atoms.The second-order Douglas-Kroll-Hess Hamiltonian was used to account for scalar relativistic effects.Cholesky decomposition was used, with the high decomposition threshold.CASPT2 and MS-CASPT2 calculations used an imaginary shift of 0.2 in addition to the default IPEA shift of 0.25.Mulliken composition of the active natural orbitals was analysed with Molpy. 22Quantum Theory of Atoms-in-Molecules (QTAIM) 23 analyses were performed with AIMALL, 24 Natural Bond Orbital (NBO) analyses were performed with NBO 6.0.

S8
Computational 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.

Figure S1 .
Figure S1.Variable-temperature magnetic data of a powdered sample of 2. a) µeff vs T, b) χT vs T, c)

Figure S2 .
Figure S2.Variable-temperature magnetic data of a powdered sample of 5. a) µeff vs T, b) χT vs T, c)

Table S2 .
Absolute and relative energies of 2 and 2A with the functional PBE, for several multiplicities at the quintet optimised geometry.* = failed to converge.

Table S5 .
The absolute (Ha) and relative (eV) energies of the MS-RASPT2 calculations on 2A-XRD, for each space symmetry and spin multiplicity.Absolute energies are shifted up by 58695 Hartree.

Table S6 .
The absolute (Ha) and relative (eV) energies of the SA-RASSCF calculations on 2A-XRD, for each space symmetry and spin multiplicity.Absolute energies are shifted up by 58695 Hartree.

Table S7 .
The occupation numbers of the natural orbitals for each root of the 1 Ag SA-RASSCF calculation on 2A-XRD.

Table S8 .
Composition analysis of the RAS1 active orbitals of the SA-RASSCF state which most contributes (66.3%) to the 1 Ag MS-RASPT2 ground state of 2A.

Table S9 .
Composition analysis of the RAS2 active orbitals of the SA-RASSCF state which most contributes (66.3%) to the 1 Ag MS-RASPT2 ground state of 2A.

Table S10 .
Composition analysis of the RAS2 active orbitals of the SA-RASSCF state which most contributes (66.3%) to the 1 Ag MS-RASPT2 ground state of 2A.