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

Treatment of [U-IV(N3)(Tren(TIPS))] (1, Tren(TIPS) = {N(CH2CH2NSiPr3i)(3)}(3-)) with excess Li resulted in the isolation of [{U-IV(mu-NLi2)(Tren(TIPS))}(2)] (2), which exhibits a diuranium(IV) 'diamond-core' dinitride motif. Over-reduction of 1 produces [U-III(Tren(TIPS))] (3), and together with known [{U-V(mu-NLi)(Tren(TIPS))}(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 [{U-IV(Tren(TIPS))}(2)(mu-NH)(mu-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.

Automated summary

In this study, a unique diuranium(IV) dinitride compound [{U-IV(mu-NLi2)(Tren(TIPS))}(2)] was synthesized by treating [U-IV(N3)(Tren(TIPS))] with Li. The reduction sequence 1 -> 4 -> 2 -> 3 was proposed based on the obtained products. It was found that lithium plays a crucial role in stabilizing the charge accumulation in the nitride. The magnetic data analysis of the synthesized compounds 2 and 5 suggests a doubly degenerate ground state with strong crystal field effects dominating over spin-orbit coupling.