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

Treatment of [U^IV(N₃)(Tren^TIPS)] (1, Tren^TIPS = {N(CH₂CH₂NSiPrⁱ₃)₃}^3-) with excess Li resulted in the isolation of [{U^IV(μ-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(μ-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(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.