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

Treatment of [U IV (N 3 )(Tren TIPS )] ( 1 , Tren TIPS = {N(CH 2 CH 2 NSiPr i 3 ) 3 } 3− ) with excess Li resulted in the isolation of [{U IV (μ-NLi 2 )(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-NLi 2 )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 U 2 N 2 . Sequential reduction of a uranium-azide through all the nitride steps is revealed. Experimentally determined U-U coupling in the diuranium( iv )-dinitride is shown by ab initio calculations to stem from the nitride bridges, not U-U bonding.