Thorium- and uranium-azide reductions: a transient dithorium-nitride versus isolable diuranium-nitrides

Molecular uranium-nitrides are now well known, but there are no isolable molecular thorium-nitrides outside of cryogenic matrix isolation experiments. We report that treatment of [M(Tren^DMBS)(I)] (M = U, 1; Th, 2; Tren^DMBS = {N(CH₂CH₂NSiMe₂Bu )₃}^3-) with NaN₃ or KN₃, respectively, affords very rare examples of actinide molecular square and triangle complexes [{M(Tren^DMBS)(μ-N₃)} ] (M = U, n = 4, 3; Th, n = 3, 4). Chemical reduction of 3 produces [{U(Tren^DMBS)}₂(μ-N)][K(THF)₆] (5) and [{U(Tren^DMBS)}₂(μ-N)] (6), whereas photolysis produces exclusively 6. Complexes 5 and 6 can be reversibly inter-converted by oxidation and reduction, respectively, showing that these UNU cores are robust with no evidence for any C-H bond activations being observed. In contrast, reductions of 4 in arene or ethereal solvents gives [{Th(Tren^DMBS)}₂(μ-NH)] (7) or [{Th(Tren^DMBS)}{Th(N[CH₂CH₂NSiMe₂Bu ]₂CH₂CH₂NSi[μ-CH₂]MeBu )}(μ-NH)][K(DME)₄] (8), respectively, providing evidence unprecedented outside of matrix isolation for a transient dithorium-nitride. This suggests that thorium-nitrides are intrinsically much more reactive than uranium-nitrides, since they consistently activate C-H bonds to form rare examples of Th-N(H)-Th linkages with alkyl by-products. The conversion here of a bridging thorium(iv)-nitride to imido-alkyl combination by 1,2-addition parallels the reactivity of transient terminal uranium(iv)-nitrides, but contrasts to terminal uranium(vi)-nitrides that produce alkyl-amides by 1,1-insertion, suggesting a systematic general pattern of C-H activation chemistry for metal(iv)- vs. metal(vi)-nitrides. Surprisingly, computational studies reveal a σ > π energy ordering for all these bridging nitride bonds, a phenomenon for actinides only observed before in terminal uranium nitrides and uranyl with very short U-N or U-O distances.