Thorium- and uranium-azide reductions: a transient dithorium-nitride 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)(I)] (M = U, ; Th, ; Tren = {N(CHCHNSiMeBu )}) with NaN or KN, respectively, affords very rare examples of actinide molecular square and triangle complexes [{M(Tren)(μ-N)} ] (M = U, = 4, ; Th, = 3, ). Chemical reduction of produces [{U(Tren)}(μ-N)][K(THF)] () and [{U(Tren)}(μ-N)] (), whereas photolysis produces exclusively . Complexes and 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 in arene or ethereal solvents gives [{Th(Tren)}(μ-NH)] () or [{Th(Tren)}{Th(N[CHCHNSiMeBu ]CHCHNSi[μ-CH]MeBu )}(μ-NH)][K(DME)] (), 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)- 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.