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

• Published in: Chemical science (Cambridge) 2019-04, Vol.10 (13), p.3738-3745
• Main Authors: Du, Jingzhen, King, David M, Chatelain, Lucile, Lu, Erli, Tuna, Floriana, McInnes, Eric J L, Wooles, Ashley J, Maron, Laurent, Liddle, Stephen T
• Format: Article
• Language: English
• Subjects: Actinides; Amides; Chemical Physics; Chemical reduction; Computation; Crystallography; Hydrogen bonds; Nitrides; Organic chemistry; Oxidation; Photolysis; Physics; Sodium azides; Thorium; Uranium
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c8sc05473h

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(CH CH NSiMe Bu ) } ) 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[CH CH NSiMe Bu ] CH CH NSi[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.