Abstract
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(TrenDMBS)(I)] (M = U, 1; Th, 2; TrenDMBS = {N(CH2CH2NSiMe2Bu t )3}3-) with NaN3 or KN3, respectively, affords very rare examples of actinide molecular square and triangle complexes [{M(TrenDMBS)(μ-N3)} n ] (M = U, n = 4, 3; Th, n = 3, 4). Chemical reduction of 3 produces [{U(TrenDMBS)}2(μ-N)][K(THF)6] (5) and [{U(TrenDMBS)}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(TrenDMBS)}2(μ-NH)] (7) or [{Th(TrenDMBS)}{Th(N[CH2CH2NSiMe2Bu t ]2CH2CH2NSi[μ-CH2]MeBu t )}(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.
Highlights
In recent years there have been major developments in uraniumligand multiple bonding,[1] and arguably none more so than U–N multiple bonds.[2]
Computational studies reveal a s > p energy ordering for the bridging nitride linkages in this study, a phenomenon so far only found in terminal uranium-nitrides and uranyl complexes with very short U–N/–O distances
It would appear that when the C3v symmetry of TrenDMBS is lowered to Cs the cle that opens up allows two azides to enter the coordination sphere of uranium in 3 at an approximate right angle ($85) whereas for the larger thorium in 4 the azides approach at a slightly more acute N–Th–N angle ($79), which seems to be enough to switch from tetramer to trimer
Summary
In recent years there have been major developments in uraniumligand multiple bonding,[1] and arguably none more so than U–N multiple bonds.[2]. Reduction of the uranium-azide complex generates diuraniumnitrides, with two charge states of a UNU core being accessible, and interchangeable, with no evidence of C–H activation chemistry even under photolytic conditions.
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