Uranium versus Thorium: A Case Study on a Base-Free Terminal Uranium Imido Metallocene.

The structure of and bonding in two base-free terminal actinide imido metallocenes, [η ⁵ -1,2,4-(Me ₃ C) ₃ C ₅ H ₂ ] ₂ An═N( p -tolyl) (An = U ( 1 ), Th ( 1' )) are compared and connected to their individual reactivity. While structurally rather similar, the U(IV) derivative 1 is slightly more sterically crowded. Furthermore, density functional theory (DFT) studies imply that the 5f orbital contribution to the bonding within the individual actinide imido An═N( p -tolyl) moieties is significantly larger for 1 than for 1' , which makes the bonds between the [η ⁵ -1,2,4-(Me ₃ C) ₃ C ₅ H ₂ ] ₂ U ²⁺ and [( p -tolyl)N] ^2- fragments more covalent. Therefore, steric and electronic factors impact the reactivity of these imido complexes. For example, complex 1 is inert toward internal alkynes, but it readily forms Lewis base adducts [η ⁵ -1,2,4-(Me ₃ C) ₃ C ₅ H ₂ ] ₂ U═N( p -tolyl)(L) (L = OPMe ₃ ( 6 ), dmap ( 9 ), PhCN ( 14 ), and 2,6-Me ₂ PhNC ( 17 )) with Me ₃ PO, 4-dimethylaminopyridine (dmap), nitrile, PhCN, or isonitrile 2,6-Me ₂ PhNC. It may also react as a nucleophile or undergo a [2 + 2] cycloaddition with CS ₂ , isothiocyanates, thio-ketones, ketones, lactides, and acyl nitriles, forming the four- or five-membered metallaheteroacycles, terminal sulfido, or oxido complexes, and cyanide amidate complexes, respectively. In contrast, after the addition of aldehyde p -tolylCHO, the tetranuclear complex [η ⁵ -1,2,4-(Me ₃ C) ₃ C ₅ H ₂ ] ₄ [OCH( p -tolyl)CH( p -tolyl)O] ₂ U ₄ O ₄ ( 10 ) is isolated. However, while 1 is unreactive toward dicyclohexylcarbodiimide (DCC), an equilibrium exists in benzene solution between N , N' -diisopropylcarbodiimide (DIC), 1 , and the four-membered metallaheterocycle [η ⁵ -1,2,4-(Me ₃ C) ₃ C ₅ H ₂ ] ₂ U[N( p -tolyl)C(═N ⁱ Pr)N( ⁱ Pr)] ( 12 ). Furthermore, 1 may also engage in single- and two-electron transfer processes. It is singly oxidized by Ph ₃ CN ₃ , CuI, Ph ₂ S ₂ , and Ph ₂ Se ₂ , yielding the uranium(V) imido complexes [η ⁵ -1,2,4-(Me ₃ C) ₃ C ₅ H ₂ ] ₂ U═N( p -tolyl)(X) (X = N ₃ ( 20 ), I ( 22 ), PhS ( 23 ), and PhSe ( 24 )), or is doubly oxidized by organic azides (RN ₃ ) and 9-diazofluorene, forming the uranium(VI) bis-imido metallocenes [η ⁵ -1,2,4-(Me ₃ C) ₃ C ₅ H ₂ ] ₂ U═N( p -tolyl)(=NR) (R = p -tolyl ( 18 ), mesityl ( 19 )) and [η ⁵ -1,2,4-(Me ₃ C) ₃ C ₅ H ₂ ] ₂ U=N( p -tolyl)[=NN=(9-C ₁₃ H ₈ )] ( 21 ), respectively.