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, [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂An═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 [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U²⁺ 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 [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U═N(p-tolyl)(L) (L = OPMe₃ (6), dmap (9), PhCN (14), and 2,6-Me₂PhNC (17)) with Me₃PO, 4-dimethylaminopyridine (dmap), nitrile, PhCN, or isonitrile 2,6-Me₂PhNC. It may also react as a nucleophile or undergo a [2 + 2] cycloaddition with CS₂, 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 [η⁵-1,2,4-(Me₃C)₃C₅H₂]₄[OCH(p-tolyl)CH(p-tolyl)O]₂U₄O₄ (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 [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U[N(p-tolyl)C(═NPr)N(Pr)] (12). Furthermore, 1 may also engage in single- and two-electron transfer processes. It is singly oxidized by Ph₃CN₃, CuI, Ph₂S₂, and Ph₂Se₂, yielding the uranium(V) imido complexes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U═N(p-tolyl)(X) (X = N₃ (20), I (22), PhS (23), and PhSe (24)), or is doubly oxidized by organic azides (RN₃) and 9-diazofluorene, forming the uranium(VI) bis-imido metallocenes [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U═N(p-tolyl)(=NR) (R = p-tolyl (18), mesityl (19)) and [η⁵-1,2,4-(Me₃C)₃C₅H₂]₂U=N(p-tolyl)[=NN=(9-C₁₃H₈)] (21), respectively.