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-1,2,4-(MeC)CH]An═N(-tolyl) (An = U (), Th ()) are compared and connected to their individual reactivity. While structurally rather similar, the U(IV) derivative 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(-tolyl) moieties is significantly larger for than for , which makes the bonds between the [η-1,2,4-(MeC)CH]U and [(-tolyl)N] fragments more covalent. Therefore, steric and electronic factors impact the reactivity of these imido complexes. For example, complex is inert toward internal alkynes, but it readily forms Lewis base adducts [η-1,2,4-(MeC)CH]U═N(-tolyl)(L) (L = OPMe (), dmap (), PhCN (), and 2,6-MePhNC ()) with MePO, 4-dimethylaminopyridine (dmap), nitrile, PhCN, or isonitrile 2,6-MePhNC. It may also react as a nucleophile or undergo a [2 + 2] cycloaddition with CS, 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 -tolylCHO, the tetranuclear complex [η-1,2,4-(MeC)CH][OCH(-tolyl)CH(-tolyl)O]UO () is isolated. However, while is unreactive toward dicyclohexylcarbodiimide (DCC), an equilibrium exists in benzene solution between ,-diisopropylcarbodiimide (DIC), , and the four-membered metallaheterocycle [η-1,2,4-(MeC)CH]U[N(-tolyl)C(═NPr)N(Pr)] (). Furthermore, may also engage in single- and two-electron transfer processes. It is singly oxidized by PhCN, CuI, PhS, and PhSe, yielding the uranium(V) imido complexes [η-1,2,4-(MeC)CH]U═N(-tolyl)(X) (X = N (), I (), PhS (), and PhSe ()), or is doubly oxidized by organic azides (RN) and 9-diazofluorene, forming the uranium(VI) bis-imido metallocenes [η-1,2,4-(MeC)CH]U═N(-tolyl)(=NR) (R = -tolyl (), mesityl ()) and [η-1,2,4-(MeC)CH]U=N(-tolyl)[=NN=(9-CH)] (), respectively.