Synthesis and electronic structure analysis of the actinide allenylidenes, [{(NR2)3}An(CCCPh2)]- (An = U, Th; R = SiMe3).

The reaction of [AnCl(NR2)3] (An = U, Th, R = SiMe3) with in situ generated lithium-3,3-diphenylcyclopropene results in the formation of [{(NR2)3}An(CH[double bond, length as m-dash]C[double bond, length as m-dash]CPh2)] (An = U, 1; Th, 2) in good yields after work-up. Deprotonation of 1 or 2 with LDA/2.2.2-cryptand results in formation of the anionic allenylidenes, [Li(2.2.2-cryptand)][{(NR2)3}An(CCCPh2)] (An = U, 3; Th, 4). The calculated 13C NMR chemical shifts of the Cα, Cβ, and Cγ nuclei in 2 and 4 nicely reproduce the experimentally assigned order, and exhibit a characteristic spin-orbit induced downfield shift at Cα due to involvement of the 5f orbitals in the Th-C bonds. Additionally, the bonding analyses for 3 and 4 show a delocalized multi-center character of the ligand π orbitals involving the actinide. While a single-triple-single-bond resonance structure (e.g., An-C[triple bond, length as m-dash]C-CPh2) predominates, the An[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]CPh2 resonance form contributes, as well, more so for 3 than for 4.