Synthesis and electronic structure analysis of the actinide allenylidenes, [{(NR(2))(3)}An(CCCPh(2))](−) (An = U, Th; R = SiMe(3))

The reaction of [AnCl(NR(2))(3)] (An = U, Th, R = SiMe(3)) with in situ generated lithium-3,3-diphenylcyclopropene results in the formation of [{(NR(2))(3)}An(CH[double bond, length as m-dash]C[double bond, length as m-dash]CPh(2))] (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)][{(NR(2))(3)}An(CCCPh(2))] (An = U, 3; Th, 4). The calculated (13)C NMR chemical shifts of the C(α), C(β), and C(γ) nuclei in 2 and 4 nicely reproduce the experimentally assigned order, and exhibit a characteristic spin–orbit induced downfield shift at C(α) 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–CPh(2)) predominates, the An[double bond, length as m-dash]C[double bond, length as m-dash]C[double bond, length as m-dash]CPh(2) resonance form contributes, as well, more so for 3 than for 4.