The reaction of UCl₄ or ThCl₄(DME)₂ (DME = dimethoxyethane) with 1,4,7,10-tetrazacyclododecane-1,4,7,10-tetraacetic acid (H₄DOTA), and 6 equiv of triethylamine, in dimethyl sulfoxide (DMSO) results in the formation of [An^IV(κ⁸-DOTA)(DMSO)] (1, An = U; 2, An = Th), which can be isolated in moderate yields after workup. Complexes 1 and 2 are the first structurally characterized actinide-DOTA complexes to feature the κ⁸ binding mode for the DOTA ligand. In addition, we isolated a few crystals of [U(κ⁴-H₂DOTA)(DMSO)₄][Cl]₂ (3). Crystallographic characterization of this material reveals that the [H₂DOTA]^2- ligand in 3 is only coordinated to U⁴⁺ by its four carboxylate arms, generating an overall κ⁴ binding mode. Similar complexes have been previously proposed as intermediates along the H₄DOTA complexation pathway, but this intermediate had not been structurally characterized until now.

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 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-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.