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

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&z.dbd;C&z.dbd;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&z.tbd;C-CPh 2 ) predominates, the An&z.dbd;C&z.dbd;C&z.dbd;CPh 2 resonance form contributes, as well, more so for 3 than for 4 . The actinide allenylidenes [{(NR 2 ) 3 }An(CCCPh 2 )] − (An = U, Th, R = SiMe 3 ) feature significant ligand-to-metal donation bonding and partial An&z.dbd;C double bond character.