An Actinide Complex with a Nucleophilic Allenylidene Ligand

The reaction of [Cp₃Th(3,3-diphenylcyclopropenyl)] (Cp = η⁵-C₅H₅) with 1 equiv of lithium diisopropylamide (LDA) results in cyclopropenyl ring opening and formation of the thorium allenylidene complex, [Li(Et₂O)₂][Cp₃Th(CCCPh₂)] ([Li(Et₂O)₂][1]), in good yield. Additionally, deprotonation of [Cp₃Th(3,3-diphenylcyclopropenyl)] with 1 equiv of LDA, in the presence of 12-crown-4 or 2.2.2-cryptand, results in the formation of discrete cation/anion pairs, [Li(12-crown-4)(THF)][Cp₃Th(CCCPh₂)] ([Li(12-crown-4)(THF)][1]) and [Li(2.2.2-cryptand)][Cp₃Th(CCCPh₂)] ([Li(2.2.2-cryptand)][1]), respectively. Interestingly, the complex [Li(Et₂O)₂][1] undergoes dimerization upon standing at room temperature, resulting in the formation of [Cp₂Th(μ:η¹:η³-CCCPh₂)]₂ (2), via loss of LiCp. The reaction of [Li(Et₂O)₂][1] with MeI results in electrophilic attack at the C_γ carbon atom, leading to the formation of a thorium acetylide complex, [Cp₃Th(C≡CC(Me)Ph₂)] (3), which can be isolated in 83% yield upon workup, whereas the reaction of [Li(Et₂O)₂][1] with benzophenone results in the formation of 1,1,4,4-tetraphenylbutatriene (4) in 99% yield, according to integration against an internal standard. Density functional theory (DFT) calculations performed on [1]^- and 2 reveal significant electron delocalization across the allenylidene ligand. Additionally, calculations of the ¹³C NMR chemical shifts for the C_α, C_β, and C_γ nuclei of the allenylidene ligand were in good agreement with the experimental shifts. The calculations reveal modest deshielding induced by spin-orbital effects originating at Th due to the involvement of the 5f orbitals in the Th-C bonds. According to a DFT analysis, the cyclopropenyl ring-opening reaction proceeds via [Cp₃Th(η¹-3,3-Ph₂-cyclo-C₃)]^- (IM), which features a carbanion character at C_β.