Carbene Complexes of Plutonium: Structure, Bonding, and Divergent Reactivity to Lanthanide Analogs

Organoplutonium chemistry was established in 1965, yet structurally authenticated plutonium–carbon bonds remain rare being limited to π-bonded carbocycle and σ-bonded isonitrile and hydrocarbyl derivatives. Thus, plutonium-carbenes, including alkylidenes and N-heterocyclic carbenes (NHCs), are unknown. Here, we report the preparation and characterization of the diphosphoniomethanide-plutonium complex [Pu­(BIPMTMSH)­(I)­(μ-I)]2 (1Pu, BIPMTMSH = (Me3­SiNPPh2)2­CH) and the diphosphonioalkylidene-plutonium complexes [Pu­(BIPMTMS)­(I)­(DME)] (2Pu, BIPMTMS = (Me3SiNPPh2)2C) and [Pu­(BIPMTMS)­(I)­(IMe4)2] (3Pu, IMe4 = C­(NMeCMe)2), thus disclosing non-actinyl transneptunium multiple bonds and transneptunium NHC complexes. These Pu–C double and dative bonds, along with cerium, praseodymium, samarium, uranium, and neptunium congeners, enable lanthanide–actinide and actinide–actinide comparisons between metals with similar ionic radii and isoelectronic 4f5 vs 5f5 electron-counts within conserved ligand fields over 12 complexes. Quantum chemical calculations reveal that the orbital-energy and spatial-overlap terms increase from uranium to neptunium; however, on moving to plutonium the orbital-energy matching improves but the spatial overlap decreases. The bonding picture that emerges is more complex than the traditional picture of the bonding of lanthanides being ionic and early actinides being more covalent but becoming more ionic left to right. Multiconfigurational calculations on 2M and 3M (M = Pu, Sm) account for the considerably more complex UV/vis/NIR spectra for 5f5 2Pu and 3Pu compared to 4f5 2Sm and 3Sm. Supporting the presence of PuC double bonds in 2Pu and 3Pu, 2Pu exhibits metallo-Wittig bond metathesis involving the highest atomic number element to date, reacting with benzaldehyde to produce the alkene PhC­(H)C­(PPh2NSiMe3)2 (4) and “PuOI”. In contrast, 2Ce and 2Pr do not react with benzaldehyde to produce 4.