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

Abstract

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 Nheterocyclic carbenes (NHCs), are unknown. Here, we report the preparation and characterization of the diphosphoniomethanide-plutonium complex [Pu(BIPM TMS H)(I)(μ-I)] 2 (1Pu, BIPM TMS H = (Me 3 SiNPPh 2 ) 2 -CH) and the diphosphonioalkylidene-plutonium complexes [Pu-, thus disclosing nonactinyl 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 4f 5 vs 5f 5 electron-counts within conserved ligand fields over 12 complexes. Quantum chemical calculations reveal that the orbitalenergy 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 5f 5 2Pu and 3Pu compared to 4f 5 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(PPh 2 NSiMe 3 ) 2 (4) and "PuOI". In contrast, 2Ce and 2Pr do not react with benzaldehyde to produce 4.

N-Heterocyclic Carbene to Actinide d-Based π-bonding Correlates with Observed Metal–Carbene Bond Length Shortening Versus Lanthanide Congeners

“…Such low values do indeed lead to some scatter in the ε values, but, in general, these are larger for the actinide–NHC bonds than for the lanthanide analogues, the clearest separation being 0.261/0.023 for 2Np vs 2Ce and 0.283/0.091 for 2Pu vs 2Pr . Lastly, we note that during the revision stage of this manuscript, a report was published that documented a plutonium complex with a Pu–C NHC interaction, complementing earlier analogous neptunium chemistry. , That research was focused on the characterization of PuC multiply bonded interactions through the coordination of diphosphonioalkylidene ({C­(PPh 2 NSiMe 3 ) 2 }, BIPM) ligands to the actinide metal ion. One of those complexes also contained coordinated I Me4 NHC ligands, for which shorter Np–C NHC vs Ce–C NHC and Pu–C NHC vs Pr–C NHC bonds were observed with differences on the order of ∼0.045–0.060 Å.…”

Progress in Nonaqueous Molecular Uranium Chemistry: Where to Next?

“…This naturally leads to the question, “Where to next?” While not claiming to be a definitive and exclusive list, the following emerge as obvious areas of focus: A “pure” alkylidene linkage of the form MCR 2 (R = H, alkyl, silyl) is yet to be secured in an isolable molecular actinide complex under normal experimental conditions. Actinide carbyne and carbido complexes, in particular terminal variants, are yet to be secured in an isolable molecular actinide complex under normal experimental conditions. Heavier Group 14 and 15 element bonding to uranium requires further development. U–U bonding in an isolable molecular complex under normal experimental conditions is yet to be secured. A clear-cut cis -uranyl in an isolable molecular complex under normal experimental conditions is yet to be secured. The above all emphasize a need to develop the molecular chemistry of transuranium elements. Noting recent reports on a neptunium­(V) bis­(imido) in 2015, a neptunium­(V) mono­(oxo) in 2022, and neptunium­(III) and plutonium­(III) diphosphonioalkylidenes in 2022 and 2024, , respectively, and early reports of alkyls and alkoxides that lack definitive structural authentication, many of the bonding motifs from Figure that have been delivered with uranium demand realization in transuranium chemistry. This applies to thorium as well, although to a lesser extent given recent advances in its chemistry.…”

“… The above all emphasize a need to develop the molecular chemistry of transuranium elements. Noting recent reports on a neptunium(V) bis(imido) in 2015, 232 a neptunium(V) mono(oxo) in 2022, 233 and neptunium(III) and plutonium(III) diphosphonioalkylidenes in 2022 and 2024, 234 , 235 respectively, and early reports of alkyls and alkoxides that lack definitive structural authentication, many of the bonding motifs from Figure 2 that have been delivered with uranium demand realization in transuranium chemistry. This applies to thorium as well, although to a lesser extent given recent advances in its chemistry.…”

Preparation of Neptunyl and Plutonyl Acetates To Access Nonaqueous Transuranium Coordination Chemistry

“…While the solvent-free, amorphous Np and Pu tri- and tetrachlorides have long served as a starting point for the preparation of organometallic actinide complexes, − progress toward molecular starting materials has been more brisk in recent years with renewed focus placed on their identification. Notable among these, solvent adducts of the actinide halides such as AnI 3 (THF) 3 , and AnCl 4 (DME) 2 (An = U, Np, Pu), as well as An(N(Si(CH 3 ) 3 ) 2 ) 3 , , have been reported and successfully used for preparation of nonactinyl complexes (preparations using AnI 3 (THF) 3 ; − preparations using AnCl 4 (DME) 2 ; − preparations using An(N(Si(CH 3 ) 3 ) 2 ) 3 ; , preparations using “NpCl 4 ” , ). However, fewer well-defined starting materials are known for high-valent species that are also compatible with nonaqueous conditions.…”