Andrew James Gaunt

Despite the discovery of actinide borohydride complexes over 80 years ago, no plutonium borohydride complexes have been structurally validated using single-crystal X-ray diffraction (XRD). Here we describe Pu-2((H3BPBu2BH3)-Bu-t)(6), the first example of a Pu(III) borohydride complex authenticated by XRD and NMR spectroscopy. Theoretical calculations (DFT, EDA, and QTAIM) and experimental comparisons of metal-boron distances suggest that metal-borohydride covalency in M-2((H3BPBu2BH3)-Bu-t)(6) complexes generally decreases in the order M = U(III) > Pu(III) > Ln(III).

Development of new synthetic methods to employ with transuranium organometallic complexes will help further the field and may provide a stronger understanding of their distinct reactivity, electronic structures, and chemical bonding. Although the organoneptunium field has grown over the last decade, just a handful of complexes have been synthesized, most of which are found in the trivalent oxidation state. Few reactions with neptunium (Np) have considered the stereochemical outcome, thus we employed a diastereoselective template protocol to synthesize meso (or: C-2) complexes of neptunium(iv). Following previous success with the diastereoselective synthesis of ((ONO)-O-(tBu2P))UCl2(dtbpy) (1), adaptation to NpCl4(dme)(2) led to the isolation of diastereopure ((ONO)-O-(tBu2P))NpCl2(dtbpy) (2), ((ONO)-O-(tBu2P) = 2,6-bis((di-tert-butylphosphino)-methanolato)pyridine). The addition of a bulky indenide ligand, derived from the in situ deprotonation of 4,7-dimethyl-1,3-bis(1-methylethyl)-1H-indene (1,3-(Pr-i)(2)-4,7-Me-2-C9H3), was pursued to synthesize the organoactinide complexes ((ONO)-O-(tBu2P))U(1,3-(Pr-i)(2)-4,7-Me-2-C9H3)Cl (3) and ((ONO)-O-(tBu2P))Np(1,3-(Pr-i)(2)-4,7-Me-2-C9H3)Cl (4). Bonding analyses consisting of orbital localization and energy decomposition methods show resemblances in covalent character between uranium (1 and 3) and neptunium (2 and 4) systems. However, an increased orbital overlap was identified in the metal - indenide with respect to the metal-dtbpy interaction, mainly driven by 5f electrons. This was attributed to the better symmetry match between the organic ligand and metal orbitals that allowed the participation of the 5f orbitals.

Organoplutonium chemistry was established in 1965, yet structurally authenticated plutonium-carbon bonds remain rare being limited to pi-bonded carbocycle and sigma-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((BIPMH)-H-TMS)(I)(mu-I)](2) (1Pu, (BIPMH)-H-TMS = (Me3SiNPPh2)(2)CH) and the diphosphonioalkylidene-plutonium complexes [Pu(BIPMTMS)(I)(DME)] (2Pu, BIPMTMS = (Me3SiNPPh2)(2)C) and [Pu(BIPMTMS)(I)(I-Me4)(2)] (3Pu, I-Me4 = 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 4f(5) vs 5f(5) 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 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(PPh2NSiMe3)(2) (4) and "PuOI". In contrast, 2Ce and 2Pr do not react with benzaldehyde to produce 4.

Reaction of the terphenyl bis(anilide) ligand [{K(DME) } L ] (L = {C H [(2,6- Pr C H )NC H ] } ) with trivalent chloride "MCl " salts (M = Ce, U, Np) yields two distinct products; neutral L M(Cl)(THF) (1 ) (M = Np, Ce), and the "-ate" complexes [K(DME) ][(L )Np(Cl) ] (2 ) or ([L M(Cl) (μ-K(X) )]) (2 , 2 ) (M = Ce, U) (X = DME or Et O) (2 ). Alternatively, analogous reactions with the iodide [MI (THF) ] salts provide access to the neutral compounds L M(I)(THF) (3 ) (M = Ce, U, Np, Pu). All complexes exhibit close arene contacts suggestive of -interactions with the central arene ring of the terphenyl backbone, with 3 comprising the first structurally characterized Pu -arene moiety. Notably, the metal-arene bond metrics diverge from the predicted trends of metal-carbon interactions based on ionic radii, with the uranium complexes exhibiting the shortest M-C distance in all cases. Overall, the data presents a systematic study of f-element M- -arene complexes across the early actinides U, Np, Pu, and comparison to cerium congeners.

In nuclear forensic analyses, measurements of actinideelementsin a sample can assist with identifying interdicted or unknown materials.While these radiochemical signatures have been extensively investigatedin uranium materials, less is known about bulk neptunium samples.This paper describes the measurement of trace actinide concentrationsand isotopic profiles in a Np-237 oxide sample. Uranium,plutonium, americium, and curium concentrations and isotopic profilesin the sample were determined and deemed potentially useful for distinguishingdifferent sources of Np-237. Several different potentialradiochronometry systems were also investigated; discordant resultsindicate that the Np sample was never completely purified of otheractinide elements, or that subsequent contamination of the sampleoccurred. Few prior studies of neptunium materials have been reported,and these data suggest that trace actinide constituents could provideunique signatures to identify material out of regulatory control.