Communication Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
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Identification of the Formal +2 Oxidation State of Neptunium: Synthesis and Structural Characterization of {NpII[C5H3(SiMe3)2]3}1− Jing Su,#,† Cory J. Windorff,#,‡,§ Enrique R. Batista,*,† William J. Evans,*,§ Andrew J. Gaunt,*,‡ Michael T. Janicke,‡ Stosh A. Kozimor,*,‡ Brian L. Scott,∥ David H. Woen,§ and Ping Yang*,† †
Theoretical Division, ‡Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States § Department of Chemistry, University of California−Irvine, Irvine, California 92697-2025, United States ∥ Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States S Supporting Information *
electronic configuration or the mixed principal quantum number 5f46d1 configuration. Accessing Np2+ is even more challenging than Pu2+ in a redox sense.7 Indeed, in parallel to our own studies with Pu, two reports from Walter, Arnold, and co-workers appeared in the literature detailing attempts to isolate Np2+ complexes.8 Alkali metal reductions of Np3+ starting materials ligated with a transcalix[2]benzene[2]pyrrole (LAr) supporting ligand,8a or with three Cp′ (Cp′ = C5H4SiMe3) anions,8b generated intensely colored solutions. The product upon reaction with reductant in the case of LAr was stable enough to allow UV/vis/NIR spectra to be recorded but too unstable in the case of Cp′ to allow any characterization. These observations suggested that Np2+ may be accessible but, unfortunately, structural evidence of Np2+ was elusive in both cases. Our synthetic approach relied on utilizing neptunium metal as a starting material and Cp″ as an ancillary ligand, which has been found to give more stable +2 complexes for U compared to the Cp′ ligand.2b Oxidation of Np0 with iodine in diethyl ether generated a brown powder, assigned as putative “NpI3(Et2O)x”, similar to analogous Pu chemistry.9 Direct treatment of this powder with 3.1 equiv of KCp″ in diethyl ether led to an n-hexane soluble species isolated as a browngreen solid that we attribute as NpIIICp″3 (1, 59% crude yield) (Scheme 1), based on analogy to the same reaction for other actinides and lanthanides.1,2,6 Single-crystals of 1 could not be isolated on the small synthetic scales employed. Complex 1 exhibits a 1H NMR spectrum consistent with assignment as NpIIICp″3 [−0.18 ppm, 54 H, Si(CH3)3; −5.44 ppm, 3 H, and −10.31 ppm, 6 H, ring protons]. These shifts are similar in magnitude to those reported for NpIIICp′3.8b The 29Si{1H} NMR spectrum of 1 contained a single resonance at −76 ppm, compared to the +8.4 ppm observed for the Pu3+ analog.6 The UV/vis/NIR spectrum of 1 is discussed later in conjunction with TDDFT calculations. Treatment of an Et2O solution of 1 (and crypt) with 1.1 equiv of KC8, followed by filtration of the solution through a KC8 column resulted in a dark brown filtrate (all reagents and glassware were precooled to −35 °C). After removal of volatiles in vacuo, the dark brown product was washed with n-hexane and n-pentane to remove any unreacted NpIIICp″3 (1) and
ABSTRACT: We report a new formal oxidation state for neptunium in a crystallographically characterizable molecular complex, namely Np2+ in [K(crypt)][NpIICp″3] [crypt = 2.2.2-cryptand, Cp″ = C5H3(SiMe3)2]. Density functional theory calculations indicate that the ground state electronic configuration of the Np2+ ion in the complex is 5f46d1.
O
ver the past decade, the formal +2 oxidation state has been identified in isolable molecular compounds for all of the 4f metal ions (except Pm) and subsequently uranium and thorium.1,2 Among the lanthanide (Ln) Ln2+ series, ten elements (La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Lu) were found to have the unusual 4fn5d1 ground states, instead of the traditional 4fn+15d0 electron configurations found for Eu, Yb, Sm, and Tm.1a,3 The suggested rationale for this unusual state was that, in the C3-symmetric tris(cyclopentadienyl) coordination environment, a 5d-orbital was comparable in energy to the 4f-orbitals.1,3 Characterization of the first U2+ and Th2+ complexes, which were also tris(cyclopentadienyl) complexes, pointed toward dorbital occupancy with 5f36d1 (U2+) and 5f06d2 (Th2+) electron configurations.2a,c Other ligand frameworks have since been shown to provide access to molecular U2+ complexes as well.2d At some point in the actinide (An) series, around plutonium, it is predicted that An2+ ions will switch from 5fn6d1 to 5fn+16d0 configuration, due to stabilization of the 5f orbitals that occurs as atomic number increases in the actinide series.4 However, syntheses, isolation, and characterization of transuranic molecules containing An2+ ions is extremely challenging because it combines highly air-sensitive chemistry with high specific-activity α-particle emitting radionuclides.5 Recently, we overcame these technical challenges and reported [K(crypt)][PuIICp″3] [crypt = 2.2.2-cryptand, Cp″ = C5H3(SiMe3)2], the first structural confirmation of a molecule formally containing a Pu2+ ion.6 Analysis by UV/vis/NIR spectroscopy and density functional theory (DFT) calculations suggested that although the 5f56d1 state was energetically accessible, [PuIICp″3]1− adopted a 5f66d0 ground state. Intrigued by these results, we set out to prepare [NpIICp″3]1−, as it was unclear if the Np2+ ion would adopt the conventional 5f56d0 ground state © XXXX American Chemical Society
Received: April 11, 2018
A
DOI: 10.1021/jacs.8b03907 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society Scheme 1. Synthesis of [K(crypt)][NpIICp″3], 2a
a
Only the anion shown for clarity.
broad features at −2.58 (3 H) and −3.44 (6 H) ppm. Resonances for the [K(crypt)]+ cation are observed at 3.53, 3.49, and 2.51 ppm. Treatment of 2 with the one-electron oxidant AgBPh4 regenerates the parent NpIIICp″3 complex 1, as indicated by 1 H NMR resonances of the product being consistent with those assigned to 1. This observation lends additional evidence to the assignment of 2 as formally containing Np2+. Further NMR and UV/vis/NIR spectra, along with reactivity/decomposition properties, are in the Supporting Information. DFT calculations were used to calculate the ground-state electronic structures of NpIIICp″3, 1, the [NpIICp″3]1− anion in 2 and the theoretical Np3+ hydride complex [HNpIIICp″3]1− (to provide further validation of 2 as containing Np2+ and not a crystallographically undetected Np3+−hydride moiety). DFT/ PBE calculations were conducted to optimize the geometric structures of NpIIICp″3, [HNpIIICp″3]1−, and [NpIICp″3]1− (Figure S10 and Table S1). The calculations reveal a ground state of 5f46d0 electronic configuration for both NpIIICp″3 and [HNpIIICp″3]1−. The predicted average metal-(ring centroid) distance is 2.499 Å for NpIIICp″, which is 0.05 Å shorter than that in [HNpIIICp″3]1−, and very close to the experimental value of 2.506 Å for PuIIICp″3.6 Similar to published DFT calculations on the relative energies of 4f n+1 5d 0 versus 4f n 5d 1 configurations in [LnIICp′3]1−,3 more advanced functionals over the GGA PBE functional are required to evaluate the ground-state electronic structure of [NpIICp″3]1−. All employed functionals including meta-GGA TPSS, hybrid meta-GGA TPSSH and hybrid PBE0, B3LYP, and BHandHLYP predict that the ground state of [NpIICp″3]1− has a 5f46d1 configuration consistent with CASSCF/NEVPT2 results (see page S17), which is more than 8 kcal/mol more stable than the alternative 5f56d0 configuration. In contrast, calculations with GGA functionals predict the alternative 5f56d0 configuration as the ground state of [NpIICp″3]1− (Table S2). Consistently, DFT/PBE calculated NpII(5f46d1)−Cpcentroid distances are in excellent agreement with experimental results with average values differing by 0.011 Å. However, it should be noted that the calculated averaged NpII(5f56d0)−Cpcentroid distance is also only 0.018 Å shorter than the experimental values. The calculated NpIII−Cpcentroid distance in [HNpIIICp″3]1− is 0.023 Å longer than experimental distances of compound 2. The calculated 0.017 Å difference in Np−Cpcentroid distance between 1 and 2 is in good agreement with that observed between UIIICp′3 and [UIICp′3]1− (∼0.013 Å), and is also very close to the 0.03 Å difference generally observed for 4fn5d1 Ln2+ complexes compared to their 4fn5d0 Ln3+ counterparts.2a,10
dried in vacuo to afford [K(crypt)][NpIICp″3] (2) in 94% crude yield (Scheme 1). Single-crystals suitable for X-ray diffraction studies were obtained from an ether/hexanes solution of 2 stored at −35 °C. Visually, the crystals were of similar appearance and morphology to the Pu2+ complex. The solid-state structure provided confirmation of the product to be [K(crypt)][NpIICp″3] (2) (Figure 1). For comparison,
Figure 1. Molecular structure of [K(crypt)][NpIICp″3], 2, shown with 50% probability ellipsoids and hydrogen atoms omitted for clarity.
previous attempts to isolate [K(crypt)][NpIICp′3] were thwarted by thermal instability (solutions of the unconfirmed reaction product decompose above −10 °C).8b Solutions of 2 are also susceptible to decomposition, but on the time scale of sample preparation and analysis could be handled at ambient temperature, consistent with Cp″ conferring greater stability to An2+ molecules than Cp′.2b Crystals of 2 occupy the P1̅ space group and are isomorphous with [K(crypt)][ThIICp″3], [K(crypt)][PuIICp″3], and [K(crypt)][NdIICp″3].1d,2c,6 Note that although several salts of the [UIICp″3]1− anion have been synthesized, only the [K(18-crown-6)(THF)2]+ salt was crystallographically characterized.2a,b In 2, the three Np-(ring centroid) distances of 2.513, 2.525, and 2.542 Å are similar to those in the Pu2+ (2.509, 2.521, and 2.536 Å), Th2+ (2.512, 2.519, and 2.533 Å), and Nd2+ (2.530, 2.543, and 2.559 Å) isomorphs. Similar to the Pu2+ complex, the UV/vis/NIR spectrum of 2 (in THF solution) exhibits an intense broad band at 437 nm (molar absorptivity of ∼2000 M−1 cm−1), a feature characteristic of the [AnIICp″3]1−/[AnIICp′3]1− complexes.2,6 For example, the Pu2+ analog has a broad band with maxima at 469 nm and molar absorptivity of ∼2700 M−1 cm−1. The 1H NMR spectrum of 2 in THF-d8 is distinct from that observed for 1, exhibiting proton signals for the SiMe3 groups at 0.28 ppm (54 H) and ring protons tentatively assigned to B
DOI: 10.1021/jacs.8b03907 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society Mulliken population analysis from non-GGA functionals at PBE optimized [NpIICp″3]1− geometry with 5f46d1 configuration shows a net 5f spin density of ∼3.90, 6d spin density of ∼0.70, and 7s spin density of ∼0.23 (Table S2). This result suggests that the [NpIICp″3]1− HOMO is best described as a 6dz2 orbital (with a small 7s orbital hybridization) that is nonbonding with respect to the Np−Cp″ interaction (Figure S11). The orbital composition is similar to that reported previously for the HOMO of [UIICp′3]1− (5f36d1), as well as those from the 4fn5d1 Ln2+ complexes.2a,3,10 In contrast, the HOMO of the alternative (higher energy) configuration 5f56d0−[NpIICp″3]1− has predominant 5f character with a small (∼10%) 6d component. However, it is important to point out that this higher energy “5f56d0” configuration is not “pure”. The Mulliken population at the non-GGA functional levels shows a net 5f spin density of ∼4.40 and 6d spin density of ∼0.35, which is intermediate between 5f56d0 and 5f46d1. For both NpIIICp″3 (5f46d0) and [HNpIIICp″3]− (5f46d0), the DFT calculations show that the Mulliken net spin densities are almost exclusively distributed on the 5f-orbitals (Table S3) and that the dz2 dominated molecular orbital is unoccupied. Thus, the calculations suggest that [NpIICp″3]1− adopts the unusual fnd1 configuration observed for the Ln2+ (Ln = La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu), U2+ and Th2+ compounds with d occupation, and is different from the [PuIICp″3]1− compound, for which the ground state is dominated by the 5f66d0 configuration.1,2,6 The UV/vis/NIR spectrum of NpIIICp″3, 1, in n-hexane solution contains a broad and weak band (with numerous peaks) in the range of 600−1000 nm. Time-dependent DFT (TDDFT) calculations using the TPSS functional suggest that this band originates from 5f→5f and 5f→6d transitions (Table S4 and Figure 2). Below 500 nm, the intense transitions are assigned to ligand-to-metal charge transfer (LMCT), i.e., ligand→5f/6d, and metal-to-ligand charge transfer (MLCT), i.e., 5f→ligand. Reduction of 1 to 2 imparts marked changes in the UV/vis/ NIR spectrum. The spectrum of 2 in THF solution is dominated by a broad feature with absorption maximum at 437 nm and extending past 700 nm (Figure 2). With molar absorptivity of ∼2000 M−1 cm−1, this band is considerably more intense than metal-to-metal transitions with 5f→5f/6d and 6d→5f character usually observed in this region for An3+containing molecules. TDDFT calculations on [NpIICp″3]1− attribute these strong absorptions to metal-to-ligand charge transfer excitations originating from Np 6d and 5f orbitals to Cp″ orbitals of π* character (Table S5). The red shift of these transitions, compared with those in 1, may be rationalized by an increase in 5f orbital energy due to increased electron repulsion in negatively charged [NpIICp″ 3 ]1− and by additional transitions from the HOMO 6dσ orbital in [NpIICp″3]1−. A red shift of these intense transitions was also observed in the comparison of the UV/vis/NIR spectra of [UIICp′3]1− with that of UIIICp′3.2a The geometry does not appear to cause significant differences in the observed features (see Table S1 and Figure S15). The TDDFT predicted electronic absorption spectrum of hypothetical [HNpIIICp″3]1− (Figure S12) does not exhibit intense bands in this region, but has the typical charge transfer bands at lower wavelengths, assigned to predominantly 5f→ligand MLCT transitions and ligand→5f/ 6d LMCT transitions (Table S6). The Laporte-forbidden 5f→ 5f transitions occur in the energy range of >700 nm. Therefore,
Figure 2. Solution phase UV/vis/NIR experimental data of (a) NpIIICp″3, 1, and (b) [K(crypt)][NpIICp″3], 2 (black traces). The orange bars represent the energy and oscillator strength for TDDFT calculated UV/vis/NIR spectra (orange traces).
comparison of simulated/experimental spectra is consistent with 2 containing Np2+ and not Np3+. In summary, we have synthesized, isolated, and characterized an organoneptunium complex that formally contains a Np2+ metal ion, representing confirmation of a new oxidation state in molecular chemistry for element 93. This extends the accessible range of actinide(II) ions within the [AnIICp″3]1− series to now comprise Th2+, U2+, Np2+, and Pu2+. Further studies are needed to understand the interplay between 5fn6d1 and 5fn+16d0 configurations and the impact of ligand changes/modifications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.8b03907. Complete experimental and calculational details, NMR and UV/vis/NIR spectra (PDF) X-ray crystallographic details (CIF)
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AUTHOR INFORMATION
Corresponding Authors
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[email protected] ORCID
Jing Su: 0000-0002-6895-2150 Enrique R. Batista: 0000-0002-3074-4022 William J. Evans: 0000-0002-0651-418X Andrew J. Gaunt: 0000-0001-9679-6020 David H. Woen: 0000-0002-5764-1453 Ping Yang: 0000-0003-4726-2860 C
DOI: 10.1021/jacs.8b03907 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Communication
Journal of the American Chemical Society Author Contributions #
Authors contributed equally to the work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the U.S. Department of Energy (DOE), Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences, Heavy Element Chemistry program (W.J.E., contract DE-SC0004739; A.J.G., S.A.K., B.L.S., E.R.B., and P.Y., contract DE-AC52-06NA25396), the Office of Workforce Development for Teachers and Scientists, Office of Science Graduate Student Research (SCGSR) program (administered by the Oak Ridge Institute for Science and Education (ORISE) for the DOE) (C.J.W., contract DE-AC0506OR23100), the Glenn T. Seaborg Institute for a postdoctoral Fellowship (J.S.), and Dr. Benjamin W. Stein for supplying dry THF.
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REFERENCES
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DOI: 10.1021/jacs.8b03907 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
