A Uranium Tri-Rhenium Triple Inverse Sandwich ... - ACS Publications

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A Uranium Tri-Rhenium Triple Inverse Sandwich Compound Michael A. Boreen,†,‡ Trevor D. Lohrey,†,‡ Guodong Rao,§ R. David Britt,§ Laurent Maron,# and John Arnold*,†,‡ †

Department of Chemistry, University of California, Berkeley, Berkeley, California 94720, United States Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Department of Chemistry, University of California, Davis, Davis, California 95616, United States # LPCNO, Université de Toulouse, INSA Toulouse, 135 Avenue de Rangueil, 31077 Toulouse, France Downloaded via IDAHO STATE UNIV on March 20, 2019 at 19:49:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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core.17 Given this intriguing diversity of binding modes, we were curious to investigate the interaction of this metalloligand with actinides. In particular, the rhenium moiety provided an opportunity to construct the first inverse sandwich compound involving an actinide and a transition metal. Because of the strongly reducing nature of Na[Re(η5Cp)(BDI)], we decided to probe reactivity using a uranium(III) precursor. UI3(1,4-dioxane)1.5 reacted quickly with 3 equiv of Na[Re(η5-Cp)(BDI)] and excess THF to form (THF)U[(μ-η5:η5-Cp)Re(BDI)]3 (1·THF), which was isolated as dark red crystals in 83% yield (Scheme 1). The

ABSTRACT: Salt metathesis between the anionic rhenium(I) compound, Na[Re(η5-Cp)(BDI)] (BDI = N,N′-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate), and the uranium(III) salt, UI3(1,4-dioxane)1.5, generated the triple inverse sandwich complex, U[(μη5:η5-Cp)Re(BDI)]3, which was isolated and structurally characterized as the Lewis base adducts, (L)U[(μ-η5:η5Cp)Re(BDI)]3 (1·L, L = THF, 1,4-dioxane, DMAP). The assignment as one uranium(III) and three rhenium(I) centers was supported by X-ray crystallography, NMR and EPR spectroscopies, and computational studies. An unusual shortening of the rhenium−Cp bond distances in 1·L relative to Na[Re(η5-Cp)(BDI)] was observed in the solid-state and reproduced in calculated structures of 1·THF and the anionic fragment, [Re(η5-Cp)(BDI)]−. Calculations suggest that the electropositive uranium center pulls electron density away from the electron-rich rhenium centers, reducing electron−electron repulsions in the rhenium−Cp moieties and thereby strengthening those interactions, while also making uranium−Cp bonding more favorable.

Scheme 1. Synthesis of (L)U[(μ-η5:η5-Cp)Re(BDI)]3a

T

he discovery and structural elucidation of ferrocene, the first sandwich compound, gave rise to a massive increase in interest in organometallic chemistry owing to the unique properties and electronic structure of this molecule.1−7 Similarly, carbocyclic ligands have been critical for the development of actinide chemistry, with compounds like uranocene key for enabling the study of actinide electronic structure and the development of novel reactivity.8−11 Inverse sandwich compounds, in which a carbocyclic ring bridges two metal centers, have also provided access to novel structures in organometallic compounds;12−15 uranium inverse sandwich compounds were reviewed recently.16 Recently, our group discovered a highly reactive anionic rhenium(I) compound, Na[Re(η5-Cp)(BDI)] (BDI = N,N′bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate), which displays a bridging Cp ligand bound η5 both to a sodium and a rhenium atom in the solid-state.17 Furthermore, the anionic fragment [Re(η5-Cp)(BDI)]− was found to act as a metalloligand for zinc(I), forming the complex [ZnRe(η5Cp)(BDI)]2, in which rhenium is directly bonded to zinc, resulting in a nearly linear, tetrametallic Re−Zn−Zn−Re © XXXX American Chemical Society

a

(1·L, L = THF, 1,4-dioxane, DMAP).

structure of 1·THF was determined using X-ray crystallography, confirming that three [Re(η5-Cp)(BDI)]− metalloligands bond to uranium through bridging Cp ligands (Figure 1). Complex 1·THF therefore represents the first structurally characterized combination of an actinide and a transition metal in an inverse sandwich complex.18 The synthetic method to prepare 1·THF is also notable, as diuranium inverse sandwich compounds have generally been prepared by reduction of an arene, commonly also acting as a solvent, by low-valent species that are often generated in situ.19−30 However, the synthesis of 1·THF involves the direct assembly of different metal fragments through salt metathesis, potentially allowing for a modular approach to the preparation of similar species with appropriate metal-containing starting materials. Received: February 3, 2019

A

DOI: 10.1021/jacs.9b01331 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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the solid-state. Typically, bonds from metals to bridging μη5:η5-Cp ligands are longer than bonds to corresponding terminal or alkali metal-bound η5-Cp ligands.35−43 As these prior examples mostly contain the same or very similar metals bridged by the Cp ligand, the observation of the opposite effect in 1·THF may be related to the use of two electronically very different metals. We hypothesize that the Lewis acidic uranium center reduces Coulombic repulsion in the highly electron-rich rhenium-Cp moiety. To confirm the trivalent oxidation state of uranium in 1· THF, as bond lengths are not a conclusive metric, we examined 1·THF using EPR spectroscopy (Figure 2). The X-

Figure 1. X-ray crystal structure of 1·THF with 50% probability ellipsoids. Aryl groups are displayed as wireframes; hydrogen atoms and positional disorder are omitted for clarity. Structural metrics are shown in Table S1.

Multiple effects likely contribute to the difference in metalloligand binding in 1·THF (through η5-Cp ligands) compared to that in [ZnRe(η5-Cp)(BDI)]2 (through direct metal−metal bonding to rhenium), though the relative impacts of these effects presently cannot be determined. The greater size of uranium relative to zinc makes direct bonding to rhenium less favorable on steric grounds,31 and the greater electropositivity and more contracted nature of the valence orbitals of uranium relative to zinc decrease the potential to form favorable covalent interactions in direct bonding to rhenium. Additionally, the Cp ligands more effectively saturate the large coordination sphere of uranium than would direct bonding to rhenium. While 1·THF is paramagnetic, the 1H NMR spectrum of this complex shows relatively sharp BDI resonances in the diamagnetic region (Figure S2). This suggested minimal unpaired spin density on the rhenium centers, making it most likely that they remain rhenium(I). In contrast, the Cp signal is relatively broad and shifted far upfield to −31.7 ppm due to its proximity to the uranium(III) f 3 center (see below).32 The solid-state structure of 1·THF displays U−C and Re−C bond ranges of 2.695(5)−2.879(5) and 2.085(5)−2.229(5) Å, respectively, consistent with η5 hapticity for all metal−Cp interactions.33 The U−O distance of 2.586(4) Å and average U−Cp(centroid) distance of 2.501(2) Å in 1·THF are similar to corresponding distances of 2.55(1) and 2.523 Å, respectively, in UCp3(THF).34 The [Re(η5-Cp)(BDI)]− metalloligand does not appear to have a great effect on the coordination geometry at the uranium center as compared to UCp3(THF). The Re−N bonds in 1·THF range from 2.032(3) to 2.052(3) Å and are comparable to the Re−N distance of 2.031(3) Å in Na[Re(η5-Cp)(BDI)].17 This supports the assignment of monovalent rhenium in 1·THF, consistent with NMR spectra suggesting no unpaired electrons on rhenium. Intriguingly, the Re−Cp(centroid) distances in 1·THF, ranging from 1.759(2) to 1.766(2) Å, are all shorter than the Re−Cp(centroid) distance of 1.816(2) Å in Na[Re(η5Cp)(BDI)], in which the Cp ligand is capped by a Na+ ion that also binds to the conjugated BDI π-system of an adjacent [Re(η5-Cp)(BDI)]− anion, forming one-dimensional chains in

Figure 2. X-band EPR frozen solution spectrum of 1·THF (7 mM in toluene) recorded at 10 K. The top, black trace shows the raw spectrum, and the bottom, blue trace shows the spectrum with the contribution from a small amount of Re(η5-Cp)(BDI) subtracted for clarity. See SI for details.

band continuous wave EPR spectrum of 1·THF recorded below 20 K exhibits a broad signal overlapped with a multiline signal that was previously shown to be the small amount of Re(η5-Cp)(BDI) in the sample (Figure S7).44 The fast-relaxing nature of the broad peak and its g-values are consistent with those of other uranium(III) complexes with 4I9/2 ground terms.45,46 Because of the broad nature of the uranium(III) EPR signal, we would not expect it to be possible to resolve any hyperfine interactions, if present, with rhenium. With a better understanding of the oxidation states in 1· THF, we have begun to explore reactivity. Addition of 1 equiv of Ph3CCl to a C6D6 solution of 1·THF led mostly to formation of Re(η5-Cp)(BDI). Additionally, when 3 equiv of Na[Re(η5-Cp)(BDI)] were allowed to react with UCl4 in THF, 1·THF and small amounts of Re(η5-Cp)(BDI) were observed after an initial crystallization. These results suggest that attempts to oxidize 1·THF lead to the oxidation of a rhenium(I) center, followed by the dissociation of neutral Re(η5-Cp)(BDI). Similarly, stoichiometric formation of Re(η5Cp)(BDI) was observed upon oxidation of the heterotetrametallic zinc(I) compound, [ZnRe(η5-Cp)(BDI)]2.17 In contrast, neither the rhenium(I) nor the uranium(III) centers in 1·THF appear to be susceptible to chemical reduction, although we did not try any reductants stronger than KC8. We observed 1·DMAP (see below) to react with any electrolyte we used, including [nBu4N][B(C6F5)4], so we could not obtain solution electrochemical measurements. Varying the L-type donor in step 2 of Scheme 1 led to the isolation of two new adducts: (1,4-dioxane)U[(μ-η5:η5-Cp)Re(BDI)]3 (1·diox) when no L-type ligand was added, and (DMAP)U[(μ-η5:η5-Cp)Re(BDI)]3 (1·DMAP) when 1 equiv of 4-(dimethylamino)pyridine was added. The structures of 1· diox and 1·DMAP were determined by X-ray crystallography (Figure S5 and S6, respectively), and selected structural details B

DOI: 10.1021/jacs.9b01331 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Figure 3. Renderings of selected calculated molecular orbitals of 1·THF: (a) SOMO−5, (b) SOMO−4, and (c) SOMO−3 show both uranium and rhenium contributions in Cp bonding molecular orbitals; (d) SOMO−2, (e) SOMO−1, and (f) SOMO show contributions from uranium f orbitals and rhenium d orbitals. Isosurface value = 0.03.

quite consistent with 1H NMR spectrum of 1·THF discussed above. The three highest energy SOMOs of 1·THF were also found to contain contributions from both uranium and rhenium, each consisting mainly of uranium f orbitals with smaller amounts from rhenium d orbitals (Figure 3d−f). Atomic charges were computed using Natural Population Analysis (NPA). In 1·THF, an average value of −0.87 was calculated for the charge of each Cp ligand, values of approximately −0.5 were calculated for the charge of each nitrogen and oxygen, and a charge of roughly zero was found for each rhenium center versus +0.7 for the much more electropositive uranium center. By comparison, charges of −0.79, −0.5, and −0.13 were calculated for the Cp ligand, nitrogen, and rhenium, respectively, in the parent anionic complex, [Re(η5-Cp)(BDI)]−, indicating that in 1·THF electron density is shifted away from the rhenium centers to facilitate the bonding interactions between the Cp ligands and uranium. These data are consistent with our rationalization of the contraction of the rhenium−Cp bonds in 1·THF relative to Na[Re(η5-Cp)(BDI)], an effect also observed in the optimized structures of 1·THF versus [Re(η5-Cp)(BDI)]−. Specifically, uranium pulls electron density from the electron-rich rhenium centers, reducing electron−electron repulsions within the rhenium−Cp bonds while strengthening the uranium−Cp interactions. Although inverse sandwich compounds with 4- to 8membered carbocyclic rings bridging two actinide centers have been prepared, no complex has been observed with a Cp ligand bridging two uranium centers,16 and only one such example with thorium has been reported.50 The lack of such diuranium complexes has been ascribed to the electrondeficiency of Cp− relative to other carbocycles.16 The computational data above supports this explanation by suggesting a more electron-rich metal-Cp fragment leads to stronger interactions with uranium. In conclusion, we isolated and structurally characterized three Lewis base adducts of the triple inverse sandwich complex, (L)U[(μ-η5:η5-Cp)Re(BDI)]3 (1·L, L = THF, 1,4-

are shown in Table S1 alongside those of 1·THF. Overall, little difference is observed in the metrics of the three adducts. The application of dynamic vacuum to solid 1·diox or 1·THF at room temperature led to the very slow loss of 1,4-dioxane or THF with concurrent gradual decomposition to form the neutral rhenium(II) complex, Re(η5-Cp)(BDI), and the rhenium(III) hydride, Re(H)(η5-Cp)(BDI).17 Therefore, we were unable to isolate the species absent an L-type donor using this strategy. Attempts to prepare the donor-free complex by the reaction of 3 equiv of Na[Re(η5-Cp)(BDI)] with UI3 in diethyl ether were also unsuccessful, and crystallizations from hexane instead yielded small amounts of material comprised of Re(η5-Cp)(BDI) and Re(H)(η5-Cp)(BDI). We therefore conclude that Lewis bases appear both to stabilize the product and facilitate crystallization. To further investigate electronic structure, we performed DFT calculations with corrections applied for dispersion forces (B3PW91-GD3BJ,47−49 see SI for details) on 1·THF and the anionic fragment, [Re(η5-Cp)(BDI)]−. In general, the calculated structures closely resemble the X-ray crystal structures (see bond length comparison in Table S5). The calculated Re−Cp(centroid) distance in [Re(η5-Cp)(BDI)]−, 1.857 Å, is slightly longer than that measured in the crystal structure of Na[Re(η5-Cp)(BDI)], 1.816(2) Å. The average U−Cp(centroid) distance was shorter in the calculated structure than in the crystal structure with values of 2.440 versus 2.501(2) Å, respectively. Importantly, the contraction of the Re−Cp(centroid) distances in 1·THF relative to [Re(η5Cp)(BDI)]− was reproduced computationally. In both 1·THF and [Re(η5-Cp)(BDI)]−, bonding with the Cp fragments was found to be roughly 50% metal and 50% Cp, with a 25% uranium and 75% rhenium ratio to the metal contribution in 1·THF. The contribution of both rhenium and uranium to the same Cp bonding molecular orbitals is exhibited in the SOMO−5, SOMO−4, and SOMO−3 (Figure 3a−c). However, calculations showed the spin density to be effectively localized to the uranium center (3.0), as displayed in an unpaired spin density plot of 1·THF (Figure S8), a result C

DOI: 10.1021/jacs.9b01331 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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thank the reviewers for their insightful comments and suggestions toward improving this paper.

dioxane, DMAP), via salt metathesis between the rhenium(I) compound, Na[Re(η5-Cp)(BDI)], and the uranium(III) salt, UI3(1,4-dioxane)1.5. Structural metrics, NMR and EPR spectroscopies, and DFT calculations all suggest that rhenium and uranium remain monovalent and trivalent, respectively, in 1·L. While 1·THF was not susceptible to reduction with KC8, oxidation led mainly to the formation of the rhenium(II) compound, Re(η5-Cp)(BDI). Computational data suggest that uranium−Cp bonding in 1·L is facilitated by redistribution of electron density away from the electron-rich rhenium centers of [Re(η 5-Cp)(BDI)]− toward uranium. The resulting decrease in Coulombic repulsion between rhenium and the Cp ligands leads to contracted rhenium−Cp bonds, which is atypical of bonding to bridging Cp ligands in inverse sandwich compounds and highlights the potential to discover unique electronic structures when connecting electron-rich transition metals and Lewis acidic f-block metals through bridging ligands.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b01331. Experimental procedures, NMR data, crystallographic data, EPR data, and computational data (PDF) Crystallographic data for 1·diox (CIF) Crystallographic data for 1·THF (CIF) Crystallographic data for 1·DMAP (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Michael A. Boreen: 0000-0001-7325-2717 Trevor D. Lohrey: 0000-0003-3568-7861 Guodong Rao: 0000-0001-8043-3436 R. David Britt: 0000-0003-0889-8436 Laurent Maron: 0000-0003-2653-8557 John Arnold: 0000-0001-9671-227X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences Heavy Element Chemistry Program of the U.S. Department of Energy (DOE) at LBNL under Contract DE-AC02-05CH11231. M.A.B. acknowledges support from the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE 1106400. T.D.L. acknowledges the U.S. DOE Integrated University Program for a graduate research fellowship. EPR spectroscopic studies were funded by the National Institutes of Health (1R35GM126961-01 to R.D.B.). L.M. is a member of the Institut Universitaire de France and acknowledges the Chinese Academy of Science and the Humboldt Foundation for support. The Advanced Light Source (ALS) is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. DOE under Contract No. DE-AC02-05CH11231. Dr. Simon J. Teat is thanked for training and guidance throughout our crystallography work at the ALS. We also D

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DOI: 10.1021/jacs.9b01331 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX