Synthesis, Electrochemistry, and Reactivity of the Actinide Trisulfides

Communication pubs.acs.org/IC

Synthesis, Electrochemistry, and Reactivity of the Actinide Trisulfides [K(18-crown-6)][An(η3‑S3)(NR2)3] (An = U, Th; R = SiMe3) Danil E. Smiles, Guang Wu, and Trevor W. Hayton* Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106, United States S Supporting Information *

Scheme 1. Synthesis of Complex 2

ABSTRACT: The reaction of [Th(I)(NR2)3] (R = SiMe3) with [K(18-crown-6)]2[S4] results in the formation of [K(18-crown-6)][Th(η3-S3)(NR2)3] (2). Oxidation of 2, or its uranium analogue, [K(18-crown-6)][U(η3-S3)(NR2)3] (1), with AgOTf, in an attempt to generate an [S3]•− complex, results in the formation of [K(18-crown6)][An(OTf)2(NR2)3] (3, An = U; 4, An = Th) as the only isolable products. These results suggest that the putative [S3]•− ligand is only weakly coordinating and can be easily displaced by nucleophiles.

that the [Th(NR2)3]+ binding pocket is too sterically demanding to accommodate all four S atoms, and one S atom is ejected as elemental sulfur. The 1H NMR spectrum of 2 exhibits two sharp resonances at 0.73 and 3.16 ppm, in a 54:24 ratio, which are assignable to the methyl groups of the silylamide ligands and the methylene groups of the 18-crown-6 moiety, respectively (Figure S1). Complex 2 crystallizes in the triclinic space group P1̅ as an Et2O solvate, 2·Et2O, and its solid-state molecular structure is shown in Figure 1. Complex 2 is isostructural to its uranium analogue 1 and features a distorted pseudotetrahedral geometry about the Th center along with an [η3-S3]2− ligand. The Th−S

hile the trisulfur radical anion [S3]•− is best known for imparting the blue color to the mineral lapis lazuli,1−3 this fragment has actually been observed in a wide variety of settings.4,5 For example, [S3]•− is likely formed during the discharge of Li−S batteries.6−10 Additionally, [S3]•− is likely present in sulfur-bearing hydrothermal solutions and may play a role in metal solubilization and transport.11−13 [S3]•− has also been invoked as an S atom source in several organic transformations, including the synthesis of benzothiazenes.14−16 Surprisingly, despite the intense interest in this species and its fundamental importance, no crystallographic data are available for this fragment.4,17 Given the venerable history of stabilizing reactive species at metal centers in organometallic chemistry,18 we reasoned that we could similarly stabilize the [S3]•− anion by ligation to a metal ion. In this regard, we recently reported the synthesis of [K(18-crown-6)][U(η3-S3)(NR2)3] (1; R = SiMe3),19 which contains the closely related [S3]2− fragment. Herein, we describe an investigation into the electrochemical properties of both 1 and its thorium analogue, [K(18-crown6)][Th(η3-S3)(NR2)3] (2), along with an exploration of their reactivity with chemical oxidants, in an effort to generate the [S3]•− moiety. We recently reported the synthesis of the uranium trisulfide complex 1 via reaction of the tetrasulfide dianion [K(18-crown6)]2[S4]19 with the uranium(IV) iodide complex [U(I)(NR2)3].20 We have since endeavored to synthesize the analogous thorium complex, with the hope of studying the reactivity of these two complexes in concert. Thus, the addition of 1 equiv of [K(18-crown-6)]2[S4] to a tetrahydrofuran (THF) solution of [Th(I)(NR2)3]21 affords a pale-green-yellow solution. Crystallization of the resulting green-yellow solid from diethyl ether affords the thorium trisulfide 2 as pale-greenyellow crystals in 53% yield (Scheme 1). Interestingly, only three of the four S atoms of [K(18-crown-6)]2[S4] are incorporated into the final product.19 To explain this observation, we suggest

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© 2016 American Chemical Society

Figure 1. Solid-state molecular structure of 2·Et2O, with 50% probability ellipsoids. Et2O solvate and H atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): Th1−S1 = 2.9224(9), Th1−S2 = 2.8679(9), Th−S3 = 2.811(1), S1−S2 = 2.062(1), S2−S3 = 2.072(1), K1−S1 = 3.172(1), K1−S2 = 3.760(1); N1−Th1−N2 = 96.2(1), N2−Th1−N3 = 107.4(1), N1−Th1−Th3 = 122.0(1). Received: July 6, 2016 Published: September 6, 2016 9150

DOI: 10.1021/acs.inorgchem.6b01618 Inorg. Chem. 2016, 55, 9150−9153

Communication

Inorganic Chemistry

We next endeavored to chemically oxidize 1 and 2 in an effort to isolate a complex containing the elusive [S3]•− ligand. Thus, oxidation of complex 1 with 1 equiv of AgOTf in THF results in the formation of a black precipitate and an orange solution, from which we were able to isolate [K(18-crown-6)][U(OTf)2(NR2)3] (3) as the only identifiable product. Similarly, oxidation of 2 with 1 equiv of AgOTf in THF affords [K(18crown-6)][Th(OTf)2(NR2)3] (4) as the only isolable thoriumcontaining product. Upon crystallization, complexes 3 and 4 can be isolated as colorless crystalline solids in 16% and 21% yields (based on An), respectively (Scheme 2). Complexes 3 and 4 are

distances in 2 range from Th1−S3 = 2.811(1) Å to Th1−S1 = 2.9224(9) Å. These values are in line with other Th−S single bonds.22 We attribute the asymmetry of the Th−S distances, along with the large differences in the N−Th−N angles [N1− Th1−N2 = 96.2(1)°, N2−Th1−N3 = 107.4(1)°, and N1−Th1− N3 = 122.0(1)°], to the steric mismatch between the large [η3S3]2− moiety and the [Th(NR2)3]+ binding pocket. Finally, the S−S bond distances in 2 [S1−S2 = 2.062(1) Å and S2−S3 = 2.072(1) Å] are similar to those of 1 and other actinide polysulfide complexes,19,22−29 as well as those in Cp*2M(κ2-S3) (M = Ti, Zr, Hf).30−32 We hypothesized that the ability of the [An(NR 2 ) 3] + framework to stabilize the [S3]2− ligand might extend to the extremely rare [S3]•− radical anion, and we postulated that this species could be readily accessed via 1e− oxidation of the [S3]2− ligand in complexes 1 and 2. Consequently, we investigated the electrochemistry of complexes 1 and 2 with cyclic voltammetry. The cyclic voltammogram of complex 1 in THF reveals a quasireversible oxidation feature at −0.61 V (vs Fc/Fc+; Figure 2). In

Scheme 2. Oxidation of [K(18-crown-6)][An(η3-S3)(NR2)3]

formally generated by exchange of the [S3]•− fragment with [OTf]− and the incorporation of 1 equiv of KOTf. Thus, their maximum yields can only be 50%. The fate of the putative [S3]•− species is unknown; however, we note that the characteristic blue color of [S3]•− is not observed over the course of the reaction. That said, polysulfides are known to be prone to disproportionation, and it is possible that [S3]•− decomposes upon release from the metal.34−36 Also of note, other oxidants, such as I2, [Fc][PF6], and [Fc][BPh4] also failed to generate an [S3]•− complex upon reaction with 1 or 2. For example, the reaction of I2 with 2 led to the formation of [K(18-crown-6)][Th(I)2(NR2)3] as the only product isolated, while the reaction of [Fc][PF6] with 1 led to the formation of [K(18-crown6)][U(F)2(NR2)3], where the F− ligands likely result from fluoride abstraction from the PF6− anion. Given the similarity of these two complexes to 3 and 4 their complete characterization was not pursued. Complex 3 crystallizes in the monoclinic space group P21/n, while complex 4 crystallizes in the triclinic space group P1.̅ Both crystallize as diethyl ether solvates, 3·Et2O and 4·Et2O, and their solid-state molecular structures are shown in Figure 3. In the solid state, complexes 3 and 4 exist as one-dimensional coordination polymers, wherein the two triflate moieties are bridged by the [K(18-crown-6)]+ moiety. Each complex features a trigonal-bipyramidal geometry about the metal center [3, O1− U1−O4 = 174.0(3)°, N−U−N (av.) = 120.0°, N−U−O (av.) = 90.2°; 4, O1−Th1−O4 = 171.7(4)°, N−Th−N (av.) = 120.0°, N−Th−O (av.) = 90.2°]. The U−N (av. 2.25 Å) and Th−N (av. 2.34 Å) distances are similar to those of other uranium(IV) and thorium(IV) silylamide complexes.21,37−39 In addition, the U−O distances in 3 [2.374(7) and 2.402(7) Å] are ca. 0.05 Å shorter than the Th−O distances in 4 [2.440(9) and 2.457(9) Å], consistent with the smaller ionic radii of U4+ vs Th4+.40 In summary, our cyclic voltammetry results reveal that coordination of the [S3]2− fragment to a metal center has a profound effect on its electrochemical properties. Additionally, these results, in combination with the chemical oxidation experiments, suggest that the [S3]•− ligand, if formed, is only weakly coordinating and can be easily displaced by weak nucleophiles, such as [OTf]−. A similar conclusion was reached by Tossell in his density functional theory study of Cu(S3).12 Likewise, Paris and Plichon determined that [S3]•− is not

Figure 2. Cyclic voltammograms of complexes 1 and 2 (200 mV/s, vs Fc/Fc+). Measured in THF with 0.1 M [NBu4][BPh4] as the supporting electrolyte.

contrast, the cyclic voltammogram of complex 2 exhibits an irreversible oxidation feature at −0.52 V (vs Fc/Fc+ at 200 mV/s; Figure 2). This feature is irreversible at all scan rates. Because complex 2 cannot undergo a metal-based oxidation, we tentatively attribute this feature to an [S3]2−/[S3]•− oxidation event. Accordingly, we have assigned the quasi-reversible redox couple observed for 1 to the UV/UIV redox event. That said, it is possible that this feature is due to an [S3]2−/[S3]•− oxidation event, but we prefer the UV/UIV assignment because of its quasireversibility, behavior that is not observed in 2. For comparison, the [S3]2−/[S3]•− redox couple was previously reported at a substantially lower potential [−1.77 V vs Fc/Fc+ in dimethylacetamide (DMA)].33 The large decrease in the [S3]2−/[S3]•− potential (ca. 1.2 V) that occurs upon coordination to the Th4+ center can be rationalized by the ability of the [Th(NR2)3]+ cation to stabilize the negative charge of the trisulfide fragment, making the [S3]2− moiety less reducing. Finally, complex 1 also features an irreversible redox feature at −2.99 V (vs Fc/Fc+ at 200 mV/s; Figure S11), which we attribute to a UIV/UIII reduction process. This feature is not observed for complex 2, which further solidifies this assignment. 9151

DOI: 10.1021/acs.inorgchem.6b01618 Inorg. Chem. 2016, 55, 9150−9153

Inorganic Chemistry

Communication

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Biosciences, and Geosciences Division, under Contract DE-SC-0001861.

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Figure 3. Solid-state molecular structures of 3·Et2O (above) and 4·Et2O (below), with 50% probability ellipsoids. Et2O solvate and H atoms are omitted for clarity.

protonated by benzoic acid in DMA and concluded that it is a relatively poor base.33 The apparent steric clash between the [An(NR2)3]+ fragment and the putative [S3]•− ligand also helps to explain its displacement from the An center. We believe these results identify a significant challenge in the synthesis of an authentic [S3]•− complex, namely, that [S3]•− is less strongly coordinating than [S3]2− by virtue of its lower charge and can be readily displaced from a metal center. This hypothesis may also explain why an [S3]•− complex has remained so elusive, despite the long history of polysulfide coordination chemistry.41 Going forward, isolation of an [S3]•− complex will likely require better tuning of the metal’s binding pocket, in combination with the use of weakly nucleophilic solvents and oxidants, to prevent its dissociation from the metal center.

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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/acs.inorgchem.6b01618. Crystallographic details for 2−4 (CIF) Experimental procedures and spectral data for complexes 2−4 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest. 9152

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Communication

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