Chalcogen Atom Transfer to Uranium(III): Synthesis and

Nov 30, 2012 - Chalcogen Atom Transfer to Uranium(III): Synthesis and. Characterization of [(R2N)3U]2(μ-E) and [(R2N)3U]2(μ‑η2:η2‑S2) (R = SiM...
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Chalcogen Atom Transfer to Uranium(III): Synthesis and Characterization of [(R2N)3U]2(μ-E) and [(R2N)3U]2(μ‑η2:η2‑S2) (R = SiMe3; E = S, Se, Te) Jessie L. Brown, Guang Wu, and Trevor W. Hayton* Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States S Supporting Information *

ABSTRACT: Addition of 0.0625 equiv of S8 to U(NR2)3 (R = SiMe3) in Et2O generates [(R2N)3U]2(μ-S) (1), which can be isolated in moderate yield by crystallization from cold Et2O. Interestingly, if the U(NR2)3 starting material is contaminated with the U(IV) metallacycle U(CH2SiMe2NSiMe3)(NR2)2, then a second product is also formed in the reaction with S8, namely, [(R2N)3U]2(μ-η2:η2-S2) (2). This species can be separated from 1, in low yield, by virtue of its insolubility in Et2O. Finally, addition of 0.5 equiv of E (E = Se, Te) to U(NR2)3 (R = SiMe3) results in the formation of [(R2N)3U]2(μ-E) (E = Se (3), Te (4)) in moderate yields. Complexes 1−4 were fully characterized, including analysis by X-ray crystallography.

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INTRODUCTION The synthesis and reactivity of metal chalcogenides has been of longstanding interest to organometallic chemists, in part because of their relevance to hydrodesulfurization and bioinorganic chemistry.1−3 Consequently, the group 16 chemistry of the transition-metal,4−14 main-group,15,16 and lanthanide17−19 elements is well established. In contrast, however, the chalcogen chemistry of the actinides is substantially less developed,20−29 but there has been a resurgence of interest in this area over the past few years.28,30−32 In many actinide systems, the introduction of the chalcogenide ligand to the actinide center requires oxidative atom transfer to a low-valent precursor. Under these conditions bridging chalcogenide architectures are typically generated. For example, Burns and co-workers reported the isolation of [(ArO)3U]2(μ-E) (E = O, S) by the addition of a variety of chalcogen donors to the uranium(III) aryloxide complex U(OAr)3 (Ar = 2,6-tBu2C6H3).20 Deployment of similar synthetic methodologies by both Andersen and Meyer also led to the formation of chalcogenide-bridged complexes.21,32,33 Likewise, Boncella and co-workers isolated a tetraselenidebridged complex, [U(NtBu)2(I)(tBu2bpy)]2(μ-η2:η2-Se4), by oxidation of [U(NtBu)2(I)(tBu2bpy)]2 (tBu2bpy = 4,4′-di-tertbutyl-2,2′-bipyridyl) with 4 equiv of elemental selenium.29 Also of note is the isolation of the tetrametallic cubane-like cluster [U(py)2(SePh)(μ3-Se)(μ2-SePh)]4, generated by the oxidation of uranium metal by 1.5 equiv of diphenyl diselenide, in the presence of 0.75 equiv of Se in pyridine.22 Finally, our laboratory reported the synthesis and isolation of an oxobridged complex, [(R2N)3U]2(μ-O), by addition of 1 equiv of trimethylamine N-oxide to U(NR2)3 (R = SiMe3).34 In contrast to the above results, we recently reported the characterization of a series of terminal uranium monochalco© 2012 American Chemical Society

genide complexes, [H3CPPh3][U(E)(NR2)3] (E = S, Se, Te; R = SiMe3),30 synthesized by the addition of the elemental chalcogen to the U(III)−ylide adduct U(H2CPPh3)(NR2)3.35 These complexes represent extremely rare examples of terminal monochalcogenides of the f elements. We postulated that, during the reaction, the ylide ligand in the U(III) starting material promoted the formation of the terminal chalcogenide ligand by limiting the availability of U(NR2)3 in solution, thereby disfavoring the generation of the more common bridging chalcogenide ligand. In an attempt to shed more light on the formation of the terminal monochalcogenides and evaluate the hypothesis that the coordinated ylide favors terminal chalcogenide formation, we explored the reactivity of the parent U(III) complex U(NR2)3 (R = SiMe3)36 with a series of elemental chalcogens.

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RESULTS AND DISCUSSION

Addition of 0.0625 equiv of S8 to an Et2O solution of U(NR2)3 results in a rapid color change from dark purple to bright orange. Crystallization from Et2O results in the isolation of [(R2N)3U]2(μ-S) (1) as orange crystalline blocks in 41% yield (Scheme 1). Complex 1 is extremely soluble in hexane, toluene, and ethereal solvents, a factor that likely impedes its isolation in higher yields. The room-temperature 1H NMR spectrum of 1 in C6D6 consists of a broad singlet at −6.72 ppm, assignable to the protons of the SiMe3 substituents. This resonance has a nearly identical chemical shift (−6.71 ppm) in py-d5. Additionally, the UV−vis−near-IR spectrum of 1 in THF is Special Issue: Recent Advances in Organo-f-Element Chemistry Received: October 24, 2012 Published: November 30, 2012 1193

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Scheme 1.

consistent with the presence of a U(IV) center.30,34,37,38 Complex 1 can also be isolated in comparable yield via addition of the S atom transfer reagent Ph3PS to U(NR2)3, confirming the results of an earlier report.39 More importantly, the characterization of 1 by 1H NMR spectroscopy provides insight into the recently reported formation of a terminal monosulfide complex, [H3CPPh3][U(S)(NR2)3].30 Specifically, crude samples of [H3CPPh3][U(S)(NR2)3] often contain a minor resonance at −6.79 ppm in their 1H NMR spectra in pyd5 (Figure S10, Supporting Information). This was previously assigned to an unidentified impurity. However, the chemical shift nearly identical with that observed for 1 suggests that 1 is also generated during the formation of [H3CPPh3][U(S)(NR2)3], and suggests that multiple S atom transfer pathways are operative upon reaction of S8 with U(H2CPPh3)(NR2)3. Interestingly, monitoring the reaction of U(NR2)3 with either 0.125 or 0.375 equiv of S8 by 1H NMR spectroscopy reveals clean formation of complex 1 (Figures S7 and S8, Supporting Information). Thus, despite the presence of excess sulfur in these reactions, only 0.5 equiv of sulfur (per uranium) is incorporated into the product. The isolation of the chalcogenide-bridged complex lies in striking contrast with the terminal chalcogenide complex observed in the reaction between S8 and U(H2CPPh3)(NR2)3.30 Previously, we argued that the terminal U(V) chalcogenide U(E)(NR2)3 was transiently formed in the reaction between U(H2CPPh3)(NR2)3 and elemental chalcogens, before being converted to the final product, [H3CPPh3][U(E)(NR2)3].30 Similarly, in the reaction between S8 and U(NR2)3, we suggest that U(S)(NR2)3 is also generated; however, in the absence of the ylide H2C PPh3, the U(V) monosulfide rapidly conproportionates with unreacted U(NR2)3 to give complex 1. Notably, the latter transformation is known for the oxo analogue.34 Complex 1 crystallizes in the monoclinic space group P21 as an Et 2 O solvate, 1·Et 2 O. Additionally, there are two independent molecules in the asymmetric unit. Each U(IV) center exhibits a distorted-tetrahedral geometry comprised of three silylamide ligands and a bridging S2− group (Figure 1). The U−N bond lengths, which range from 2.21(1) to 2.26(2) Å (Table 1), are similar to those observed for other U(IV) amides.34,30 The U−S distances range from 2.640(4) to 2.680(4) Å and are comparable to the U−S bond lengths observed for [((tBuArO)3tacn)U]2(μ-S) (U1−S1 = 2.592(6) Å) and [((AdArO)3N)U]2(μ-S) (average U−S = 2.72 Å)32 but much longer than the U−S distance exhibited by the terminal sulfido complexes [H3CPPh3][U(S)(NR2)3] (2.4805(5) Å) and [Na(18-crown-6)][Cp*2U(S)(StBu)] (2.477(2) Å).24,30 Interestingly, the two independent molecules in the unit cell of 1 exhibit slightly different U−S−U bond angles (U1−S1−U2 = 171.3(2)° and U3−S2−U4 = 165.2(2)°). The 6° difference

Figure 1. Solid-state molecular structure of [(R2N)3U]2(μ-S) (1·Et2O) with 30% probability ellipsoids. The solvate molecule and hydrogen atoms are removed for clarity.

Table 1. Selected Bond Distances (Å) and Angles (deg) for [(R2N)3U]2(μ-E) (E = O, S (1), Se (3), Te (4); R = SiMe3)

av U−E av U−N U1−E1−U2 U3−E2−U4 av E−U−N a

[(R2N)3U]2(μO)a

1

3

4

2.14 2.28 179.2(4) 179.0(4) 113.0

2.662 2.24 171.3(2) 165.2(2) 109.4

2.735 2.22 168.77(5) 162.33(5) 107.6

3.015 2.24 171.12(2) 178.42(2) 107.9

Taken from ref 34.

between these two molecules is likely due to crystal packing and suggests that the potential energy surface for U−S−U bending is relatively flat. The average N−U−S angle in 1 is 109°, but there is considerable variance in these angles. For example, the smallest N−U−S angle is N11−U4−S2 = 96.5(4)°, while the largest is N10−U4−S2 = 125.1(4)°. This large difference appears to result from steric repulsion caused by the close proximity of six hexamethyldisilylamide substituents in the molecule. Interestingly, in a few instances, addition of S8 to U(NR2)3 also resulted in the deposition of a small amount of orange solid. This solid was isolated by filtration and identified as [(R2N)3U]2(μ-η2:η2-S2) (2). The formation of 2 was observed to be highly dependent on the batch of U(NR2)3 used in the synthesis, and in most instances, complex 2 was not formed at all. We have previously observed a similar batch dependence with reactions involving U(NR2)3.34 In the previous example, the presence of residual NaI in the U(NR2)3 was revealed to alter the reaction outcome; however, addition of exogenous NaI to the S8 reaction did not result in the formation of 2. Instead, we have observed a correlation between the formation of 2 and the presence of the U(IV) metallacycle U(CH2SiMe2NSiMe3)(NR2)2,40,41 in samples of U(NR2)3 (see the Supporting Information for further details). U(CH2SiMe2NSiMe3)(NR2)2 is a common impurity in U(NR2)3, and we have observed that batches of U(NR2)3 typically contain around 3−5% of this material. However, some batches can contain as much as 20% of the U(IV) metallacycle, and it is from these samples that the formation of 2 is observed upon reaction with S8. While it is not 1194

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clear what role the U(IV) metallacycle is playing in the formation of complex 2, we note that independently prepared samples of U(CH2SiMe2NSiMe3)(NR2)2 react with S8 to provide a new, as yet unidentified species (Figure S16, Supporting Information), likely via S atom insertion into the U−C bond. Evans and co-workers observed similar reactivity upon reaction of (η5-C5Me4SiMe2CH2-κC)2U with S8, forming (η5:η2-C5Me4SiMe2CH2S2)2U.42 However, we are not certain what role the putative “U(SxCH2SiMe2NSiMe3)(NR2)2” plays in the formation of 2. Finally, it should be noted that separation of U(NR2)3 from U(CH2SiMe2NSiMe3)(NR2)2 by recrystallization has proven a challenge, as both species exhibit similar solubilities. Furthermore, sublimation of U(NR2)3 does not remove U(CH2 SiMe2 NSiMe 3)(NR2 ) 2. In fact, we have observed that sublimation can actually lead to an increase in the relative amount of U(CH2SiMe2NSiMe3)(NR2)2, possibly due to the thermal instability of U(NR2)3. Despite the variability of the S atom transfer reaction, we were able to complete the characterization of complex 2. Complex 2 can be isolated as a dark orange crystalline solid by crystallization from a concentrated THF solution. It is quite soluble in THF or toluene, but it exhibits limited solubility in Et2O or hexane, which permits its separation from complex 1. Its room-temperature 1H NMR spectrum in C6D6 consists of a broad singlet at −7.40 ppm, assignable to the protons of the SiMe3 substituents. This resonance is shifted upfield to −8.27 ppm in pyridine-d5. Finally, the UV−vis−near-IR spectrum of complex 2 is nearly identical with that of complex 1 and the previously characterized [(R2N)3U]2(μ-O).34 Notably, the 1H chemical shift observed for the SiMe3 groups in 2 matches a resonance observed in crude samples of the terminal monosulfide complex [H3CPPh3][U(S)(NR2)3] (Figure S10, Supporting Information),30 suggesting that 2 is also generated during the formation of [H3CPPh3][U(S)(NR2)3]. Crystals of 2 suitable for X-ray diffraction analysis were grown from a concentrated THF solution stored at −25 °C. Complex 2 crystallizes in the triclinic space group P1̅, and its solid-state molecular structure is shown in Figure 2. The solidstate molecular structure of 2 consists of two [U(N{SiMe3}2)3]+ fragments bridged by an (μ-η2:η2-S2)2− ligand. The S−S bond length in 2 (S1−S2 = 2.1051(19) Å) is similar to those observed in other (μ-η2:η2-S2)2− complexes of the transition metals and lanthanides.43−47 Interestingly, the U−S bond lengths in 2 exhibit a notable asymmetry. For example, the two shortest U−S bond distances are U1−S1 = 2.7062(16) Å and U2−S2 = 2.7366(16) Å, which are comparable to those of complex 1 and [Na(DME)3]2[((AdArO)3N)U)2(μ-S)2] (av U−S = 2.701(2) Å).32 However, the remaining two U−S distances are notably longer (U1−S2 = 2.8205(14) Å, U2−S1 = 2.9228(15) Å). This difference can be rationalized by invoking one anionic and one dative U−S interaction per sulfur atom within the U2(μ-η2:η2-S2) core.29 To our knowledge, complex 2 is only the fourth reported actinide complex featuring the (S2)2− ligand.23 For comparison, US3 features a (μ-η2:η2-S2)2‑ ligand with an S−S distance of 2.086(4) Å and U−S bond lengths ranging from 2.753(2) Å to 2.772(2) Å,48,49 while the uranyl disulfide Cs4[UO2(η2-S2)3] exhibits average U−S and S−S distances of 2.77 and 2.08 Å, respectively.50 Also of note is K4USe8, which features four (η2-Se2)2− ligands.51 We also investigated the ability of 2 to act as an S atom transfer reagent. Monitoring the addition of 1 equiv of Ph3P to a C6D6 solution of complex 2 by both 1H and 31P NMR spectroscopy reveals the formation of only a small amount of

Figure 2. Solid-state molecular structure of [(R2N)3U]2(μ-η2:η2-S2) (2) with 30% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): U1−S1 = 2.7062(16), U1−S2 = 2.8205(14), U2−S1 = 2.9228(15), U2−S2 = 2.7366(16), S1−S2 = 2.1051(19); U1−S1−U2 = 138.30(5), U1−S2− U2 = 134.87(5).

Ph3PS over the course of 24 h (Figures S12 and S13, Supporting Information). Similarly, addition of excess U(NR2)3 to a C6D6 solution of 2 results in formation of only small amounts of complex 1, even at extended reaction times (Figure S14, Supporting Information). These results suggest that complex 2 is not easily converted into complex 1 and therefore is likely not an intermediate along the reaction pathway to complex 1. We also probed the reactivity of 1 with a series of S atom donors, including S8, Ph3PS, and ethylene sulfide. However, no reaction between 1 and these reagents was observed under any conditions. Overall, these results suggest that, once formed, 1 and 2 cannot interconvert. This lack of reactivity may not be surprising, considering the steric protection afforded by the bulky hexamethyldisilylamide ligands. Finally, we explored the reactivity of U(NR2)3 with the heavier chalcogens. Thus, addition of 0.5 equiv of either elemental selenium or tellurium to an Et2O solution of U(NR2)3 results in a rapid color change from dark purple to orange. Upon workup, [(R 2 N) 3 U] 2 (μ-Se) (3) and [(R2N)3U]2(μ-Te) (4) are isolable in 50−60% yields (Scheme 1). The 1H NMR spectra of complexes 3 and 4 in C6D6 exhibit broad singlets at −6.44 and −5.97 ppm, respectively, assignable to the protons on the SiMe3 substituents. Monitoring the reaction of U(NR2)3 with 1 equiv of Se in C6D6 by 1H NMR spectroscopy reveals clean formation of complex 3 (Figure S15, Supporting Information). No evidence for any other Secontaining product is observed, despite the use of excess chalcogen in the reaction. Notably, the 1H chemical shift observed for 3 matches a resonance observed in crude samples of the terminal monoselenide complex [H3CPPh3][U(Se)(NR2)3] (Figure S11, Supporting Information),30 demonstrating that 3 is also generated during the formation of [H3CPPh3][U(Se)(NR2)3]. Crystals of 3 suitable for X-ray diffraction analysis were grown from a concentrated toluene solution stored at −25 °C, while crystals of 4 were grown from a cold Et2O solution. Complex 3 crystallizes as a toluene solvate, 3·C7H8, in the 1195

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Figure 3. Solid-state molecular structures of [(R2N)3U]2(μ-Se) (3·C7H8) (left) and [(R2N)3U]2(μ-Te) (4) (right) with 30% probability ellipsoids. Solvent molecules and hydrogen atoms are omitted for clarity.

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monoclinic space group P21 with two independent molecules in the asymmetric unit. Complex 4 crystallizes in the monoclinic space group P21/c, also with two independent molecules in the asymmetric unit. As with 1, each U(IV) center in 3 and 4 exhibits a pseudotetrahedral geometry comprised of a bridging E2− (E = Se, Te) group and three silylamide ligands (Figure 3). For 3, the U−Se distances range from 2.727(2) to 2.751(2) Å, while for 4 the U−Te distances range from 2.9914(5) to 3.0420(5) Å (Table 1). These bond distances are longer than those exhibited by the terminal monochalcogenides [U(Se)(NR2)3]− (U−Se = 2.6463(7) Å) and [U(Te)(NR2)3]− (U−Te = 2.866(2) Å).30 However, they are comparable to those exhibited by [((tBuArO)3tacn)U]2(μ-Se) (U1−Se1 = 2.7188(4) Å), [((AdArO)3N)U]2(μ-Se) (average U−Se = 2.823(1) Å), and [Na(DME)3]2[{((AdArO)3N)U}2(μ-Te)2] (average U−Te = 3.072(1) Å).32 Finally, there are slight differences in the U− E−U angles observed for the two independent molecules in the unit cells of 3 and 4 (3. U1−Se1−U2 = 168.77(5)° and U3− Se2−U4 = 162.33(5)°; 4, U1−Te1−U2 = 171.12(2)° and U3− Te2−U4 = 178.42(2)°). A similar effect was observed for complex 1.

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EXPERIMENTAL SECTION

General Considerations. All reactions and subsequent manipulations were performed under anaerobic and anhydrous conditions under an atmosphere of nitrogen or argon. Tetrahydrofuran (THF), hexanes, and diethyl ether (Et2O) were dried using a Vacuum Atmospheres DRI-SOLV solvent purification system and stored over 3 Å sieves for 24 h before use. Pyridine, pyridine-d5, and C6D6 were dried over 3 Å molecular sieves for 24 h before use. U[N(SiMe3)2]3,36 U(CH2SiMe2NSiMe3)(NR2)2,41 and Ph3PS52 were synthesized according to the previously reported procedures. All other reagents were purchased from commercial suppliers and used as received. NMR spectra were recorded on a Varian UNITY INOVA 400 or Varian UNITY INOVA 500 spectrometer. 1H NMR spectra were referenced to external SiMe4 using the residual protio solvent peaks as internal standards. 31P{1H} NMR spectra were referenced to external 85% H3PO4. IR spectra were recorded on a Nicolet 6700 FT-IR spectrometer with a NXR FT Raman Module. UV−vis−near-IR experiments were performed on a UV-3600 Shimadzu spectrophotometer. Elemental analyses were performed by the Micro-Mass Facility at the University of California, Berkeley. Synthesis of [U(N(SiMe3)2)3]2(μ-S) (1). Method A. To a solution of U[N(SiMe3)2]3 (143 mg, 0.199 mmol) in Et2O (2 mL) was added 0.0625 equiv of S8 (3.2 mg, 0.012 mmol). A color change from dark purple to orange was observed within 5 min following the addition. After 5 min, the solution was filtered through a Celite column (0.5 cm × 2 cm) supported on glass wool and the volume of the solution was reduced by half in vacuo. Storage of this solution at −25 °C for 24 h resulted in the deposition of dark orange crystals (60.2 mg, 41% yield). Crystals suitable for X-ray crystallography were grown from a concentrated Et2O solution stored at −25 °C for 24 h. Note that the batch of U[N(SiMe3)2]3 used in this reaction contained 3.5% U(CH2SiMe2NSiMe3)(NR2)2, as determined by 1H NMR spectroscopy. 1H NMR (400 MHz, 25 °C, C6D6): δ −6.72 (br s, 108H, CH3). 1 H NMR (400 MHz, 25 °C, py-d5): δ −6.71 (br s, 108H, CH3). Anal. Calcd for C36H108N6Si12SU2: C, 29.41; H, 7.40; N, 5.71. Found: C, 29.43; H, 7.12; N, 6.11. UV−vis−near-IR (OC4H8, 4.9 mM, 25 °C, nm (L mol−1 cm−1)): 622 (ε = 36.2), 698 (ε = 51.3), 822 (ε = 16.6), 916 (ε = 13.0), 954 (ε = 13.2), 1086 (ε = 52.0), 1164 (ε = 43.0), 1386 (ε = 24.8), 1530 (ε = 32.0), 1586 (ε = 33.4), 1626 (ε = 33.0). IR (KBr pellet, cm−1): 1406 (w), 1252 (s), 1184 (w), 1065 (w), 932 (sh m), 887 (s), 847 (s), 775 (m), 764 (sh m), 687 (sh w), 660 (m), 615 (m). Method B. To a solution of U[N(SiMe3)2]3 (139 mg, 0.193 mmol) in hexanes (3 mL) was added Ph3PS (29.0 mg, 0.099 mmol). A color change from dark purple to orange, concomitant with the deposition of a white precipitate, was observed within 2 min following the addition. After 10 min, the solution was filtered through a Celite column (0.5 cm × 2 cm) supported on glass wool and the volume of the solution was reduced by half in vacuo. Storage of this solution at −25 °C for 24 h resulted in the deposition of white microcrystalline material (presumed to be Ph3P). The solution was then filtered

CONCLUSIONS

The highly reducing U(III) tris(amide) U(NR2)3 (R = SiMe3) readily activates S8, providing [(R2N)3U]2(μ-S) in moderate yield. Interestingly, in some cases, a second product was also isolated from this reaction: namely, the bridging disulfide [(R2N)3U]2(μ-η2:η2-S2). Its formation appeared to be correlated to the presence of the U(IV) metallacycle U(CH2SiMe2NSiMe3)(NR2)2, which is a common impurity in batches of the U(NR2)3 starting material. Chalcogen atom transfer reactivity was also observed with elemental selenium and tellurium, permitting the isolation of [(R2N)3U]2(μ-E) (E = Se, Te) in moderate yields. Notably, in all instances, chalcogenide-bridged complexes were isolated, regardless of the stoichiometry used in the reaction. This point is perhaps the most important, as it reveals the critical role that the coordinating ylide plays in the previously reported syntheses of the terminal monochalcogenides [H3CPPh3][U(E)(NR2)3] (E = S, Se, Te) from U(CH2PPh3)(NR2)3.30 With this recognition in hand, it may be possible to rationally choose coligands sets that promote the formation of terminal chalcogenide ligands in other systems. 1196

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Table 2. X-ray Crystallographic Data for Complexes 1·Et2O, 2, 3·C7H8, and 4 empirical formula cryst habit, color cryst size (mm) cryst syst space group V (Å3) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) Z formula wt calcd density (Mg/m3) abs coeff (mm−1) F000 total no. of rflns no. of unique rflns Rint final R indices (I > 2σ(I)) largest diff peak, hole (e Å−3) GOF

1·Et2O

2

3·C7H8

4

U2OC40H118N6Si12S block, orange 0.20 × 0.20 × 0.20 monoclinic P21 7020.8(11) 11.5726(10) 21.379(2) 28.376(3) 90.00 90.026(6) 90.00 4 1544.60 1.461 4.873 3104 77675 34650 0.0399 R1 = 0.0907, wR2 =0.2119 6.727, −6.657 1.214

U2C36H108N6Si12S2 block, orange 0.20 × 0.10 × 0.10 triclinic P1̅ 3286.2(2) 11.6382(4) 11.6617(4) 27.9827(11) 92.123(3) 97.662(3) 118.498(2) 2 1502.54 1.519 5.233 1500 31404 13422 0.0704 R1 = 0.0328, wR2 = 0.0655 2.073, −2.036 0.980

U2C43H116N6Si12Se block, orange 0.25 × 0.20 × 0.10 monoclinic P21 6876(6) 11.600(6) 21.067(11) 28.140(14) 90.00 90.827(5) 90.00 4 1608.00 1.553 5.475 3202 29181 25135 0.0339 R1 = 0.0509, wR2 = 0.1232 1.970, −1.936 0.972

U2C36H108N6Si12Te block, orange 0.40 × 0.30 × 0.10 monoclinic P21/c 13303.3(13) 32.0769(15) 11.8700(8) 38.0147(19) 90.00 113.205(2) 90.00 8 1566.02 1.564 5.537 6160 75016 32744 0.0357 R1 = 0.0482, wR2 = 0.0826 2.820, −1.431 1.144

through a Celite column (0.5 cm × 2 cm) supported on glass wool, and the volatiles were removed in vacuo. The resulting solid was dissolved in Et2O (2 mL), and the solution was refiltered through a Celite column (0.5 cm × 2 cm) supported on glass wool. The volume of the solution was then reduced by half in vacuo. Storage of this solution at −25 °C for 24 h resulted in the deposition of dark orange crystals (54.3 mg, 38% yield). This material was spectroscopically identical with the material prepared by method A. Synthesis of [U(N(SiMe3)2)3]2(μ-η2:η2-S2) (2). To a solution of U[N(SiMe3)2]3 (107 mg, 0.149 mmol) in Et2O (2 mL) was added 0.125 equiv of S8 (4.9 mg, 0.019 mmol). A color change from dark purple to dark orange concomitant with the deposition of an orange solid was observed immediately following the addition. After 5 min, the solution was filtered through a Celite column (0.5 cm × 2 cm) supported on glass wool to give an orange filtrate, leaving an orange solid on the Celite. The solid was extracted into THF (2 mL), filtered through a Celite column (0.5 cm × 2 cm) supported on glass wool, and concentrated in vacuo. Storage of this solution at −25 °C for 24 h resulted in the deposition of an orange powder (27.8 mg, 25% yield). Crystals suitable for X-ray crystallography were grown from a dilute THF solution stored at −25 °C for 24 h. Note that the batch of U[N(SiMe 3 ) 2 ] 3 used in this reaction contained 20% U(CH2SiMe2NSiMe3)(NR2)2, as determined by 1H NMR spectroscopy. 1 H NMR (400 MHz, 25 °C, C6D6): δ −7.40 (br s, 108H, CH3). 1H NMR (400 MHz, 25 °C, py-d5): δ −8.27 (br s, 108H, CH3). Anal. Calcd for C36H108N6Si12S2U2: C, 28.78; H, 7.24; N, 5.59. Found: C, 28.41; H, 7.09; N, 5.24. UV−vis−near-IR (OC4H8, 2.9 mM, 25 °C, nm (L mol−1 cm−1)): 696 (ε = 117.5), 1084 (ε = 77.8), 1156 (ε = 64.5), 1392 (ε = 39.1), 1558 (ε = 38.6). IR (KBr pellet, cm−1): 1446 (w), 1410 (w). 1252 (s), 1184 (w), 1103 (w), 1070 (w), 1018 (w), 935 (sh s), 889 (s), 847 (s), 775 (m), 760 (m), 687 (w), 660 (m), 615 (m). Synthesis of [U(N(SiMe3)2)3]2(μ-Se) (3). To a solution of U[N(SiMe3)2]3 (117 mg, 0.163 mmol) in Et2O (2 mL) was added 0.5 equiv of selenium (7.4 mg, 0.094 mmol). A color change from dark purple to red-orange was observed within 5 min following the addition. After 5 min, the solution was filtered through a Celite column (0.5 cm × 2 cm) supported on glass wool and the volume of the solution was reduced by half in vacuo. Storage of this solution at −25 °C for 4 h resulted in the deposition of dark orange crystals (67.3 mg, 55% yield). Crystals suitable for X-ray crystallography were grown

from a concentrated toluene solution stored at −25 °C for 24 h. 1H NMR (500 MHz, 25 °C, C6D6): δ −6.44 (br s, 108H, CH3). 1H NMR (400 MHz, 25 °C, py-d5): δ −6.56 (br s, 108H, CH3). Anal. Calcd for C36H108N6Si12SeU2: C, 28.50; H, 7.17; N, 5.54. Found: C, 28.34; H, 7.17; N, 5.37. UV−vis−near-IR (OC4H8, 3.9 mM, 25 °C, nm (L mol−1 cm−1)): 617 (ε = 70.8), 698 (ε = 87.0), 812 (ε = 31.4), 906 (ε = 24.4), 954 (ε = 22.8), 1088 (ε = 63.8), 1156 (ε = 55.6), 1388 (ε = 33.6), 1512 (ε = 38.6), 1586 (ε = 42.6), 1626 (ε = 42.3). IR (KBr pellet, cm−1): 1408 (w), 1253 (s), 1182 (w), 1099 (sh w), 1074 (sh w), 1022 (m), 933 (sh s), 881 (s), 850 (s), 775 (s), 762 (sh s), 683 (sh m), 663 (s), 611 (s). Synthesis of [U(N(SiMe3)2)3]2(μ-Te) (4). To a solution of U[N(SiMe3)2]3 (98.0 mg, 0.136 mmol) in Et2O (2 mL) was added 0.5 equiv of tellurium (9.0 mg, 0.071 mmol). A color change from dark purple to dark red was observed within 10 min following the addition. After 25 min, the solution was filtered through a Celite column (0.5 cm × 2 cm) supported on glass wool and the volume of the solution was reduced by half in vacuo. Storage of this solution at −25 °C for 24 h resulted in the deposition of orange crystals, which were suitable for X-ray crystallography (62.4 mg, 58% yield). 1H NMR (500 MHz, 25 °C, C 6 D 6 ): δ −5.97 (br s, 108H, CH 3 ). Anal. Calcd for C36H108N6Si12TeU2: C, 27.61; H, 6.95; N, 5.37. Found: C, 27.25; H, 6.65; N, 5.70. UV−vis−near-IR (OC4H8, 4.6 mM, 25 °C, nm (L mol−1 cm−1)): 814 (ε = 25.4), 908 (ε = 19.4), 950 (ε = 19.8), 1088 (ε = 62.8), 1152 (ε = 53.6), 1390 (ε = 30.6), 1500 (ε = 31.6), 1606 (ε = 40.6). IR (KBr pellet, cm−1): 1406 (w), 1252 (s), 1182 (w), 1018 (w), 933 (s), 885 (sh s), 847 (s), 773 (m), 762 (sh m), 687 (sh m), 662 (m), 613 (m). X-ray Crystallography. Data for 1−4 were collected on a Bruker KAPPA APEX II diffractometer equipped with an APEX II CCD detector using a TRIUMPH monochromator with a Mo Kα X-ray source (α = 0.71073 Å). The crystals of 1·Et2O, 2, 3·C7H8, and 4 were mounted on a cryoloop under Paratone-N oil, and all data were collected at 100(2) K using an Oxford nitrogen gas cryostream system. Frame exposures of 2 s were used for 2. Frame exposures of 4 s were used for 4. Frame exposures of 10 s were used for 3·C7H8, and frame exposures of 15 s were used for 1·Et2O. Data collection and cell parameter determination were conducted using the SMART program.53 Integration of the data frames and final cell parameter refinement were performed using SAINT software.54 Absorption 1197

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Organometallics

Article

correction of the data was carried out using the multiscan method SADABS. 55 Subsequent calculations were carried out using SHELXTL.56 Structure determination was done using direct or Patterson methods and difference Fourier techniques. All hydrogen atom positions were idealized and rode on the atom of attachment. Structure solution, refinement, graphics, and creation of publication materials were performed using SHELXTL.56 A summary of relevant crystallographic data for complexes 1·Et2O, 2, 3·C7H8, and 4 is presented in Table 2.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, and CIF files giving experimental details, crystallographic data, and spectral data for 1−4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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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 No. DEFG02-09ER16067.

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REFERENCES

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