Use of Alkylsodium Reagents for the Synthesis of Trivalent Uranium

Jun 1, 2012 - Cory J. Windorff and William J. Evans. Organometallics 2014 ... Caleb J. Tatebe , Sara A. Johnson , Matthias Zeller , Suzanne C. Bart. J...
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Use of Alkylsodium Reagents for the Synthesis of Trivalent Uranium Alkyl Complexes Ellen M. Matson, William P. Forrest, Phillip E. Fanwick, and Suzanne C. Bart* H.C. Brown Laboratory, Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: A family of rare uranium(III) alkyl complexes, Tp*2UR (R = CH2SiMe3 (3-CH2SiMe3), CH3 (4-CH3), (CH2)3CH3 (5-(CH2)3CH3); Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), was synthesized by salt metathesis with alkylsodium reagents and Tp*2UI (2). All compounds were fully characterized using 1H NMR, infrared, and electronic absorption spectroscopies. Compounds 3-CH2SiMe3 and 4-CH3 were structurally characterized using X-ray crystallography and have U−C bond distances of 2.601(9) and 2.54(3) Å, respectively.

(CH2)CH3]−.18 Another entry to this family of trivalent uranium species was recently synthesized in our laboratory, Tp*2UCH2Ph (Tp* = hydrotris(3,5-dimethylpyrazolyl)borate), which is stabilized by the sterically bulky bis(hydrotris(3,5dimethylpyrazolyl)borate) ligand framework.23 As for transition-metal alkyls, the uranium−carbon bond in this species readily undergoes insertion of carbon dioxide and carbon disulfide. Furthermore, this uranium(III) compound is a convenient synthon to access uranium(III) acetylides, amides, and thiolates.24 Given this utility of the uranium(III)−carbon bond, we set out to explore a general synthetic technique to access further species. Herein we report the synthesis and full characterization of a series of uranium(III) alkyls, Tp*2UR (R = CH2SiMe3, CH3, (CH2)3CH3), which were formed from the corresponding alkylsodium reagents. These reagents are an effective alternative to alkyllithiums, whose salts can be difficult to separate from the products, and alkylpotassiums, which can also serve as reductants.

T

ransition-metal alkyl species are well-established and have important roles in the synthesis of polymers,1 as precursors for atomic layer deposition,2 and in biological systems, such as the cobalt−methyl species found in vitamin B12.3 The elements in the f block, and more specifically uranium, in comparison have not benefitted from such thorough studies. Although uranium alkyls were first explored in the 1950s as a potential route for isotope separation,4 studies of these species have lagged behind those of their transitionmetal counterparts. A survey of the literature reveals the earliest and most common examples of uranium alkyl compounds are those in the +4 oxidation state. Ongoing studies of such complexes have developed these from unstable intermediates5 to well-defined, fully characterized species.6,7 The first examples, reported by Marks in the early 1970s, described the synthesis of a family of tris(cyclopentadienyl)uranium(IV) alkyls, Cp3UR (R = Me, nBu, allyl, neopentyl, pentafluorophenyl, iPr, tBu, Ph, vinyl, 2cis-butenyl, 2-trans-butenyl).6,8 Although they were characterized by 1H NMR spectroscopy, these species proved to be unstable, decomposing via homolytic cleavage of the uranium− carbon σ bond. Stable uranium(IV) alkyl species have been synthesized by taking advantage of large ancillary ligands,7,9 large alkyl groups,10−12 or multiple alkyl groups13−16 for steric protection. Since the earliest examples of uranium−carbon bonds, this area has expanded to encompass a variety of alkyl groups installed using alkyllithium,6,8,10,13,14,17,18 organogold,19 or Grignard6,8,16 reagents with uranium halides. Despite the prevalence of uranium(IV) alkyls, the uranium(III) counterparts are rare. Previous examples of isolable uranium(III) alkyl compounds include U(CH(SiMe3)2)3,20 Cp*2U(CH(SiMe3)2)21 (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide), [Cp*UCH(SiMe3)2]2(μ-η6:η6-C6H6),22 and [Cp3U© 2012 American Chemical Society

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RESULTS AND DISCUSSION Initial efforts were aimed at a general synthetic route toward a family of U(III) alkyls stabilized by the bulky Tp* ligand framework. Alkyllithium reagents were used for this purpose, given their commercial availability, steric and electronic variability, and prevalence in organometallic chemistry.25−27 The alkylation of Tp*UI2(THF)228 was attempted using 2 equiv of LiCH2SiMe3 in a solution of THF at −35 °C (eq 1). Upon addition, a color change from bright purple to deep blue was observed; however, a purple solid (1) was isolated after filtration and workup. Analysis by 1H NMR spectroscopy Received: April 5, 2012 Published: June 1, 2012 4467

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Table 1. Structural Parameters for 1, 3-CH2SiMe3, and 4CH3a 1 U1−N11 U1−N21 U1−N31 U1−N61 U1−N71 U1−N81 U1−C41 U1−C41−Si41 U1−I1 U1−I2 U1−I3

revealed a paramagnetically broadened and shifted spectrum consistent with a C3v-symmetric molecule. Two resonances integrating to nine protons each are found at −16.32 and 3.58 ppm and correspond to the endo and exo Tp* CH3, respectively. Additionally, respective signals for the pyrazole and the B−H protons of the Tp* ligand are located at 8.25 and 17.83 ppm. Two larger, broad resonances at 2.00 and 4.72 ppm integrate to 16 protons each and indicate that the desired product was not formed, as the integration value of these signals does not match those expected for two trimethylsilyl groups. Instead, these resonances are consistent with four THF ligands. Analysis by infrared spectroscopy confirms the presence of one Tp* ligand, with a single, characteristic B−H stretch at 2559 cm−1.29−33 To elucidate the identity of the product, analysis by X-ray crystallography was employed. Purple crystals of 1 were grown from a concentrated solution of diethyl ether and THF (1:1 ratio). Refinement of the data revealed an ion pair with a solvated lithium cation and a six coordinate uranium anion in a pseudo octahedral geometry with a single hydrotris(3,5dimethylpyrazolyl)borate ligand and three iodide substituents, [Li(THF)2(Et2O)][Tp*UI3] (1) (Figure 1, Table 1). Crystal-

a

2.563(8) 2.515(8) 2.568(9)

3-CH2SiMe3

4-CH3

2.656(7) 2.670(7) 2.594(7) 2.640(8) 2.724(8) 2.547(7) 2.601(9) 131.5(5)

2.669(8) 2.611(9) 2.630(9) 2.700(8) 2.560(9) 2.542(9) 2.54(3)

3.0892(8) 3.1023(10) 3.1126(9)

Distances are given in Å and angles in deg.

complexes with pyrazole-based ligands that have bond distances ranging from 3.087 to 3.238 Å.30,33 The isolation of 1 demonstrates that lithium alkylating agents are ineffective for the synthesis of uranium(III) alkyl complexes from U−I bonds. At −35 °C, the deep blue color signifies formation of an intermediate, possibly the desired alkyl species. When the compound is warmed to room temperature, however, the color reverts to bright purple, indicating further reactivity in solution. The product, 1, has three iodide ligands, suggesting that dissolved lithium iodide reacts to cause ligand substitution and rearrangement. Utilizing noncoordinating solvents also does not produce the desired uranium(III) alkyl but instead results in a mixture of unidentified, brown decomposition products and starting material after prolonged stirring. Although alkyllithiums have proven unsuccessful with Tp*UI2(THF)2, productive reactivity of these species with other precursors to form uranium(III) alkyls has been successful. Sattelberger et al. have shown alkylation of the uranium(III) species U(O-2,6-tBu2C6H3)3 with 3 equiv of LiCH(SiMe3)2 to form U(CH(SiMe3)2)3.20 Precipitation of the product was accomplished by trituration with hexanes, leaving the lithium aryloxide dissolved in solution. Marks and coworkers were able to synthesize the uranium(III) monoalkyl Cp*2UCH(SiMe3)2 by addition of LiCH(SiMe3)2 to the trimeric species [UCp*2Cl]3.21 Recrystallization of Cp*2UCH(SiMe3)2 from diethyl ether facilitated separation of the desired alkyl derivative from lithium chloride. U(CH(SiMe3)2)3 and Cp*2UCH(SiMe3)2 were generated prior to the advent of the common trivalent starting material, UI3(THF)4;34,35 thus, new methods are required for successful alkylation of the uranium− iodide bond. More recently, Evans et al circumvented this by synthesizing [Cp*UCH(SiMe 3 ) 2 ] 2 (μ-η 6 :η 6 -C 6 H 6 ) from [Cp*2U]2(μ-η6:η6-C6H6) and LiCH(SiMe3)2, with facile separation of LiCp*.22 Synthesis and Characterization of Uranium(III) Monoalkyl Complexes. The alkylation of Tp*2UI33 (2) was explored in analogy to the previously synthesized Tp*2UCH2Ph,33 to determine if the bulkier ligand framework had an effect on the stabilization of the uranium−carbon σ bonds. Treating a solution of 2 in thawing THF (−108 °C) with either LiCH3 or LiCH2SiMe3 causes an immediate color change from deep purple to green, indicating that a reaction has occurred. Warming the reaction mixture to room temperature again causes the solution to return to bright purple. Characterization by 1H NMR spectroscopy revealed the

Figure 1. Molecular structure of 1 shown with 30% probability ellipsoids. The lithium counterion, selected hydrogen atoms, and solvent molecules have been omitted for clarity.

lization from diethyl ether resulted in a lithium cation coordinated by two ether and two THF molecules in a trans geometry rather than the four chemically equivalent THFs observed in the 1H NMR spectrum. The U−N bond distances, ranging from 2.515(8) to 2.568(9) Å, are consistent with those previously reported for mono-Tp* uranium complexes.30 Additionally, the U−I distances of 3.0892(8), 3.1023(10), and 3.1126(9) Å resemble the distances for trivalent uranium 4468

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and −13 ppm, while the analogous resonance for the exo Tp* CH3 groups appears between −3 and 0 ppm and for Tp* CH between 7 and 8 ppm. Interestingly, protons of the carbon coordinated directly to the paramagnetic uranium center in 3CH2SiMe3 and 4-CH3 are present as broad resonances, located near 64 ppm, significantly downfield from the benzylic protons reported for Tp*2UCH2Ph (21.35 ppm). Compound 4-CH3, to the best of our knowledge, is the first neutral uranium(III) methyl complex. 1H NMR spectroscopic data for [Li(THF)][Cp3U(CH3)], which contains a trivalent uranium anion, was reported by Folcher et al. and displays a singlet for the uranium methyl far downfield at 101 ppm (3H).18 Treating 2 with 1 equiv of NanBu at −35 °C produces the same immediate color change as observed for 3-CH2SiMe3 and 4-CH3, which is indicative of the formation of the desired uranium(III) n-butyl product Tp* 2 U(CH 2 ) 3 CH 3 (5(CH2)3CH3). However, a subsequent color change to greenbrown occurs upon warming to room temperature, supporting the notion that further reactivity of the desired complex occurs. Characterization of the products at the end of the reaction showed no free Tp* ligand, indicating that the Tp*2U framework is still intact. Analysis of the volatiles showed predominantly butane with the formation of small amounts of butenes and octane, indicating that U−C homolytic cleavage similar to that observed by Marks is operative.8 Variable-temperature 1H NMR spectroscopy (toluene-d8) was used to characterize the temperature-sensitive, dark green intermediate 5-(CH2)3CH3. Analysis at −15 °C showed chemical shifts for 5-(CH2)3CH3 that compare favorably to 3-CH2SiMe3 and 4-CH3 (Table 2). Two large signals at −12.41 and −3.02 ppm correspond to the endo and exo Tp* methyl resonances, respectively. A signal for the pyrazole protons integrating to 6H is located at 6.98 ppm, while the broad resonance for the equivalent BH protons is found at 5.23 ppm. All resonances for the n-butyl alkyl group are located downfield at 23.84 (−CH3), 29.23 (−CH2), 66.57 (−CH2), and 73.52 ppm (−CH2), and this downfield shift is inversely proportional to the proximity of the protons to the paramagnetic uranium. Uranium complexes with n-butyl substituents are scarce in the literature. Typically, these complexes are cited as unstable intermediates, whose decomposition pathways include βhydrogen and C−H reductive elimination pathways.6,8 To date, only two examples have been characterized crystallographically, including Folcher’s [LiC14H28N2O4][Cp3U(nC4H9)]18 and Raymond’s Cp3U(n-C4H9),40 whose structural parameters were subsequently determined by Farina and coworkers.41 The 1H NMR data for [Cp3U(n-C4H9)]− contains resonances for the n-butyl substituent at 12.0 (3H), 14.0 (2H), 15.8 (2H), and 98.5 ppm(2H) at 25 °C (referenced to

formation of 1, supporting that lithium iodide in solution causes further reactivity at the uranium−carbon bond and Tp* ligand rearrangement. Changing to Tp*2UCl32 under the same conditions did not result in alkylation but only yielded decomposition products. Because alkyllithium reagents proved unsuccessful for generating isolable uranium(III) alkyl species, sodium and potassium alkylating agents were explored. It was hypothesized that the NaI and KI produced in these reactions would precipitate from solution more effectively on the short reaction time scales, facilitating removal. Given the success of benzylpotassium for the synthesis of Tp*2UCH2Ph from 2,23 the preparations of KPh36 and KnBu37 were attempted. These alkylpotassium reagents readily decomposed in solution at cold temperatures, preventing isolation and further use. Thus, NaCH2SiMe3, NaCH3, and Na(CH2)3CH3 were prepared using a modified literature procedure,38 which entailed mixing sodium tert-butoxide with 1 equiv of the corresponding alkyllithium reagent, LiR (R = CH2SiMe3, CH3, (CH2)3CH3). The formation of Na(CH2)3CH3 was verified using variabletemperature 1H NMR spectroscopy in THF-d8 at −75 °C by comparison to literature values.39 Both NaCH2SiMe3 and NaCH3 were characterized under the same conditions and display spectroscopic features similar to those of Na(CH2)3CH3 (see Experimental). With the alkylsodium reagents in hand, formation of the desired trivalent uranium alkyl species was attempted. Treating THF solutions of 2 with NaR (R = CH2SiMe3 (3), CH3 (4)) at −35 °C resulted in color changes from purple to green (eq 2).

After workup, the products were isolated in high yields (78% and 80%, respectively) and assigned as Tp*2UCH2SiMe3 (3CH2SiMe3) and Tp*2UCH3 (4-CH3). 1H NMR spectroscopic data for 3-CH2SiMe3 and 4-CH3 as well as a comparison to Tp*2UCH2Ph are summarized in Table 2. All compounds display similar spectroscopic features. For the endo Tp* CH3 groups, a signal integrating to 18H is observed between −11

Table 2. 1H NMR Data for Tp*2UCH2Ph, 3-CH2SiMe3, and 4-CH3 Measured in Benzene-d6 at 25 °C and 5-(CH2)3CH3 in Toluene-d8 at −15 °Ca

Tp* CH3 Tp* CH BH CH2 phenyl SiMe3 n Bu a

Tp*2UCH2Ph

3-CH2SiMe3

4-CH3

−11.55, −1.53 7.49 0.28 21.35 11.21, 13.47, 20.89

−11.44, −2.45 7.00 −1.97 64.03

−12.52, −0.94 7.39 3.80 64.01 (−CH3)

5-(CH2)3CH3 −12.41, −3.02 6.98 5.23 73.52

15.62 23.84, 29.23, 66.57 1

Chemical shifts are given in ppm relative to Si(CH3)4) using H chemical shifts of residual C6H6 as a secondary standard. 4469

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Figure 2. Molecular structures of 3-CH2SiMe3 and 4-CH3 shown with 30% probability ellipsoids. Selected hydrogen atoms and solvent molecules are omitted for clarity.

C6H6),18 the same pattern and downfield shift observed for 5(CH2)3CH3, providing further support for the formation of the bis-Tp* n-butyl complex. As limited structural data exist for uranium(III) alkyl complexes,20,23 X-ray crystallography was utilized to ascertain the structural features of 3-CH2SiMe3 and 4-CH3. Green crystals of 3-CH2SiMe3 suitable for analysis were grown from a concentrated solution of pentane with a trace amount of toluene at −35 °C. Refinement of crystallographic data confirmed the assignment of 3-CH2SiMe3 and showed a seven-coordinate uranium center with two hydrotris(3,5dimethylpyrazolyl)borate ligands and a neosilyl group coordinated in an η1 fashion through the methylene carbon (Figure 2). The two Tp* ligands are coordinated in a κ3 fashion, with U−N bond distances ranging from 2.547(7) to 2.724(8) Å. The U1−C40 methylene bond distance in 3-CH2SiMe3 of 2.601(8) Å is within experimental error of the U−C distances of Tp*2UCH2Ph (2.57(2) Å) and the [Cp3U(CH2)3CH3]− anion (2.557(9) Å). These U−C distances in 3-CH2SiMe3 and [Cp3U(CH2)3CH3]− are significantly longer than for U(CH(SiMe3)2)3 (2.48(2) Å) and [Cp*UCH(SiMe3)2]2(μ-η6:η6C6H6) (2.508(2) Å)22 and are attributed to greater steric congestion around the uranium imparted by the large ancillary Tp* ligands. The U1−C40−Si41 bond angle of 131.1(5)° compares favorably to that of Tp*2UCH2Ph (132.8(14)°). Additionally, no agostic interactions are observed with the C− H bonds of the trimethylsilyl groups, supporting η 1 coordination of the alkyl substituent. 3-CH2SiMe3 is the first crystallographically characterized uranium(III) alkyl complex containing a neosilyl substituent, although uranium(IV) derivatives have been synthesized, including Tp 2 U(CH2SiMe3)242 (Tp = hydrotris(pyrazolyl)borate), Tp2U(CH2SiMe3)Cl,42 and Tp*U(CH2SiMe3)xCl3−x.43 Crystallographic characterization has been performed for the uranium(IV) diamido bis(alkyl) [DIPPNCOCN]U(CH2Si(CH3)3)217 (DIPPNCOCN = [2,6-iPr2PhNH(CH2CH2]2O), which has U− C bond distances of 2.44(2) and 2.40(2) Å and acute U−C−Si angles of 129.7(10) and 127.0(11)°. Additionally, Hayton et al. recently reported the crystal structure of the homoleptic uranium anion [U(CH2SiMe3)5]−, which has U−C bond lengths ranging from 2.445(6) to 2.485(6) Å and U−C−Si

bond angles ranging from 125.7(3) to 128.9(3)°.15 These structural parameters compare favorably to those of 3CH2SiMe3 and [DIPPNCOCN]U(CH2Si(CH3)3)2 and indicate η1 coordination in all cases. 4-CH3 was also characterized using X-ray crystallography by analysis of green crystals grown from a concentrated toluene− THF (2:1) solution. In analogy to Tp*2UCH2Ph and 3CH2SiMe3, the structure of 4-CH3 was established as sevencoordinate with two κ3-Tp* ligands and a methyl group. The U−N(pz) distances range from 2.542(9) to 2.700(8) Å and are consistent with those in other bis(Tp*) uranium complexes. The U1−C40 bond length of 2.54(3) Å is within error of the uranium−carbon bond lengths for Tp*2UCH2Ph and 3CH2SiMe3. Upon analysis of the crystal, a small amount of 2 was discovered in the asymmetric unit. The disordered fragments were refined with the occupancies of the methyl group and the iodine atom set to sum to 1. Final refinement resulted in 99.75% carbon and 0.25% iodine. This phenomenon of cocrystallization of starting material and product has also been observed in the structural refinement of Tp*2UCH2Ph.23 With the exception of [LiC14H28N2O4][Cp3U(n-C4H9)], [Li(THF)][Cp3U(CH3)], and Tp*2UCH2Ph, reports of stable uranium(III) alkyl complexes highlight the importance of utilizing the sterically bulky −CH(SiMe3)2 substituent with the synthesis of U[CH(SiMe 3 ) 2 ], Cp* 2 UCH(SiMe 3 ) 2 , and [Cp*UCH(SiMe3)2]2(μ-η6:η6-C6H6).22 The studies presented herein demonstrate that use of a sterically bulky ancillary ligand framework allows for variation of the alkyl substituent and a general synthetic route. In response to variation in the size of the alkyl substituent, slight changes in the coordination angles of the Tp* ligands are noted in the Tp*2UR family. The benzyl group is flat, allowing it to fit easily in the pocket between the pyrazole rings, and the methyl group is sterically small. This affects the C41−U1−NX angles of Tp*2UCH2Ph and 4-CH3, which have similar respective values of 75.6(6)° (N21), 84.7(5)° (N31), 75.2(6) ° (N61) and 74.6(6)° (N81), 83.3(5)° (N11), 71.5(6)° (N71). In contrast, the pyrazole rings shift to accommodate the sterically larger trimethylsilyl group in 3-CH2SiMe3, creating C41−U1−NX angles of 75.3° (N11), 100.2(3)° (N81), and 74.7(3)° (N71). 4470

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monoalkyl complexes by salt metathesis of the corresponding uranium monoiodide with an alkylsodium reagent has been presented. Full characterization of these species by 1H NMR, infrared, and electronic absorption spectroscopies as well as Xray crystallography has been reported for this family and helps to support the +3 oxidation state of the uranium center. Synthesis of trivalent uranium alkyls highlights the importance of selecting the appropriate alkylation reagents. Using alkyllithiums proves to be ineffective for this chemistry, as the eliminated salts are soluble in the polar solvents required to handle these uranium(III) compounds, and further reactivity of this salt prevents isolation of the desired product. Precipitation of sodium iodide occurs much more readily on the time scale of the reaction, making alkylsodium reagents the ideal choice for installing uranium−carbon σ bonds. Furthermore, although benzylpotassium was successful in the synthesis of Tp*2UCH2Ph, other alkylpotassium reagents were unisolable, precluding their use in a general alkylation technique.

With the identities of the desired uranium(III) alkyls established, further studies were aimed at attempting to sequester the lithium iodide during the reaction with alkyllithium reagents by using an additive. One equivalent of TMEDA was added to a solution of Tp*2UI (2) prior to the addition of the alkyllithium salt (R = CH2SiMe3, CH3). Upon workup of the product, 2 and the alkyl complex remained in a 1:1 ratio, verified by 1H NMR spectroscopy, indicating equilibrium between the two complexes. Magnetic studies performed on several independently synthesized solid samples of 3-CH2SiMe3 and 4-CH3 resulted in effective magnetic moments (μeff) of 2.6 μB (23 °C) for both compounds, while the magnetic moment of Tp*2UCH2Ph was found to be slightly lower, with a value of 2.5 μB (23 °C). Due to the thermal instability of 5-(CH2)3CH3, magnetic moments could not be determined. These room-temperature magnetic moments are within the region expected for uranium(III) complexes, which typically range from 2.5 to 3.0 μB.20,44−47 Experimentally determined magnetic moments are much lower than the theoretically calculated value of 3.69 μB for a 4I9/2 5f 3 ground-state uranium ion.48 To further establish the alkyl complexes as trivalent, electronic absorption spectroscopic data were recorded in THF between 300 and 2000 nm for 3-CH2SiMe3 and 4-CH3 and compared to the data for Tp*2UCH2Ph and Tp*2UI (Figure 3). All three alkyl compounds display intense, color-

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EXPERIMENTAL SECTION

General Considerations. All air- and moisture-sensitive manipulations were performed using standard Schlenk techniques or in an MBraun inert atmosphere drybox with an atmosphere of purified nitrogen. The MBraun drybox was equipped with a coldwell designed for freezing samples in liquid nitrogen as well as two −35 °C freezers for cooling samples and crystallizations. Solvents for sensitive manipulations were dried and deoxygenated using literature procedures with a Seca solvent purification system.1 Benzene-d6 and toluene-d8 were purchased from Cambridge Isotope Laboratories, dried with molecular sieves and sodium, and degassed by three freeze− pump−thaw cycles. THF-d8 was purchased from Cambridge Isotope Laboratories and used as received. Elemental analyses were performed by Midwest Microlab, LLC in Indianapolis, IN. Uranium turnings were purchased from Manufacturing Sciences Corp. in Oak Ridge, TN, and cleaned and amalgamated before use.34 (Trimethylsilyl)methyllithium, methyllithium, n-butyllithium, and sodium Tp* butoxide were purchased from Sigma Aldrich and used as received. Tp*2UI (2),33 Tp*UI2(THF)2,28 and NanBu38 were prepared according to literature procedures. 1 H NMR spectra were recorded on a Varian Inova 300 spectrometer operating at a frequency of 299.992 MHz. All chemical shifts were reported relative to the peak for SiMe4 using 1H (residual) chemical shifts of the solvent as a secondary standard. The spectra for paramagnetic molecules were obtained using an acquisition time of 0.5 s; thus, the peak widths reported have an error of ±2 Hz. For paramagnetic molecules, the 1H NMR data are reported with the chemical shift, followed by the peak width at half-height in hertz, the integration value, and (where possible) the peak assignment. Solidstate infrared spectra were recorded using a Perkin-Elmer FT-IR Spectrum RX I spectrometer. Samples were made by crushing the solids, mixing with dry KBr, and pressing into a pellet. Electronic absorption measurements were recorded at 294 K in THF in sealed 1 cm quartz cuvettes with a Jasco V-670 spectrophotometer. Single crystals for X-ray diffraction were coated with poly(isobutylene) oil in a glovebox and quickly transferred to the goniometer head of a Rigaku Rapid II image plate diffractometer equipped with a MicroMax002+ high-intensity copper X-ray source with confocal optics. Preliminary examination and data collection were performed with Cu Kα radiation (λ = 1.541 84 Å). Cell constants for data collection were obtained from least-squares refinement. The space group was identified using the program XPREP.50 The structures were solved using the structure solution program PATTY in DIRDIF99.51 Refinement was performed on a LINUX PC using SHELX-97.50 The data were collected at a temperature of 150(1) K. Preparation of UI3(dioxanes)1.5. The literature preparation of UI3(dioxanes)1.5 was followed with minor modifications.52 A 100 mL

Figure 3. Electronic absorption spectra of Tp*2UI, Tp*2UCH2Ph, 3CH2SiMe3, and 4-CH3 in THF at ambient temperature. The inset shows the near-infrared region of the spectra. Solvent overtones are present between 1600 and 1800 nm.

giving d−f transitions in the visible part of the spectrum.49 Low-intensity f−f absorption bands are spread throughout the near-infrared region as well, with four mid-intensity electronic transitions ranging from 1150 to 1400 nm for 3-CH2SiMe3 (1188, 1228, 1261, 1381 nm), 4-CH3 (1188, 1225, 1272, 1381 nm), and Tp*2UCH2Ph (1186, 1222, 1263, and 1381 nm). Characteristic absorptions are also observed for Tp*2UCH2Ph at 842 and 512 nm, the latter of which is assigned to a metal to ligand charge transfer to the benzyl substituent.44,49 The similarity of the spectra for compounds 3-CH2SiMe3 and 4CH3 indicate that their electronic structures are analogous.

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CONCLUSIONS In summary, a general synthetic route to a family of bis(hydrotris(3,5-dimethylpyrazolyl)borate)uranium(III) 4471

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Organometallics

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Schlenk flask was charged with uranium turnings (10.00 g, 0.042 mol) and 70 mL of dioxanes. On the Schlenk line, 1.1 equiv of I2 (12.19 g, 0.46 mol) was added under an argon purge and the mixture was stirred for 3 days, over which time a dark purple precipitate formed. The solid was isolated via filtration and dried under reduced pressure for 4 h (23.30 g, 0.031 mol, 73%). Preparation of UI3(THF)4. The preparation of UI3(THF)4 from UI3(dioxanes)1.5 was modified from the literature procedure.52 A 100 mL Schlenk flask was charged with UI3(dioxanes)1.5 (23.30 g, 0.031 mol), approximately 60 mL of THF, and a stir bar. The dark purple slurry was stirred at room temperature for 3 days. After this time, the reaction was concentrated to half the original volume. A dark blue solid precipitated and was isolated by filtration (27.58 g, 0.030 mol, 97%) and identified as UI3(THF)4 by comparison to literature values.34 Preparation of [Li(OEt 2 )(THF)2 ][Tp*UI3 ] (1). A 20 mL scintillation vial was charged with Tp*UI2(THF)2 (0.100 g, 0.107 mmol), 5 mL of THF, and a stir bar and was cooled to −35 °C. A separate 20 mL scintillation vial was charged with 2 equiv of LiCH2SiMe3 (0.020 g, 0.213 mmol) and 2 mL of THF. The resulting clear solution containing the lithium alkyl was added dropwise into the dark purple uranium-containing solution with vigorous stirring. An immediate color change from purple to dark blue was observed. After the mixture was stirred for 10 min at −35 °C, solvents were removed in vacuo, resulting in a bright purple residue. Recrystallization from a pentane−THF solution (1:1) at −35 °C afforded the product [Li(THF)4][Tp*UI3] as a bright purple powder (0.055 g, 0.045 mmol, 64%). Crystals suitable for X-ray analysis were grown from a concentrated solution of diethyl ether. Anal. Calcd for C31H56BI3N6ULiO4: C, 30.64; H, 4.81; N, 6.92. Found: C, 30.49; H, 4.66; N, 6.87. 1H NMR (C6D6, 25 °C): δ −16.32 (28, 9H, Tp* CH3), 1.35 (74, 16H, −CH2), 2.30 (18, 9H, Tp* CH3), 3.58 (28, 16H, −CH2), 8.25 (16, 3H, Tp* CH), 17.83 (249, 1H, BH). IR: 2559 cm−1 (BH). Preparation of NaR (R = −CH2SiMe3, −CH3). The procedure has been adapted from that for Na(CH2)3CH3.38 A 20 mL scintillation vial was charged with NaOtBu (1.000 g, 0.010 mol) and approximately 10 mL of pentane. In a separate 20 mL scintillation vial, solid LiR (R = −CH2SiMe3, 0.940 g, 0.010 mol; R = −CH3, 0.396 g, 0.010 mol) was dissolved in approximately 7 mL of pentane. Both solutions were cooled to −35 °C, followed by addition of the lithium alkyl solution dropwise to the NaOtBu, solution with vigorous stirring. Immediately, a thick white precipitate formed. After complete addition, the vial was cooled for 1 h, followed by stirring at room temperature for another 1 h. The white solid was collected by vacuum filtration. After it was washed three times with pentane, the solid was dried under vacuum for 3 h, yielding a white powder as the product in quantitative yield. 1H NMR (THF-d8, −75 °C): NaCH2SiMe3, δ −2.185 (s, 3, 2H, −CH2), −1.89 (s, 4, 9H, −Si(CH3)3); NaCH3, δ 0.21 (s, 4, 3H, −CH3). Preparation of Tp*2U(CH2SiMe3) (3-CH2SiMe3). A 20 mL scintillation vial was charged with Tp*2UI (2; 0.100 g, 0.104 mmol), approximately 10 mL of THF, and a stir bar. This purple solution was cooled to −35 °C. Solid NaCH2SiMe3 (0.011 g, 0.100 mmol) was weighed by difference and added to the cooled uranium solution with vigorous stirring. Immediately, a color change to blue-green was observed. After 5 min of stirring, solvents were removed under reduced pressure. The resulting residue was dissolved in approximately 10 mL of pentane and this solution was filtered over Celite to remove NaI and dried in vacuo, leaving the product, Tp*2U(CH2SiMe3), as a green powder (0.076 g, 0.082 mmol, 78%). Single crystals for X-ray diffraction were grown from a concentrated solution of pentane and toluene (20:1) at −35 °C. Anal. Calcd for C35H53N12SiB2U: C, 44.41; H, 6.03; N, 18.28. Found: C, 44.08; H, 5.89; N, 18.09. 1H NMR (C6D6, 25 °C): δ −11.44 (29, 18H, Tp* CH3), −2.45 (8, 18H, Tp* CH3), −1.97 (200, 2H, BH), 7.00 (9, 6H, Tp* CH), 15.62 (16, 9H, −Si(CH3)3), 64.03 (140, 2H, CH2). IR: 2523, 2552 cm−1 (BH). Preparation of Tp*2U(CH3) (4-CH3). A 20 mL scintillation vial was charged with Tp*2UI (2; 0.100 g, 0.104 mmol), approximately 5 mL of THF, and a stir bar and cooled to −35 °C. Solid NaCH3 (1.05 equiv, 0.004 g, 0.109 mmol) was weighed by difference and added to

the purple solution with vigorous stirring, causing an immediate color change to bright green. After the mixture was stirred for 30 min, volatiles were removed in vacuo. The resulting brown-green residue was washed with pentane until the decant ran clear. Toluene was added to dissolve the remaining green oil, and this solution was filtered over Celite to remove NaI. Removal of the solvent left a dark green powder assigned as Tp*2UCH3 (0.040 g, 0.047 mmol, 80%). Crystals suitable for X-ray analysis were grown from a toluene−THF solution (2:1) at −35 °C. Anal. Calcd for C31H45N12B2U: C, 44.01; H, 5.36; N, 19.87. Found: C, 44.09; H, 5.57; N, 19.78. 1H NMR (C6D6, 25 °C): δ −12.52 (40, 18H, Tp* CH3), −0.94 (21, 18H, Tp* CH3), 3.80 (198, 2H, BH), 7.39 (37, 6H, Tp* CH), 66.64 (99, 3H, CH3). IR: 2519, 2557 cm−1 (BH). Preparation of Tp*2U(CH2)3CH3 (5-(CH2)3CH3). A 20 mL scintillation vial was charged with 2 (0.025 g, 0.026 mmol), approximately 5 mL of toluene-d8, and a stir bar. The solution was cooled to −35 °C. Solid Na(CH2)3CH3 (0.002 g, 0.025 mmol) was weighed by difference and added to the solution with vigorous stirring. Immediately a color change from dark purple to green was observed. The reaction mixture was kept at −35 °C for approximately 4 h, after which time the cooled solution was filtered over Celite to remove NaI. The filtrate was transferred to a J. Young tube and immediately transferred to a cooled Varian 300 probe for spectroscopic analysis. 1H NMR (C7D8, −15 °C): δ −12.41 (30, 18H, Tp* CH3), −3.02 (12, 18H, Tp* CH3), 5.23 (249, 2H, BH), 6.98 (26, 6H, Tp* CH), 23.84 (16, 3H, CH3), 29.23 (33, 2H, CH2), 66.57 (84, 2H, CH2), 73.52 (189, 2H, CH2). Infrared spectroscopic and elemental analysis data for this sample were not obtained due to our inability to isolate 5(CH2)3CH3 at ambient temperature.

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ASSOCIATED CONTENT

S Supporting Information *

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 We wish to acknowledge Purdue University for funding and Prof. Tong Ren for access to a magnetic susceptibility balance and electronic absorption spectrometer.

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REFERENCES

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