Synthesis and Reactivity of Trivalent Tp*U(CH2Ph)2(THF): Insertion vs

The synthesis of a rare uranium(III) bis(benzyl), Tp*U(CH2Ph)2(THF) (2), was achieved by salt metathesis of Tp*UI2(THF)2 with 2 equiv of KCH2Ph at low...
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Synthesis and Reactivity of Trivalent Tp*U(CH2Ph)2(THF): Insertion vs Oxidation at Low-Valent Uranium−Carbon Bonds 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: The synthesis of a rare uranium(III) bis(benzyl), Tp*U(CH2Ph)2(THF) (2), was achieved by salt metathesis of Tp*UI2(THF)2 with 2 equiv of KCH2Ph at low temperature. This was characterized by 1H NMR, infrared, and electronic absorption spectroscopy as well as X-ray crystallography. Addition of benzophenone to 2 forms the uranium(IV) radical coupling product Tp*U(CH2Ph)2(OC(Ph)2CH2Ph) (3), whereas N3Mes produces the imido derivative Tp*U(NMes)(CH2Ph)(THF) (4). Adding a further 1 equiv of N3Mes to tetravalent 4 results in U−C insertion to form the U(IV) triazenido species Tp*U(NMes)[(CH2Ph)N3(Mes)-κ2N1,2](THF) (5). Compound 2 also reacts with the redox-active 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]quinone (DIPPiq) to form the oxidized amido(phenolate) Tp*U(CH2Ph)(DIPPap) (6).

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INTRODUCTION Migratory insertion into metal−carbon bonds is an essential step in organometallic catalysis, as this is the event where new bonds are formed. Tetravalent organouranium species supported by a variety of substituted cyclopentadienyl ligands,1 amides,2−5 alkoxides,6 and phosphines7,8 as well as stable homoleptic examples9,10 have been synthesized, prompting studies of insertion at the uranium(IV)−carbon bonds. Organic azides are interesting substrates, since they can either insert into U−C bonds or oxidize the uranium center. In the case of Cp*2UMe2 (Cp* = 1,2,3,4,5-pentamethylcyclopentadienide), 1-azidoadamantane inserts into the uranium−carbon σ-bond, forming the corresponding triazenido species Cp*2UMe[(Me)NNN(Ad)-κ2N1,3].11 The related tethered uranium(IV) compound [(η5-C5Me4SiMe2CH2-κC)2U] reacts analogously to form the tethered triazenido species.12 Carbonyl insertion has been demonstrated with carbon monoxide to convert Cp3UR (R = Me, Et, iPr, nBu, tBu) into the corresponding acyl derivatives.13 Similarly, the uranium(IV) alkyl ((Me3Si)2N)3UCH314 and cyclometalated ((Me3Si)2N)2UCH2Si(Me)2NSiMe315 insert ketones and aldehydes into their U−C bonds. Insertion of acetone was successfully demonstrated for Tp*2UCl(CH2SiMe3)16 and Tp*2U(CH3)2,17 which is significant given that protonation of alkyl groups by the acidic C−H’s can be a competing side reaction.18 Insertion of small molecules such as CO2,19 acetone,20 carbodiimides,11 nitriles,21 and diazoalkanes22 has b e e n d em o n st r a t e d f o r C p * 2 U M e 2 , w h il e [ ( η 5 C5Me4SiMe2CH2-κC)2U] inserts CO, carbodiimides, and carbon disulfide12 and the uranium allyl derivative Cp*2U(η3CH2CHCH2)(η1-CH2CHCH2) inserts carbon dioxide.23 © 2013 American Chemical Society

In comparison to uranium(IV) compounds, trivalent derivatives have not undergone thorough insertion studies, presumably due to the fact that these are rare in comparison to their tetravalent counterparts. Seminal examples of uranium(III) alkyl species include [Cp 3 U(CH 2 )CH 3 ] − , 2 4 [Cp 3 UCH 3 ] − , 2 5 Cp* 2 U(CH(SiMe 3 ) 2 ), 2 6 [Cp*UCH(SiMe3)2]2(μ-η6:η6-C6H6),27 and U(CH(SiMe3)2)3.28 To our knowledge, most of these early examples have limited reactivity reported, with the exception of [Cp*UCH(SiMe3)2]2(μ-η6:η6C6H6), which has been shown by Evans to form Cp*2U(NAd)2, Cp*(η5:κN-C5Me4CH2NAd)U(NAd), and the insertion product Cp*(AdN3CH(SiMe3)2-κ2N1,2)U(NAd)2 in the presence of 1-azidoadamantane. This example highlights simultaneous insertion and oxidation of the uranium center in the presence of strongly oxidizing organic azides.27 More recently, we reported a family of uranium(III) alkyl derivatives, Tp* 2UR′ (R′ = CH 2 Ph, CH 2 SiMe3 , CH 3 , (CH2)3CH3),29,30 with reactivity studies centering on the benzyl analogue. Like transition-metal alkyls, this species readily undergoes insertion of carbon dioxide and disulfide.30 In the presence of more oxidizing substrates such as organic azides, N3R″ (R″ = Mes (Mes = 2,4,6-trimethylphenyl), Ph, Ad),31 and diazoalkanes, N2CR1R2 (R1 = R2 = Ph; R1 = H, R2 = SiMe3), the respective uranium(IV) terminal imido and hydrazonido species were formed. The formation of uranium−nitrogen multiple bonds is accompanied by extrusion of 1/2 equiv of bibenzyl, indicating that Tp*2U(CH2Ph) readily undergoes U− Special Issue: Recent Advances in Organo-f-Element Chemistry Received: November 27, 2012 Published: February 26, 2013 1484

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cooled to −35 °C. To this slurry was added 2 equiv of the orange solid KCH2Ph (0.028 g, 0.214 mmol), which was weighed by difference. After 5 min of stirring, the reaction mixture was placed back into the freezer for 3 h with frequent agitation. After this time, KI had precipitated as a white salt and was removed by filtering over Celite. The filtrate was concentrated in vacuo. Recrystallization from a concentrated THF solution layered with pentane produced a dark blue solid (0.051 g, 0.065 mmol, 62%). Large blue, block-shaped crystals suitable for X-ray diffraction were grown from a concentrated solution of THF, diethyl ether, and pentane in a 2/1/1 ratio. Due to the thermal instability of 2, elemental analysis could not be obtained. 1H NMR (THF-d8, 25 °C): −13.22 (86, 9H, Tp*-CH3), 0.04 (48, 4H, −CH2), 1.27 (4, 9H, Tp*-CH3), 1.23 (7, 2H, p-CH), 4.19 (78, 4H, mCH), 6.79 (9, 4H, o-CH), 7.58 (10, 3H, Tp*-CH), 10.42 (225, 1H, BH). IR: 2555 cm−1 (B−H). Synthesis of Tp*U(CH2Ph)2(OC(Ph)2CH2Ph) (3). A 20 mL scintillation vial was charged with 2 (0.050 g, 0.064 mmol) and approximately 2 mL of THF and was cooled to −35 °C. In a separate 20 mL scintillation vial, 1 equiv of benzophenone (0.012 g, 0.065 mmol) was dissolved in approximately 2 mL of diethyl ether. This clear solution was added dropwise to the dark blue solution of 2 with stirring. Upon addition of benzophenone the solution turned magenta transiently, followed by a progression to brown-orange. Solvents were removed in vacuo. The product was recrystallized from a concentrated solution of diethyl ether layered with pentane, affording the product as an orange-red solid (0.034 g, 0.035 mmol, 54%). Orange, chunkshaped crystals suitable for X-ray crystallography were grown from a concentrated solution of diethyl ether cooled to −35 °C for 2 days. Elemental analysis of 3 was not obtained due to instability. 1H NMR (C6D6, 25 °C): −78.91 (30, 2H, CH2), −73.29 (29, 2H, CH2), −10.80 (5, 9H, Tp*-CH3), −7.07 (d, J = 7.2, 4H, o- CH), −1.70 (3, 9H, Tp*CH3), 4.69 (184, 1H, B-H), 4.82 (t, J = 6.9, 4H, m-CH), 7.29 (3, 3H, Tp*-CH), 8.44 (t, J = 6.9, 4H, m- CH′), 8.66 (t, J = 8.1, 1H, p- CH′), 9.00 (t, J = 6.9, 2H, p-CH), 12.80 (d, J = 7.5, 2H, o-CH′), 19.77 (d, J = 7.2, 2H, m-CH), 20.79 (6, 4H, o-CH), 25.77 (7, 2H, −CH2′). IR: 2550 cm−1 (B−H). Synthesis of Tp*U(NMes)(CH 2 Ph)(THF) (4). A 20 mL scintillation vial was charged with 2 (0.050 g, 0.064 mmol) and approximately 4 mL of THF. This dark blue solution was cooled until frozen in a cold well. In a separate vial, 0.9 equiv of N3Mes (0.009 g, 0.056 mmol) was dissolved in approximately 2 mL of THF and also cooled until frozen. The thawing organic azide solution was added to the thawing uranium solution dropwise. Upon addition, this solution changed from dark blue to red-orange. After the mixture was stirred for 5 min, solvents were removed in vacuo. Once dry, the dark red solid was washed with pentane to remove bibenzyl and subsequently extracted with diethyl ether and filtered over Celite. Removing the solvents under reduced pressure resulted in the isolation of the product Tp*U(NMes)(CH2Ph)(THF) (4) as a red-orange powder (0.032 g, 0.039 mmol, 61%). Crystals suitable for X-ray analysis were grown from a concentrated solution of THF and toluene (1/2 ratio). Anal. Calcd for C35H48N7BOU: C, 50.55; H, 5.82; N, 11.80. Found: C, 50.39; H, 5.58; N, 11.52. 1H NMR (C6D6, 25 °C): −141.40 (38, 2H, −CH2), −43.65 (13, 3H, Tp*-CH3), −20.36 (64, 2H), −18.15 (17, 1H), −15.92 (12, 3H, Tp*-CH3), −11.25 (67, 3H, Tp*-CH3), −10.80 (18, 1H), −7.71 (14, 1H), −7.24 (5, 1H), −0.96 (5, 3H, Tp*-CH3), 3.31 (156, 1H, B-H), 4.70 (t, J = 8, 1H, p-CH), 6.97 (5, 3H, Tp*CH3), 14.63 (10, 3H, Tp*-CH3), 15.71 (5, 2H), 15.81 (7, 2H), 20.30 (7, 2H), 23.43 (93, 3H), 32.68 (13, 3H, Tp*-CH3), 50.07 (80, 3H), 58.34 (91, 1H), 66.94 (142, 3H). IR (KBr): 2543 cm−1 (B−H). Synthesis of Tp*U(NMes)[(CH2Ph)NNN(Mes)-κ2N1,2](THF) (5). A 20 mL scintillation vial was charged with 2 (0.050 g, 0.064 mmol) and approximately 4 mL of THF. This dark blue solution was cooled to −35 °C. In a separate scintillation vial, 2 equiv of N3Mes (0.020 g, 0.124 mmol) dissolved in approximately 1 mL of THF was cooled to −35 °C. The pale orange solution of the azide was added dropwise to the uranium-containing solution with vigorous stirring. The solution immediately turned dark orange-brown. Solvents were removed in vacuo. The resulting brown oil was dissolved in diethyl ether and filtered. The product was recrystallized from a concentrated solution of

C homolytic cleavage and oxidation in addition to the observed insertion chemistry. These results prompted an investigation to determine if the observed insertion vs oxidation chemistry was due to the sterically crowded uranium center in Tp*2U(CH2Ph). In this work, the synthesis and characterization of the less sterically hindered trivalent uranium bis(alkyl), Tp*U(CH2Ph)2(THF), is presented along with its reactivity toward benzophenone, mesityl azide, and an imino(quinone). The difference in reactivity between uranium(III) and uranium(IV) alkyl complexes is highlighted, as well as characterization by multiple spectroscopic methods and X-ray crystallography.

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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.32 Benzene-d6 was purchased from Cambridge Isotope Laboratories, dried with molecular sieves and sodium, and degassed by three freeze−pump−thaw cycles. Tetrahydrofuran-d8 was purchased from Cambridge Isotope Laboratories and used as received. Benzophenone was purchased from Sigma Aldrich, recrystallized from anhydrous diethyl ether, and dried on a Schlenk line overnight prior to use. Elemental analyses were performed by Midwest Microlab, LLC, in Indianapolis, IN. Tp*UI2,33 KCH 2 Ph, 34 4,6-di-tert-butyl-2-[(2,6-diisopropylphenyl)imino]quinone,35 and (2,4,6-trimethyl)phenylazide (N3Mes)36 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 in ppm, followed by the peak width at half-height in hertz, the integration value, and, where possible, the peak assignment. Solid-state 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 of Tp*U(CH2Ph)2(THF) (2) were coated with polybutenes oil in a glovebox and quickly transferred to the goniometer head of a Nonius KappaCCD image plate diffractometer equipped with a graphite crystal, incident beam monochromator. Preliminary examination and data collection were performed with Mo Kα radiation (λ = 0.71073 Å). Single crystals of Tp*U(CH2Ph)2[OC(Ph)2(CH2Ph)] (3), Tp*U(NMes)(CH2Ph)(THF) (4), and Tp*U(NMes)[(CH2Ph)NNN(Mes)-κ2N1,2](THF) (5) 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 = 1.54184 Å). Cell constants for data collection were obtained from least-squares refinement. The space group was identified using the program XPREP.37 The structures were solved using the structure solution program PATTY in DIRDIF99.38 Refinement was performed on a LINUX PC using SHELX-97.37 The data were collected at a temperature of 150(1) K. Synthesis of Tp*U(CH2Ph)2(THF) (2). A 20 mL scintillation vial was charged with Tp*UI2(THF)2 (1; 0.100 g, 0.106 mmol), approximately 3 mL of THF, and a stir bar. The purple slurry was 1485

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diethyl ether layered with pentane (1/1 ratio). Decanting the resulting solution and removing traces of residual solvents left the product, 5, as small brown-orange crystals (0.055 g, 0.055 mmol, 87%). Orange plates suitable for X-ray analysis were grown from a pentane/ether/ THF solution (4/4/1, respectively). Anal. Calcd for C44H59N10BOU: C, 53.22; H, 6.00; N, 14.11. Found: C, 52.98; H, 5.79; N, 13.97. 1H NMR (C6D6, 25 °C): −70.07 (117, 2H, −CH2), −36.54 (152, 3H), −34.00 (103, 1H, B-H), −27.78 (117, 2H), −25.50 (78, 2H), −21.55 (44, 1H), −19.53 (60, 3H), −19.53 (60, 3H), −12.53 (53, 3H), −11.91 (57, 3H), −6.61 (333, 1H), −6.17 (8, 3H), −5.37 (10, 3H), −1.73 (31, 1H), −1.46 (6, 1H), −1.30 (10, 1H), 23.06 (33, 3H), 33.20 (18, 3H), 44.48 (60, 6H), 50.91 (22, 2H), 59.63 (45, 1H), 74.95 (75, 3H). IR (KBr): 2551 cm−1 (B−H). Tp*U(CH2Ph)(DIPPap) (6). A 20 mL scintillation vial was charged with 2 (0.050 g, 0.064 mmol) and approximately 3 mL of THF and cooled to −35 °C. In a separate vial, 1 equiv of DIPPiq (0.024 g, 0.064 mmol) was dissolved in approximately 3 mL of THF. The brown-red solution was added dropwise to the blue uranium solution, resulting in an instantaneous color change to red-orange. After solvents were removed in vacuo, the product was recrystallized as large red blocks from a concentrated solution of diethyl ether (0.062, 0.057, 89%). Anal. Calcd for C47H66NOBU: C, 57.31; H, 6.61; N, 9.75. Found: C, 57.28; H, 6.84; N, 9.52. 1H NMR (C6D6, 25 °C): −22.38 (d, J = 9, 1H, CH), −14.35 (m, J = 6, 1H, CH(CH3)2), −11.12 (4, 3H, Tp*-CH3), −8.87 (d, J = 9, 1H, CH), −3.34 (d, J = 6, 3H, CH(CH3)2), −2.47 (d, J = 6, 3H, CH(CH3)2), −1.40 (4, 3H, Tp*-CH3), 0.60 (4, 3H, Tp*CH3), 0.70 (d, J = 6, 3H, CH(CH3)2), 0.87 (d, J = 7, 3H, CH(CH3)2), 1.59 (4, 9H, C(CH3)3), 1.61 (3, 1H, Tp*-CH), 1.80 (d, J = 7, 2H, phenyl-CH), 2.16 (3, 3H, Tp*-CH3), 2.96 (d, J = 8, 1H, m-CH), 3.20 (3, 1H, Tp*-CH), 3.39 (t, J = 8, 1H, p-CH), 3.71 (4, 3H, Tp*-CH3), 3.99 (m, J = 6, 1H, CH(CH3)2), 4.33 (3, 3H, Tp*-CH3), 4.47 (t, J = 8, 1H, p-CH), 4.99 (d, J = 8, 1H, m-CH), 5.30 (4, 9H, C(CH3)3), 6.92 (t, J = 8, 1H, p-CH), 7.10 (276, 1H, B-H), 8.12 (3, 1H, Tp*-CH), 10.34 (d, J = 8, 2H, phenyl-CH). IR (KBr): 2553 cm−1 (B−H).

Figure 1. 1H NMR spectrum of 2 recorded in THF-d8 at 25 °C.

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RESULTS AND DISCUSSION Successful reactivity of the uranium(III) iodide Tp*2UI with benzylpotassium to form the benzyl derivative, Tp*2UCH2Ph, inspired the attempted formation of the corresponding bis(benzyl) species, Tp*U(CH2Ph)2, using analogous methods (eq 1). Treating a purple THF solution of Tp*UI2(THF)233,39

Figure 2. Molecular structure of 2 shown with 30% probability ellipsoids. Selected hydrogen atoms and solvent molecules are omitted for clarity.

Table 1. Structural Parameters (Distances in Å and Angles in deg) for Tp*U(CH2Ph)2(THF) (2) Bond Distances U1−N11 U1−N21 U1−N31 U1−C40 U1−C41

(1) at −35 °C with 2 equiv of benzylpotassium produced a blue-green solution of Tp*U(CH2Ph)2. Concentration and layering of this solution with pentane precipitated the deep blue product in moderate yields (62%). Upon dissolution of Tp*U(CH2Ph)2 in benzene-d6 for analysis by 1H NMR spectroscopy, an immediate change to brown was noted. Acquisition of the spectrum showed the formation of toluene accompanying decomposition, indicating protonation of the benzyl groups. Similar observations were made during dissolution in pentane and toluene over temperatures ranging from −196 to 25 °C, eliminating thermal decomposition as the cause. Thus, the presence of the coordinating THF solvent is responsible for stabilization of Tp*U(CH2Ph)2. The 1H NMR spectrum of blue Tp*U(CH2Ph)2 was successfully obtained in THF-d8 (Figure 1). Two signals integrating to nine protons each are visible at −13.22 and 1.27

2.559(6) U1−C46 2.629(7) U1−C50 2.649(6) U1−C51 2.615(7) U1−O61 2.952(5) Bond Angles

U1−C40−C41 U1−C50−C51

3.038(7) 2.604(9) 2.964(12) 2.552(5)

88.3(4) 89.7(6)

ppm and correspond to the endo and exo Tp* methyls, respectively, while those for the pyrazole and B−H protons are found at 7.58 and 10.42 ppm. Additionally, a broad peak integrating to four protons (0.04 ppm) is observable for the symmetric methylene protons of the two chemically equivalent benzyl groups in solution. Resonances corresponding to the phenyl protons are located at 1.23, 4.19, and 6.79 ppm. As uranium(III) bis(alkyl) complexes are rare, X-ray crystallography was employed to confirm formation of Tp*U(CH 2 Ph) 2 and gain structural insights into the 1486

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Table 2. Hapticity Parameters for 2 compd (dmpe)U(CH2Ph)3Me Cp*U(CH2Ph)3 U(CH2Ph)4 Tp*U(CH2Ph)2(THF) (2) Tp*U(=NMes)(CH2Ph)(THF) (4)

MCi − MCH2a

MCo − MCH2b

MCo′ − MCH2c

Δd

Δ′e

ref

0.22 0.34

0.55 0.89

0.91 0.94

0.33 0.55

0.69 0.61

0.34 0.38 0.29

0.86 0.97 0.52

1.31 1.00 0.80

0.52 0.59 0.23

0.97 0.62 0.51

7 40 8 this work this work

a

Average metal to ipso carbon bond length minus metal to methylene carbon bond length. bAverage metal to shorter ortho carbon bond length minus metal to methylene carbon bond length. cAverage metal to longer ortho carbon bond length minus metal to methylene carbon length. d [MCortho − MCH2] − [MCipso − MCH2]. e[MCortho′ − MCH2] − [MCipso − MCH2].

uranium−benzyl interactions. Blue, block crystals grown from a concentrated mixture of THF and diethyl ether (2:1) layered with pentane were analyzed, revealing a six-coordinate uranium compound with one Tp* ligand, two benzyl groups, and a THF molecule, Tp*U(CH2Ph)2(THF) (2) (Figure 2, Table 1). The uranium center is distorted trigonal prismatic, with N11, N21, and N31 comprising one face of the prism and O61 of THF and the methylene carbons C40 and C50 comprising the other face. The U−C bond distances of 2.615(7) and 2.604(9) Å are on the order of those in Tp*2UCH2Ph (2.57(2) Å),30 Tp*2UCH2SiMe3 (2.601(9) Å),29 Tp*2UCH3 (2.54(3) Å),29 U(CH(SiMe3)2)3 (2.48(2) Å),28 [Cp*UCH(SiMe3)2]2(μ-η6:η6C6H6) (2.508(2) Å),27 and [Cp3U(CH2)CH3]− (2.557(9) Å).24 The U1−O61 distance of 2.6552(5) Å and the U−N distances ranging from 2.559(6) to 2.649(6) Å are consistent with Tp*UI2(THF)2 (1).33 Unlike the structure of Tp*2UCH2Ph, which has an η1coordinated benzyl group with a U1−C41−C42 bond angle of 132.8(14)°,30 the structure of 2 has acute U−Cmethylene−Cipso bond angles, 88.3(4) and 89.7(6)°, similar to those observed for homoleptic U(CH2Ph)4.8 To further understand the coordination of the benzyl substituents to uranium, bond length calculations to determine the hapticity (ηn) were performed using literature procedures and tabulated (Table 2),7 where Δ = [MCo − CH2] − [MCipso − MCH2] and Δ′ = [MCo′ − CH2] − [MCipso − MCH2]. Values of Δ and Δ′ that are similar in magnitude indicate η4-hapticity, while large differences in Δ values indicate a lower coordination number. The respective Δ and Δ′ values for 2 are 0.52 and 0.97 for one alkyl substituent and 0.59 and 0.62 for the other, supporting η4 coordination for both. The high coordination number of the benzyl substituents is most likely a factor in the stability of this molecule, as these data compare favorably to those in U(CH2Ph)4, where this is also the case.8 To determine the influence of η4-benzyl substituents on the stability of complex 2, the synthesis of the uranium bis(neosilyl) compound Tp*U(CH2SiMe3)2 was attempted. Addition of 2 equiv of NaCH2SiMe329 to a −35 °C solution of 1 in THF resulted in an initial color change to blue. During

Figure 3. Electronic absorption spectra of Tp*2UCH2Ph (blue) and Tp*U(CH2Ph)2(THF) (2) (red) in THF at ambient temperature. The inset shows the near-infrared region of the spectrum. Solvent overtones are present between 1600 and 1800 nm.

workup and isolation, decomposition was denoted by progression of the reaction color to brown. By analogy to Tp*2U(CH2)3CH3 and Tp*2UCH3, syntheses of the bis(methyl) and bis(n-butyl) derivatives were attempted but proved unsuccessful.29 To unambiguously discern the oxidation state and electronic features of Tp*U(CH2Ph)2(THF), (2), electronic absorption spectroscopy in the UV−vis and near-IR regions was utilized. The absorption spectra of Tp*U(CH 2 Ph) 2 (THF) and Tp*2UCH2Ph collected in THF at ambient temperature are presented in Figure 3. Both spectra have an interesting feature that reveals a similarity in the electronic environment of the uranium center due to the presence of the benzyl group. The characteristic peak around 1200 nm (ε = 150 M−1 cm−1) is indicative of intensity stealing that occurs as a result of an arylbased electronic transition, which is parity forbidden, stealing charge density from a strong, metal-based electronic transition. A color-producing band for complex 2 is observed at 582 nm (ε = 600 M−1 cm−1), corresponding to the intense blue hue of the

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bis(benzyl) complex, and has intensity similar to that reported for Tp*2UCH2Ph (506 nm, ε ≈ 1000 M −1 cm−1).30 Importantly, the broad and weak absorptions in the region between 1400 and 2000 cm−1 are characteristic of f−-f transitions of uranium(III) complexes, confirming assignment of the +3 oxidation state of 2.30,41,42 Reactivity of Tp*U(CH2Ph)2(THF). Results from our laboratory have shown multiple character traits of the uranium−carbon bond in Tp*2UCH2Ph, revealing both traditional metal−alkyl reactivity, as in the insertion of carbon dioxide,30 and the tendency of the alkyl to homolytically cleave and couple, resulting in an oxidized uranium species.31 Thus, the reactivity of complex 2 toward carbonyl insertion was tested by treating a solution of 2 with 1 equiv of benzophenone at −35 °C (eq 2) in order to probe the nature of the U−C bond. Initially the reaction mixture turned purple, but this color quickly dissipated, leaving an orange-brown solution. After workup and isolation, the resulting orange powder, 3, was recrystallized from a concentrated diethyl ether solution layered with pentane. Analysis of the product by 1H NMR spectroscopy revealed a paramagnetically shifted and broadened spectrum with 16 resonances ranging from −78.91 to 25.77 ppm. Signals integrating to 9 protons at −10.80 and −1.70 ppm correspond to the endo and exo methyl groups of the Tp* ligand, respectively; a resonance at 7.29 ppm for the pyrazole protons and a broad signal at 4.69 ppm for the B−H of the ligand also appear. Two resonances assigned to the chemically inequivalent methylene protons of two benzyl groups7,8,40 are observed upfield at −78.91 and −73.29 ppm, showing that the desired insertion had not occurred. Using X-ray crystallography, the molecular structure of compound 3 was determined by analysis of orange crystals grown from a concentrated diethyl ether solution at −35 °C. Refinement of the data revealed the structure as Tp*U(OC(Ph)2CH2Ph)(CH2Ph)2, (3), which features a six-coordinate pseudooctahedral uranium with a Tp* ligand, two benzyl groups, and an aryloxide (Figure 4, Table 3). The aryloxide

Table 3. Structural Parameters (Distances in Å and Angles in deg) for 3 Bond Distances U1−N11 U1−N21 U1−N31 U1−O40

2.518(4) O40−C41 2.563(3) U1−C50 2.549(4) U1−C60 2.079(3) Bond Angles

U1−C50−C51 U1−C60−C61

1.434(5) 2.480(5) 2.507(5)

119.9(8) 124.8(3)

ligand results from addition of a benzyl group to the carbonyl carbon of benzophenone, causing a reduction of the carbon− oxygen bond order and oxidation of the uranium from +3 to +4. The U−C distances of 2.480(5) and 2.507(5) Å are consistent with other U(IV) benzyl distances.8,10,21,40,43−46 Additionally, the respective U1−C50−C51 and U1−C60−C61 bond angles of 119.9(8) and 124.8(3)° compare favorably to those of the η1-benzyl substituent in Tp*2UCH2Ph.30 This change from the η4 coordination of the benzyl rings in complex 2 is in part due to the presence of the sterically bulky aryloxide ligand which serves to protect the uranium center. The U1− O40 distance of 2.079 Å is substantially shorter than that of the datively coordinated THF in 2, as would be expected for an anionic aryloxide group.46−48 The U−N bond distances of the κ3-Tp* ligand, ranging from 2.518(4) to 2.563(3) Å, are consistent with previously reported bond distances for monoTp* uranium(IV) complexes.16 The formation of 3 presumably proceeds by the displacement of THF by benzophenone with subsequent reduction of the OC bond by the electron-rich uranium(III) center, resulting in the formation of a transient benzophenone radical containing a uranium(IV) center. This is supported by observation of the characteristic purple color for the ketyl radical, showing that radical rearrangement rather than insertion at the uranium−carbon bond occurs (eq 2). The benzophenone radical is immediately trapped by a benzyl radical, which must derive from facile homolytic scission of the U−C bond. Benzyl radical formation from 2 results in an unstable, low-valent byproduct that subsequently decomposes to U0 and free Tp* ligand, the latter being observed in the 1H NMR spectrum of a crude sample of 3. Precedent for ketyl radical formation by uranium(III) has been reported by Meyer et al., who found that exposure of [((t‑BuArO)3tacn)UIII] to 1 equiv of 4,4-di-tert-butylbenzophenone resulted in the isolation of the uranium(IV) ketyl radical species, as confirmed by electronic absorption spectropscopy as well as X-ray crystallography.47 Additional support for formation of uranium(IV) benzophenone radical intermediates with subsequent radical coupling has been reported. Formation of Cp*2U(OC(Ph2)(C10H8N2)) results when benzophenone is added to Cp*2U(bipy), which contains a radical bipyridine ligand.48 Benzophenone reduction forms a uranium(IV) biradical intermediate which undergoes radical coupling to form a new carbon−carbon bond, resulting in a metallacyclohexane. Similarly, addition of 1 equiv of benzophenone to [((AdArO)3tacn)UIII] results in the formation of the radical intermediate [((AdArO)3tacn)UIV(OC•Ph2)], which in the presence of cyclohexadiene abstracts a hydrogen atom, resulting in the formation of the uranium(IV) diphenyl methoxide species [((AdArO)3tacn)UIV(OCHPh2)].47 Treating 2 with benzophenone in the presence of excess cyclohexadiene

Figure 4. Molecular structures of 3 shown with 30% probability ellipsoids. Selected hydrogen atoms and solvent molecules have been omitted for clarity. 1488

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Figure 5. Molecular structures of 4 (left) and 5 (right) shown with 30% probability ellipsoids. Selected hydrogen atoms, solvent molecules, and coordinated THF (5) have been omitted for clarity.

did not form a new product by 1H NMR spectroscopy, indicating that benzyl radical coupling with the benzophenone radical is the kinetically favored process and is responsible for the formation of 3. The reactivity of N3Mes with 2 was examined to determine whether insertion or oxidation is the favored pathway for organic azide reactivity. Exposing 2 to 0.9 equiv of N3Mes at thawing THF temperature (−108 °C) caused a change from deep blue to brown-red with concomitant bubbling of N2. After workup and recrystallization from a concentrated diethyl ether/ pentane solution (1/1), the product was isolated as an orangered solid, 4, in moderate yields (61%, eq 3). Initial characterization of 4 by IR spectroscopy revealed a single B− H stretch at 2543 cm−1 characteristic of the uranium-

coordinated Tp* ligand, and the lack of an azide stretch was noted, indicating the reduction of the NN multiple bonds of N3Mes. Analysis and integration by 1H NMR spectroscopy of the crude reaction mixture showed a singlet at 2.78 ppm with multiplets in the aromatic region corresponding to 1/2 equiv of bibenzyl. Extrusion of bibenzyl and dinitrogen upon addition of the azide indicated formation of the tetravalent imido species Tp*U(NMes)(CH 2 Ph)(THF) (4), by analogy to Tp*2UNR″.31 Also in the spectrum were 22 resonances shifted and broadened by the paramagnetic uranium. A sharp singlet at −141.34 ppm is comparable with the upfield-shifted benzylCH2 peaks for complex 3 and is therefore indicative of methylene protons of a benzyl group in 4. Due to the complexity of the 1H NMR spectrum of 4, X-ray analysis was necessary for structural confirmation. Red crystals suitable for diffraction were grown from a toluene/THF mixture (2/1) at −35 °C, and their analysis revealed a sixcoordinate uranium(IV) center with a distorted-trigonalprismatic geometry with a Tp* ligand, benzyl group, and imido substituent (Figure 5, left, and Table 4). The top face of the prism is composed of the three N(pz) atoms of the Tp* ligand, while the bottom face is made up of the THF, benzyl, and imido ligands. The U1−N41 distance of 1.990(5) Å for the imido along with its nearly linear U1−N41−C40 bond angle of 177.7(5)° is as expected for uranium−nitrogen multiple bonding, comparing favorably with Tp*2UNMes, which has a U−N bond length of 1.988(5) Å and U−N−C angle of 176.6°.31 These data are consistent with the interaction of the lone pair on N of the imido ligand with the uranium center. Finally, the U−C bond length of 2.578(8) Å is consistent with 3. Interestingly, the uranium center appears to have significant interaction with the Cipso of the benzyl group, with a U−C

Table 4. Structural Parameters (Distances in Å and Angles in deg) for 4 and 5 distance or angle

4

5

U1−N11 U1−N21 U1−N31 U1−N41 U1−N41−C40 U1−N51 U1−N52 N51−N52 N52−N53 U1−C50 U1−C51 U1−C50−C51 U1−O61

2.542(5) 2.643(6) 2.587(6) 1.990(5) 177.7(5)

2.581(5) 2.509(5) 2.633(5) 1.986(5) 175.5(4) 2.416(5) 2.509(5) 1.310(7) 1.277(8)

2.576(8) 2.866(7) 85.8(5) 2.501(4)

2.481(4) 1489

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uranium(IV) performs the desired insertion chemistry, this may be the preferred oxidation state for future catalytic applications, as radical chemistry is avoided. Attempts to compare the reactivity of the uranium(IV) tris(benzyl) species Tp*U(CH2Ph)3 to 2 were thwarted due to the inability to isolate and characterize this molecule. The reactivity of trivalent 2 toward 4,6-di-tert-butyl-2-[(2,6diisopropylphenyl)imino]quinone (DIPPiq)35 was also tested, as this class of bidentate ligands can be redox-active through formation of amido phenolate (ap) derivatives.50,51 Exposure of 2 to 1 equiv of DIPPiq results in an immediate color change to orange, similar to the formation of tetravalent 3 (eq 4). Following workup, analysis of the crude mixture shows the presence of 1/2 equivalent of free bibenzyl, indicating loss of one benzyl substituent per uranium via homolytic scission. The paramagnetically shifted spectrum of the isolated product, 6, also shows 26 resonances ranging from −22.38 to 10.34 ppm. Six resonances integrating to three protons each correspond to the methyls of the Tp* ligand, whereas the isopropyl methyl groups of the aryl substituent appear as four doublets at −3.34, −2.47, 0.70, and 0.87 ppm. Additionally, two signals integrating to 9H are found at 1.59 and 5.30 ppm, corresponding to two tert-butyl groups. Infrared spectroscopy confirms the presence of the coordinated Tp* ligand with a B−H absorption at 2553 cm−1. Although the presence of DIPPiq is supported by its 1H NMR spectrum, electronic absorption spectroscopy was performed to determine the oxidation state of the uranium center and thereby the nature of the potentially redox-active ligand. The spectrum of 6 in the UV−vis and near-IR regions was collected from its solution in THF at ambient temperature and is presented Figure S3 (Supporting Information). The most significant feature of the spectrum is a series of sharp bands with weak molar absorptivities, ranging from 10 to 80 M−1 cm−1 in the near-IR region, in contrast to the analogous spectra of 2 and Tp*2UCH2Ph. These signals are consistent with the f−f transitions of U(IV) complexes with 5f2 electronic configuration. On the basis of the above data and the presence of both the Tp* ligand and single benzyl group, the orange product is assigned as the amido(phenolate) compound Tp*U(CH2Ph)(DIPPap) (6). The formation of 6 is consistent with the previous observation that uranium(III) benzyl complexes are prone to oxidation via bibenzyl extrusion.31,52 This is favored over intramolecular reductive elimination of the two alkyl moieties from a single uranium to produce the trivalent “Tp*U(DIPPap)” and over carbonyl insertion. The former reaction course is in contrast to recent work from our laboratory highlighting the exposure of the uranium(IV) tetrabenzyl complex U(CH2Ph)4 to the α-diimine ligand MesDABMe (MesDABMe = [ArN

bond length of 2.866(7) Å. The U1−C50−C51 bond angle of 85.8(5)° and small difference in calculated values of Δ and Δ′ (Table 2) support an η4 coordination of the benzyl group. A dative U−O bond of 2.501(4) Å is observed, as well as a κ3-Tp* ligand with U−N distances ranging from 2.542(5) to 2.643(6) Å. Treating 4 with a further equiv of N3Mes caused no obvious color change or effervescence, but following workup, recrystallization from diethyl ether afforded an orange powder (5) in high yields (87%). Complex 5 was also attainable by the addition of 2 equiv of N3Mes to a THF solution of 2 at −35 °C. Analysis by 1H NMR spectroscopy revealed a complicated spectrum, with paramagnetically broadened and shifted resonances ranging from −70 to 75 ppm, which could not be used for unambiguous structural assignment. IR spectroscopy showed a single B−H stretch at 2551 cm−1, supporting retention of the Tp* ligand and absence of the azide stretch for free N3Mes. Since no loss of bibenzyl or N2 was noted, it was hypothesized that azide insertion occurred. Analysis of dark red crystals grown from a concentrated toluene/pentane solution at −35 °C by X-ray diffraction showed the molecular structure of 5 to be the uranium(IV) triazenido imido species Tp*U(NMes)[(CH2Ph)NNN(Mes)κ2N1,2](THF) (5) (Figure 5, right, and Table 4). The second equivalent of N3Mes inserts along the U−C bond to form a monoanionic triazenido ligand, which has respective U1−N51 and U1−N52 distances of 2.416(5) and 2.509(5) Å. The N51− N52 distance (1.310(7) Å) is intermediate of single and double bonds. The imido functionality in 5 remains intact after addition of the second equivalent of N3Mes, with no significant structural changes. Evans has reported uranium(IV) κ2N1,3triazenido derivatives, Cp*2[(Me)NNN(Ad)-κ2N1,3]U(X)] (X = OTf, Br), which have U−N bond lengths of 2.415(5) and 2.452(5) Å for the triflate derivative and 2.435(2) and 2.460(2) Å for the bromide species, both of which are on the order of those for observed 5.49 We believe that the κ2N1,2 coordination for the triazenido ligand in 5 as opposed to κ2N1,3 coordination in Cp*2[(Me)NNN(Ad)-κ2N1,3]U(X)] is due to steric constraints. If this ligand in 5 were indeed κ2N1,3, it would actually bring the bulky mesityl and benzyl groups closer to the large bis-Tp* framework, creating steric hindrance. The generation of complexes 4 and 5 allows a comparison of the reactivity between uranium(III) and uranium(IV) benzyl complexes toward organic azides. While addition of N3Mes to trivalent 2 results in oxidation and imido formation with extrusion of bibenzyl to form 4, the same addition of N3Mes to tetravalent 4 results in insertion to form the κ2-triazenido species 5. In the latter case, the uranium(IV) oxidation state is preserved and no loss of bibenzyl or oxidation to uranium(V) or (VI) is noted, indicating that uranium(IV) is the thermodynamically preferred oxidation state. Given that 1490

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(10) Fortier, S.; Melot, B. C.; Wu, G.; Hayton, T. W. J. Am. Chem. Soc. 2009, 131, 15512−15521. (11) Evans, W. J.; Walensky, J. R.; Ziller, J. W.; Rheingold, A. L. Organometallics 2009, 28, 3350−3357. (12) Siladke, N. A.; LeDuc, J.; Ziller, J. W.; Evans, W. J. Chem. Eur. J. 2012, 18, 14820−14827. (13) Paolucci, G.; Rossetto, G.; Zanella, P.; Yunlu, K.; Fischer, R. D. J. Organomet. Chem. 1984, 272, 363−383. (14) Dormond, A.; Aaliti, A.; Elbouadili, A.; Moise, C. J. Organomet. Chem. 1987, 329, 187−199. (15) Dormond, A.; Elbouadili, A.; Moise, C. J. Organomet. Chem. 1989, 369, 171−185. (16) Domingos, A.; Marques, N.; Pires de Matos, A.; Santos, I.; Silva, M. Organometallics 1994, 13, 654−662. (17) Campello, M. P. C.; Calhorda, M. J.; Domingos, A.; Galvao, A.; Leal, J. P.; Pires de Matos, A.; Santos, I. J. Organomet. Chem. 1997, 538, 223−240. (18) Matson, E. M.; Fanwick, P. E.; Bart, S. C. Organometallics 2011, 30, 5753−5762. (19) Moloy, K. G.; Marks, T. J. Inorg. Chim. Acta 1985, 110, 127− 131. (20) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.; Marks, T. J. J. Am. Chem. Soc. 1981, 103, 6650−6667. (21) Jantunen, K. C.; Burns, C. J.; Castro-Rodriguez, I.; Da Re, R. E.; Golden, J. T.; Morris, D. E.; Scott, B. L.; Taw, F. L.; Kiplinger, J. L. Organometallics 2004, 23, 4682−4692. (22) Cantat, T.; Graves, C. R.; Jantunen, K. C.; Burns, C. J.; Scott, B. L.; Schelter, E. J.; Morris, D. E.; Hay, P. J.; Kiplinger, J. L. J. Am. Chem. Soc. 2008, 130, 17537−17551. (23) Webster, C. L.; Ziller, J. W.; Evans, W. J. Organometallics 2012, 31, 7191−7197. (24) Arnaudet, L.; Charpin, P.; Folcher, G.; Lance, M.; Nierlich, M.; Vigner, D. Organometallics 1986, 5, 270−274. (25) Foyentin, M.; Folcher, G.; Ephritikhine, M. J. Organomet. Chem. 1987, 335, 201−206. (26) Manriquez, J. M.; Fagan, P. J.; Marks, T. J.; Vollmer, S. H.; Day, C. S.; Day, V. W. J. Am. Chem. Soc. 1979, 101, 5075−5078. (27) Evans, W. J.; Traina, C. A.; Ziller, J. W. J. Am. Chem. Soc. 2009, 131, 17473−17481. (28) Van der Sluys, W. G.; Burns, C. J.; Sattelberger, A. P. Organometallics 1989, 8, 855−857. (29) Matson, E. M.; Forrest, W. P.; Fanwick, P. E.; Bart, S. C. Organometallics 2012, 31, 4467−4473. (30) Matson, E. M.; Forrest, W. P.; Fanwick, P. E.; Bart, S. C. J. Am. Chem. Soc. 2011, 133, 4948−4954. (31) Matson, E. M.; Crestani, M. G.; Fanwick, P. E.; Bart, S. C. Dalton Trans. 2012, 41, 7952−7958. (32) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518−1520. (33) McDonald, R.; Sun, Y.; Takats, J.; Day, V. W.; Eberspracher, T. A. J. Alloys Compd. 1994, 213/214, 8−10. (34) Lochmann, L.; Lim, D. J. Organomet. Chem. 1971, 28, 153−158. (35) Abakumov, G. A.; Cherkasov, V. K.; Piskunov, A. V.; Meshcheryakova, I. N.; Maleeva, A. V.; Poddel’skii, A. I.; Fukin, G. K. Dokl. Chem. 2009, 427, 168−171. (36) Spencer, L. P.; Altwer, R.; Wei, P.; Gelmini, L.; Gauld, J.; Stephan, D. W. Organometallics 2003, 22, 3841−3854. (37) Sheldrick, G. M. Acta Crystallogr. 2008, 112, A64. (38) Beurskens, P. T.; Beurskens, G.; de Gelder, R.; Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M. DIRDIF99; Crystallography Laboratory, University of Nijmegen, Nijmegen, The Netherlands, 2008. (39) Kraft, S. J.; Fanwick, P. E.; Bart, S. C. Inorg. Chem. 2010, 49, 1103−1110. (40) Kiplinger, J. L.; Morris, D. E.; Scott, B. L.; Burns, C. J. Organometallics 2002, 21, 5978−5982. (41) Boudreaux, E. A.; Mulay, L. N. Theory and Applications of Molecular Paramagnetism; Wiley: New York, 1976.

C(Me)C(Me)NAr]; Ar = 2,4,6-trimethylphenyl (Mes)), which results in concerted reductive elimination.8

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CONCLUSION The synthesis and characterization of a rare uranium(III) bis(alkyl) complex, Tp*U(CH2Ph)2(THF), that features η4benzyl substituents and a labile THF ligand supported by a hydrotris(3,5-dimethylpyrazolyl)borate ligand are presented. Spectroscopic and structural data support the assignment of 2 as a trivalent species. The reactivity of 2 has been explored with a variety of organic substrates, specifically benzophenone, mesityl azide, and an iminoquinone. In all cases, oxidation of the uranium from +3 to +4 is observed with participation from a benzyl radical, supporting the notion that there is a significant driving force in favor of the tetravalent oxidation state. Reactivity with the azide and quinone was accompanied by loss of 1/2 equiv of bibenzyl that was easily quantified. The oxidation of Tp*U(CH2Ph)2(THF) to form the corresponding uranium(IV) alkyl 4 provided the opportunity to compare the reactivity of the trivalent and tetravalent organometallic species. Compound 4 reacts with mesityl azide via migratory insertion to produce the triazenido species, in contrast to 2, which can undergo facile homolytic scission to produce benzyl radicals that participate in further coupling chemistry. This difference in reactivity demonstrates that while uranium(III) can be an effective reducing agent, radical reactivity is predominant and divergent from what is observed for tetravalent uranium.

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, cifs, and 1H NMR spectra for 3−6, an electronic absorption spectrum for 6, and crystallographic data for 2−5. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We wish to acknowledge the National Science Foundation (No. CHE-1149875) for funding this work.

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REFERENCES

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