Article pubs.acs.org/Organometallics
Synthesis and CO2 Insertion Reactivity of Allyluranium Metallocene Complexes Christopher L. Webster, Joseph W. Ziller, and William J. Evans* Department of Chemistry, University of California, Irvine, California 92697-2025, United States S Supporting Information *
ABSTRACT: The U4+ metallocene allyl chloride complexes (C5Me5)2U[η3-CH2C(R)CH2]Cl (R = H, Me) can be synthesized by reaction of (C5Me5)2UCl2 with 1 equiv of the corresponding allyl Grignard reagents, [CH2C(R)CH2]MgCl, in hydrocarbon solvents. Bis(allyl)uranium complexes can also be obtained in this manner using 2 equiv of the corresponding allyl Grignard, and X-ray crystallographic studies reveal the presence of both η3- and η1-allyl ligands: (C5Me5)2U[η3CH2C(R)CH2][η1-CH2C(R)CH2]. Sodium amalgam reduction of (C5Me5)2U[η3-CH2C(R)CH2]Cl generates the U3+ metallocene allyl complexes (C5Me5)2U[η3-CH2C(R)CH2]. Carbon dioxide reacts with the U4+ allyl complexes to form the U−C insertion products (C5Me5)2U[κ2O,O′-O2CCH2CHCH2]2−xClx (x = 0, 1). The dicarboxylate (C5Me5)2U[κ2O,O′O2CCH2CHCH2]2, which has a 171.98(5)° O−U−O angle, reacts with Me3SiCl to regenerate (C5Me5)2UCl2 and liberate Me3SiOC(O)CH2CHCH2.
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chemistry vis-à-vis the (C5Me5)2Ln(η3-CH2CHCH2) lanthanide compounds, the synthesis of bis(pentamethylcyclopentadienyl) uranium allyl compounds has been pursued. Surprisingly, the commonly used [(C5Me5)2U]n+ (n = 1, 2) metallocene platform had not been employed with allyl ligands, even though it is broadly useful with a variety of ligand systems.27−35 We report here the synthesis of U4+ and U3+ metallocenes of formulas (C5Me5)2U[CH2C(R)CH2]X (R = H, Me; X = Cl, CH2C(R)CH2) and (C5Me5)2U[CH2C(R)CH2], respectively. The latter complexes were of particular interest, since potential access to allyl η1 coordination would make them synthons for rare η1-alkyl U3+ complexes. To our knowledge, the only isolable and structurally verified examples of this class are (C 5 Me 5 ) 2 U[CH(SiMe 3 ) 2 ], 30 U[CH(SiMe3)2]3,36 {(C5Me5)[(Me3Si)2CH]U}2(μ-η6:η6-C6H6),37 and [hydrotris(3,5-dimethylpyrazolyl)borate]2U(R′) (R′ = (CH2Ph)−, (CCPh)−, (CCSiMe3)−).38
INTRODUCTION Allyl ligands are valuable in organometallic chemistry, since they can interconvert between η1 and η3 coordination modes to generate and stabilize open sites at the metal center as needed for synthesis and catalysis.1−6 Although steric control of reactivity is very important in f-element chemistry, allyl ligands have not been extensively developed for the actinide metals. In contrast, rare-earth metallocene allyl complexes, (C5Me5)2M(η3-CH2CHCH2) (M = Sc,7 Y,8 lanthanides8−13) have been utilized broadly, since they function well as synthons for traditional η1-alkyl metallocenes. Reports on uranium allyl complexes first appeared in 1969 and 1974 on complexes formulated as (CH2CHCH2)3UX (X = CH2CHCH2,14 Cl,15 Br,15 I15). These complexes were unstable above 253 K and were not structurally characterized by X-ray diffraction, but they were found to react with CO216−18 and to be effective ethylene and butadiene polymerization catalysts when X was a halide.14,15 Uranium allyl complexes with cyclopentadienyl ancillary ligands were subsequently reported, such as (C5H5)3U[η1-CH2C(R)CH2] (R = H,19 Me20) and (C5Me5)U[η3-CH2C(Me)CH2]3,21 as well as cyclopentadienylfree complexes such as ( i PrO) 2 U(η 3 -CH 2 CHCH 2 ) 2 , 22 {[ t BuNSi(Me) 2 ] 2 O-κ 3 N,O,N′}U(CH 2 CHCH 2 ) 2 , 2 3 and (tBu3CO)2U(CH2CHCH2)2.24 The only U3+ allyl complex reported in the literature, to our knowledge, is the highly unstable U(CH2CHCH2)3.25 Reactivity studies on uranium allyl complexes have been limited to the polymerization and CO2 chemistry mentioned above and CO2 insertion with (C5H5)3U(η1-CH2CHCH2).26 Definitive X-ray crystallographic studies have been limited to (C5H5)3U[η1-CH2C(Me)CH2]20 and (C5Me5)U[η3-CH2C(Me)CH2]3.21 To further develop this potentially useful class of uranium allyl complexes and to explore their utility in organoactinide © 2012 American Chemical Society
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EXPERIMENTAL SECTION
All syntheses and manipulations described below were conducted under dinitrogen, except for reactions involving Na(Hg), which were done under argon, with rigorous exclusion of air and water using glovebox, Schlenk, and vacuum line techniques. Solvents used were sparged with UHP argon and dried over columns containing Q-5 and molecular sieves purged with argon. C6D6 was dried over sodium− potassium alloy, degassed using three freeze−pump−thaw cycles, and vacuum-transferred before use. Alkyl Grignard reagents were purchased from Sigma-Aldrich, (CH2CHCH2)MgCl (2.0 M in THF) and [CH2C(Me)CH2]MgCl (0.5 M in THF), and were used as received. CO2 (99.98%) was purchased from AirGas and used as received. Me3SiCl (98%+) was purchased from Alfa Aesar, dried over Received: August 6, 2012 Published: October 2, 2012 7191
dx.doi.org/10.1021/om3007536 | Organometallics 2012, 31, 7191−7197
Organometallics
Article
4A molecular sieves, and degassed via three freeze−pump−thaw cycles. (C5Me5)2UCl227 and [(C5Me5)2U][(μ-Ph)2BPh2]39 were synthesized as previously described. 1H NMR spectra were recorded with a Bruker DRX500 MHz spectrometer. Infrared spectra were recorded as KBr pellets on a Varian 1000 FT-IR spectrometer. Elemental analyses were performed on a Perkin-Elmer 2400 Series II CHNS analyzer. (C5Me5)2U(η3-CH2CHCH2)Cl (1). Rapid addition of a 2.0 M solution of (CH2CHCH2)MgCl (92 μL, 0.18 mmol) in THF to a stirred dark red solution of (C5Me5)2UCl2 (107 mg, 0.19 mmol) in hexane (20 mL) caused an immediate color change to red-orange. After 10 min, the solvent was removed via reduced pressure and the solids were extracted with hexane. The white and gray insoluble solids were removed via centrifugation and filtration, and the solvent was removed from the filtrate via reduced pressure to yield a microcrystalline red-orange solid, 1 (95 mg, 91%). Crystals could be grown from concentrated toluene, hexane, and THF solutions at 238 K but were too small for X-ray crystallographic analysis. Variable-temperature 1H NMR studies from 220 to 340 K were conducted (see the Supporting Information) and are discussed in the Results. 1H NMR (C6D6, 298 K): δ 3.0 (s, C5Me5, 30H), −59.4 (br s, CH2CHCH2, 1H). The terminal allylic protons could not be located due to the paramagnetic nature of uranium.40 IR: 2899 s, 2856 s, 2729 w, 1526 w, 1433 m, 1378 s, 1255 w, 1020 m, 821 s cm−1. Anal. Calcd for C23H35ClU: C, 47.22; H, 6.03. Found: C, 47.55; H, 5.90. (C5Me5)2U[η3-CH2C(Me)CH2]Cl (2). Following the procedure for 1, a 0.5 M solution of [CH2C(Me)CH2]MgCl (370 μL, 0.185 mmol) in THF was combined with (C5Me5)2UCl2 (107 mg, 0.185 mmol) to yield 2 as a microcrystalline brown solid (103 mg, 94%). Rectangular brown plates suitable for X-ray crystallography were grown from a concentrated hexane solution at 243 K. 1H NMR (C6D6, 298 K): δ 3.1 (s, C5Me5, 30H), −25.1 (s, CH2C(Me)CH2, 3H). The terminal allylic protons could not be located due to the paramagnetic nature of uranium.40 IR: 2896 s, 2856 s, 2725 w, 1436 m, 1378 s, 1313 w, 1021 m, 818 s cm−1. Anal. Calcd for C24H37UCl: C, 48.12; H, 6.23. Found: C, 48.50; H, 6.01. (C5Me5)2U(η3-CH2CHCH2)(η1-CH2CHCH2) (3). Following the procedure for 1, a 2.0 M solution of (CH2CHCH2)MgCl (184 μL, 0.368 mmol) in THF was combined with (C5Me5)2UCl2 (107 mg, 0.185 mmol) to yield a black microcrystalline solid, 3 (96 mg, 90%). Black blocks suitable for X-ray crystallography were grown from a concentrated hexane solution at 243 K. Variable-temperature 1H NMR studies from 200 to 370 K were conducted (see the Supporting Information) as discussed in Results. 1H NMR (C6D6, 298 K): δ 0.4 (s, C5Me5). The terminal allylic protons could not be located due to the paramagnetic nature of uranium.40 IR: 3060 w, 2980 w, 2860 w, 2726 w, 1592 s, 1434 m, 1379 m, 1188 m, 1019 m, 964 m, 817 s, 536 s, 445 s cm−1. Anal. Calcd for C26H40U: C, 52.87; H, 6.83. Found: C, 52.30; H, 7.10. (C5Me5)2U[η3-CH2C(Me)CH2][η1-CH2C(Me)CH2] (4). Following the procedure for 1, a 0.5 M solution of [CH2C(Me)CH2]MgCl (690 μL, 0.346 mmol) in THF was combined with (C5Me5)2UCl2 (100 mg, 0.173 mmol) to yield black crystals of 4 (55 mg, 52%) after workup and crystallization in a concentrated 243 K solution of hexane. Black blocks suitable for X-ray crystallography were grown from a concentrated hexane solution at 243 K. Variable-temperature 1H NMR studies from 200 to 350 K were conducted as discussed in Results. 1H NMR (C6D6, 298 K): δ 30.5 (b, CH2(Me)CH2, 6 H), 1.0 (s, C5Me5, 30H), −12.1 (b, CH2(Me)CH2, 6H). IR: 3069 w, 2969 m, 2911 s, 2723 w, 1592 m, 1437 m, 1378 m, 1272 m, 1018 w, 950 m, 889 w, 851 w, 827 m, 811, 773 w cm−1. Anal. Calcd for C28H44U: C, 54.36; H, 7.17. Found: C, 53.86; H, 7.25. (C5Me5)2U(η3-CH2CHCH2)(THF) (5·THF) from [(C5Me5)2U][(μPh)2BPh2]. A 2.0 M solution of (CH2CHCH2)MgCl (58 μL, 0.12 mmol) in THF was added via syringe to a rapidly stirred slurry of [(C5Me5)2U][(μ-Ph)2BPh2] (100 mg, 0.117 mmol) in hexane (20 mL). Green and white solids were immediately deposited, and the slurry was stirred for 5 min. The slurry was centrifuged, and the solids were removed via filtration. Removal of solvent from the filtrate via reduced pressure yielded a microcrystalline blue-green solid. A
concentrated solution of this solid in hexane at 243 K yielded bluish green crystals of 5·THF (10 mg, 15%) suitable for X-ray crystallography. 1H NMR (C6D6, 298 K): δ −9.4 (br s, C5Me5). Samples slowly lost THF while drying, which complicated the elemental analysis. Analytical data for the desolvate are given in the next paragraph. (C5Me5)2U(η3-CH2CHCH2) (5) from (C5Me5)2U(η3-CH2CHCH2)Cl (1). Na (61 mg, 2.7 mmol) was smeared onto the inside of a vial containing a dark red solution of 1 (250 mg, 0.442 mmol) in hexane (20 mL). Enough Hg to make a 1 wt % amalgam (6 g) was pipetted into the solution, and the mixture was stirred vigorously. After approximately 3 h, the red solution became dark green and white solids were observed. After 18 h total, the mixture was centrifuged and the solids were removed via filtration. The solvent was removed from the filtrate via reduced pressure to yield (C5Me5)2U(η3-CH2CHCH2) (212 mg, 90%) as a microcrystalline yellow-green solid. 1H NMR (C6D6, 298 K): δ −11.3 (s, C5Me5, 15H), −13.1 (s, C5Me5, 15H). IR: 2904 s, 2863 s, 1439 m, 1385 s, 1255 w, 1015 m, 823 s cm−1. Anal. Calcd for C23H35U: C, 50.27; H, 6.42. Found: C, 50.56; H, 6.40. (C5Me5)2U[η3-CH2C(Me)CH2] (6) from (C5Me5)2U[η3-CH2C(Me)CH2]Cl (2). Following the procedure for 5, 2 (212 mg, 0.356 mmol) was combined with 1 wt % NaHg (55 mg of Na, 2.49 mmol) and stirred for 18 h to yield 6 as a yellow-green microcrystalline solid (180 mg, 90%). 1H NMR (C6D6, 298 K): δ −7.2 (br s, C5Me5, 30H), −19.0 (s, CH2C(Me)CH2, 3H). IR: 2903 s, 2861 s, 1442 m, 1389 s, 1254 w, 1015 m, 822 s cm−1 . Anal. Calcd for C24H37U: C, 51.15; H, 6.62. Found: C, 51.40; H, 7.02. (C5Me5)2U[κ2O,O′-O2CCH2CHCH2]Cl (7). A red-orange solution of 1 (100 mg, 0.171 mmol) in toluene (15 mL) was degassed via three freeze−pump−thaw cycles. CO2 (1 atm) was introduced, and the stirred solution rapidly turned bright orange. This mixture was stirred for 1 h and then dried via reduced pressure to leave an orange solid. A concentrated hexane mixture at 243 K yielded dark orange crystals of 7 (104 mg, 97%). 1H NMR (C6D6, 298 K): δ 8.7 (s, C5Me5, 30H), −0.6 (d, 9 Hz, CH2CHCH2, 1H), −2.8 (d, 16 Hz, CH2CH CH2, 1H), −9.1 (m, CH2CHCH2, 1H), −23.8 (s, CH2CHCH2, 2H). IR: 2725 m, 2906 s, 2858 m, 2725 w, 1592 m, 1508 m, 1440 s, 1379 m, 1390 m, 1262 m, 1021 m, 992 m, 918 m, 805 w, 751 m cm−1. Anal. Calcd for C24H35ClO2U: C, 45.83; H, 5.61. Found: C, 45.99; H, 5.20. (C5Me5)2U[κ2O,O′-O2CCH2C(Me)CH2]Cl (8). A brown-red solution of 2 (100 mg, 0.168 mmol) in toluene (15 mL) was degassed via three freeze−pump−thaw cycles. CO2 (1 atm) was introduced, and the stirred solution rapidly turned bright orange. After 1 h, solvent was removed via reduced pressure to yield 8 (108 mg, 99%) as an orange semicrystalline powder. 1H NMR (C6D6, 298 K): δ 8.7 (s, C5Me5, 30H), −1.1 (s, CH2C(Me)CH2, 1H), −3.9 (s, CH2C(Me)CH2, 1H), −8.7 (s, CH2C(Me)CH2, 3H), −23.8 (s, CH2CHCH2, 2H). IR: 3079 s, 2905 s, 2726 w, 1592 s, 1502 m, 1433 s, 1302 w, 1253 m, 1093 w, 1022 m, 894 s, 760 s, 675 m, 533 w cm−1. Anal. Calcd for C25H37ClO2U: C, 46.69; H, 5.80. Found: C, 47.23; H, 6.12. (C5Me5)2U[κ2O,O′-O2CCH2CHCH2]2 (9). A black solution of 3 (100 mg, 0.169 mmol) in toluene (15 mL) was degassed via three freeze−pump−thaw cycles. CO2 (1 atm) was introduced, and the stirred solution rapidly turned bright orange. After 1 h, the solvent was removed via reduced pressure to leave an orange oil. A concentrated hexane mixture at 243 K yielded dark orange crystals of 9 (109 mg, 97%). 1H NMR (C6D6, 298 K): δ 14.3 (m, 7 Hz, CH2CHCH2, 2H), 11.6 (d, 6 Hz, CH2CHCH2, 4H), 10.1 (d, 17.5 Hz, CH2CHCH2, 2H), 8.7 (d, 9.2 Hz, CH2CHCH2, 2H), 4.3 (s, C5Me5, 30H). IR: 2905 s, 1534 s, 1444 s, 1379 w, 1263 w, 1023 w, 933 w, 915 m, 767 w cm−1. Anal. Calcd for C28H40O4U: C, 49.55; H, 5.94. Found: C, 50.02; H, 5.80. Reaction of 9 with Me3SiCl. Excess Me3SiCl was added dropwise to a 0.25 mL C6D6 solution of 9 (10 mg, 0.015 mmol) in an NMR tube. 1H NMR spectroscopy indicated no reaction after 1 h. After 12 h in a 313 K oil bath, the solution had turned from bright orange to dark red. The 1H NMR spectrum indicated complete consumption of 9 and formation of (C5Me5)2UCl227 along with resonances appropriate for 7192
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Table 1. X-ray Data and Collection Parameters on (C5Me5)2U[η3-CH2C(Me)CH2]Cl (2), (C5Me5)2U(η3-CH2CHCH2)(η1CH2CHCH2) (3), (C5Me5)2U[η3-CH2C(Me)CH2][η3-CH2C(Me)CH2] (4), and (C5Me5)2U[κ2O,O′-O2CCH2CHCH2]2 (9) empirical formula temp (K) cryst syst space froup a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalcd (Mg/m3) μ (mm−1) R1a (I > 2.0σ(I)) wR2a (all data) a
2
3
4
9
C24H37ClU 143(2) monoclinic P21/n 8.4590(8) 17.0606(16) 16.0282(15) 90 101.3709(10) 90 2267.7(4) 4 1.755 7.281 0.0214 0.0527
C26H40U 88(2) monoclinic P21/c 16.6441(6) 17.5923(6) 16.9473(6) 90 109.8633(4) 90 4667.1(3) 8 1.681 6.963 0.0210 0.0514
C28H44U 88(2) monoclinic C2/c 16.4323(8) 14.4904(7) 21.0299(10) 90 97.6487(5) 90 4962.9(4) 8 1.656 6.553 0.0136 0.0330
C28H40O4U 143(2) monoclinic P21/n 8.4244(5) 20.0795(12) 15.6383(10) 90 97.9538(6) 90 2619.9(3) 4 1.721 6.226 0.0159 0.0395
Definitions: R1 = ∑||Fo| − |Fc||/∑|Fo|, wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2.
η3-allyl protons were not observed in previously reported U[η3CH2C(R)CH2] complexes, and only the vinyl protons are observable in (C5H5)3U(η1-CH2CHCH2).19 Protons on carbon atoms α to U4+ are often not observed due to the paramagnetic nature of 5f2 U4+.40 For example, no resonances are observed for the α carbon protons in (η5-C5Me4SiMe2CH2κC)2U, whereas they appear in derivatives in which a single atom has been inserted between the α carbons and the metal.41 In addition to the paramagnetic effects of U4+, the NMR spectra can be complicated by η1/η3 fluxional processes involving the allyl ligand.8,12,43 The [CH2C(Me)CH2]− methyl protons in 2 were located at −25.1 ppm. Variable-temperature 1H NMR spectroscopy studies of 1 from 220 to 340 K were conducted to study allyl fluxionality. These studies did not reveal any resonance for the terminal allyl protons, but two (C5Me5)− resonances (3.0 and 1.5 ppm) were observed at low temperature. Their coalescence temperature of approximately 250 K leads to a calculated ΔG⧧ barrier of 13 kcal/mol.42 As discussed previously with the rare-earth metallocene allyl complexes (C 5 Me 5 ) 2 M(η 3 -CH 2 CHCH2),8,12,43 the spectra can be explained by rotation about the U−(η3-allyl) C2 axis, η1−η3 interconversions, or both. Crystals of 1 were too small for X-ray analysis, but a single crystal of 2 established the η3 coordination mode in the solid state (Figure 1 and Table 2). The [η3-CH2C(Me)CH2]− ligand has U−C(allyl) distances of 2.569(3) Å [U−C(21)], 2.777(3) Å [U−C(22)], and 2.706(3) Å [U−C(23)]. These can be compared with distances of 2.66(1), 2.80(1), and 2.66(1) Å in (C5Me5)U[η3-CH2C(Me)CH2]3.21 The latter complex and the (C5Me5)2M(η3-CH2CHCH2) compounds (M = Sm,12 Y,8 Lu8) have similar M−C distances for the terminal allyl carbon atoms, which is in contrast with the situation in 2. The disparate M−C distances in 2 are more similar to those in the substituted allyl complexes (C 5 Me 5 ) 2 Sm(η 3 -CH 2 CHCHMe) at 2.55(2), 2.69(2), and 2.71(1) Å and [(C5Me5)2Sm(η3CH2CHCHCH2−)]2, 2.57(2), 2.68(2), 2.73(2) Å. A reviewer has pointed out that the long U−C(23) distance is in the direction of η1 coordination. All other distances and angles in 2 are within the expected ranges for a (C5Me5)2UX2 complex.45
the formation of 2 equiv of Me3SiOC(O)CH2CHCH2. 1H NMR (C6D6, 298 K): δ 13.6 (br s, C5Me5UCl2, 30H), 6.0 (br s, CH2CH CH2, 2H), 4.9 (s, CH2CHCH2, 2H), 3.9 (s, CH2CHCH2, 2H), 2.9 (s, CH2CHCH2, 4H), 0.2 (s, Me3Si, 18 H). Thermal Stability of 1−6. Complexes 1, 2, 5, and 6 are stable in C6D6 solution at 323 K over a 12 h period by 1H NMR spectroscopy but decompose over several days at this temperature. Complex 3 is stable at 370 K for a period at least 10 min but will decompose overnight. Complex 4 begins to slowly decompose at 340 K and decomposes over 10 min at 350 K. X-ray Data Collection, Structure Determination, and Refinement. Crystallographic information for complexes 2, 3, 4, and 9 (CCDC Nos. 891621−891624) is summarized in Table 1 and in the Supporting Information.
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RESULTS (C5Me5)2U[η3-CH2C(R)CH2]Cl (R = H (1), Me (2)). (C5Me5)2UCl2 reacts with (CH2CHCH2)MgCl in a 1:1 ratio at room temperature within 5 min to generate (C5Me5)2U[η3CH2CHCH2]Cl (1) in >90% yield as an analytically pure redorange solid (eq 1). The 2-methylallyl analogue (C5Me5)2U[η3-
CH2C(Me)CH2]Cl (2) can be made similarly. Both 1 and 2 can be prepared in THF, toluene, or hexane, but reactions in hexane provided reaction mixtures with the fewest byproducts by 1H NMR spectroscopy. The infrared spectra of both 1 and 2 show absorptions at 1433 and 1436 cm−1, respectively, and no significant signals in the 1550−1700 cm−1 range corresponding to an uncoordinated CC bond. This spectroscopic signature for 1 and 2 is consistent with η3-allyl coordination.14,20,22 The room-temperature 1H NMR spectrum of 1 contains a single resonance for the (C5Me5)− protons at 3.0 ppm and 2 had an analogous single resonance at 3.1 ppm. The terminal allyl protons in 1 and 2 were not observable by 1H NMR spectroscopy. The terminal 7193
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Figure 2. ORTEP44 representation of (C5Me5)2U[η3-CH2C(R)CH2][η1-CH2C(R)CH2] (R = H, 3, left; R = Me, 4, right) with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Only one of the crystallographically independent molecules from the unit cell of 3 is shown.
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Figure 1. ORTEP representation of (C5Me5)2U[η3-CH2C(Me)CH2]Cl (2) with ellipsoids drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.
Table 2. Selected Bond Distances and Angles for (C5Me5)2U[η3-CH2C(Me)CH2]Cl (2), (C5Me5)2U(η3CH2CHCH2)(η1-CH2CHCH2) (3), and (C5Me5)2U[η3CH2C(Me)CH2][η1-CH2C(Me)CH2] (4) U(1)−(C5Me5 centroid)av (Å) (C5Me5 centroid)−U(1)−(C5Me5 centroid) (deg) U(1)−Cl(1) (Å) U(1)−C(21) (Å) U(1)−C(22) (Å) U(1)−C(23) (Å) U(1)−C(25) (Å)
2
3
2.489 130.9
2.485 133.8
2.512 130.39
2.592(3) 2.718(3) 2.732(3) 2.526(3)
2.563(2) 2.794(2) 2.718(2) 2.538(1)
2.643(1) 2.569(3) 2.777(3) 2.706(3)
methyl protons in CH2C(CH3)CH2. Since only one set of allyl resonances is observed, the two allyl groups must be equivalent on the NMR time scale. The broadness of the low-field resonances can be explained by an η1/η3 interconversion in the presence of a paramagnetic center. In the solid state, the metrical parameters of the [(C5Me5)2U]2+ metallocene components as well as the [η3CH2C(R)CH2]− ligands of complexes 3 and 4 are very similar to those of 2 (Table 2). The average U−(C5Me5 centroid) distances and the (C5Me5 centroid)−U−(C5Me5 centroid) angles are similar, and each [η3-CH2C(R)CH2]− ligand is attached to uranium with one short connection (2.563(2)− 2.592(3) Å) and two longer ones (2.706(3)−2.794(2) Å). The longest U−C(η3-allyl) distance involves the middle carbon, C(22), in the case of 2 and 4. In 3, an end carbon has the longest distance: U(1)−C(23) is 2.732(3) Å. As mentioned above, this pattern of U−C(η3-allyl) distances may facilitate access to η1 coordination. The similarities in 2−4 show that a chloride with a 2.643(1) Å U−Cl distance or an [η1CH2C(R)CH2] group with a 2.526(3) or 2.538(1) Å U−C distance gives similar structural parameters to the {(C5Me5)2U[η3−CH2C(R)CH2]}1+ unit. Even the 98.01(9) and 97.28(5)° C(22)−U−C(25) angles in 3 and 4, respectively, are similar to the 97.09(8)° C(23)−U−Cl(1) angle in 2. The U−C(η1-allyl) distances in 3 (2.526(3) Å) and 4 (2.538(1) Å) are significantly longer than the 2.424(7) Å U−C analogues in (C5Me5)2UMe2.51 The η1-allyl ligands in 3 and 4 contain 1.332(5) and 1.348(3) Å C−C distances, as expected for localized CC double bonds and the observed 1592 cm−1 IR absorptions. (C5Me5)2U[η3-CH2C(R)CH2] [R = H (5), Me (6)]. Initial attempts to make a U3+ metallocene allyl complex involved the reaction of [(C5Me5)2U][(μ-Ph)2BPh2] with (CH2CHCH2)MgCl in hexane at room temperature. Short reaction times, e.g. 5 min, fully consumed the starting material but produced only ∼15% crystalline yield of the THF adduct (C5Me5)2U(η3CH2CHCH2)(THF) (5·THF), which could be characterized by 1H NMR spectroscopy (vide infra) and by X-ray crystallography (eq 3 and Figure 3).
4
(C5Me5)2U[η3-CH2C(R)CH2][η1-CH2C(R)CH2] (R = H (3), Me (4)). When (C5Me5)2UCl2 is treated with [CH2C(R)CH2]MgCl in a 1:2 ratio in hexane at room temperature, a rapid color change occurs and bis(allyl) complexes can be isolated: (C5Me5)2U[η3-CH2C(R)CH2][η1-CH2C(R)CH2] (R = H, 3, 90% yield; R = Me, 4, 50% yield) (eq 2).
The infrared spectra of 3 and 4 both show absorptions at 1592 cm−1 that indicate the presence of an η1-allyl group in each complex.14,20,22 X-ray crystallography confirmed that η3and η1-allyl ligands are present in both 3 and 4 (Figure 2). These are the only structural examples of f-element complexes containing both η3- and η1-allyl ligands. Several transition-metal examples of mixed allyl ligation are known, 46−50 including (C 5 H 5 )Zr(η 3 -CH 2 CHCH 2 ) 2 (η 1 CH 2 CHCH 2 ) 46 and (Ph 3 P)Ir(η 3 -CH 2 CHCH 2 ) 2 (η 1 CH2CHCH2).49 The 1H NMR spectra of 3 and 4 show a single resonance for the two (C5Me5)− rings in each complex from 200 to 350 K, indicating fluxional behavior in solution. Resonances assignable to the terminal allylic protons are observable for 3 above 300 K and for 4 above 255 K, but only as a very broad resonance (ν1/2 = 400 Hz) at low field: 13.5 ppm for 3 and 30.5 ppm for 4. Complex 3 has an additional resonance at −8.0 ppm integrating to 2H vs 30 H for the (C5Me5)− resonance that is assignable to the CH2CHCH2 protons. In 4, a resonance at −12.2 ppm a integrating to 6H vs 30 H for (C5Me5)− can be assigned to the 7194
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color changes to orange occur within 1 min. Although the 1H NMR and IR spectra of the products of reactions with 1 and 2 suggest CO 2 insertion to form (C 5 Me 5 ) 2 U[κ 2 O,O′O2CCH2(R)CH2]Cl (R = H (7) Me (8)), crystallographic confirmation remains elusive. This was not the case with 3, which formally reacts via double CO2 insertion to form the structurally characterizable bis(allylcarboxylate) (C5Me5)2U[κ2O,O′-O2CCH2CHCH2]2 (9; eq 5, Figure 4).
Figure 3. Ball and stick diagram drawn with ORTEP44 of an ordered model of (C5Me5)2U(η3-CH2CHCH2)THF (5·THF). As described in the Supporting Information, the structure is disordered and the data established connectivity only.
Further study showed that 5·THF is not stable in the presence of additional Grignard reagent. Isolated samples of 5·THF and 5 (vide infra) react with additional (CH2CHCH2)MgCl to form a hexane-insoluble material that might result from incorporation of magnesium, as has been observed with allyl complexes of other electropositive metals.52 The 1H NMR spectrum of 5·THF in C6D6 shows a single broad resonance at −9.4 ppm assigned to (C5Me5)−. Neither free THF nor coordinated THF is observable by 1H NMR spectroscopy for this paramagnetic U3+ complex. To obtain higher yields of a U3+ allyl complex, the more classical method of forming U3+ metallocenes by reducing U4+ complexes with Na(Hg) was attempted.29,53 Na(Hg) reduction of 1 and 2 in hexane forms (C5Me5)2U[η3-CH2C(R)CH2] (R = H (5), Me (6)) at room temperature over 18 h in >90% yield as analytically pure products (eq 4). Addition of THF to C6D6
Figure 4. ORTEP 44 representation of (C 5 Me 5 ) 2 U[κ 2 O,O′O2CCH2CHCH2]2 (9), plotted at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (deg): U(1)−O(1), 2.465(2); U(1)−O(2), 2.45(1); U(1)−O(3) 2.449(2); U(1)−O(4), 2.42(2); U(1)−C(21), 2.830(2); U(1)−C(25), 2.812(2); C(21)−O(1), 1.260(3); C(21)− O(2), 1.256(3); O(3)−C(25), 1.258(3); O(4)−C(25), 1.257(3); C(22)−C(23), 1.484(3); C(22)−C(23), 1.484(3); C(23)−C(24), 1.300(4); C(25)−C(26), 1.515(3); C(26)−C(27), 1.486(4); C(27)− C(28), 1.304(4); O(1)−U(1)−O(3), 171.98(5); U−(C5Me5 centroid)av, 2.478; (C5Me5 centroid)−U(1)−(C5Me5 centroid), 137.4.
solutions of 5 forms 5·THF by 1H NMR spectroscopy, a reaction that could be reversed by repeatedly dissolving 5·THF in hexane and drying under reduced pressure. This reduction is also effected in 4 h using K(Hg), but the Na(Hg) reactions repeatedly produced fewer side products by 1H NMR spectroscopy. Similar attempts using KC8 as the reductant in THF were not successful: KC8 reactions yielded solids that matched (C5Me5)2UCl(THF) by 1H NMR spectroscopy in THF-d8. The infrared spectra of 5 and 6 contain no characteristic signal in the 1500−1600 cm−1 range attributable to η1-allyl ligands. 1H NMR spectroscopic analysis of 5 showed two resonances of equal intensity at −11.3 and −13.1 ppm at room temperature, assigned to (C5Me5)−. These coalesce into a single broad resonance at 310 K, which matches the fluxionality observed in solution for 1−4 as well as that found for the rareearth (C5Me5)2M(η3-CH2CHCH2) compounds (M = Sm,12 Y,8 Lu8), and similar explanations apply as described above. A barrier of 13 kcal/mol is calculated for this process for 5.42 CO2 Insertion Reactivity. To compare the reactivity of the new uranium allyl complexes with the reactivity of (C5H5)3U(CH2CHCH2)54 with CO2,26 1−3 were exposed to carbon dioxide (1 atm) at room temperature. In all three cases, rapid
Complex 9 is a rare example of a structurally characterized 10-coordinate uranium complex with a bis(pentaalkylcyclopentadienyl) framework. The only other examples to our knowledge are (C5Me5) 2U[κ2N,O-OC(CCPh)NPh]2, 55 (η5:η2-C5Me4SiMe2CH2S2)2U,56 and [(η5:η2C5Me4SiMe2CH2CNtBu)2U].41 Given the ubiquitous nature of (C5Me5)− and acetate ligands in uranium chemistry, it is surprising to find that 9 is the first crystallographically characterized uranium complex containing both a κ2-carboxylate and a (C5Me5)− ligand. The 1H NMR spectrum of 9 contains a single resonance for (C5Me5)− at 4.3 ppm and a single set of allyl resonances at 14.3, 11.6, 10.1, and 8.7 ppm corresponding to 2, 4, 2, and 2 protons, respectively, vs 30 H for the (C5Me5)− resonance with the coupling expected for an allyl substituent. The IR spectrum contains absorptions for the symmetric and asymmetric stretches of the carboxylates at 1534 and 1444 cm−1, as expected for the nonbridging bidentate coordination mode found in the X-ray crystal structure.57 An absorption at 1589 cm−1 is observed corresponding to the uncoordinated terminal alkene. The metrical parameters of the metallocene portion of 9 are not unusual,45 but the complex does contain a 171.98(5)° 7195
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Interestingly, the reaction of 3 with CO2 gas generated a rare example of a crystallographically characterizable bidentate carboxylate complex of an actinide metallocene, (C5Me5)2U[κ2O,O′-O2CCH2CHCH2]2 (9). The following complexes have been spectroscopically characterized: (C 5 H 5 ) 2 U(O 2 CCH 3 ) 2 , 5 7 (C 5 Me 5 ) 2 U(O 2 CCH 3 ) 2 , 6 3 (C 5 H 5 ) 3 U(O2CCH2CHCH2),26 (C5H5)3Th(O2CCH3),64 (C9Me7)2Th(O2CCH3)265 (UC5Me4SiMe2CH2CO2)2U,56 and (C5Me5)2U(O 2 CCCPh) 2 , 5 5 but o nly the acetate clusters [(C5H5)U(O2CCH3)2]4O2,66 and [(C5H5)U(O2CCH3)5O]267 have been characterized by X-ray crystallography. The lack of crystallographic data on uranium metallocene carboxylates seems unusual, since carboxylate is a small-bite, chelating, oxygen donor ligand. The reaction of the bis(carboxylate) 9 with Me3SiCl to form (C5Me5)2UCl2 and the allyl silyl ester Me3SiOC(O)CH2CH CH2 is another example58−61 showing that uranium carboxylates can be removed from organoactinide complexes to generate uranium halides and the corresponding silyl ester. The result with 9 shows that this cycle can be run even with a 10coordinate U4+ metallocene.59
O(1)−U(1)−O(3) bond angle involving the outer oxygen atoms of the two carboxylates. This is a large O−U−O angle for a complex that is not a uranyl [OUO]n+ species. In comparison, the X−U−X angles in the aforementioned 10coordinate complexes are 177.80° in (C5Me5)2U[κ2N,OOC(CCPh)NPh]2,55 156.96° in (η5:η2C 5 Me 4 SiMe 2 CH 2 S 2 ) 2 U, 56 and 151.09(7)° in [(η 5 :η 2 C5Me4SiMe2CH2CNtBu)2U].55 Another unusual characteristic of 9 is that both allyl moieties are oriented in the same direction with respect to the plane that bisects the metallocene wedge. Hence, 9 is not symmetric in this respect in the solid state, although this may lead to better packing in the extended crystal. Complex 9 was treated with Me3SiCl to determine if the CO2 insertion product could be removed from the metal. Removal of oxygen-bound ligands from uranium by Me3SiCl has previously been reported.58−61 This reaction occurs overnight at 313 K and generates the starting material (C5Me5)2UCl2 and 2 equiv of Me3SiOC(O)CH2CHCH2 by 1H NMR spectroscopy. This completes a cycle and is similar to the reaction of [hydrotris(3,5-dimethylpyrazolyl)borate]2U(κ2O,O′-O2CCH2Ph) with Me3SiX (X = Cl, I) reported by Bart et al.59
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CONCLUSIONS The bis(pentamethylcyclopentadienyl) ligand set has allowed the isolation of room temperature stable uranium allyl complexes involving both U3+ and U4+. The bis(allyl) complexes (C 5 Me 5 ) 2 U[η 3 -CH 2 C(R)CH 2 ][η 1 -CH 2 C(R) CH2] (R = H (3), Me (4)) provide rare examples of two allyl coordination modes in a single structure. Complex 3 allowed access to a double carbon dioxide insertion product, the bis(carboxylate) (C5Me5)2U[κ2O,O′-O2CCH2CHCH2]2 (9), which is a rare example of a uranium metallocene carboxylate characterizable by X-ray diffraction. The carboxylates in 9 can be removed from the uranium center with Me3SiCl to regenerate the original (C5Me5)2UCl2 starting material and form a cycle of allyl coordination, CO2 insertion, and derivatization.
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DISCUSSION The syntheses of the U4+ allyl complexes of the bis(pentamethylcyclopentadienyl)uranium metallocene unit, namely (C5Me5)2U[η3-CH2C(R)CH2]Cl [R = H (1), Me (2)] and (C5Me5)2U[η3-CH2C(R)CH2][η1-CH2C(R)CH2] (R = H (3), Me (4)) are straightforward (eqs 1 and 2). The complexes are thermally stable above room temperature, in contrast to the monocyclopentadienyl complex (C5Me5)U[η3CH2C(Me)CH2]3, reported earlier.21 This is consistent with the stabilizing influence of the [(C5Me5)2]2− ligand set often seen in f-element chemistry. Complexes 3 and 4 are unusual examples of bis(allyl) compounds in which single crystals contain both η1 and η3 allyl groups. Although compounds of this type are known with transition metals,46−50 this had not been observed before with the f elements. Hence, the dichotomy of binding for the allyl ligand in the solid state exists even with the large electrophilic U4+ ion. In contrast, the synthesis of the U3+ metallocene allyl complexes (C5Me5)2U[η3-CH2C(R)CH2] [R = H (5), Me (6)] via the commonly used halide-free [(C 5 Me 5 ) 2 U][(μPh)2BPh2]39 precursor (eq 3) did not produce high yields due to instability toward additional Grignard reagent. However, the traditional method of Na(Hg) reduction of U4+ metallocene halides (eq 4) was effective without the formation of ate salts, as is sometimes observed in such reactions.62 The U3+ metallocene allyl complexes appear to have thermal stabilities comparable to those of their U4+ allyl chloride analogues. This will enhance their synthetic utility, given the potential for 5 and 6 to behave like U3+ alkyl synthons. Preliminary reactivity studies with the U4+ allyl complexes focused on CO2 as a substrate, since there is precedent for using this substrate in uranium allyl chemistry. Insertion of CO2 into the U−allyl linkage is facile, which suggests access to η1allyl binding not only in 3, where it is observed by X-ray crystallography, but also with the η3-allyl complexes 1 and 2. The variable-temperature 1H NMR spectra of 2−5 indicated that these complexes are fluxional in solution, a process that could involve η1-allyl intermediates and facilitate insertion.
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ASSOCIATED CONTENT
S Supporting Information *
Text, tables, figures, and CIF files giving X-ray data collection, structure solution, and refinement and X-ray diffraction details of compounds 2−4 and 9 (CCDC Nos. 891621−891624) and variable-temperature NMR spectra of 3 and 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 We thank the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences of the Department of Energy (DE-FG02-10ER16161) for support and Ryan Zarkesh for assistance with X-ray crystallography.
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