Synthesis and Insertion Chemistry of a Cyclooctatetraenyl Uranium

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Organometallics 2011, 30, 458–465 DOI: 10.1021/om100700p

Synthesis and Insertion Chemistry of a Cyclooctatetraenyl Uranium “Tuck-in” Metallocene, (η8-C8H8)(η5:η1-C5Me4CH2)U Michael K. Takase, Nathan A. Siladke, Joseph W. Ziller, and William J. Evans* Department of Chemistry, University of California, Irvine, California 92697-2025, United States Received July 15, 2010

[(C5Me5)(C8H8)U(μ-OTf)]2, 1 (OTf = OSO2CF3), reacts with LiMe to form the metathesis product (C5Me5)(C8H8)UMe, 2, as well as the product of a methyl C-H activation, (C8H8)(C5Me4CH2)U, 3. Methane was observed in the 1H NMR spectrum of the reaction mixture, which is consistent with the metalation of a (C5Me5)- ligand by the uranium methyl group in 2. Complex 2 was identified in this reaction mixture by addition of iPrNdCdNiPr to form the previously reported (C5Me5)(C8H8)U[iPrNC(Me)NiPr-κ2N,N0 ], 4. The reactions of 1 with LiEt and LiCH2CMe3 similarly form mixtures at room temperature that contain 3 and compounds with 1H NMR spectra consistent with (C5Me5)(C8H8)UEt, 5, and (C5Me5)(C8H8)U(CH2CMe3), 6, respectively. Complexes 2, 5, and 6 convert to 3 after 24 h at 100 °C. Complex 3 can also be synthesized at 100 °C from (C5Me5)(C8H8)UPh, 7, and was characterized as the THF adduct of a “tuck-in” complex, (η8-C8H8)(η5:η1-C5Me4CH2)U(THF), 8, by X-ray crystallography. Complex 3 allowed the insertion chemistry of an isolable f element (η5:η1C5Me4CH2)2- metallocene complex to be studied for the first time and provides access to new heteroleptic tethered metallocenes. iPrNdCdNiPr inserts into the U-C(η1-C5Me4CH2) bond of 3 to form the amidinate complex (C8H8)[C5Me4CH2C(NiPr)2]U, 9. tBuNtC reacts in a series of steps through intermediates such as (C8H8)(C5Me4CH2CdNtBu)U, 10, and (C8H8)(C5Me4CH2CdNtBu)U(CtNtBu) that leads to the isolation of the isocyanide double-insertion product, (η8-C8H8)[η5C5Me4CH2C(dCdNtBu)NtBu-κN]U, 11.

Introduction Recent studies of the heteroleptic U4þ metallocene [(C5Me5)(C8H8)U]2(μ-C8H8) have shown that it reacts readily with AgOTf (OTf = OSO2CF3) to form a convenient precursor for generating [(C5Me5)(C8H8)U]þ complexes, namely, [(C5Me5)(C8H8)U(μ-OTf)]2, 1, eq 1.1 Reactions of 1 with alkali metal amide, alkyl, and aryl salts lead to triflate substitution

(C5Me5)(C8H8)UR, as shown for R = CH(SiMe3)2 in eq 2.1 Since the (C8H8)2- and (C5Me5)- ligands are similar in size,2,3

and formation of mixed ligand U 4þ metallocenes with a single monodentate ligand, i.e., (C5Me5)(C8H8)UX and

this provides U4þ metallocenes sterically similar to the common bis(ligand) pentamethylcyclopentadienyl metallocenes (C5Me5)2UX2 and (C5Me5)2UR2, but with a single monodentate ligand since the [(C5Me5)(C8H8)]3- ligand combination carries a 3- charge. Although the reaction of 1 with LiCH(SiMe3)2 in eq 2 provided a high-yield synthesis of a monoalkyl U4þ mixed ligand metallocene,1 analogous reactions with LiMe, LiEt, and LiCH2CMe3 were more complicated. We report here

*To whom correspondence should be addressed. Fax: 949-824-2210. E-mail: [email protected]. (1) Evans, W. J.; Takase, M. K.; Ziller, J. W.; Rheingold, A. L. Organometallics 2009, 28, 5802–5808.

(2) Evans, W. J.; Clark, R. D.; Ansari, M. A.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 9555–9563. (3) Evans, W. J.; Johnston, M. A.; Greci, M. A.; Ziller, J. W. Organometallics 1999, 18, 1460–1464.

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that this complication arises due to activation of a methyl C-H bond of the (C5Me5)- ligand to form products containing an (η5:η1-C5Me4CH2)2- “tuck-in”4 unit. Tuck-in metallocene complexes, such as “(C5Me5)(η5:η1C5Me4CH2)Lu”,5 were proposed as key intermediates in the mechanistic schemes involving lanthanide and actinide C-H activation reported in the 1980s, eq 3.6,7 However, during the next two decades no crystallographic evidence for f element

tuck-in metallocene complexes was obtainable. Crystallographically characterized tuck-in complexes were reported for transition metals,8-11 i.e., (C5Me5)(η5:η1-C5Me4CH2)Ti,12,13 and bimetallic lanthanide “tuck-over” complexes involving Ln(μ-η5:η1-C5Me4CH2)Ln units were structurally characterized,14-18 but the closest structural information on actinide tuck-in metallocene species involved an adamantylimido insertion product, (C5Me5)(η5:η1-C5Me4CH2NAd)U, presumably formed from a transient tuck-in or tuck-over intermediate, eq 4.19

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12, is shown in eq 5.20 More recently, two actinide tuck-in metallocene structures have been reported in metallocene

complexes that have an additional bidentate ligand, either the amidinate, [iPrNC(Me)NPri]-, eq 6,21 or a bicyclic guanidinate, (hpp)- = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato, eq 7.22 In addition, studies of the N(CH2CH2NSiMe3)3 chemistry of uranium have led to a diuranium complex, [Me3SiNCH2CH2N(CH2CH2NSiMe2CH2)2]U2[N(CH2CH2NSiMe3)3], that contains a dimetalated ligand that has bridging methylene groups described as tucked-in and tucked-over using the metallocene language.23

In 2008, the first f element tuck-in metallocene complex was structurally characterized. This tuck-in tuck-over dihydride, (C5Me5)U[μ-η5:η1:η1-C5Me3(CH2)2](μ-H)2U(C5Me5)2, (4) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203–219. (5) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491–6493. (6) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51–56. (7) Bruno, J. W.; Smith, G. M.; Marks, T. J.; Fair, C. K.; Schultz, A. J.; Williams, J. M. J. Am. Chem. Soc. 1986, 108, 40–56. (8) Schock, L. E.; Brock, C. P.; Marks, T. J. Organometallics 1987, 6, 232–241. (9) Fischer, J. M.; Piers, W. E.; Young, V. G., Jr. Organometallics 1996, 15, 2410–2412. (10) Bulls, A. R.; Schaefer, W. P.; Serfas, M.; Bercaw, J. E. Organometallics 1987, 6, 1219–1226. (11) Kreindlin, A. Z.; Dolgushin, F. M.; Yanovsky, A. I.; Kerzina, Z. A.; Petrovskii, P. V.; Rybinskaya, M. I. J. Organomet. Chem. 2000, 616, 106–111. (12) Bercaw, J. E. J. Am. Chem. Soc. 1974, 96, 5087–5095. (13) Luinstra, G. A.; Teuben, J. H. J. Am. Chem. Soc. 1992, 114, 3361–3367. (14) Evans, W. J.; Perotti, J. M.; Ziller, J. W. Inorg. Chem. 2005, 44, 5820–5825. (15) Evans, W. J.; Champagne, T. M.; Ziller, J. W. J. Am. Chem. Soc. 2006, 128, 14270–14271. (16) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. Organometallics 1991, 10, 134–142. (17) Booij, M.; Meetsma, A.; Teuben, J. H. Organometallics 1991, 10, 3246–3252. (18) Evans, W. J.; Perotti, J. M.; Ziller, J. W. J. Am. Chem. Soc. 2005, 127, 3894–3909. (19) Peters, R. G.; Warner, B. P.; Scott, B. L.; Burns, C. J. Organometallics 1999, 18, 2587–2589. (20) Evans, W. J.; Miller, K. A.; DiPasquale, A. G.; Rheingold, A. L.; Stewart, T. J.; Bau, R. Angew. Chem., Int. Ed. 2008, 47, 5075–5078.

The results presented here suggest that the heteroleptic metallocene unit [(C5Me5)(C8H8)U]þ also appears to foster tuck-in formation. This generates a simple two-ring f element tuck-in metallocene species similar to those originally postulated in the 1980s. The synthesis of this tuck-in complex and the structure of its THF adduct are reported as well as the insertion reactivity of the U-C bond in the tuck-in moiety. This is the first insertion chemistry reported for an isolated f element tuck-in metallocene complex.

Experimental Section The syntheses and manipulations described below were conducted under argon with rigorous exclusion of air and water using glovebox, vacuum line, and Schlenk techniques. A glovebox free of coordinating solvents was used except where noted. Elemental analyses were obtained with a Perkin-Elmer 2400 Series II CHNS elemental analyzer. Infrared spectra were recorded as KBr pellets on a Perkin-Elmer Spectrum One FT-IR spectrometer. 1H NMR spectra were recorded in C6D6 on a Bruker DRX 500 MHz spectrometer unless otherwise noted. Due to the paramagnetism (21) Evans, W. J.; Walensky, J. R.; Ziller, J. W.; Rheingold, A. L. Organometallics 2009, 28, 3350–3357. (22) Montalvo, E.; Ziller, J. W.; DiPasquale, A. G.; Rheingold, A. L.; Evans, W. J. Organometallics 2010, 29, 2104–2110. (23) Gardner, B. M.; McMaster, J.; Lewis, W.; Blake, A. J.; Liddle, S. T. J. Am. Chem. Soc. 2009, 131, 10388–10389.

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of uranium, only resonances that could be unambiguously identified are reported. Solvents were dried over columns containing Q-5 and molecular sieves. NMR solvents were dried over sodium potassium alloy, degassed, and vacuum transferred prior to use. [(C5Me5)(C8H8)U(μ-OTf)]2, 1, and (C5Me5)(C8H8)UPh, 7, were prepared as previously described.1 LiMe (2 M in diethyl ether) and LiEt (0.5 M in 90/10 benzene/cyclohexane) were obtained from Aldrich, transferred by cannula into a Schlenk flask, and isolated as white powders upon removal of solvent under vacuum. t BuNtC (Aldrich) was dried over molecular sieves and degassed prior to use. (C5Me5)(C8H8)UMe, 2. LiMe (4 mg, 0.18 mmol) was added as a white powder to a brown solution of [(C5Me5)(C8H8)U(μOTf)]2, 1 (100 mg, 0.16 mmol), in toluene (6 mL). After 30 min, an aliquot was removed, the solids were filtered from the aliquot, and the solvent was removed under vacuum, leaving a light brown solid that contained 1 and (C5Me5)(C8H8)UMe, 2. On the basis of the 1H NMR resonances assigned to (C5Me5)- for each compound, the ratio of 1:2 was 5:1. After stirring the bulk solution for an additional 36 h, solids were removed by centrifugation and the solvent was removed under vacuum, leaving a light brown solid (60 mg). The 1H NMR spectrum contained several resonances, which could be assigned to the two organoactinide complexes, 2 and (C8H8)(C5Me4CH2)U, 3, described below. On the basis of the resonances assigned to (C5Me5)- and (C5Me4CH2)2- for 2 and 3, respectively, the ratio of 2:3 was 1:2. Heating this mixture to 100 °C in toluene causes the resonances for 2 to disappear and those for 3 to grow. After 12 h, only the resonances for 3 are present. When the reaction was conducted in a sealed NMR tube in benzene-d6, a resonance at 0.15 ppm was observed by 1H NMR spectroscopy, consistent with the presence of methane. 1H NMR for 2 (C6D6): δ -1.1 (s, C5Me5, Δν1/2 = 108 Hz, 15H), -35.8 (s, C8H8, Δν1/2 = 46 Hz, 8H). The resonances for the methyl protons could not be found in the 1H NMR spectrum. A colorless solution of iPrNdCdNiPr (17 μL, 0.11 mmol) was added to 130 mg of a brown mixture of 2 and 3, obtained from a 36 h reaction above, in toluene (6 mL) and stirred for 1 h. The solvent was removed under vacuum, leaving a red solid (62 mg). The 1H NMR resonances assigned to 2 and 3 in the brown starting solution disappeared, and a set of resonances identical to those found for the previously characterized complex (C5Me5)(C8H8)U[iPrNC(Me)NiPr], 4,1 were present. A second set of new resonances were present and identified as (C8H8)[C5Me4CH2C(NiPr)2]U, 9, the product of reacting 3 with iPrNdCdNiPr described below. On the basis of the resonances assigned to C8H8 for each compound, the ratio of 4:9 in the red solid was 1:1.8. (C8H8)(C5Me4CH2)U, 3. (C5Me5)(C8H8)UPh, 7 (170 mg, 0.31 mmol), was heated in toluene (12 mL) for 16 h at 100 °C. The solvent was removed under vacuum, leaving 3 as a brown solid (143 mg, 98%). When the reaction was performed in a sealed NMR tube in toluene-d8, benzene was observed in the 1H NMR spectrum at 7.15 ppm. 1H NMR (C6D6): δ 23.6 (s, C5Me4CH2, Δν1/2=9 Hz, 6H), -36.1 (s, C8H8, Δν1/2=10 Hz, 8H), -47.0 (s, C5Me4CH2, Δν1/2 = 9 Hz, 6H). The resonances for the CH2 protons on the cyclopentadienyl ligand could not be found in the 1H NMR spectrum. 13C NMR: δ 15 (s, C5Me4CH2), 167 (s, C5Me4CH2), 236 (s, C8H8). IR: 3023m, 2902s, 2855m, 1437m, 1379m, 1022m, 899m, 747w, 713s, 616w, 569m cm-1. Anal. Calcd for C18H22U: C, 45.38; H, 4.65. Found: C, 44.77; H, 4.58. (η8-C8H8)(η5:η1-C5Me4CH2)U(THF), 8. Recrystallization of 3 from a THF/hexane solution at -35 °C gave single crystals of 8, identified by X-ray crystallography. Data collection parameters and full details of data collection are in the Supporting Information. 1H NMR (C6D6): δ 19.3 (s, C5Me4CH2, Δν1/2 = 12 Hz, 6H), -1.0 (s, THF, Δν1/2 = 28 Hz, 4H), -5.4 (s, THF, Δν1/2=74 Hz, 4H), -35.0 (s, C8H8, Δν1/2=14 Hz, 8H), -38.5 (s, C5Me4CH2, Δν1/2=28 Hz, 6H), -67.4 (s, C5Me4CH2, Δν1/2=114 Hz, 2H). 13C NMR (C6D6): δ 0 (s, C5Me4CH2), 19 (s, THF), 38 (s, THF), 154 (s, C5Me4CH2), 235 (s, C8H8). Anal. Calcd for partially desolvated

Takase et al. 8, i.e., (C8H8)(C5Me4CH2)U(THF)0.2: C, 46.00; H, 4.86. Found: C, 45.96; H, 4.90. (C5Me5)(C8H8)UEt, 5. LiEt (6 mg, 0.16 mmol) was added as a white powder to a brown solution of 1 (100 mg, 0.16 mmol) in toluene (6 mL). After 30 min, an aliquot was removed, the solids were removed by filtration, and the solvent was removed under vacuum, leaving an orange solid that contained 1 and 5. On the basis of the 1H NMR resonances assigned to (C5Me5)- for each compound, the ratio of 1:5 was 5:1. The bulk of the solution was stirred for an additional 36 h, the solids were removed by centrifugation, and the solvent was removed under vacuum, leaving a light brown solid (70 mg). On the basis of the resonances assigned to (C5Me4CH2)2- and (C5Me5)- for 3 and 5, respectively, the ratio of 3:5 was 1.6:1. When the reaction was conducted in a sealed NMR tube (C6D6), ethane was observed in the 1H NMR spectrum at 0.79 ppm. 1H NMR for 5 (C6D6): δ 87 (s, Et, Δν1/2 = 110 Hz, 2H), 48.5 (s, Et, Δν1/2 = 41 Hz, 3H), -5.2 (s, C5Me5, Δν1/2=23 Hz, 15H), -35.6 (s, C8H8, Δν1/2=46 Hz, 8H). (C5Me5)(C8H8)U(CH2CMe3), 6. LiCH2CMe3 (13 mg, 0.16 mmol) was added as a white powder to a brown solution of 1 (100 mg, 0.16 mmol) in toluene (6 mL). After 30 min, an aliquot was removed, the solids were separated by filtration, and the solvent was removed under vacuum, leaving a brown solid that contained 3 and 6. On the basis of the 1H NMR resonances assigned to (C8H8)2- for each compound, the ratio of 3:6 was 4.5:1. The bulk of the solution was stirred for an additional 36 h, the solids were separated by centrifugation, and the solvent was removed under vacuum, leaving a brown solid (57 mg) containing a 5.2:1 mixture of 3:6 on the basis of the (C8H8)2- resonances in the 1H NMR spectrum. When the reaction was performed in a sealed NMR tube (C6D6), neopentane was observed in the 1H NMR spectrum at 0.99 ppm. 1H NMR for 6 (C6D6): δ 10.0 (s, CH2CMe3, Δν1/2=24 Hz, 9H), -1.5 (s, C5Me5, Δν1/2 = 24 Hz, 15H), -36.2 (s, C8H8, Δν1/2 = 37 Hz, 8H). The resonances for the CH2 protons on the CH2CMe3 ligand could not be found in the 1H NMR spectrum. (C8H8)[C5Me4CH2C(NiPr2)2]U, 9. A colorless solution of i PrNdCdNiPr (48 μL, 0.31 mmol) was added to a brown solution of 3 (148 mg, 0.31 mmol) in toluene (6 mL) and stirred overnight. The solvent was removed under vacuum, leaving 9 as an orange solid (184 mg, 98%). 1H NMR (C6D6): δ 24.5 (s, CHMe2, Δν1/2= 13 Hz, 2H), 8.5 (s, CHMe2, Δν1/2 = 111 Hz, 12H), 3.5 (s, C5Me4CH2, Δν1/2 = 5 Hz, 2H), -1.7 (s, C5Me4CH2, Δν1/2 = 10 Hz, 6H), -20.6 (s, C5Me4CH2, Δν1/2 = 13 Hz, 6H), -36.7 (s, C8H8, Δν1/2=21 Hz, 8H). This 1H NMR spectrum was identical to one of the components in the 1/LiMe/iPrNdCdNiPr reaction above. 13C NMR (C 6D 6): δ -98 (s, C5 Me 4CH 2), -35 (s, C 5Me 4CH 2), -28 (s, C5 Me 4CH 2), -18 (s, CHMe 2), 73 (s, CHMe2), 239 (s, C8H8). IR: 2958s, 2906s, 2857s, 2361m, 2343w, 2113m, 1594s, 1439m, 1372m, 1254m, 1221w, 1158w, 1116w, 1024w, 995w, 900w, 783w, 722s, 570w cm-1. Anal. Calcd for C25H36N2U: C, 49.83; H, 6.02; N, 4.65. Found: C, 49.53; H, 5.61; N, 3.69. Repeated attempts to analyze this complex gave low N values. The reason for this is not known, but it occasionally is observed in f element chemistry.24-27 (C8H8)(C5Me4CH2CdNtBu)U, 10. A colorless solution of t BuNtC (24 μL, 0.21 mmol) was added to a brown solution of 3 (101 mg, 0.21 mmol) in toluene (8 mL) and stirred overnight. The solvent was removed under vacuum, leaving 10 as an orange solid (118 mg, 99%). 1H NMR (C6D6): δ 21.6 (s, C5Me4CH2, Δν1/2 = 8 Hz, 3H), 21.4 (s, CMe3, Δν1/2 = 12 Hz, 9H), 0.08 (s, C5Me4CH2, Δν1/2 = 5 Hz, 3H), -0.05 (s, C5Me4CH2, Δν1/2 = 5 Hz, 2H), -23.7 (s, C5Me4CH2, Δν1/2 = 16 Hz, 3H), -36.4 (s, C8H8, Δν1/2 = 17 Hz, (24) Siladke, N. A.; Evans, W. J.; Ziller, J. W. Z. Anorg. Allg. Chem. 2010, 636, 2347–2351. (25) Gaunt, A. J.; Scott, B. L.; Neu, M. P. Inorg. Chem. 2006, 45, 7401–7407. (26) Lee, J.; Freedman, D.; Melman, J. H.; Brewer, M.; Sun, L.; Emge, T. J.; Long, F. H.; Brennan, J. G. Inorg. Chem. 1998, 37, 2512–2519. (27) Freedman, D.; Emge, T. J.; Brennan, J. G. Inorg. Chem. 1999, 38, 4400–4404.

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8H), -49.1 (s, C5Me4CH2, Δν1/2 = 24 Hz, 3H). 13C NMR (C6D6): δ -106 (s, CMe3), -63 (s, C5Me4CH2), -33 (s, C5Me4CH2), 2 (s, C5Me4CH2), 70 (s, C5Me4CH2), 144 (s, C5Me4CH2), 207 (s, C8H8). IR: 2968s, 2902s, 2004w, 1981m, 1439m, 1378m, 1240w, 1195m, 1021w, 950w, 900m, 716s, 568m cm-1. Anal. Calcd for C23H31NU: C, 49.37; H, 5.58; N, 2.50. Found: C, 49.72; H, 5.54; N, 2.50. (η8 -C8 H8 )[η5 -C5 Me 4CH 2C(dCdN tBu)Nt Bu-KN]U, 11. A colorless solution of tBuNtC (2 μL, 0.18 mmol) was added to an orange solution of 10 (10 mg, 0.18 mmol) in C6D6 (1 mL). The 1 H NMR spectrum of the mixture taken after 10 min was consistent with the formation of the adduct, (C8H8)(C5Me4CH2CdNtBu)U(CtNtBu). 1H NMR (C6D6): δ 19.0 (s, CMe3, Δν1/2 = 6 Hz, 9H), 18.5 (s, C5Me4CH2, Δν1/2 = 7 Hz, 3H), 1.1 (s, C5Me4CH2, Δν1/2 = 27 Hz, 3H), -5.5 (s, CMe3, Δν1/2 = 58 Hz, 9H), -20.4 (s, C5Me4CH2, Δν1/2 = 18 Hz, 3H), -30.9 (s, C8H8, Δν1/2 = 8 Hz, 8H), -39.8 (s, C5Me4CH2, Δν1/2 = 12 Hz, 3H). The resonances for the CH2 protons on the cyclopentadienyl ligand could not be found in the 1H NMR. The 1H NMR spectrum taken after 12 h showed complete conversion of this solution to 11, which is prepared in bulk as described below. A colorless solution of tBuNtC (82 μL, 0.72 mmol) was added to a brown solution of 3 (172 mg, 0.36 mmol) in toluene (6 mL) and stirred overnight. The solvent was removed under vacuum, leaving 11 as an orange solid (230 mg, 99%). Crystals of 11 suitable for X-ray diffraction were grown at -35 °C from a concentrated toluene solution. 1H NMR (C6D6): δ 27.4 (s, C5Me4CH2, Δν1/2 = 10 Hz, 3H), 26.7 (s, CMe3, Δν1/2=12 Hz, 9H), -3.0 (s, CMe3, Δν1/2 = 2 Hz, 9H), -11.1 (s, C5Me4CH2, Δν1/2 =9 Hz, 3H), -17.1 (s, C5Me4CH2, Δν1/2=14 Hz, 3H), -24.5 (s, C5Me4CH2, Δν1/2=10 Hz, 3H), -36.3 (s, C8H8, Δν1/2=17 Hz, 8H), -51.9 (s, C5Me4CH2, Δν1/2=25 Hz, 2H). 13C NMR (C6D6): δ -142 (s, C5Me4CH2), -95 (s, C5Me4CH2), -43 (s, C5Me4CH2), 7 (s, C5Me4CH2), 22 (s, CMe3), 114 (s, CMe3), 188 (s, C8H8). IR: 2966s, 2934s, 2902s, 2163w, 1986s, 1619m, 1545w, 1453m, 1359m, 1236m, 1192s, 1118w, 1065w, 947w, 909m, 718s, 569w cm-1. Anal. Calcd for C28H40N2U: C, 52.33; H, 6.27; N, 4.36. Found: C, 51.93; H, 6.12; N, 3.71. X-ray Crystallographic Data. Information on X-ray data collection, structure determination, and refinement for 8 and 11 are given in the Supporting Information.

Results Reaction of 1 with LiR (R = Me, Et, CH2CMe3). The reactions of 1 with LiMe, LiEt, and LiCH2CMe3 did not exclusively give the expected metathesis products as found in the analogous reaction of 1 with LiCH(SiMe3)2, eq 2. The 1H NMR spectra obtained after 36 h showed resonances attributable to two organoactinide metallocene products as well as methane, ethane, and neopentane from LiMe, LiEt, and LiCH2CMe3, respectively. Each spectrum contained a set of resonances at -23.6, -36.1, -57.0, and -93.8 ppm attributable to 3 described below, as well as resonances attributable to the expected metathesis products (C5Me5)(C8H8)UR (R = Me, 2; Et, 5; CH2CMe3, 6), eq 8.

In attempts to trap the (C5Me5)(C8H8)UMe complex, 2, that was thought to be forming in the reaction of 1 with LiMe, the carbodiimide iPrNdCdNiPr was added to the reaction

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mixture. Carbodiimide insertions had previously been used to derivatize U-C bonds to form crystallographically characterizable products.21 The insertion of carbodiimides has been observed in the heteroleptic metallocene phenyl complex (C5Me5)(C8H8)UPh, 7, eq 9,1 and the anticipated product of carbodiimide insertion with (C5Me5)(C8H8)UMe, 2, namely, (C5Me5)(C8H8)U[iPrNC(Me)NiPr-κ2N,N0 ], 4, eq 10, has been previously synthesized by metathesis and structurally characterized, as shown in eq 11.1

Addition of iPrNdCdNiPr to the 1/LiMe reaction mixture after centrifugation to remove any unreacted LiMe gave two new sets of 1H NMR resonances, one of which matched that of 4. This provided evidence that the reaction of 1 and LiMe did produce the expected metathesis product, 2. The second set of resonances matched the product of the reaction of 3, obtained in pure form as described below, with iPrNdCdNiPr, a reaction discussed in a later section. Scheme 1 shows the reaction chemistry that is more fully documented below. (C8H8)(C5Me4CH2)U, 3. Heating the mixtures formed from 1 and LiR (R = Me, Et, CH2CMe3) led to a decrease in the (C5Me5)(C8H8)UR resonances in the 1H NMR spectra along with an increase in the resonances for 3. Complex 3 can be formed cleanly from (C5Me5)(C8H8)UPh, 7, by heating, eq 12. This is the preferred route to 3 since it involves a single organometallic precursor and no lithium salts. (C5Me5)(C8H8)U[CH(SiMe3)2]1 does not convert to 3 even after heating at 110 °C for three days.

Although X-ray quality crystals of solvate-free 3 have not yet been isolated, in the presence of THF, the solvate (η8C8H8)(η5:η1-C5Me4CH2)U(THF), 8, could be isolated and fully

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Takase et al. Scheme 1

Table 1. Selected Distances (A˚) and Angles (deg) for (η8-C8H8)(η5:η1-C5Me4CH2)U(THF), 8, and (η8-C8H8)[η5:η1-C5Me4CH2C(dCdNtBu)-NtBu]U, 11 8 E(1) = C E(2) = O R = CH2

Figure 1. Molecular structure of one of the independent molecules of (η8-C8H8)[η5:η1-C5Me4CH2]U(THF), 8, with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms and a 1/2 of a toluene have been omitted for clarity.

defined by X-ray crystallography, eq 13, Figure 1, Table 1. On the basis of this facile solvation reaction and the reaction

chemistry described below that gives crystallographically characterized products that are exclusively monometallic, it is assumed that 3 is monomeric as shown. If 3 were dimeric, it could have a double tuck-over structure, (C8H8)U(μ-η5:η1C5Me4CH2)2U(C8H8). However, this would minimally require cleavage of two U-C bonds to get the products described below. Moreover, no evidence for dimer formation was observed by isopiestic molecular weight measurements on 3 in benzene. The unit cell of the complex contains two crystallographically independent molecules. The 1.991 and 1.988 A˚ U-(C8H8 ring centroid) distances in 8 are similar to those in other [(C5Me5)(C8H8)U]þ complexes, which range from 1.894 to (28) Evans, W. J.; Nyce, G. W.; Ziller, J. W. Angew. Chem., Int. Ed. 2000, 39, 240–242.

U(1)-Cnt1a U(1)-Cnt2a U(1)-E(1) U(1)-E(2) U(1)-C(C5Me4R) range U(1)-C(C8H8) range U(1)-C(C5Me5) av U(1)-C(C8H8) av Cnt1-U(1)-E(1)a Cnt2-U(1)-E(1)a Cnt1-U(1)-E(2)a Cnt2-U(1)-E(2)a Cnt1-U(1)-Cnt2a a

2.389, 2.378 1.991, 1.988 2.586(7), 2.596(6) 2.519(4), 2.514(5) 2.478(5)-2.798(6) 2.655(7)-2.761(6) 2.67(11) 2.70(3) 62.3, 62.4 113.3, 133.8 100.7, 101.8 112.4, 110.7 143.0, 144.6

11 E(1) = N(1) R = CH2 2.470 1.958 2.227(3) 2.663(3)-2.837(3) 2.642(3)-2.716(3) 2.75(6) 2.68(2) 97.5 125.4 137.0

Cnt1 is (C5Me4R); Cnt2 is (C8H8).

2.008 A˚.1,28-30 The 143.0° and 144.6° (C5Me4CH2 ring centroid)-U-(C8H8 ring centroid) bond angles are slightly larger than those in [(C5Me5)(C8H8)U]þ complexes, which range from 134.5° to 140.7°.1 This contrasts with the situation in the previously reported tuck-in, tuck-over complex (C5Me5)U[μ-η5:η1:η1-C5Me3(CH2)2](μ-H)2U(C5Me5)2, 12, eq 5. The (C5Me5)U[μ-η5:η1:η1-C5Me3(CH2)2] tuck-in part of 12 has a (ring centroid)-U-(ring centroid) angle of 130.4°, whereas it is 133.5° in the U(C5Me5)2 regular metallocene part of the molecule. The (ring centroid)-U-(ring centroid) angles in 8 are similar to the 144.5° and 141.8° angles in (C5Me5)(C8H6iPr2)U(THF).31,32 The 2.389 and 2.378 A˚ U-(C5Me4CH2 ring centroid) distances are slightly shorter than the U-(C5Me5 ring centroid) distance in [(C5Me5)(C8H8)U]þ complexes, which range from 2.423 to 2.522 A˚.1 This is consistent with the higher charge of the (C5Me4CH2)2- versus (C5Me5)- ligands. Similarly, in the structure of 12 the U-(C5Me3(CH2)2 ring centroid) distance in the metallocene with the tuck-in moiety is 2.373 A˚, whereas the three U-(C5Me5 ring centroid) distances are 2.485, (29) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Polyhedron 2006, 25, 484–492. (30) Berthet, J.-C.; Boisson, C.; Lance, M.; Vigner, J.; Nierlich, M.; Ephritikhine, M. J. Chem. Soc., Dalton Trans. 1995, 3027–3033. (31) Summerscales, O. T.; Cloke, F. G. N.; Hitchcock, P. B.; Green, J. C.; Hazari, N. Science 2006, 311, 829–831. (32) Frey, A. S.; Cloke, F. G. N.; Hitchcock, P. B.; Day, I. J.; Green, J. C.; Aitken, G. J. Am. Chem. Soc. 2008, 130, 13816–13817.

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2.486, and 2.496 A˚. The methylene carbon positions, C(6) in Figure 1 and its equivalent, C(28), are 0.825 and 0.797 A˚, respectively, out of the ring plane and are oriented toward the uranium center. The U(1)-C(6) and U(2)-C(28) distances are 2.586(7) and 2.596(6) A˚, respectively, compared to 2.568(8) A˚ for the corresponding distance in 12. The (C5Me4CH2)2- ligand in 8 is not symmetrically bound to uranium. The shortest U-C(C5Me4CH2) metal ring carbon distances for the two independent molecules in the unit cell of 8 arise from C(1) and C(23), which are attached to the methylene carbons C(6) and C(28). Their 2.494(6) and 2.487(5) A˚ bond lengths, respectively, are significantly shorter than those of the next closest carbons, C(2), C(5), C(24), and C(27), which have 2.627(6)-2.644(5) A˚ distances. Carbons C(3), C(4), C(25), and C(26) are furthest away from uranium, with U-C distances of 2.789(6), 2.789(6), 2.789(6), and 2.794(6) A˚, respectively. In comparison, U-C(C5Me5) distances for nine-coordinate U4þ range from 2.62(2) to 2.89(2) A˚.33 The analogous distances in 12 are even more disparate: 2.422(7) A˚ for the tuck-in ring carbon, 2.438(6) A˚ for the tuck-over ring carbon, 2.784(7) and 2.708(7) A˚ for the adjacent ring carbons, and 2.934(8) A˚ for the carbon furthest from the methylene groups. In contrast, the cyclooctatetraenyl ring in 8 is bound more symmetrically and the U-C(C8H8) distances range from 2.670(6) to 2.761(6) A˚. Insertion Reactivity of 3. One of the characteristic reactions of M-C bonds is insertion of unsaturated substrates. Isolation of 3 provided an opportunity to study this well-documented reaction with a simple bis(ring) f element tuck-in metallocene for the first time. Since insertion of iPrNdCdNiPr,1,21 t BuNtC,34-37 and CtO36,38-42 into uranium-alkyl bonds has been previously reported, these substrates were examined. Complex 3 reacts with each of these substrates, but only in the case of the isocyanide was an insertion product fully characterized by X-ray crystallography isolated as described below. Insertion of iPrNdCdNiPr. Complex 3 reacts with i PrNdCdNiPr to form a product formulated as 9, eq 14, on the basis of analytical and spectroscopic data. The structure drawn in eq 14 is consistent with the 1H NMR spectrum of 9 that contains resonances for two types of methyl

(33) Evans, W. J.; Miller, K. A.; Ziller, J. W.; Greaves, J. Inorg. Chem. 2007, 46, 8008–8018. (34) Zanella, P.; Paolucci, G.; Rossetto, G.; Benetollo, F.; Polo, A.; Fischer, R. D.; Bombieri, G. J. Chem. Soc., Chem. Commun. 1985, 96–98. (35) Dormond, A.; Elbouadili, A. A.; Moise, C. J. Chem. Soc., Chem. Commun. 1984, 749–751. (36) Simpson, S. J.; Andersen, R. A. J. Am. Chem. Soc. 1981, 103, 4063–4066. (37) Dormond, A.; El Bouadili, A. A.; Moise, C. J. Less-Common Met. 1986, 122, 159–166. (38) Fagan, P. J.; Manriquez, J. M.; Marks, T. J.; Day, V. W.; Vollmer, S. H.; Day, C. S. J. Am. Chem. Soc. 1980, 102, 5393–5396. (39) Katahira, D. A.; Moloy, K. G.; Marks, T. J. Organometallics 1982, 1, 1723–1726. (40) Manriquez, J. M.; Fagan, P. J.; Marks, T. J.; Day, C. S.; Day, V. W. J. Am. Chem. Soc. 1978, 100, 7112–7114. (41) Moloy, K. G.; Marks, T. J.; Day, V. W. J. Am. Chem. Soc. 1983, 105, 5696–5698. (42) Paolucci, G.; Rossetto, G.; Zanella, P.; Yunlu, K.; Fischer, R. D. J. Organomet. Chem. 1984, 272, 363–383.

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Figure 2. Molecular structure of (η8-C8H8)[η5-C5Me4CH2C(dCdNtBu)NtBu-κN]U, 11, with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity.

groups on the cyclopentadienyl ring and the equivalent amidinate iPr groups. Unfortunately, single crystals suitable for definitive characterization by X-ray crystallography were elusive. Insertion of tBuNtC. As has previously been observed with other uranium alkyl complexes,1,34-37,43 tert-butyl isocyanide reacts with 3. Spectroscopic and analytical evidence indicates that there is an initial insertion of one isocyanide to form an iminoacyl complex, 10, followed by coordination of a second isocyanide to generate an isocyanide adduct such as (C8H8)(C5Me4CH2CdNtBu)U(CtNtBu). After 12 h, further insertion occurs to form 11, which could be characterized by X-ray crystallography, Figure 2, Table 1. The proposed sequence is shown in eq 15.

The structure of 11 has 1.958 A˚ U-(C8H8 ring centroid) and 2.420 A˚ U-(C5Me4CH2 ring centroid) distances and a 137.0° (C5Me4CH2 ring centroid)-U-(C8H8 ring centroid) bond angle, which are similar to those in other [(C5Me5)(C8H8)U]þ complexes.1 The methylene carbon in 11, C(6), is still out of the plane of the ring pointing toward the metal center as in 8, but the displacement is less, 0.236 A˚, since there is a longer linkage to the metal. Due to the tethered nature of this cyclopentadienyl ligand, the ring carbon atoms have disparate values depending on their distance from the tether, as was found in 8. The U-C(1) distance is the shortest of all the ring carbons at (43) Evans, W. J.; Siladke, N. A.; Ziller, J. W. Chem.;Eur. J. 2010, 16, 796–800.

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2.663(3) A˚, and the two carbons adjacent to the methylene are 2.706(3), C(2) and 2.724(3) A˚, C(5). The carbons furthest from the methylene are 2.810(3), C(3), and 2.837(3) A˚, C(4). These distances are significantly longer than the U-C distances in the more reactive 8. The distances in the [η5-C5Me4CH2C(dCdNtBu)NtBuκN]2- part of 11 are consistent with the resonance form shown in eq 15. The 2.227(3) A˚ U(1)-N(1) distance in 11 is more similar to the 2.284(4) A˚ analogue in (C5Me5)(C8H8)U[N(SiMe3)2]29 than to the 2.423(6) A˚ U-N dative bond in (C5Me5)(C8H8)U[C(Ph)dNtBu].1 Hence, the U-N bond in 11 can be viewed as a uranium amide bond. The 1.539(5) A˚ C(6)-C(19) and 1.429(4) A˚ C(19)-N(1) distances in 11 are consistent with C-C and C-N single bonds, respectively. The 1.336(5) A˚ C(19)-C(20) and 1.216(4) A˚ C(20)-N(2) distances are reasonable for CdC and CdN double bonds, respectively. The infrared spectrum of 11 shows absorbances at 1619 and 1545 cm-1 consistent with the presence of CdC and CdN double bonds. The 173.0(4)° C(19)-C(20)-N(2) angle is close to linear, as expected for a heteroallene.44,45 The insertion of 2 equiv of isocyanide into a metal alkyl bond has been previously observed in both main group46 and transition metal complexes.44,46-56 The results in the literature closest to those in eq 15 involve the complexes (C5Me5)[η5:η1C5Me4CH2C(dNXyl)CdNXyl]MCl (Xyl = 2,6-Me2C6H3; M = Ti, 13; Zr, 14)53 obtained by double insertion of isocyanide into the metal carbon bonds of (C5Me4CH2)2ligands, eq 16. The structure of 14 differs from that of 11 in that the carbon of the second inserted isocyanide is attached to Zr

instead of nitrogen, as found in the U(1)-N(1) linkage in 11. This leads to two CdN double bonds in 14 instead of a heterocumulene moiety. Both complexes have three atom links between the metal and the cyclopentadienyl ring.

Discussion The reactions of 1 with LiMe, LiEt, and LiCH2CMe3 are more complicated than would be expected from the metathesis reactions previously reported with [(C 5Me5)(C8H8)U]þ complexes.1 NMR data suggest that these reactions initially produce the expected metathesis products, namely, the alkyl complexes (C5Me5)(C8H8)UMe, 2, (C5Me5)(C8H8)UEt, 5, and (C5Me5)(C8H8)U(CH2CMe3), 6, respectively, but these complexes go on to metalate the normally inert (C5Me5)ligand to form the tuck-in product, (η8-C8H8)(η5:η1-C5Me4CH2)U, 3. Evidence for the formation of 2 was obtained by trapping it with iPrNdCdNiPr to form (C5Me5)(C8H8)U[iPrNC(Me)NiPr-κ2N,N0 ], 4. (44) Yamamoto, Y.; Tsuji, M.; Igoshi, T. Inorg. Chim. Acta 1999, 286, 233–236. (45) Shapiro, P. J.; Vij, A.; Yap, G. P. A.; Rheingold, A. L. Polyhedron 1995, 14, 203–209. (46) Onitsuka, K.; Ogawa, H.; Joh, T.; Takahashi, S.; Yamamoto, Y.; Yamazaki, H. J. Chem. Soc., Dalton Trans. 1991, 1531–1536. (47) Sanchez-Nieves, J.; Royo, P.; Pellinghelli, M. A.; Tiripicchio, A. Organometallics 2000, 19, 3161–3169.

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The reactivity of the (C5Me5)(C8H8)UR complexes 2, 5, and 6 is significantly different from that of a typical metallocene alkyl complex such as (C5Me5)2UMe2. The latter complex shows no sign of alkyl C-H bond activation reactivity even upon heating.57 However, C-H bond activation reactivity is observed for (C5Me5)2UPh2,57 which is difficult to isolate since it eliminates benzene to form a “(C5Me5)2U(C6H4)” intermediate that can be trapped with reagents such as PhCtCPh57 and carbodiimides.21 In the (C5Me5)2UPh2 case, aryl C-H activation is facilitated by the presence of two adjacent phenyl groups. In contrast, monophenyl (C5Me5)(C8H8)UPh, 7, is stable at room temperature, but activates a C-H bond of the (C5Me5)- ligand upon heating to generate the uranium tuckin, 3. C-H bond activation with electropositive metals can be rationalized by both steric crowding and coordinative unsaturation. The prototypical C-H activation leading to Schrock carbenes is explained by steric crowding,58,59 and both eqs 6 and 7 can be explained in this way. Equation 6 sets up the synthesis of a highly crowded intermediate, “(C5Me5)3Th[iPrNC(Me)NPri]”, and the (C5Me5)2(hpp)UR complexes in eq 7 become more reactive to the formation of the tuck-in product as the size of R increases. In contrast, the early identification of tuck-in complexes with complexes like “(C5Me5)(η5:η1-C5Me4CH2)Lu”15 was consistent with high reactivity from coordinatively unsaturated intermediates. The propensity of 2, 5, 6, and 7 to form 3 may arise due to the coordinative unsaturation of the (C5Me5)(C8H8)UR compounds. There does not appear to be a major difference in reactivity of Me versus Et versus Np, which contrasts sharply with the (C5Me5)2(hpp)UR series, where steric crowding appears to be driving the metalation: Me is stable, Et requires heating to form a tuck-in, and Np cannot be isolated. Formation of 3 does not ameliorate the coordinative unsaturation versus the (C5Me5)(C8H8)UR precursors, but the reaction does generate a chelated alkyl, (C5Me4CH2)2- and releases RH to entropic advantage. The reactivity of 3 with isocyanides is consistent with the presence of a U-C alkyl bond in a sterically unsaturated complex. Insertion of tBuNtC into uranium alkyl bonds has been seen with a variety of ligand combinations including the [(C5Me5)(C8H8)]3- ligand combination.1,34,35 Insertion of a single equivalent of tBuNtC is usually observed even if there is more than one uranium-alkyl bond present, e.g., (48) Owen, G. R.; Vilar, R.; White, A. J. P.; Williams, D. J. Organometallics 2002, 21, 4799–4807. (49) Utz, T. L.; Leach, P. A.; Geib, S. J.; Cooper, N. J. Organometallics 1997, 16, 4109–4114. (50) Becker, T. M.; Alexander, J. J.; Bauer, J. A. K.; Nauss, J. L.; Wireko, F. C. Organometallics 1999, 18, 5594–5605. (51) Riera, V.; Ruiz, J.; Jeannin, Y.; Philoche-Levisalles, M. J. Chem. Soc., Dalton Trans. 1988, 1591–1597. (52) Adams, R. D.; Chodosh, D. F. J. Am. Chem. Soc. 1977, 99, 6544– 6550. (53) Fandos, R.; Meetsma, A.; Teuben, J. H. Organometallics 1991, 10, 2665–2671. (54) Motz, P. L.; Alexander, J. J.; Ho, D. M. Organometallics 1989, 8, 2589–2601. (55) Veya, P.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1994, 13, 441–450. (56) Aoki, K.; Yamamoto, Y. Inorg. Chem. 1976, 15, 48–52. (57) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.; Marks, T. J. J. Am. Chem. Soc. 1981, 103, 6650–6667. (58) Schrock, R. R. Chem. Rev. 2002, 102, 145–179. (59) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 4th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005.

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with (C5Me5)2UMe2. This decrease in reactivity is reported to be due to the steric bulk of the resulting iminoacyl ligand, which blocks further coordination and reaction of the monoinsertion complex.35 However, the tethered nature of the complex (η5-C5Me4SiMe2CH2-κC)2U allows for the insertion of 1 equiv of isocyanide into each of the two tethered alkyl groups.43 Complex 3 differs from all of these cases in that 2 equiv of isocyanide insert into a single U-C bond, eq 15. Equation 15 is similar to the analogous reaction with the titanium and zirconium tuck-in complexes, except that the structure of the bis(insertion) product is different.

Conclusion The [(C5Me5)(C8H8)]3- ligand combination attached to U appears to foster the formation of tuck-in complexes: the (C5Me5)- ligand is readily metalated by alkyl groups to form (C8H8)(C5Me4CH2)U, 3, characterized as the THF adduct (η8-C8H8)(η5:η1-C5Me4CH2)U(THF), 8. Complex 8 is a member of a growing list of actinide tuck-in complexes 4þ

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that have been synthesized from the normally inert (C5Me5)ligand. Complex 8 is the most closely related structurally characterized f element metallocene to the intermediate, “(C5Me5)(η5:η1-C5Me4CH2)Lu”, postulated in the activation of methane decades ago, eq 3. In contrast to some of the recent examples, tuck-in formation does not seem to be driven by steric crowding. Initial insertion reactivity studies indicate that it will be a valuable, well-defined example with which to examine tuck-in reactivity.

Acknowledgment. We thank the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences of the Department of Energy for support of this research. Supporting Information Available: X-ray data collection, structure solution, and refinement (PDF) and X-ray diffraction details of compounds 8 and 11 (CIF, CCDC No. 784356 and 784357). This material is available free of charge via the Internet at http://pubs.acs.org.