Utility of the 1,3,4,6,7,8-Hexahydro-2H-pyrimido[1 ... - ACS Publications

Apr 13, 2010 - Monoalkyl uranium chemistry has been probed by reacting the metallocene chloride complex (C5Me5)2(hpp)UCl, 1, (hpp)− = 1,3,4,6,7,8-he...
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Organometallics 2010, 29, 2104–2110 DOI: 10.1021/om100076s

Utility of the 1,3,4,6,7,8-Hexahydro-2H-pyrimido[1,2-a]pyrimidinato Ligand, (hpp)-, in Stabilizing Uranium Metallocene Mono-Alkyl and “Tuck-in” Complexes Elizabeth Montalvo,† Joseph W. Ziller,† Antonio G. DiPasquale,‡ Arnold L. Rheingold,‡ and William J. Evans*,† †

‡

Department of Chemistry, University of California, Irvine, California 92697-2025, and Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, MC 0358, La Jolla, California 92093-0358 Received January 29, 2010

Monoalkyl uranium chemistry has been probed by reacting the metallocene chloride complex (C5Me5)2(hpp)UCl, 1, (hpp)-=1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidinato, with alkyl lithium reagents. Complex 1 reacts with LiMe, LiCtCPh, LiPh, and LiEt to generate (C5Me5)2(hpp)UMe, 2, (C5Me5)2(hpp)U(CtCPh), 3, (C5Me5)2(hpp)UPh, 4, and (C5Me5)2(hpp)UEt, 5, respectively. Complexes 2-5 react with CuI to form the iodide complex (C5Me5)2(hpp)UI, 6, and methane, phenylacetylene, benzene, and ethane, respectively. Attempts to make a neopentyl analogue of 2-5 from the reaction of 1 with neopentyllithium yielded the “tuck-in” complex (C5Me5)(η5:η1-C5Me4CH2)(hpp)U, 7. Complex 7 can also be synthesized by heating 5 to 70 °C in a reaction that forms ethane as a byproduct. Introduction Recent studies of the (hpp)- ligand derived from 1,3,4,6,7,8hexahydro-2H-pyrimido[1,2-a]pyrimidine, Hhpp, with uranium have shown that this bicyclic guanidinate can combine readily with uranium metallocenes to make monosubstituted derivatives, (C5Me5)2(hpp)UX (X=Cl, Me, N3),1 e.g., eqs 1 and 2, in which the reactivity of a single X- ligand could be studied without the complication of two such reactive sites as found in the more common actinide metallocenes (C5Me5)2UX2.2

The effect of the {(C5Me5)2(hpp)}3- ligand combination on uranium alkyl reactivity is described here. The utility of the previously reported (C5Me5)2(hpp)UCl, 1,1 as a precursor to monoalkyl complexes via ionic metathesis is presented as well as the metalation reactivity of some of the alkyl complexes to *Corresponding author. E-mail: [email protected]. (1) Evans, W. J.; Montalvo, E.; Ziller, J. W.; DiPasquale, A. G.; Rheingold, A. L. Inorg. Chem. 2010, 49, 222. (2) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.; Marks, T. J. J. Am. Chem. Soc. 1981, 103, 6650. pubs.acs.org/Organometallics

Published on Web 04/13/2010

form the second example of a crystallographically characterizable uranium “tuck-in”3 complex involving a [η5:η1-C5Me4(CH2)]2- ligand. For decades, structural information on f element tuck-in complexes was elusive. In the 1980s, C-H alkyl bond activation with lutetium,4,5 scandium,3 and thorium6 metallocenes was explained with highly reactive tuck-in intermediates, but structural evidence for tuck-in complexes was found only with transition metals.7-14 Metalated ligand systems involving [N(SiMe3)2]- 15,16 and [C5H2(CMe3)3]- 17,18 were crystallographically characterized in f element complexes, and bridging [μ-η5:η1-C5Me4(CH2)]2- “tuck-over” complexes (3) 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. (4) Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491. (5) Watson, P. L.; Parshall, G. W. Acc. Chem. Res. 1985, 18, 51. (6) 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. (7) Bercaw, J. E. J. Am. Chem. Soc. 1974, 96, 5087. (8) Schock, L. E.; Brock, C. P.; Marks, T. J. Organometallics 1987, 6, 232. (9) Bulls, A. R.; Schaefer, W. P.; Serfas, M.; Bercaw, J. E. Organometallics 1987, 6, 1219. (10) Luinstra, G. A.; Teuben, J. H. J. Am. Chem. Soc. 1992, 114, 3361. (11) Fischer, J. M.; Piers, W. E.; Young, V. G., Jr. Organometallics 1996, 15, 2410. (12) Kreindlin, A. Z.; Dolgushin, F. M.; Yanovsky, A. I.; Kerzina, Z. A.; Petrovskii, P. V.; Rybinskaya, M. I. J. Organomet. Chem. 2000, 616, 106. (13) Beweries, T.; Burlakov, V. V.; Bach, M. A.; Peitz, S.; Arndt, P.; Baumann, W.; Spannenberg, A.; Rosenthal, U.; Pathak, B.; Jemmis, E. D. Angew. Chem., Int. Ed. 2007, 46, 6907. (14) Rybinskaya, M. I.; Kreindlin, A. Z.; Struchkov, Y. T.; Yanovskii, A. I. J. Organomet. Chem. 1989, 359, 233. (15) Simpson, S. J.; Andersen, R. A. J. Am. Chem. Soc. 1981, 103, 4063. (16) Gardner, B. M.; McMaster, J.; Lewis, W.; Blake, A. J.; Liddle, S. T. J. Am. Chem. Soc. 2009, 131, 10388. (17) Maron, L.; Werkema, E. L.; Perrin, L.; Eisenstein, O.; Andersen, R. A. J. Am. Chem. Soc. 2005, 127, 279. (18) Jaroschik, F.; Momin, A.; Nief, F.; Le Goff, X.-F.; Deacon, G. B.; Junk, P. C. Angew. Chem., Int. Ed. 2009, 48, 1117. r 2010 American Chemical Society

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could be structurally characterized,19-22 but only recently has crystallographic evidence been obtained on f element tuck-in complexes. The first example was the tuck-in tuckover uranium dihydride (η5-C5Me5)[η5:η1:η1-C5Me3(CH2)2]U(μ-H)2U(η5-C5Me5)2, eq 3.23 More recently, a thorium example, (η5:η1-C5Me4CH2)(η5-C5Me5)Th[iPrNC(Me)NiPr-

κ2N,N0 ],24 was obtained from a bis(pentamethylcylcopentadienyl) amidinate ligand combination, eq 4. The analogous uranium complex could not be formed in this way

since reduction to U3þ occurred when eq 4 was examined with uranium.24 We report here that the {(C5Me5)2(hpp)}3ligand combination can also provide a structurally characterizable uranium tuck-in complex.

Experimental Section The manipulations described below were conducted under argon with rigorous exclusion of air and water using Schlenk, vacuum line, and glovebox techniques. Solvents were sparged with UHP argon and dried over columns containing Q-5 and molecular sieves. NMR solvents (Cambridge Isotope Laboratories) were dried over sodium-potassium alloy, degassed, and vacuum-transferred before use. (C5Me5)2(hpp)UCl was prepared as previously described.1 CuI was obtained from SigmaAldrich and used without further purification. Neopentyllithium was prepared as previously described.25 Methyllithium (1.6 M solution in diethyl ether), ethyllithium (0.5 M solution in benzene/cyclohexane), isobutyllithium (1.7 M solution in heptane), and phenyllithium (1.8 M solution in di-n-butyl ether) were purchased from Sigma-Aldrich, and the corresponding solvent was removed under vacuum prior to use. PhCtCLi was prepared by deprotonation of HCtCPh (Sigma-Aldrich) with 1 equiv of iBuLi in hexane. 1H NMR and 13C NMR spectra were recorded on a Bruker DRX500 spectrometer at 25 °C. Due to the paramagnetism of uranium, only resonances that could be unambiguously identified are reported. Infrared spectra were recorded as KBr pellets on a Varian 1000 FTIR spectrophotometer at 25 °C. Elemental analyses were performed on a PerkinElmer 2400 Series II CHNS analyzer. (19) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. Organometallics 1991, 10, 134. (20) Booij, M.; Deelman, B. J.; Duchateau, R.; Postma, D. S.; Meetsma, A.; Teuben, J. H. Organometallics 1993, 12, 3531. (21) Evans, W. J.; Perotti, J. M.; Ziller, J. W. Inorg. Chem. 2005, 44, 5820. (22) Evans, W. J.; Champagne, T. M.; Ziller, J. W. J. Am. Chem. Soc. 2006, 128, 14270. (23) Evans, W. J.; Miller, K. A.; DiPasquale, A. G.; Rheingold, A. L.; Stewart, T. J.; Bau, R. Angew. Chem., Int. Ed. 2008, 47, 5075. (24) Evans, W. J.; Walensky, J. R.; Ziller, J. W. Chem.;Eur. J. 2009, 15, 12204. (25) Evans, W. J.; Champagne, T. M.; Ziller, J. W.; Kaltsoyannis, N. J. Am. Chem. Soc. 2006, 128, 16178.

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(C5Me5)2(hpp)UMe, 2. In an argon-filled glovebox, LiMe (2 mg, 0.09 mmol) was added to a solution of (C5Me5)2(hpp)UCl, 1 (50 mg, 0.073 mmol), in THF (5 mL) while stirring. After the reaction mixture was stirred for 1 h, an insoluble material was removed from the mixture via centrifugation and filtration. Solvent was removed under reduced pressure, the product was extracted with toluene, and the solvent was removed under vacuum to produce 2 as a dark yellow solid (44 mg, 91%). The 1H NMR spectrum of 2 matched that of the previously characterized (C5Me5)2(hpp)UMe.1 (C5Me5)2(hpp)U(CtCPh), 3. In an argon-filled glovebox, LiCtCPh (57 mg, 0.53 mmol) was added to a solution of 1 (301 mg, 0.441 mmol) in THF (15 mL) at -35 °C while stirring. After the reaction mixture was stirred for 2 h at ambient temperature, an insoluble material was removed from the mixture via centrifugation and filtration. Solvent was removed under reduced pressure, the product was extracted with toluene, and the solvent was removed under vacuum, leaving 3 as a dark yellow solid (304 mg, 92%). Yellow X-ray quality crystals of 3 were grown from a concentrated toluene solution at -35 °C. 1H NMR (C6D6): δ 38.19 (s, Δν1/2 = 25 Hz, 2H, C7H12N3), 14.75 (s, Δν1/2 = 18 Hz, 2H, C7H12N3), 12.91 (d, 2H, Ph), 10.81 (t, 1H, Ph), 7.91 (t, 2H, Ph), 5.91 (s, Δν1/2 = 15 Hz, 2H, C7H12N3), 1.99 (s, Δν1/2 = 10 Hz, 30H, C5Me5), -4.28 (s, Δν1/2 = 13 Hz, 2H, C7H12N3), -12.70 (s, Δν1/2 = 18 Hz, 2H, C7H12N3), -24.39 (s, Δν1/2 = 19 Hz, C7H12N3). 13C NMR (C6D6): δ 136.4 (Ph), 117.0 (Ph), 84.3 (C7H12N3), 84.1 (C7H12N3), 57.2 (C7H12N3), 53.3 (Ph), 35.1 (C7H12N3), 22.2 (C7H12N3), -44.86 (C5Me5), -61.7 (C7H12N3). IR: 2931s, 2883s, 2848s, 2720w, 2049m, 1595w, 1552s, 1496s, 1481 m, 1452s, 1378 m, 1358w, 1341w, 1317s, 1289m, 1260m, 1199m, 1146m, 1111w, 1061m, 1026m, 900w, 805w, 777m, 751s, 727m, 690m cm-1. Anal. Calcd for UC35H47N3: C, 56.21; H, 6.33; N, 5.62. Found: C, 55.69; H, 6.11; N, 5.85. (C5Me5)2(hpp)UPh, 4. As described for 3, 4 was obtained as a dark yellow solid (320 mg, 90%) from LiPh (45 mg, 0.54 mmol) and 1 (304 mg, 0.446 mmol) in THF (15 mL) at -35 °C. Yellow X-ray quality crystals of 4 were grown from a concentrated benzene solution at 25 °C. 1H NMR (THF-d8): δ 15.23 (s, Δν1/2 = 21 Hz, 2H, C7H12N3), 14.68 (s, Δν1/2 = 16 Hz, 1H, Ph), 10.88 (s, Δν1/2 = 21 Hz, 2H, C7H12N3), 5.16 (s, Δν1/2 = 14 Hz, 2H, Ph), 2.14 (s, Δν1/2 = 7 Hz, 30H, C5Me5), 1.88 (s, Δν1/2 = 14 Hz, 2H, C7H12N3), -1.30 (s, Δν1/2 = 18 Hz, 2H, C7H12N3), -2.57 (s, Δν1/2 = 8 Hz, 2H, C7H12N3), -4.44 (s, Δν1/2 = 11 Hz, 2H, C7H12N3). 13C NMR (THF-d8): δ 108.0 (Ph), 53.4 (C7H12N3), 52.9 (Ph), 47.9 (C7H12N3), 43.6 (C7H12N3), 21.4 (C7H12N3), -45.9 (C5Me5), -59.8 (C7H12N3), -62.7 (C7H12N3). IR: 2945s, 2898s, 2850s, 2723w, 1545s, 1500s, 1470m, 1451s, 1379m, 1358w, 1320m, 1291m, 1259m, 1200m, 1142m, 1111w, 1059m, 1026m, 945w, 912w, 800w, 723m, 706m cm-1. Anal. Calcd for UC33H47N3: C, 54.76; H, 6.54; N, 5.80. Found: C, 55.48; H, 6.79; N, 5.32. (C5Me5)2(hpp)UEt, 5. As described for 3, 5 was obtained as a dark red solid (270 mg, 89%) from LiEt (20 mg, 0.5 mmol) and 1 (305 mg, 0.447 mmol) in THF (15 mL) at -35 °C. 1H NMR (C6D6): δ 26.35 (s, Δν1/2 = 17 Hz, 2H, C7H12N3), 8.46 (s, Δν1/2 = 18 Hz, 2H, C7H12N3), 2.65 (s, Δν1/2 = 17 Hz, 2H, C7H12N3), -2.01 (s, Δν1/2 = 5 Hz, 30H, C5Me5), -7.56 (s, Δν1/2 = 13 Hz, 2H, C7H12N3), -16.97 (s, Δν1/2 = 18 Hz, 2H, C7H12N3), -26.95 (s, Δν1/2 = 16 Hz, 2H, C7H12N3), -150.0 (s, Δν1/2 = 2 Hz, 3H, CH2CH3). 13C NMR (C6D6): δ 118.6, 78.2 (C7H12N3), 61.3 (C7H12N3), 36.3 (C7H12N3), 31.7, 17.4 (C7H12N3), -18.6 (C7H12N3), -43.4 (C7H12N3), -54.8 (C5Me5), -67.0. IR: 2938s, 2888s, 2846s, 2731w, 1545s, 1502s, 1476 m, 1452s, 1438s, 1379m, 1359w, 1319m, 1289m, 1260m, 1200m, 1144m, 1111w, 1060m, 1025m, 988w, 877w, 803w, 725m, 691w, 600w cm-1. Anal. Calcd for UC29H47N3: C, 51.54; H, 6.22; N, 7.01. Found: C, 51.85; H, 6.65; N, 6.98. (C5Me5)2(hpp)UI, 6, from 2. CuI (71 mg, 0.38 mmol) was added to a solution of (C5Me5)2(hpp)UMe, 2 (206 mg, 0.311 mmol), in toluene (10 mL) while stirring. After the reaction mixture was stirred for 12 h, an insoluble material was removed

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from the mixture via centrifugation and filtration. Solvent was removed under vacuum, leaving 6 as a dark orange solid (208 mg, 86%). Orange crystals of 6 suitable for X-ray diffraction were grown from a concentrated benzene solution at 25 °C. 1 H NMR (C6D6): δ 7.41 (s, Δν1/2 = 3 Hz, 30H, C5Me5), 1.17 (s, Δν1/2 = 10 Hz, 2H, C7H12N3), 0.53 (s, Δν1/2 = 12 Hz, 2H, C7H12N3), 0.35 (s, Δν1/2 = 13 Hz, 2H, C7H12N3), -0.78 (s, Δν1/2 = 15 Hz, 2H, C7H12N3), -2.66 (s, Δν1/2 = 15 Hz, 2H, C7H12N3), -10.18 (s, Δν1/2 = 14 Hz, 2H, C7H12N3). 13C NMR (C6D6): δ 41.0 (C7H12N3), 40.7 (C7H12N3), 39.7 (C7H12N3), 38.5 (C7H12N3), -3.3 (C7H12N3), -16.4 (C7H12N3), -33.2 (C5Me5). IR: 2941s, 2897s, 2847s, 2722w, 1630w, 1554s, 1493s, 1475m, 1468s, 1450s, 1376s, 1319s, 1287m, 1260w, 1212s, 1146s, 1111m, 1058m, 1027m, 897w, 878w, 805w, 729m, 691w cm-1. Anal. Calcd for UC27H42N3I: C, 41.92; H, 5.47; N, 5.43. Found: C, 41.86; H, 6.20; N, 5.38. When this reaction was repeated in a sealed J-Young NMR tube in C6D6, a resonance consistent with the formation of methane was observed in the 1H NMR spectrum of the reaction products. (C5Me5)2(hpp)UI, 6, from 3. CuI (6 mg, 0.03 mmol) was added to a J-Young NMR tube containing 3 (15 mg, 0.020 mmol) in C6D6. The J-Young NMR tube was immediately capped, and a color change from dark yellow to dark orange was observed. 1H NMR spectroscopy showed quantitative conversion of starting material to 6, and a resonance consistent with the formation of phenylacetylene was also observed. (C5Me5)2(hpp)UI, 6, from 4. CuI (7 mg, 0.04 mmol) was added to a J-Young NMR tube containing 4 (19 mg, 0.024 mmol) in THF-d8. The J-Young NMR tube was immediately capped, and a color change from dark yellow to dark orange was observed. 1 H NMR spectroscopy showed quantitative conversion of starting material to 6, and a resonance consistent with the formation of benzene was also observed. (C5Me5)2(hpp)UI, 6, from 5. CuI (6 mg, 0.03 mmol) was added to a J-Young NMR tube containing 5 (14 mg, 0.021 mmol) in C6D6. The J-Young NMR tube was immediately capped, and a color change from dark red to dark orange was observed. 1H NMR spectroscopy showed quantitative conversion of starting material to 6, and a resonance consistent with the formation of ethane was also observed. (C5Me5)(η5:η1-C5Me4CH2)(hpp)U, 7, from 5. (C5Me5)2(hpp)UEt, 5 (200 mg, 0.3 mmol), in toluene was heated to 70 °C for 12 h with frequent venting while stirring. Solvent was then removed under vacuum, and the resulting brown oil was triturated with hexane. After the solvent was removed under reduced pressure, 7 was obtained as a brown solid (151 mg, 79%). Brown crystals of 7 suitable for X-ray diffraction were grown from a concentrated benzene solution at 25 °C. 1H NMR (C6D6): δ 3.19 (s, Δν1/2 = 129 Hz, 3H, C5Me4CH2), 1.40 (s, Δν1/2 = 60 Hz, 3H, C5Me4CH2), -0.90 (s, Δν1/2 = 6 Hz, 15H, C5Me5), -3.25 (s, Δν1/2 = 9 Hz, 2H, C7H12N3), -8.15 (s, Δν1/2 = 61 Hz, 3H, C5Me4CH2), -10.68 (s, Δν1/2 = 102 Hz, 3H, C5Me4CH2), -13.05 (s, Δν1/2 = 21 Hz, 2H, C7H12N3). 13C NMR (C6D6): δ 36.9 (C5Me4CH2), 27.1 (C5Me4CH2), 20.2 (C7H12N3), 26.8 (C5Me4CH2), -67.0 (C5Me5), -119.5 (C7H12N3). IR: 2944s, 2904s, 2849s, 2720w, 1610w, 1545s, 1496s, 1471m, 1450s, 1377s, 1321s, 1291m, 1260w, 1210s, 1146s, 1111w, 1062m, 1027m, 878w, 830w, 785m, 725s, 692w, 602w cm-1. Anal. Calcd for UC27H41N3: C, 50.23; H, 6.40; N, 6.51. Found: C, 49.60; H, 6.17; N, 6.39. X-ray Data Collection, Structure Determination, and Refinement. Crystallographic information on complexes 3, 4, 6, and 7 is summarized in the Supporting Information and Table 1.

Results (C5Me5)2(hpp)UR via Ionic Metathesis. (C5Me5)2(hpp)UCl,1 1, prepared as shown in eq 1, functions well as a precursor to the monoalkyl uranium complexes (C5Me5)2(hpp)UR (R = Me, 2; CtCPh, 3; Ph, 4; Et, 5), via ionic

Montalvo et al.

metathesis reactions with alkyl lithium reagents, eq 5. The methyl complex (C5Me5)2(hpp)UMe, 2, had previously been

made from (C5Me5)2UMe2 and Hhpp, eq 2,1 and its preparation via eq 5 was confirmed by 1H NMR spectroscopy. (C5Me5)2(hpp)U(CtCPh), 3. The phenylalkynide complex 3 was characterized by spectroscopic and analytical means and definitively identified by X-ray crystallography, Figure 1. The IR spectrum of 3 contains a C-N stretch arising from the (hpp)ligand at 1552 cm-1. Complexes 4-7, presented below, have analogous C-N stretches at 1545, 1545, 1554, and 1545 cm-1, respectively.26 The IR spectrum of 3 also has an absorption at 2049 cm-1 characteristic of CtC triple bonds that is similar to that of other uranium alkynides, e.g., 2056 and 2062 cm-1 in (C5Me5)2U(CtCPh)227 and (C5Me5)2U(CtCPh)[(iPr)NC(CtCPh)N(iPr)-κ2N,N0 ],28 respectively. The 1H NMR spectrum of 3 is similar to that of other U4þ (C5Me5)2(hpp)UX complexes (X = Cl, Me, N3)1 and has six independent resonances attributable to the (hpp)- ligand. Three are at low field, 38.19, 14.74, and 12.91 ppm, and three are at high field, -4.28, -12.70, and -24.39 ppm. Complexes 4-6, described below, display similar patterns. In addition, all the phenyl resonances are observed in 3. Complex 3 adds to the small list of crystallographically characterized U4þ alkynyl complexes, i.e., (C5Me5)2U(CtCPh)[(iPr)NC(CtCPh)N(iPr)-κ2N,N0 ],28 (C5Me5)2U(CtCPh)(NPh2),29 (C5H5)3U(CtCR) (R = H30 and Ph31), [U{N(CH2CH2NSiMe2tBu)3}(CtCC6H4Me)],32 (C5Me5)2U(CtCPh)2,33 and (C5Me4SiMe3)2U(CtCPh)2.34 (C5Me5)2(hpp)UPh, 4. The phenyl complex 4 was also characterized by spectroscopic and analytical means and definitively identified by X-ray crystallography, Figure 2. Complex 4 is the first crystallographically characterized uranium phenyl complex to our knowledge. The previously reported (C5Me5)2UPh2 readily decomposes and has not been characterized by X-ray diffraction.2 In contrast to 3, the 1H spectrum of complex 4 contains only two of the three expected phenyl resonances, which is probably due to the increased proximity of the phenyl group to the paramagnetic uranium metal center. (C5Me5)2(hpp)UEt, 5. Reaction of 1 with LiEt gave a complex that had an elemental analysis and spectroscopic characteristics consistent with the ethyl analogue (26) Wilkins, J. D. J. Organomet. Chem. 1974, 80, 349. (27) Straub, T.; Frank, W.; Reiss, G. J.; Eisen, M. S. J. Chem. Soc., Dalton Trans. 1996, 2541. (28) Evans, W. J.; Walensky, J. R.; Ziller, J. W.; Rheingold, A. L. Organometallics 2009, 28, 3350. (29) Graves, C. R.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. Organometallics 2008, 27, 3335. (30) Atwood, J. L.; Hains, C. F., Jr.; Tsutsui, M.; Gebala, A. E. J. Chem. Soc., Chem. Commun. 1973, 452. (31) Atwood, J. L.; Tsutsui, M.; Ely, N.; Gebala, A. E. J. Coord. Chem. 1976, 5, 209. (32) Boaretto, R.; Roussel, P.; Kingsley, A. J.; Munslow, I. J.; Sanders, C. J.; Alcock, N. W.; Scott, P. Chem. Commun. 1999, 1701. (33) Evans, W. J.; Walensky, J. R.; Ziller, J. W. Organometallics 2010, 29, 945. (34) Evans, W. J.; Siladke, N. A.; Ziller, J., W. C. R. Chem., 2010, in press.

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Table 1. X-ray Data Collection Parameters for (C5Me5)2(hpp)U(CtCPh), 3, (C5Me5)2(hpp)UPh, 4, (C5Me5)2(hpp)UI, 6, and (C5Me5)(η5:η1-C5Me4CH2)(hpp)U, 7 3

4

7

C27H42N3IU 773.57 100(2) monoclinic P21/n 8.9362(12) 18.274(2) 16.592(2) 90 93.455(2) 90 2704.6(6) 4 1.900 7.162 0.0431 0.0900

C27H41N3U 645.66 103(2) triclinic P1 8.6188(8) 9.2322(9) 17.4294(16) 82.0205(10) 84.9103(10) 67.1607(10) 1264.8(2) 2 1.695 6.435 0.0207 0.0550

)

)

C33H47N3U 3 C4H8O empirical formula C35H47N3U 3 C7H8 fw 839.92 795.87 temp (K) 153(2) 103(2) cryst syst monoclinic monoclinic P21/c space group P21/n a (A˚) 10.4092(11) 10.9515(6) b (A˚) 10.3444(11) 14.5882(8) c (A˚) 34.255(4) 20.7117(11) R (deg) 90 90 β (deg) 91.4437(13) 90.4525(6) γ (deg) 90 90 3687.3(7) 3308.9(3) volume (A˚3) Z 4 4 1.513 1.598 Fcalcd (mg/m3) 4.434 4.938 μ (mm-1) 0.0476 0.0158 R1a (I > 2.0σ(I)) 0.1003 0.0377 wR2b (all data) P P P 2 2 2 P 2 2 1/2 a b R1 = Fo| - |Fc / |Fo|. wR2 = [ [w(Fo - Fc ) / [w(Fo ) ]] .

6

Figure 1. Thermal ellipsoid plot of (C5Me5)2(hpp)U(CtCPh), 3, drawn at the 30% probability level with hydrogen atoms excluded for clarity.

(C5Me5)2(hpp)UEt, 5, but single crystals suitable for X-ray crystallography were not obtained. The 1H NMR resonance for the (C5Me5)- methyl groups of 5, -2.01 ppm, is similar to the chemical shift found for 2, -1.72 ppm. For the ethyl ligand in 5, only a resonance for the methyl group was observed at -150 ppm, which has a significantly different chemical shift than the resonance for the methyl group in 2 (-219.7 ppm). To obtain more evidence on the existence of 5, the reaction of 5 with CuI was investigated. CuI has recently been shown to be an excellent reagent to convert U4þ alkyls to iodides with elimination of the alkane.35 In addition, copper salts can oxidize U4þ metallocenes,29,36-38 a result that could also aid in identifying 5. The reaction of 5 with CuI forms the iodide (35) Evans, W. J.; Walensky, J. R.; Ziller, J. W. Organometallics 2010, 29, 101. (36) Graves, C. R.; Scott, B. L.; Morris, D. E.; Kiplinger, J. L. J. Am. Chem. Soc. 2007, 129, 11914. (37) Graves, C. R.; Vaughn, A. E.; Schelter, E. J.; Scott, B. L.; Thompson, J. D.; Morris, D. E.; Kiplinger, J. L. Inorg. Chem. 2008, 47, 11879. (38) Graves, C. R.; Yang, P.; Kozimor, S. A.; Vaughn, A. E.; Clark, D. L.; Conradson, S. D.; Schelter, E. J.; Scott, B. L.; Thompson, J. D.; Hay, P. J.; Morris, D. E.; Kiplinger, J. L. J. Am. Chem. Soc. 2008, 130, 5272.

Figure 2. Thermal ellipsoid plot of (C5Me5)2(hpp)UPh, 4, drawn at the 50% probability level with hydrogen atoms excluded for clarity.

(C5Me5)2(hpp)UI, 6, and ethane, eq 6. The CuI reaction was also examined with the methyl, phenylalkynide, and phenyl complexes 2, 3, and 4, and complex 6 was again obtained, eq 6. Methane, phenylacetylene, and benzene, respectively, were observed in the 1H NMR spectra of reactions carried out in sealed J-Young NMR tubes in C6D6 and THF-d8.

C-H Activation and Tuck-in Formation: (C5Me5)(η5:η1C5Me4CH2)(hpp)U, 7. Attempts to make a neopentyl derivative of 2-5, i.e., “(C5Me5)2(hpp)U(CH2CMe3)” via the ionic metathesis reaction in eq 5 between 1 and Me3CCH2Li did not give an analogous product. The 1H NMR spectrum of this reaction mixture was complicated, but one set of resonances could ultimately be assigned to the tuck-in complex (C5Me5)(η5:η1-C5Me4CH2)(hpp)U, 7, in light of the X-ray crystal structure, Figure 4, and independent synthesis

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Figure 3. Thermal ellipsoid plot of (C5Me5)2(hpp)UI, 6, drawn at the 50% probability level with hydrogen atoms excluded for clarity.

Figure 4. Thermal ellipsoid plot of (C5Me5)(η5:η1-C5Me4CH2)(hpp)U, 7, drawn at the 50% probability level with hydrogen atoms excluded for clarity.

of 7 from 5 (see below). Complex 7 apparently formed by C-H bond activation of a methyl group of one of the (C5Me5)- ligands, eq 7. If the neopentyl complex did form, presumably it quickly metalated the (C5Me5)- ligand to form 7, Figure 4. Complex 7 was also observed as a component in the reaction mixture obtained from 1 and iBuLi.

Complex 7 can be more conveniently synthesized by heating the ethyl complex 5 to 70 °C for 12 h, eq 8. Ethane is the only byproduct in this reaction. The 1H NMR spectrum

-

-

contained one (C5Me5) and only two (hpp) resonances along with four resonances, each of which integrated to three protons corresponding to the methyl groups in the (η5:η1C5Me4CH2)2- ligand. The methylene resonance could not be located in this paramagnetic system.

The methyl complex 2 also forms 7 upon heating to 70 °C, but other products are generated in addition to methane. In contrast, (C5Me5)2UMe2 has a half-life of 16 h at 100 °C.2 (C5Me5)2(hpp)U(CtCPh), 3, and (C5Me5)2(hpp)UPh, 4, do not form 7 upon heating. The latter result contrasts with the diphenyl metallocene (C5Me5)2UPh2, which decomposes via an aryl C-H bond metalation to form a (C5Me5)2U(C6H4) intermediate that can be trapped by PhCtCPh to make the metallacycle (C5Me5)2U[C2(Ph)2C6H4].2 Structural Data. The (C5Me5)2(hpp)UX complexes 3, 4, and 6 each have a formally nine-coordinate uranium metal center ligated by two pentamethylcyclopentadienyl ligands, a chelating (hpp)- ligand, and an additional anionic ligand, X [X = (CtCPh)-, 3; (Ph)-, 4; I-, 6], Figures 1-3. Complex 7 also has a nine-coordinate uranium metal center ligated, in this case, by one (C5Me5)- ligand, a tuck-in (η5:η1-C5Me4CH2)2- ligand, and a (hpp)- ligand. Table 2 compares the crystallographic data obtained on these complexes. The {(C5Me5)2(hpp)U}þ components of 3, 4, 6, and 7 have similar metrical parameters, and these also match those in the previously reported U4þ complexes (C5Me5)2(hpp)UX (X = Cl and N3).1 The (hpp)- ligand in these complexes interacts with the uranium metal center primarily through two nitrogen atoms with 2.347(7)-2.418(5) A˚ U-N(hpp) bond distances. The U-C(hpp) distances are significantly longer. In all cases except 7, in which the U-N bonds are equal within the error limits, the U-N(hpp) distances adjacent to the anionic X- ligands are 0.02-0.06 A˚ shorter than the U-N(hpp) further from X. The C(21)-N(1), C(21)-N(2), and C(21)N(3) bond lengths are shorter than the average distance expected for a C-N single bond (1.469 A˚) but longer than that expected for a CdN double bond (1.303 A˚).39 Hence, the negative charge of the monoanionic (hpp)- ligand is delocalized over the N(1)-C(21)-N(2) unit. The U-X bond lengths in (C5Me5)2(hpp)UX [X = (Ct CPh)-, 3; (Ph)-, 4; I-, 6] are within the expected ranges. The 2.434(7) A˚ U(1)-C(28) bond distance in the alkynyl complex 3 is equivalent to the corresponding bond distances in the amidinate complex (C5Me5)2U(CtCPh)[(iPr)NC(CtCPh)N(iPr)-κ2N,N0 ] [2.433(1) A˚], 8,28 and (C5Me5)2U(CtCPh)(NPh2) [2.409(4) A˚]29 and similar to that in (C5H5)3U(Ct CPh) [2.33(2) A˚].30 The 1.241(9) A˚ C(28)-C(29) bond distance in the (CtCPh)- ligand is similar to the analogous bond distances in 8 [1.214(3) A˚],28 (C5Me5)2U(CtCPh)(NPh2) [1.197(6) A˚],29 and (C5H5)3U(CtCPh) [1.25(2) A˚].30 The C(28)-C(29)-C(30) angle is almost linear, 177.1(8)°. Since complex 4 is the first crystallographically characterized actinide phenyl complex, direct comparisons with actinide complexes are not possible. However, the 2.513(2) A˚ U(1)-C(28) bond distance in 4 is similar to the 2.505(14) A˚ U-C(CH2Ph) benzyl bond distance in (C5Me5)2U(CH2Ph)[η2-(O,C)-ONC5H4]40 and the 2.424(7) and 2.414(7) A˚ U-C(Me) bond distances in (C5Me5)2UMe241 when the 0.05 A˚ difference in ionic radii between eight- and a nine-coordinate U4þ metal centers is considered.42 The U-C(Ph) bond distance in 4 is also close to the 2.511(8) A˚ Sm-C(Ph) bond (39) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1. (40) Pool, J. A.; Scott, B. L.; Kiplinger, J. L. J. Am. Chem. Soc. 2005, 127, 1338. (41) 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. (42) Shannon, R. D. Acta Crystallogr. 1976, A32, 751.

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Table 2. Bond Distances (A˚) and Angles (deg) in (C5Me5)2(hpp)U(CtCPh), 3, (C5Me5)2(hpp)UPh, 4, (C5Me5)2(hpp)UI, 6, and (C5Me5)(η5:η1-C5Me4CH2)(hpp)U, 7 bond distance/angle

3

4

6

7

U(1)-(C5Me5 ring centroid) U(1)-C(C5Me5) av U(1)-N(1) U(1)-N(2) U(1)-C(21) U(1)-X N(1)-C(21) N(2)-C(21) N(3)-C(21) Cnt1-U(1)-Cnt2 N(1)-C(21)-N(2) N(1)-U(1)-N(2) U(1)-N(1)-C(21) U(1)-N(2)-C(21) N(1)-U(1)-X

2.474, 2.483 2.75(2) 2.356(5) 2.418(5) 2.845(6) 2.434(7)a 1.345(9) 1.327(9) 1.351(8) 136.5 113.2(6) 55.67(19) 96.7(4) 94.4(4) 76.9(2)

2.524, 2.529 2.80(3) 2.385(2) 2.404(2) 2.860(2) 2.513(2)b 1.344(2) 1.340(2) 1.359(2) 129.8 112.82(16) 55.67(5) 96.09(11) 95.34(11) 76.63(6)

2.502, 2.508 2.78(2) 2.347(7) 2.408(6) 2.833(9) 3.1177(7)c 1.341(11) 1.350(11) 1.368(11) 137.9 113.2(7) 56.4(2) 96.5(5) 93.5(5) 79.7

2.398, 2.475 2.72(1) 2.383(3) 2.385(3) 2.842(3) 2.567(3)d 1.349(4) 1.343(4) 1.358(4) 138.7 113.2(3) 56.25(9) 95.16(18) 95.22(19) 130.78(10)

a

X = C(28). b X = C(28). c X = I(1). d X = C(6).

Table 3. Selected Bond Distances (A˚) and Angles (deg) in (C5Me5)2(hpp)U(CtCPh), 3, (C5Me5)2U(CtCPh)[(iPr)NC(CtCPh)N(iPr)κ2N,N0 ], 8, (C5Me5)2(hpp)UI, 6, and (C5Me5)2[iPrNC(Me)NiPr-κ2N,N0 ]UI, 9 bond distance/angle

3

8

6

9

U(1)-(C5Me5 ring centroid) U(1)-N(1) U(1)-N(2) U(1)-C(21) U(1)-X Cnt1-U(1)-Cnt2 N(1)-U(1)-N(2)

2.474, 2.483 2.356(5) 2.418(5) 2.845(6) 2.434(7)a 136.5 55.67(19)

2.484, 2.502 2.482(1) 2.422(1) 2.876(1) 2.433(1)b 134.4 54.82(5)

2.502, 2.508 2.347(7) 2.408(6) 2.833(9) 3.1177(7)c 137.9 56.4(2)

2.498, 2.500 2.371(4) 2.480(3) 2.880(5) 3.1118(4)d 133.0 54.45(14)

a

X = C(28). b X = C(36). c X = I(1). d X = I(1).

distance in (C5Me5)2Sm(Ph)(THF)43 (nine-coordinate U4þ and an eight-coordinate Sm3þ differ by 0.029 A˚42). In the tuck-in complex 7, the methylene carbon, C(6), in Figure 4 is 0.785 A˚ out of the ring plane and is oriented toward the uranium center. The 2.567(3) A˚ U(1)-C(6) bond distance in 7 is very close to the corresponding 2.564(1) A˚ bond distance in [(C5Me5)U{μ-η5:η1:η1-C5Me3(CH2)2}(μ-H)2U(C5Me5)2],23 shown in eq 3. Comparisons with the thorium tuck-in complex in eq 4 are not possible since high-quality X-ray data were not available on that compound. The U(1)-C(6) bond distance is longer than the 2.424(7) and 2.414(7) A˚ U-C(Me) bond distances in (C5Me5)2UMe241 taking into account the 0.05 A˚ difference in ionic radii between eight- and nine-coordinate U4þ metal centers.42 The 3.1177(7) A˚ U(1)-I(1) bond distance in 6 can be compared to the 3.054(1)-3.092(1) A˚ range of corresponding bond distances in (C5Me5)2UI2(NCR)44 (R = Me, tBu), the 3.1118(4) A˚ U-I bond distance in (C5Me5)2[iPrNC(Me)NiPrκ2N,N0 ]UI,35 9, and the 3.2135(4) A˚ analogous bond distance in the U3þ complex (C5Me5)2UI(2,20 -bipyridine).45 The X-ray crystal structures of complexes 3, 6, 8, and 9 allow the direct comparison of analogous guanidinate and amidinate complexes, i.e., (C5Me5)2(hpp)UX [X = (CtCPh)-, 3; I-, 6] and (C5Me5)2(amidinate)UX [X=(CtCPh)-, 8; I1-, 9], respectively. Table 3 compares selected bond distances and angles for these complexes. Overall, the structural parameters are very similar. For example, the U(1)-(C5Me5 ring centroid) bond (43) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. Organometallics 1985, 4, 112. (44) Maynadie, J.; Berthet, J.-C.; Thuery, P.; Ephritikhine, M. Organometallics 2006, 25, 5603. (45) Mehdoui, T.; Berthet, J.-C.; Thuery, P.; Salmon, L.; Riviere, E.; Ephritikhine, M. Chem.;Eur. J. 2005, 11, 6994.

distances in 3, 6, 8, and 9 are equivalent and the centroid(1)U(1)-centroid(2) angles in the guanidinate complexes 3 and 6 are only 2° and 5° larger than those in the corresponding amidinate complexes 8 and 9, respectively. As mentioned above, the U-X bond distances are equivalent for X = (CtCPh)and close for X = I-. The U-N bond distances are either equivalent or less than 0.05 A˚ longer for the amidinate complexes, which is consistent with a slightly lower basicity of amidinate versus guanidinate ligands.46 The U-C bond distances are equivalent. The N(1)-U(1)-N(2) angles are less than 1° larger for the guanidinate complexes.

Discussion The combination of the (hpp)- ligand with two (C5Me5)ligands provides a modified metallocene coordination environment for U4þ that contains only one reactive group. In this sense, it is like the amidinate complexes (C5Me5)2[(iPr)NC(R)N(iPr)-κ2N,N0 ]UR (R = Me,28 CtCPh,28 and I35), and comparisons can be made between the {(C5Me5)2(hpp)}3ligand combination described here and {(C5Me5)2(amidinate)}3-. In addition to the structural similarities described above, parallels in reactivity exist. Reduced uranium-alkyl reactivity is observed in the amidinate complexes, and a similar feature is seen here. For example, neither (C5Me5)2U[(iPr)NC(R)N(iPr)]UMe28 nor (C5Me5)2(hpp)UMe, 2, reacts at 25 °C with H2 (1 atm),1 PhSSPh,1 or HCtCPh, reagents that typically react with uranium alkyls such as (C5Me5)2UMe2.2,33,47 The reactivities are not exactly parallel, however, since a phenyl analogue of 4 has not been (46) Bailey, P. J.; Pace, S. Coord. Chem. Rev. 2001, 214, 91. (47) Evans, W. J.; Miller, K. A.; Ziller, J. W.; DiPasquale, A. G.; Heroux, K. J.; Rheingold, A. L. Organometallics 2007, 26, 4287.

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obtained with the cyclopentadienyl amidinate ligand combination and (C5Me5)2U[(iPr)NC(R)N(iPr)]UMe28 does not generate a tuck-in complex analogous to 7 upon heating.28 Consistent with the reduced reactivity of the methyl group in 2 compared to (C5Me5)2UR2 complexes,2 the {(C5Me5)2(hpp)}3- ligand combination allows the isolation of the first crystallographically characterizable uranium phenyl complex. Since the {(C5Me5)2(hpp)}3- ligand combination allows only one other ligand for U4þ, the metalation that occurs between the phenyl groups in the decomposition of (C5Me5)2U(C6H5)2 cannot occur.2 The isolation of the ethyl complex (C5Me5)2(hpp)UEt, 5, is also consistent with the fact that this ligand combination can provide a protective environment for ligands that are often difficult to isolate.2,19,48,49 The isolation of the tuck-in complex (C5Me5)(η5:η1-C5Me4CH2)(hpp)U, 7, may also occur because this coordination environment can stabilize certain types of alkyl linkages. The influence of the coordination environment on enhancing the alkyl C-H bond activation that forms 7 is not clear since 7 is only the second example of a U4þ tuck-in complex. The fact that this C-H bond activation reactivity is more facile with larger alkyl groups, neopentyl, isobutyl > ethyl, methyl, parallels the steric enhancement of R-elimination reactivity observed in the formation of Schrock carbene and carbyne complexes in systems like the prototypical TaCl5/ LiCH2CMe3 reaction.50,51 (48) Paolucci, G.; Rossetto, G.; Zanella, P.; Yunlu, K.; Fischer, R. D. J. Organomet. Chem. 1984, 272, 363. (49) Dormond, A.; Aaliti, A.; Moise, C. J. Org. Chem. 1988, 53, 1034. (50) Schrock, R. R. Chem. Rev. 2002, 102, 145. (51) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 4th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005.

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In contrast to [(C5Me5)U{μ-η5:η1:η1-C5Me3(CH2)2}( μH)2U(C5Me5)2],23 eq 3, which has hydride and tuck-over sites of reactivity in addition to a tuck-in moiety, complex 7 contains only one reactive tuck-in unit. Hence, this (hpp)modification of the metallocene ancillary ligand set provides a less complicated compound for the study of tuck-in reactivity with an isolable complex.

Conclusion Combining the (hpp)- ligand with the traditional bis(pentamethylcyclopentadienyl) ligand set allows the isolation of monoalkyl, -aryl, -alkynyl, and U4þ complexes and a simple U4þ tuck-in complex. In the phenyl case, this leads to enhanced stability compared to (C5Me5)2UPh2.2 In the neopentyl and ethyl cases, this ligand environment may enhance C-H activation reactivity for the formation of tuck-in species.

Acknowledgment. We thank the Chemical Sciences, Geosciences, and Biosciences Division of the Office of Basic Energy Sciences of the Department of Energy for support. This research was facilitated in part by a National Physical Science Consortium Fellowship and by stipend support from Los Alamos National Laboratory (to E.M.). We thank Dr. Michael K. Takase for assistance with X-ray crystallography. Supporting Information Available: X-ray diffraction data, atomic coordinates, thermal parameters, and complete bond distances and angles for complexes 3, 4, 6, and 7. This material is available free of charge via the Internet at http://pubs.acs.org.