Synthesis and Insertion Chemistry of Monoalkyl and Monoaryl

Sep 16, 2009 - Synopsis. [(C5Me5)(C8H8)U]2(C8H8) reduces silver triflate to make a convenient precursor, [(C5Me5)(C8H8)U(μ-OSO2CF3)]2, for the synthe...
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Organometallics 2009, 28, 5802–5808 DOI: 10.1021/om900620m

Synthesis and Insertion Chemistry of Monoalkyl and Monoaryl Uranium(IV) Heteroleptic Metallocene Complexes William J. Evans,*,† Michael K. Takase,† Joseph W. Ziller,† and Arnold L. Rheingold‡ †

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 July 15, 2009

The utility of [(C5Me5)(C8H8)U]2(μ-η3:η3-C8H8), 1, as a precursor to monoalkyl and monoaryl heteroleptic pentamethylcyclopentadienyl cyclooctatetraenyl U4þ metallocene complexes, (C5Me5)(C8H8)UR (R = alkyl, aryl), has been explored. Complex 1 reacts with AgOTf (OTf = OSO2CF3) to form [(C5Me5)(C8H8)U( μ-OTf)]2, 2, which contains readily displaceable triflate ligands. Complex 2 reacts with KN(SiMe3)2, LiCH(SiMe3)2, LiPh, and Li[iPrNC(Me)NiPr] to form (C5Me5)(C8H8)U[N(SiMe3)2], 3, (C5Me5)(C8H8)U[CH(SiMe3)2], 4, (C5Me5)(C8H8)UPh, 5, and (C5Me5)(C8H8)U[iPrNC(Me)NiPr-κ2N,N0 ], respectively. The insertion reactivity of 5 with tert-butyl isocyanide and the carbodiimide iPrNdCdNiPr was investigated, and (C5Me5)(C8H8)U[η2-C(Ph)dNtBu], 6, and (C5Me5)(C8H8)U[iPrNC(Ph)NiPr], 7, were isolated, respectively.

Introduction Recent results in the reductive chemistry of uranium hydrides, [(C5Me5)2UH2]2 and [(C5Me5)2UH]2,1 as shown in eq 1 have provided facile access to [(C5Me5)(C8H8)U]2( μ-η3:η3-C8H8), 1,2,3 an unusual complex that contains both

conventional planar (η8-C8H8)2- ligands as well as a nonplanar bridging ( μ-η3:η3-C8H8)2- moiety. Tetravalent 1 can react as a two-electron reductant to form derivatives of the tetravalent heteroleptic [(C5Me5)(C8H8)U]1þ unit.4-9 This in itself is a rather unusual bis(ring)-metal unit in *Corresponding author. Fax: 949-824-2210. E-mail: [email protected]. (1) Fagan, P. J.; Manriquez, J. M.; Maatta, E. A.; Seyam, A. M.; Marks, T. J. J. Am. Chem. Soc. 1981, 103, 6650–6667. (2) Evans, W. J.; Miller, K. A.; Kozimor, S. A.; Ziller, J. W.; DiPasquale, A. G.; Rheingold, A. L. Organometallics 2007, 26, 3568– 3576. (3) Evans, W. J.; Nyce, G. W.; Ziller, J. W. Angew. Chem., Int. Ed. 2000, 39, 240–242. (4) Evans, W. J.; Takase, M. K.; Ziller, J. W.; DiPasquale, A. G.; Rheingold, A. L. Organometallics 2009, 28, 236–243. (5) Schake, A. R.; Avens, L. R.; Burns, C. J.; Clark, D. L.; Sattelberger, A. P.; Smith, W. H. Organometallics 1993, 12, 1497–1498. (6) Berthet, J.-C.; Le Marechal, J.-F.; Ephritikhine, M. J. Organomet. Chem. 1994, 480, 155–161. (7) Cendrowski-Guillaume, S. M.; Le Gland, G.; Nierlich, M.; Ephritikhine, M. Eur. J. Inorg. Chem. 2003, 1388–1393. (8) Evans, W. J.; Kozimor, S. A.; Ziller, J. W. Polyhedron 2006, 25, 484–492. (9) Berthet, J.-C.; Boisson, C.; Lance, M.; Vigner, J.; Nierlich, M.; Ephritikhine, M. J. Chem. Soc., Dalton Trans. 1995, 3027–3033. (10) Evans, W. J.; Kozimor, S. A. Coord. Chem. Rev. 2006, 250, 911– 935. pubs.acs.org/Organometallics

Published on Web 09/16/2009

organouranium chemistry, where the [(C5Me5)2U]2þ moiety is predominant.1,10,11 This paper describes the use of 1 as a precursor to cyclopentadienyl cyclooctatetraenyl complexes of U4þ such that pyrophoric (C8H8)2- and (C8H6R2)2- salts are not required in the synthesis. Relatively few cyclopentadienyl cyclooctatetraenyl U4þ complexes have been reported in the literature. The precursors to these mixed metallocenes are either the tetravalent complexes (C5Me5)(C8H8)UI,6 (C5Me5)(C8H8)U( μ-Cl)UCl(C5Me5)(C8H8),8 and [(C5Me5)(C8H8)U(THF)2][BPh4]9 or the trivalent species (C5Me5)(C8H8)U(THF),5 (C5Me5)(C8H8)U[OP(NMe2)3],7 (C5Me5)[C8H6(SiiPr3)2]U(THF),12-14 and (C5Me4H)[C8H6(SiiPr3)2]U.14,15 All of these precursors were synthesized from cyclooctatetraenyl salts. To the best of our knowledge, the syntheses of 12,3 and the reaction of U(BH4)4 with C8H8 in refluxing mesitylene to form (C8H8)U(BH4)216 are the only examples in the literature in which neutral C8H8 is used directly to make a [(C8H8)U]2þ complex. Since (C8H8)2- ligands are roughly similar in size to (C5Me5)1- ligands,17,18 the homoleptic dicationic metallocene [(C5Me5)2U]2þ and the heteroleptic monocation [(C5Me5)(C8H8)U]1þ are sterically similar. However, (11) Ephritikhine, M. Dalton Trans. 2006, 2501–2516. (12) Summerscales, O. T.; Cloke, F. G. N.; Hitchcock, P. B.; Green, J. C.; Hazari, N. Science 2006, 311, 829–831. (13) 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. (14) Summerscales, O. T.; Frey, A. S. P.; Cloke, F. G. N.; Hitchcock, P. B. Chem. Commun. 2009, 198–200. (15) Summerscales, O. T.; Cloke, F. G. N.; Hitchcock, P. B.; Green, J. C.; Hazari, N. J. Am. Chem. Soc. 2006, 128, 9602–9603. (16) Baudry, D.; Bulot, E.; Ephritikhine, M.; Nierlich, M.; Lance, M.; Vigner, J. J. Organomet. Chem. 1990, 388, 279–287. (17) Evans, W. J.; Clark, R. D.; Ansari, M. A.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 9555–9563. (18) Evans, W. J.; Johnston, M. A.; Greci, M. A.; Ziller, J. W. Organometallics 1999, 18, 1460–1464. r 2009 American Chemical Society

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the difference in charge means that the homoleptic U4þ metallocene dication [(C5Me5)2U]2þ typically is found in disubstituted complexes, such as (C5Me5)2UX2 and (C5Me5)2UR2, e.g., (C5Me5)2UCl21 and (C5Me5)2UMe2.1 In contrast, the heteroleptic U4þ monocation [(C5Me5)(C8H8)U]1þ can generate monosubstituted (C5Me5)(C8H8)UX and (C5Me5)(C8H8)UR compounds. To our knowledge, the only crystallographically reported examples of the latter type are (C5Me5)(C8H8)U( μ-Cl)UCl(C5Me5)(C8H8),8 (C5Me5)(C8H8)UCl[OCNH(CH2)5],8 (C5Me5)(C8H8)U[N(SiMe3)2],8,19 and [(C5Me5)(C8H8)U(THF)2][BPh4];9 that is, no alkyl- or aryl-uranium compounds of this type have been reported. A single thorium alkyl is known, namely, (C5Me5)(C8H8)Th[CH(SiMe3)2].20 To examine differences in U4þ alkyl and aryl chemistry between the [(C5Me5)2]2- and [(C5Me5)(C8H8)]3- coordination environments, the synthesis of (C5Me5)(C8H8)UR complexes was pursued using 1 as a precursor. This was accomplished using the two-electron reduction chemistry of 14 with AgOTf to generate a triflate complex, [(C5Me5)(C8H8)U( μ-OTf)]2, 2, that reacts with amide, alkyl, and aryl reagents. The phenyl example was of particular interest since the bis(pentamethylcyclopentadienyl) metallocene bis(phenyl) complex (C5Me5)2UPh2 is unstable with respect to benzene elimination.1

Experimental Details The syntheses and manipulations described below were conducted under argon with rigorous exclusion of air and water using glovebox, vacuum line, and Schlenk techniques. All glovebox manipulations were performed in an argon-filled glovebox free of coordinating solvents. Elemental analyses were obtained with a Perkin-Elmer 2400 CHNS elemental analyzer. Infrared spectra were recorded as KBr pellets on a Varian 1000 FT-IR spectrometer. 1H and 13C NMR spectra were recorded with a Bruker DRX 500 MHz spectrometer. 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]2( μ-η3:η3-C8H8), 1, was prepared as previously described.2 AgOTf (Aldrich) was used without further purification. KN(SiMe3)2 (Aldrich) was recrystallized from toluene before use. LiCH(SiMe3)2 was prepared as previously described.21 LiMe and LiPh were obtained as 2 M solutions (Aldrich) in diethyl ether and dibutyl ether, respectively, transferred by cannulation into a Schlenk flask, and isolated as white powders upon removal of solvent under vacuum. tBuNtC and iPrNdCdNiPr were dried over 4 A˚ molecular sieves and degassed by three freeze-pump-thaw cycles. Li[iPrNC(Me)NiPr] was prepared by the reaction of LiMe and iPrNdCdNiPr in hexane followed by removal of the solvent under vacuum.22 [(C5Me5)(C8H8)U( μ-OTf)]2, 2. AgOTf (230 mg, 0.895 mmol) was added as a white powder to a dark brown solution of 1 (475 mg, 0.449 mmol) in benzene (30 mL) and stirred overnight. The solids were removed by centrifugation and the solvent was removed under vacuum, leaving 2 as a light brown solid (504 mg, 90%). In a 1H NMR experiment, free C8H8 was observed at 5.6 ppm in a 1:1 molar ratio with 2. Crystals of 2 3 C7H8 suitable (19) Fagan, P. J.; Manriquez, J. M.; Marks, T. J.; Day, C. S.; Vollmer, S. H.; Day, V. W. Organometallics 1982, 1, 170–180. (20) Gilbert, T. M.; Ryan, R. R.; Sattelberger, A. P. Organometallics 1989, 8, 857–859. (21) Davidson, P. J.; Harris, D. H.; Lappert, M. F. J. Chem. Soc., Dalton Trans. 1976, 2268–2274. (22) Villiers, C.; Thuery, P.; Ephritikhine, M. Eur. J. Inorg. Chem. 2004, 4624–4632.

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for X-ray diffraction were grown at -35 °C from a concentrated toluene solution. 1H NMR (C6D6): δ 11.5 (s, C5Me5, Δν1/2=31 Hz, 30H), -40.7 (s, C8H8, Δν1/2 = 47 Hz, 16H). 13C NMR (C6D6): δ 281 (s, C8H8), -17.2 (s, C5Me5). IR: 2911s, 2865m, 1495m, 1438m, 1380s, 1308s, 1220s, 1186s, 1024s, 733s, 626s cm-1. Anal. Calcd for 2, C38H46F6O6S2U2: C, 36.43; H, 3.70. Found: C, 37.10; H, 3.45. (C5Me5)(C8H8)U[N(SiMe3)2], 3. KN(SiMe3)2 (10 mg, 0.14 mmol) was added to a brown solution of 2 (10 mg, 0.0094 mmol) in C6D6 (1 mL). 1H NMR spectroscopy showed quantitative conversion of starting material to the previously characterized 3.8,19 (C5Me5)(C8H8)U[CH(SiMe3)2], 4. A colorless solution of LiCH(SiMe3)2 (27 mg, 0.16 mmol) in toluene (2 mL) was added to a brown solution of 2 (100 mg, 0.16 mmol) in toluene (3 mL) and stirred overnight. The solids were removed by centrifugation and the solvent was removed under vacuum, leaving 4 as an orange solid (96 mg, 94%). Crystals of 4 suitable for X-ray diffraction were grown at -35 °C from a concentrated toluene solution. 1H NMR (C6D6): δ 3.1 (s, C5Me5, Δν1/2 = 5 Hz, 15H), -12.2 (s, CH(SiMe3)2, Δν1/2 = 46 Hz, 18H), -35.3 (s, C8H8, Δν1/2 = 8 Hz, 8H). 13C NMR (C6D6): δ 239 (s, C8H8), -4 (s, CH(SiMe3)2), -30 (s, C5Me5). The methine 1H NMR resonances could not be located with certainty. IR: 3027m, 2905s, 2123m, 1736w, 1435m, 1381m, 1020m, 900m, 710s, 616s, 567s cm-1. Anal. Calcd for C25H42Si2U: C, 47.15; H, 6.65. Found: C, 46.85; H, 6.48. (C5Me5)(C8H8)UPh, 5. LiPh (27 mg, 0.32 mmol) was added as a white powder to a brown solution of 2 (200 mg, 0.32 mmol) in toluene (8 mL) and stirred overnight. The solids were removed by centrifugation and the solvent was removed under vacuum, leaving 5 as an orange solid (142 mg, 80%). 1H NMR (C6D6): δ 17.5 (t, Ph, Δν1/2 =18 Hz, 1H), 14.7 (d, Ph, Δν1/2 =17 Hz, 1H), 1.2 (s, Ph, Δν1/2 = 15 Hz, 2H), -0.6 (s, C5Me5, Δν1/2 = 6 Hz, 15H), -35.7 (s, C8H8, Δν1/2 = 10 Hz, 8H). 13C NMR (C6D6): δ 238 (s, C8H8), 233 (s, Ph), 93 (s, Ph), 80 (s, Ph), -36 (s, C5Me5). IR: 3039m, 3025m, 2902s, 2855s, 1492w, 1432m, 1376m, 1222s, 1025s, 901s, 716s, 700s, 616w cm-1. Anal. Calcd for C24H28U: C, 51.98; H, 5.09. Found: C, 52.37; H, 5.13. (C5Me5)(C8H8)U[η2-C(Ph)dNtBu], 6. A colorless solution of t BuNtC (27 μL, 0.23 mmol) was added to an orange solution of 5 (130 mg, 0.23 mmol) in toluene (6 mL) and stirred overnight. The solvent was removed under vacuum, leaving 6 as a red solid (149 mg, 99%). Crystals of 6 suitable for X-ray diffraction were grown at -35 °C from a concentrated toluene solution. 1H NMR (C6D6): δ 24.1 (d, Ph, Δν1/2= 15 Hz, 2H), 14.3 (t, Ph, Δν1/2 = 16 Hz, 2H), 11.2 (s, tBu, Δν1/2 = 9 Hz, 45H), 9.5 (t, Ph, Δν1/2 = 16 Hz, 1H), -8.0 (s, C5Me5, Δν1/2 = 9 Hz, 15H), -35.2 (s, C8H8, Δν1/2 = 26 Hz, 8H). 13C NMR (C6D6): δ 207 (s, C8H8), 148 (s, Ph), 141 (s, Ph), 130 (s, Ph), 45 (s, tBu), -54 (s, C5Me5). IR: 3044m, 3024m, 2970s, 2903s, 2857s, 1594s, 1557s, 1435s, 1360s, 1221w, 1191s, 1024m, 909m, 898m, 760s, 740s, 714s, 569s, 462w, 432w cm-1. Anal. Calcd for C29H37NU: C, 54.62; H, 5.85; N, 2.20. Found: C, 53.87; H, 5.51; N, 1.99. (C5Me5)(C8H8)U[iPrNC(Ph)NiPr], 7. A colorless solution of i PrNdCdNiPr (41 μL, 0.26 mmol) in toluene (2 mL) was added to an orange solution of 5 (130 mg, 0.26 mmol) in toluene (8 mL) and stirred overnight. The solvent was removed under vacuum, leaving 7 as a red solid (135 mg, 80%). 1H NMR (C6D6): δ 25.1 (d, Ph, Δν1/2 = 13 Hz, 1H), 12.1 (t, Ph, Δν1/2 = 15 Hz, 1H), 8.8 (t, Ph, Δν1/2 = 15 Hz, 1H), 7.5 (t, Ph, Δν1/2 = 14 Hz, 1H), 6.6 (s, CHMe2, Δν1/2 =24 Hz, 2H), 4.2 (s, Ph, Δν1/2 =12 Hz, 1H), 3.5 (s, C5Me5, Δν1/2 = 4 Hz, 15H), -11.8 (s, CHMe2, Δν1/2 =12 Hz, 6H), -13.7 (d, CHMe2, Δν1/2=13 Hz, 6H), -30.5 (s, C8H8, Δν1/2 = 10 Hz, 8H). 13C NMR (C6D6): δ 217 (s, C8H8), 132 (s, Ph), 129 (s, Ph), 127 (s, Ph), 117 (s, Ph), 116 (s, Ph), 43 (s, CHMe2), -1.7 (s, CHMe2), -4.7 (s, CHMe2), -27 (s, C5Me5). IR: 3037w, 2959s, 2923s, 1631w, 1600w, 1419s, 1379m, 1360m, 1323s, 1205m, 1161w, 1134w, 1009m, 903w, 777m, 717s cm-1. Anal. Calcd for C31H42N2U: C, 54.70; H, 6.22; N, 4.12. Found: C, 54.07; H, 6.39; N, 3.80.

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Figure 1. Molecular structure of [(C5Me5)(C8H8)U(μ-OTf)]2 3 (C7H8), 2 3 C7H8, with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms and toluene solvent molecule have been omitted for clarity. (C5Me5)(C8H8)U[iPrNC(Me)NiPr-K2N,N0 ], 8. A colorless solution of Li[iPrNC(Me)NiPr] (24 mg, 0.16 mmol) in toluene (2 mL) was added to a brown solution of 2 (100 mg, 0.16 mmol) in toluene (3 mL) and stirred overnight. The solids were removed by centrifugation and the solvent was removed under vacuum, leaving 8 as a light orange solid (89 mg, 90%). Crystals of 8 suitable for X-ray diffraction were grown at -35 °C from a concentrated toluene solution. 1H NMR 500 MHz (C6D6): δ 16.1 (t, CHMe2, Δν1/2 = 21 Hz, 2H), 12.9 (s, Me, Δν1/2 = 8 Hz, 3H), 2.1 (s, C5Me5, Δν1/2 =3 Hz, 15H), -7.1 (d, CHMe2, Δν1/2 = 11 Hz, 6H), -23.3 (d, CHMe2, Δν1/2 = 11 Hz, 6H), -30.3 (s, C8H8, Δν1/2 = 5 Hz, 8H). 13C NMR (C6D6): δ 210 (s, C8H8), 53 (s, CHMe2), 29 (s, Me), 4 (s, CHMe2), -29 (s, C5Me5), -14 (s, CHMe2). IR: 3037w, 2973s, 2952s, 2904m, 2858s, 1464s, 1419m, 1375s, 1326s, 1307m, 1200s, 1169w, 1114w, 1024w, 904w, 813m, 783w, 757w, 718s cm-1. Anal. Calcd for C26H40N2U: C, 50.48; H, 6.52; N, 4.53. Found: C, 49.63; H, 6.42; N, 4.18.

Evans et al.

has a single bridging chloride, presumably because a structure with two bridging chlorides would bring the four rings too close together. In contrast, the longer three-atom bridge of the ( μ-OTf)1- ligands in 2 allows a symmetrically bridged complex to form. The 2.452 A˚ U-(C5Me5) and 1.957 A˚ U-(C8H8) centroid distances and the 138.1° (C5Me5)-U-(C8H8) centroid angle, Table 2, are similar to those in other [(C5Me5)(C8H8)U]1þ complexes as shown in Table 3. The 2.499(3) and 2.518(3) A˚ bridging U-O bond distances in 2 are similar to the 2.496(5)-2.543(5) A˚ range of U-O( μ-OTf) distances found in the syn- and anti-isomers of the [(C5Me5)2UMe( μ-OTf)]2 metallocene dimers that have similar bridging triflate ligands.23 (C5Me5)(C8H8)U[N(SiMe3)2], 3. To test the ease of displacement of the (OTf)1- ligands in 2, the reaction with KN(SiMe3)2 was examined, eq 3. This reagent was chosen since

the anticipated product, (C5Me5)(C8H8)U[N(SiMe3)2], 3, has been previously characterized.8,19 A 1H NMR experiment involving 1 equiv of dimeric 2 with 2 equiv of KN(SiMe3)2 showed quantitative conversion of 2 to 3. (C5Me5)(C8H8)U[CH(SiMe3)2], 4. The first alkyl ligand examined was [CH(SiMe3)2]1- because of its similar size to [N(SiMe3)2]1-.24-29 As shown in eq 4, the reaction of 1 equiv of 2 with 2 equiv of LiCH(SiMe3)2 forms the desired monoalkyl

Results [(C5Me5)(C8H8)U( μ-OTf)]2, 2. [(C5Me5)(C8H8)U]2( μ-η3: η -C8H8), 1, readily reduces 2 equiv of AgOTf to form 2 as 3

shown in eq 2. A metallic solid was isolated as a byproduct by centrifugation, and in an NMR-scale experiment, 1 equiv of C8H8 was observed by 1H NMR spectroscopy. Complex 2 was characterized by 1H and 13C NMR spectroscopy, IR spectroscopy, elemental analysis, and definitively identified by X-ray crystallography, Figure 1. The 11.5 and -40.7 ppm 1H NMR resonances observed for 2 are typical for U4þ complexes of (C5Me5)1- and (C8H8)2- ligands, respectively.2-9 X-ray data collection parameters for 2 3 C7H8 are listed in Table 1. The structure of 2 has two formally 10-coordinate uranium centers with a trans arrangement of (C8H8)2- and (C5Me5)1- rings. The only other structurally characterized bridged bimetallic complex containing [(C5Me5)(C8H8)U]1þ is the asymmetric uranium chloride complex (C5Me5)(C8H8)U( μ-Cl)UCl(C5Me5)(C8H8).8 The chloride complex

compound 4, which was characterized by spectroscopic and analytical methods and was definitively identified by X-ray crystallography, Figure 2. In addition to resonances at 3.1 and -35.3 ppm assignable to (C5Me5)1- and (C8H8)2-, respectively, the 1H NMR spectrum of 4 contains a resonance at -12.2 ppm assignable to the methyl protons on [CH(SiMe3)2]1-. No resonance definitely assignable to the methine proton was found. Complex 4 crystallizes in the orthorhombic space group Pnma, whereas (C5Me5)(C8H8)U[N(SiMe3)2], 3,8 and (C5Me5)(C8H8)Th[CH(SiMe3)2], 9,20 crystallize in the monoclinic (23) Kiplinger, J. L.; John, K. D.; Morris, D. E.; Scott, B. L.; Burns, C. J. Organometallics 2002, 21, 4306–4308. (24) Den Haan, K. H.; De Boer, J. L.; Teuben, J. H.; Spek, A. L.; Kojic-Prodic, B.; Hays, G. R.; Huis, R. Organometallics 1986, 5, 1726– 1733. (25) Avent, A. G.; Caro, C. F.; Hitchcock, P. B.; Lappert, M. F.; Li, Z.; Wei, X.-H. Dalton Trans. 2004, 1567–1577. (26) Rees, W. S., Jr.; Just, O.; Van Derveer, D. S. J. Mater. Chem. 1999, 9, 249–252. (27) Hitchcock, P. B.; Lappert, M. F.; Smith, R. G.; Bartlett, R. A.; Power, P. P. J. Chem. Soc., Chem. Commun. 1988, 1007–1009. (28) Brady, E. D.; Clark, D. L.; Gordon, J. C.; Hay, P. J.; Keogh, D. W.; Poli, R.; Scott, B. L.; Watkin, J. G. Inorg. Chem. 2003, 42, 6682– 6690. (29) Sundermeyer, J.; Khvorost, A.; Harms, K. Acta Crystallogr., Sect. E 2004, E60, m1117–m1119.

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Table 1. X-ray Data Collection Parameters for [(C5Me5)(C8H8)U(μ-OTf)]2 3 (C7H8), 2 3 C7H8, (C5Me5)(C8H8)U[CH(SiMe3)2], 4, (C5Me5)(C8H8)U[η2-C(Ph)dNtBu], 6, and (C5Me5)(C8H8)U[iPrNC(Me)NiPr-κ2N,N0 ], 8 2 3 C7H8

4

C38H46F6O6S2U2 3 C7H8 C25H42Si2U 1437.20 636.80 203(2) 153(2) monoclinic orthorhombic Pnma P21/c 15.109(3) 18.6588(18) 17.305(4) 14.6630(14) 9.825(2) 9.2540(9) 90 90 96.590(3) 90 90 90 2552.0(10) 2531.8(4) 2 4 1.870 1.671 6.489 6.514 0.0303 0.0143 0.0799 0.0352 P P P a 2 2 2 P 2 2 1/2 Definitions: wR2 = [ [w(Fo - Fc ) ]/ [w(Fo ) ]] , R1 = ||Fo| - |Fc||/ |Fo|.

empirical formula fw temperature (K) cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) volume (A˚3) Z Fcalcd (mg/m3) μ (mm-1) R1 [I > 2.0σ(I )]a wR2 (all data)a

6

8

C29H37NU 637.63 103(2) orthorhombic P212121 13.3518(15) 13.5779(15) 13.6379(15) 90 90 90 2472.4(5) 4 1.713 6.581 0.0306 0.0844

C26H40N2U 618.63 163(2) monoclinic P21/n 9.0390(6) 16.8194(11) 16.1651(10) 90 97.4160(10) 90 2437.0(3) 4 1.686 6.674 0.0189 0.0468

Table 2. Selected Bond Distances (A˚) and Angles (deg) for [(C5Me5)(C8H8)U(μ-OTf)]2 3 (C7H8), 2 3 C7H8, (C5Me5)(C8H8)U[CH(SiMe3)2], 4, (C5Me5)(C8H8)U[η2-C(Ph)dNtBu], 6, and (C5Me5)(C8H8)U[iPrNC(Me)NiPr-κ2N,N0 ], 8

U(1)-Cnt1(C5Me5)a U(1)-Cnt2(C8H8)a U(1)-C(C5Me5) range U(1)-C(C8H8) range U(1)-C(C5Me5) avg U(1)-C(C8H8) avg U(1)-E(1) U(1)-E(2) Cnt1-U(1)-Cnt2a Cnt1-U(1)-E(1)a Cnt2-U(1)-E(1)a Cnt1-U(1)-E(2)a Cnt2-U(1)-E(2)a a

2 3 C7H8 E(1) = O(1) E(2) = O(30 )

4 E(1) = C(19)

6 E(1) = C(19) E(2) = N(1)

8 E(1) = N(1) E(2) = N(2)

2.452 1.957 2.699(4)-2.773(5) 2.660(5)-2.703(5) 2.73(3) 2.68(2) 2.499(3) 2.518(3) 138.1 97.7 115.5 95.3 117.8

2.499 1.969 2.763(2)-2.783(2) 2.643(2)-2.754(3) 2.775(9) 2.81(33) 2.469(3)

2.489 1.985 2.724(6)-2.805(6) 2.676(6)-2.722(6) 2.77(3) 2.71(2) 2.390(4) 2.423(6) 137.9 101.1 119.8 101.1 118.9

2.516 2.008 2.785(2)-2.802(2) 2.693(2)-2.750(2) 2.791(6) 2.72(2) 2.417(2) 2.420(2) 136.4 100.3 118.4 100.1 117.9

138.2 100.7 121.0

Cnt1 is C(1)-C(5); Cnt2 is C(11)-C(18)

Table 3. Selected Bond Distances (A˚) and Angles (deg) in [(C5Me5)(C8H8)]3- Complexes of U4þ in Order of Increasing U-(C5Me5) Ring Centroid Distance complex (C5Me5)(C8H8)U(μ-Cl)UCl(C5Me5)(C8H8)8 [(C5Me5)(C8H8)U(μ-OTf)]2 3 (C7H8), 2 3 C7H8 [(C5Me5)(C8H8)U(THF)2][BPh4]9 (C5Me5)(C8H8)U[η2-C(Ph)dNtBu], 6 (C5Me5)(C8H8)U[CH(SiMe3)2], 4 (C5Me5)(C8H8)UCl[OCNH(CH2)5]8 (C5Me5)(C8H8)U[N(SiMe3)2]8 (C5Me5)(C8H8)U[iPrNC(Me)NiPr], 8 [(C5Me5)(C8H8)U]2(C8H8)3

U-(C5Me5 ring centroid)

U-(C8H8 ring centroid)

(C5Me5 ring centroid)-U(C8H8 ring centroid)

2.423 2.491 2.452 2.479 2.489 2.499 2.512 2.515 2.516 2.522

1.894 1.980 1.957 1.963 1.985 1.969 1.997 1.989 2.008 1.983

138.0 140.7 138.1 139.7 137.9 138.2 138.2 134.7 136.4 134.5

space groups P21/c and P21/n, respectively. The structure of 4 can be compared to that of 9 and the (C5Me5)2Ln[CH(SiMe3)2] complexes where Ln = Ce,30 Nd,31,32 and Y.24 The overall metallocene alkyl structures of these complexes are (30) Heeres, H. J.; Renkema, J.; Booij, M.; Meetsma, A.; Teuben, J. H. Organometallics 1988, 7, 2495–2502. (31) Mauermann, H.; Swepston, P. N.; Marks, T. J. Organometallics 1985, 4, 200–202. (32) Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 8091–8103.

similar, but details of the specific angles between ligands differ between the actinides and the lanthanides. The 138.2° (C5Me5)-U-(C8H8) centroid angle in 4 is similar to the 137.8° analogue in the thorium complex, and these values are slightly larger than the 135.1°, 134.6°, and 134.4° angles for the Ce, Nd, and Y examples, respectively. Since 4 has a mirror plane that contains the U-C(alkyl) bond, the UC(12)-Si(1) and U-C(12)-Si(10 ) angles are crystallographically equivalent at 117.13(8)°. This is similar to the structure of 9, which has 115.6(8)° and 116.9(5)° angles. In contrast,

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1.2 ppm assignable to a phenyl group. Unfortunately, single crystals suitable for X-ray diffraction were not obtained.

Figure 2. Molecular structure of (C5Me5)(C8H8)U[CH(SiMe3)2], 4, with thermal ellipsoids drawn at the 50% probability level. Cyclooctatetraenyl and methyl hydrogen atoms have been omitted for clarity.

the lanthanide metallocenes have disparate Ln-C-Si angles that allow agostic interactions between one of the silyl methyl groups and the metal. For example, in (C5Me5)2Ce[CH(SiMe3)2] the angles are 99.1(2)° and 139.2(2)°.30 Hence, in the actinides, no silyl methyl agostic interactions are evident. Complexes 4 and 9 also differ from the lanthanide complexes in that the methine carbon atoms of the [CH(SiMe3)2]1ligands are 0.331 A˚ (4) and 0.332 A˚ (9) out of the plane of the metal and two silicon atoms, whereas for the lanthanide compounds, the Ln, C, Si, Si atom set is coplanar to within 0.031 A˚ for Ce30 and 0.034 A˚ for Nd.31,32 The structure of the Y complex is intermediate between those of the An and Ln complexes in this regard with the methine carbon 0.248 A˚ out of the Y, Si, Si plane.24 The 2.469(3) A˚ U-C[CH(SiMe3)2] bond distance in 4 is similar to the 2.54(1) A˚ bond distance in (C5Me5)(C8H8)Th[CH(SiMe3)2]20 when the difference in ionic radius of ninecoordinate U4þ (1.05 A˚) vs Th4þ (1.09 A˚) is considered.33 On the basis of the seven-coordinate ionic radii for the Ln3þ ions (Ce, 1.07 A˚; Nd, 1.05 A˚; Y, 0.96 A˚),33 the Ln-C bond for Nd would be expected to best match that of 4. However, the (C5Me5)2Ln[CH(SiMe3)2] distances (Ln = Ce, 2.535(5) A˚;30 Nd, 2.517(7) A˚;31,32 Y, 2.468(7) A˚24) show that the U-C bond distance in 4 is shorter than that in the neodymium complex and closer to that in the yttrium compound. The U-C[CH(SiMe3)2] bond distance is longer than the 2.284(4) A˚ U-N[N(SiMe3)2] bond distance in 3, as is typical for [CH(SiMe3)2]1vs [N(SiMe3)2]1- complexes. For example, the Y-C distance is 2.468(7) A˚ in (C5Me5)2Y[CH(SiMe3)2] vs the 2.274(5) and 2.253(5) A˚ Y-N distances in the two crystallographically independent molecules in the unit cell of (C5Me5)2Y[N(SiMe3)2].24 (C5Me5)(C8H8)UPh, 5. The reaction of 2 with 2 equiv of LiPh forms a product that is formulated as (C5Me5)(C8H8)UPh, 5, eq 5, based on 1H and 13C NMR spectroscopy, IR spectroscopy, and elemental analysis. The 1H NMR spectrum of 5 has resonances at -0.6 and -35.7 ppm, in the range expected for (C5Me5)1- and (C8H8)2- rings attached to U4þ, along with resonances at 17.5, 14.7, and (33) Shannon, R. D. Acta Crystallogr., Sect. A 1976, A32, 751–767.

(C5Me5)(C8H8)U[η2-C(Ph)dNtBu], 6. To obtain structural data related to 5, insertion reactions were examined to determine if crystallographically characterizable derivatives could be generated. Complex 5 reacts with tBuNtC by insertion into the U-C(Ph) bond to form (C5Me5)(C8H8)U[η2-C(Ph)dNtBu], 6, eq 6. Complex 6 was characterized spectroscopically and analytically and was definitively identified by X-ray crystallography, Figure 3.

As shown in Table 3, the U-(C5Me5 ring centroid) and U-(C8H8 ring centroid) distances and the (C5Me5 centroid)-U-(C8H8 centroid) angle in 6 are similar to those in previously reported U4þ complexes.3,8,9 The metrical parameters of the tert-butylphenyliminoacyl ligand formed in the insertion reaction in eq 6 are very similar to those in (C5Me5)2Y[C(CH2C6H3Me2-3,5)dN(C6H3Me2-2,6)], 10, formed analogously by insertion of (C6H3Me2-2,6)NtC into the Y-C σ-bond in (C5Me5)2Y(CH2C6H3Me2-3,5).34 The 1.276(7) A˚ C(19)-N(1) bond in 6 is similar to the 1.299(4) A˚ analogue in 10, and both values are similar to the 1.279(8) A˚ bond distance for a Caryl-CdN-Csp3 carbon nitrogen double bond.35 The 2.390(4) A˚ U-N(1) and 2.423(6) A˚ U-C(19) distances in 6 are similar to each other and close to the 2.407(3) A˚ Y-N and 2.392(3) A˚ Y-C analogues in 10. Hence, complex 6 has bond distances that match an yttrium metallocene like those of 4 and (C5Me5)2Y[CH(SiMe3)2]. Since M-N bonds are generally shorter than analogous M-C bonds (see above), the near equivalence of the U-N and U-C bond in 6 suggests that the iminoacyl ligand can be viewed as a σ-bonded carbon ligand with an adjacent nitrogen to metal dative bond. A similar conclusion was made about 10 and the (C5H5)3U[C(Me)dNCy] (Cy = C6H11) product of the reaction of CyNtC with (C5H5)3UMe, which has a 1.25(2) A˚ C-N, 2.40(2) A˚ U-N, and 2.36(2) A˚ U-C bond distance.36 (C5Me5)(C8H8)U[iPrNC(Ph)NiPr], 7. Since the insertion of the carbodiimide, iPrNdCdNiPr, into uranium-carbon bonds in (C5Me5)2UMe2 has been recently reported to form crystallographically characterizable complexes,37 the reaction (34) Den Haan, K. H.; Luinstra, G. A.; Meetsma, A.; Teuben, J. H. Organometallics 1987, 6, 1509–1515. (35) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1–S19. (36) Zanella, P.; Paolucci, G.; Rossetto, G.; Benetollo, F.; Polo, A.; Fischer, R. D.; Bombieri, G. J. Chem. Soc., Chem. Commun. 1985, 96– 98. (37) Evans, W. J.; Walensky, J. R.; Ziller, J. W.; Rheingold, A. L. Organometallics 2009, 28, 3350–3357.

Article

Figure 3. Molecular structure of (C5Me5)(C8H8)U[η2-C(Ph)d NtBu], 6, with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity.

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Figure 4. Molecular structure of (C5Me5)(C8H8)U[iPrNC(Me)NiPr-κ2N,N0 ], 8, with thermal ellipsoids drawn at the 50% probability level. Hydrogen atoms have been omitted for clarity.

of 5 with this substrate was also examined. Although this reaction generated 7 in high yield with spectroscopic and analytical properties consistent with (C5Me5)(C8H8)U[iPrNC(Ph)NiPr], eq 7, crystals suitable for X-ray diffraction were not obtained. The 1H NMR spectrum of 7 had resonances

at 3.5 and -30.5 ppm consistent with (C5Me5)1- and (C8H8)2- ligands, respectively, as well as resonances expected for the amidinate ligand. (C5Me5)(C8H8)U[iPrNC(Me)NiPr-K2N,N0 ], 8. In order to obtain crystallographic data on an amidinate complex with the [(C5Me5)(C8H8)U]1þ moiety, the synthesis of the analogue of 7 with the phenyl group in the amidinate ligand substituted by methyl was pursued. Again, the displacement reactivity of 2 was used. As shown in eq 8, 2 reacts with 2

equiv of LiiPrNC(Me)NiPr to form (C5Me5)(C8H8)U[iPrNC(Me)NiPr-κ2N,N0 ], 8, which could be characterized by X-ray crystallography, Figure 4. In this case, substitution of Me for Ph in the amidinate backbone made a substantial difference in the formation of single crystals suitable for X-ray diffraction. Complex 8 had a 1H NMR spectrum very similar to that of 7 except the resonances assigned to phenyl groups were absent and a resonance assignable to a methyl group was present at 12.9 ppm. The structure of the [(C5Me5)(C8H8)U]1þ unit of 8 is similar to those of previously characterized [(C5Me5)(C8H8)U]1þ complexes, Table 3. The U-N(amidinate) distances of 2.416(2) and 2.420(2) A˚ in 8 are similar to those in the neutral U4þ amidinate complex (C5Me5)2U[iPrNC(Me)NiPr-κ2N, N0 ]Me (2.453(2) and 2.461(2) A˚),37 but they are longer than

Figure 5. Ball and stick overlay of (C5Me5)(C8H8)U[iPrNC(Me)NiPr-κ2N,N0 ], 8 (solid bonds and atoms), and {(C5Me5)2U[iPrNC(Me)NiPr-κ2N,N0 ]}{BMePh3} (dashed bonds and atoms) with the [(C5Me5)U]3þ unit aligned. The [BMePh3]1and hydrogens have been omitted for clarity.

those in the cationic complex {(C5Me5)2U[iPrNC(Me)NiPrκ2N,N0 ]}{BMePh3} (2.350(1) and 2.345(1) A˚).38 The C(25)N(1) and C(25)-N(2) bond distances are equivalent at 1.334(3) and 1.332(3) A˚ and the 2.865 A˚ U-C(25) distance is too long for a significant interaction. Similar features are found in the [(C5Me5)2U]2þ amidinates.37 The X-ray crystal structures of 8 and the {(C5Me5)2U[iPrNC(Me)NiPr-κ2N,N0 ]}1þ cation provide an opportunity to directly examine the similarities of the [(C5Me5)(C8H8)]3and [(C5Me5)2]2- ligand sets. Figure 5 shows an overlay of the two structures with the [(C5Me5)U]3þ components aligned.

Discussion This study provides a general approach for the synthesis of heteroleptic pentamethylcyclopentadienyl cyclooctatetraenyl uranium metallocenes without the need to use highly reactive (38) Evans, W. J.; Walensky, J. R.; Ziller, J. W. Chem. Eur. J., in press.

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(C8H8)2- salts. The reaction of [(C5Me5)(C8H8)U]2( μ-η3:η3C8H8), 1, with AgOTf, eq 2, provides a convenient reagent, [(C5Me5)(C8H8)U( μ-OTf)]2, 2, that can be converted to other heteroleptic metallocenes containing the [(C5Me5)(C8H8)U]1þ moiety, eqs 3-5 and 8. Complex 2 reacts readily with alkali metal amide and amidinate salts such as K[N(SiMe3)2] and LiiPrNC(Me)NiPr to form [(C5Me5)(C8H8)U]1þ complexes 3 and 8. The reaction of 2 with LiCH(SiMe3)2 and LiPh shows that monoalkyl and monoaryl uranium complexes of the type (C5Me5)(C8H8)UR can also be accessed in this way. The insertion reactivity observed between (C5Me5)(C8H8)UPh, 5, and tBuNtC and iPrNdCdNiPr, eqs 6 and 7, respectively, shows that the [(C5Me5)(C8H8)]3- ancillary ligand set is not too sterically bulky to prevent subsequent reaction chemistry with the single U-C bond that it supports. In the case of the monophenyl complex, 5, this means that the chemistry of the U4þ-phenyl linkage can be investigated in a metallocene environment without the complicating instability observed with (C5Me5)2UPh2 due to benzene elimination.1 The synthesis of the amidinate complexes 7 and 8 by insertion and substitution, respectively, is also consistent with the view that the [(C5Me5)(C8H8)]3- ligand environment is not sterically restrictive. Figure 5 shows directly the similarities of this ligand set to the [(C5Me5)2]2- analogue that supports a wide range of chemistry.

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Conclusion [(C5Me5)(C8H8)U]2( μ-η3:η3-C8H8), 1, is a good cyclooctatetraene-based synthetic precursor for heteroleptic (C5Me5)(C8H8)UX and (C5Me5)(C8H8)UR compounds. Reduction of AgOTf generates [(C5Me5)(C8H8)U( μ-OTf)]2, 2, which is readily converted to (C5Me5)(C8H8)U[N(SiMe3)2], 3, (C5Me5)(C8H8)U[CH(SiMe3)2], 4, (C5Me5)(C8H8)UPh, 5, and (C5Me5)(C8H8)U[iPrNC(Me)NiPr-κ2N,N0 ], 8. These reactions indicate that this approach is applicable to a variety of ligands. The insertion chemistry demonstrated with 5 to form (C5Me5)(C8H8)U[η2-C(Ph)dNtBu], 6, and (C5Me5)(C8H8)U[iPrNC(Ph)NiPr], 7, shows that these (C5Me5)(C8H8)UR complexes can allow investigation of single U-R bonds. Hence, the [(C5Me5)(C8H8)]3- ligand set provides the opportunity to investigate U4þ reaction chemistry with one reactive ligand at a time in a metallocene environment.

Acknowledgment. We thank the National Science Foundation for support of this research. Supporting Information Available: X-ray data collection, structure solution, and refinement (PDF) and X-ray diffraction details of compounds 2, 4, 6, and 8 (CIF, CCDC Nos. 739279739282). This material is available free of charge via the Internet at http://pubs.acs.org.