Octa- and Nonamethylfluorenyl Complexes of ... - ACS Publications

Reaction of mixed-ligand zirconocene dichlorides containing either octa- or nonamethylfluorenyl ligands (Flu′′ = C13Me8H or Flu* = C13Me9) with iB...
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Organometallics 2009, 28, 2285–2293

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Octa- and Nonamethylfluorenyl Complexes of Zirconium(IV): Reactive Hydride Derivatives and Reversible Hydrogen Migration between the Metal and the Fluorenyl Ligand Patrick Bazinet and T. Don Tilley* Department of Chemistry, UniVersity of California at Berkeley, Berkeley, California 94720-1460, and Chemical Sciences DiVision, Ernest Orlando Lawrence Berkeley National Laboratory, One Cyclotron Road, Berkeley, California 94720-1460 ReceiVed January 20, 2009

Reaction of the zirconocene dichloride Cp′′Flu*ZrCl2 (Cp′′ ) 1,3-(SiMe3)2C5H3, Flu* ) C13Me9) with BuLi (iBuLi ) LiCH2CHMe2) resulted in elimination of isobutylene and formation of Cp′′(η5:η3C13Me9H)ZrH (1-syn-1,2-DHF*D), possessing an η5:η3-dihydrofluorenediyl ligand derived from a metalto-benzo ring hydride transfer. This species undergoes reversible hydride transfer and exists in equilibrium with only one of its three other possible isomers (1-syn-3,4-DHF*D). Compound 1-syn-1,2-DHF*D catalyzes the cyclization of 1,5-hexadiene to methylenecyclopentane, and its reaction with excess isobutylene leads to the elimination of isobutane and formation of the cyclometalated zirconocene isobutyl species (η5:η1-C5H3-1-SiMe2CH2-3-SiMe3)(η5-C13Me9)ZriBu (2). Reaction of Cp′′Flu′′ZrCl2 with iBuLi directly generated the cyclometalated zirconocene species (η5:η1-C5H3-1-SiMe2CH2-3-SiMe3)(η5C13Me8H)ZriBu (3); however, reaction of the dichloride Cp′′Flu′′ZrCl2 with iBuLi in the presence of hydrogen generated the dihydrofluorenediyl monohydride derivative Cp′′(η5:η3-C13Me8H2)ZrH (4). Treatment of the cyclometalated isobutyl species 3 with H2 led to partial hydrogenation of the Flu′′ ligand and formation of the monohydride Cp′′(η5:η3-C13Me8H6)ZrH (5), which contains a hexahydrofluorenediyl ligand. Partial hydrogenation of the Flu′′ ligand proceeded exclusively via an intramolecular pathway, as evidenced by the all-exo configuration of the methyl groups on the saturated benzo ring. Structural characterization of 1-syn-1,2-DHF*D, 2, 3, and 5 revealed a highly strained η5:η3-coordination mode for the dihydro- and hexahydrofluorenediyl ligands. i

Introduction The tremendous interest in group 4 metallocenes has largely been focused on the chemistry of high-valent, M(IV) or d0, derivatives and their applications in catalytic transformations such as olefin polymerization.1-3 The principal characteristic of importance for such catalysts is their ability to undergo rapid and sequential olefin insertions. The reverse transformation, β-H elimination, is also an intrinsic step in the polymerization process that leads to chain termination and the formation of metal * Corresponding author. E-mail: [email protected]. (1) (a) For leading reviews on metallocene chemistry, see: The Metallocenes: Synthesis, ReactiVity and Applications; Togni, A., Halterman, R. L., Eds.; Wiley-VCH: Weinheim, 1998. (b) For a series of reviews on “Metallocene Complexes as Catalysts for Olefin Polymerization” see: Alt, H. G.; Licht, E. H.; Licht, A. I.; Schneider, K. J. Coord. Chem. ReV. 2006, 250, 2. (c) Mo¨hring, P. C.; Coville, N. J. Coord. Chem. ReV. 2006, 250, 18. (d) Erker, G.; Kehr, G.; Fro¨hlich, R. Coord. Chem. ReV. 2006, 250, 36. (e) Dong, J.-Y.; Hu, Y. Coord. Chem. ReV. 2006, 250, 47. (f) Janiak, C. Coord. Chem. ReV. 2006, 250, 66. (g) Zhang, J.; Wang, X.; Jin, G.-X. Coord. Chem. ReV. 2006, 250, 95. (h) Kaminsky, W.; Sperber, O.; Werner, R. Coord. Chem. ReV. 2006, 250, 110. (i) Ferreira, M. J.; Martins, A. M. Coord. Chem. ReV. 2006, 250, 118. (j) Prashar, S.; Antin˜olo, A.; Otero, A. Coord. Chem. ReV. 2006, 250, 133. (k) Razavi, A.; Thewalt, U. Coord. Chem. ReV. 2006, 250, 155. (l) Focante, F.; Mercandelli, P.; Sironi, A.; Resconi, L. Coord. Chem. ReV. 2006, 250, 170. (m) Cobzaru, C.; Hild, S.; Boger, A.; Troll, C.; Rieger, B. Coord. Chem. ReV. 2006, 250, 189. (n) Tritto, I.; Boggioni, L.; Ferro, D. R. Coord. Chem. ReV. 2006, 250, 212. (o) Wang, B. Coord. Chem. ReV. 2006, 250, 242. (p) Xie, Z. Coord. Chem. ReV. 2006, 250, 259. (2) (a) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (b) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255. (c) Kaminsky, W. Macromol. Chem. Phys. 1996, 197, 3907.

hydride species and constitutes a possible catalyst deactivation pathway.4 The relative rates of olefin insertion and β-H elimination strongly influence polymer chain length and the resulting physical properties of the polymer. Detailed studies have demonstrated that insertion and β-H elimination are both sensitive to the steric properties of the olefin and the ancillary ligand set of the catalyst.5,6 The rates of both reactions are slowed by an increase in steric encumbrance, though olefin insertion is affected to a greater extent.5 These studies illustrate how the nature and size of the Cp-type ligands can greatly influence the outcome of metallocene-catalyzed polymerizations. Metallocene hydride complexes are also important due to their usefulness as reagents and catalysts for hydrogenations and related reductions.7 Zirconocene dihydride derivatives are typically accessed via the hydrogenolysis of corresponding alkyl (3) Examples of other applications of group 4 metallocenes include: Organic coupling reactions: (a) Negishi, E.; Takahashi, T. Acc. Chem. Res. 1994, 27, 124. (b) Rosenthal, U.; Pellny, P.-M.; Kirchbauer, F. G.; Burlakov, V. V. Acc. Chem. Res. 2000, 33, 119. Small molecule activation: (c) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Nature 2004, 427, 527. (d) MacKay, B. A.; Fryzuk, M. D. Chem. ReV. 2004, 104, 385. (4) Resconi, L.; Camurati, I.; Sudmeijer, O. Top. Catal. 1999, 7, 145. (5) Chirik, P. J.; Bercaw, J. E. Organometallics 2005, 24, 5407. (6) (a) Hajela, S.; Bercaw, J. E. Organometallics 1994, 13, 1147. (b) Alelyunas, Y. W.; Guo, Z.; LaPointe, R. E.; Jordan, R. F. Organometallics 1993, 12, 544. (c) Guo, Z.; Swenson, D. C.; Jordan, R. F. Organometallics 1994, 13, 1424. (7) For a reviews on hydrozirconation see: (a) Schwartz, J.; Labinger, J. A. Angew. Chem., Int. Ed. Engl. 1976, 15, 333. (b) Wipf, P.; Kendall, C. Top. Organomet. Chem. 2004, 8, 1, and references therein. (c) For examples of asymmetric hydrogenation reactions see: Hoveyda, A. H.; Morken, J. P. Angew. Chem., Int. Ed. Engl. 1996, 35, 1262, and references therein.

10.1021/om900047x CCC: $40.75  2009 American Chemical Society Publication on Web 03/11/2009

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complexes,8 but can also be prepared from zirconocene dichlorides by reaction with a boron or aluminum hydride source.9 Zirconocene dihydride species are susceptible to deactivation via dimerization, to form dinuclear species through bridging hydride ligands; however, sterically demanding cyclopentadienyl-based ligands can be used to inhibit dimerization and favor more reactive, monomeric species.8 In rare cases, dihydride species have been observed to undergo reductive elimination to produce molecular dihydrogen and low-valent species.10 However, a far more commonly observed reaction is the elimination of an alkane from zirconocene alkyl hydride species. For example, the widely utilized Negishi’s reagent (a synthon for Cp2Zr) is obtained from reaction of zirconocene dichloride (Cp2ZrCl2, Cp ) C5H5) with 2 equiv of BuLi.3a The initially formed dibutyl intermediate undergoes a series of transformations, including the elimination of butane, to yield a zirconocene-olefin complex that behaves as the source of “zirconocene”.11 The reductive elimination of alkanes from well-defined group 4 metallocene derivatives has recently been studied in detail and has proven to be an efficient method for the preparation of low-valent zirconium species isolated as dinitrogen complexes. For example, Cp*2Zr(H)CH2CMe3 eliminates neopentane to produce the known dinitrogen compound [Cp*2Zr(N2)]2(µ2-N2),12,13 and the related compound Cp′′2Zr(H)iBu (Cp′′ ) 1,3-(SiMe3)2C5H3) eliminates isobutane to generate the bridging, side-on bound dinitrogen species [Cp′′2Zr]2(µ2-η2,η2-N2).14 Examples of early transition metal hydride species containing indenyl (Ind) or fluorenyl (Flu) ligand derivatives, fused-ring analogues of Cp, are very rare.15,16 Correspondingly, very little is known about how Ind- and Flu-based ancillary ligands influence the rates of olefin insertion and β-H elimination in metallocene derivatives. This is particularly noteworthy considering the widespread use of Ind and Flu complexes for the stereospecific polymerization of R-olefins.2a,16-18 Recently, zirconocene hydrido alkyl complexes containing substituted indenyl ligands were shown to eliminate alkane to generate lowvalent sandwich compounds with indenyl ligands that feature (8) Chirik, P. J.; Day, M. W.; Bercaw, J. E. Organometallics 1999, 18, 1873, and references therein. (9) (a) Jones, S. B.; Peterson, J. L. Inorg. Chem. 1981, 20, 2889. (b) Larsonneur, A.; Choukroun, R.; Jaud, J. Organometallics 1993, 12, 3216. (c) Grossman, R. B.; Doyle, R. A.; Buchwald, S. L. Organometallics 1991, 10, 1501. (d) Cuenca, T.; Galakhov, M.; Royo, E.; Royo, P. J. Organomet. Chem. 1996, 515, 33. (10) Bradley, C. A.; Lobkovsky, E.; Keresztes, I.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128, 6454. (11) Dioumaev, V. K.; Harrod, J. F. Organometallics 1997, 16, 1452. (12) Manriquez, J. M.; Bercaw, J. E. J. Am. Chem. Soc. 1974, 96, 6229. (13) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. Organometallics 2003, 22, 2797. (14) Pool, J. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003, 125, 2241. (15) Examples of early transition metal hydrides containing fluorenyl derivatives include: (a) Kirillov, E.; Lehmann, C. W.; Razavi, A.; Carpentier, J.-F. Eur. J. Inorg. Chem. 2004, 943. (b) Kirillov, E.; Lehmann, C. W.; Razavi, A.; Carpentier, J.-F. Organometallics 2004, 23, 2768. (16) For reviews on early transition metal fluorenyl compounds see: (a) Kirillov, E.; Saillard, J.-Y.; Carpentier, J.-F. Coord. Chem. ReV. 2005, 249, 1221. (b) Alt, H. G.; Samuel, E. Chem. Soc. ReV. 1998, 27, 323. (17) For examples of stereospecific polymerization of R-olefins see: (a) Wild, F. R. W. P.; Zsolnai, L.; Huttner, G.; Brintzinger, H. H. J. Organomet. Chem. 1982, 232, 233. (b) Kaminsky, W.; Ku¨lper, K.; Brintzinger, H. H.; Wild, F. R. W. P. Angew. Chem., Int. Ed. 1985, 24, 507. (c) Ewen, J. A.; Jones, R. L.; Razavi, A.; Ferrara, J. D. J. Am. Chem. Soc. 1988, 110, 6255. (d) Razavi, A.; Bellia, V.; De Brauwer, Y.; Hortmann, K.; Peters, L.; Sirole, S.; Van Belle, S.; Thewalt, U. Macromol. Chem. Phys. 2004, 205, 347. (18) (a) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Chem. ReV. 2000, 100, 1253. (b) Coates, G. W. Chem. ReV. 2000, 100, 1223.

Bazinet and Tilley

unique η9-bonding modes.19 These η9,η5-bis(indenyl)zirconium sandwich complexes were subsequently shown to undergo facile oxidative addition of H2 to yield bis(indenyl)zirconium dihydride species (η5-C9H5-1,3-R2)2ZrH2 (R ) SiMe3, SiMe2Ph, iPr).10 These complexes were observed to rearrange to monohydride complexes via a metal-to-benzo ring hydride transfer, resulting in formation of complexes bearing an η3:η5-dihydroindenediyl ligand. This chemistry for indenyl-supported zirconium hydride complexes is particularly significant considering the widespread use of polymerization catalysts containing indenyl and fluorenyl ligand derivatives and the importance of β-H elimination and metal hydride intermediates in the polymerization process. We recently reported the synthesis of mixed-ligand zirconocene derivatives containing either octa- or nonamethylfluorenyl ligands (Flu′′, C13Me8H or Flu*, C13Me9).20,21 The large size and highly electron-donating natures of Flu′′ and Flu* should promote the generation of unsaturated metal centers and highly reactive hydride derivatives. Fluorenyl-supported zirconocene hydride complexes are of interest for investigation of the ligand’s influence on fundamental transformations such as olefin insertion, β-H elimination, and reductive elimination. Additionally, such hydride complexes might offer interesting comparisons to the previously reported indenyl derivatives, with respect to the participation of fluorenyl-based ligands in hydride migration chemistry. Such studies are important considering the well-known use of fluorenyl ligands for R-olefin polymerization catalysts, particularly syndiospecific Cs symmetric catalysts of the general type Me2C(C5H4)(C13H8)MCl2 (M ) Zr, Hf).17c,17d Here we present the synthesis and crystallographic characterizations of Flu′′- and Flu*-containing zirconocene monohydride derivatives. Hydrogen migrations to the fluorenyl ligands are readily observed, to form the corresponding η3,η5-dihydrofluorenediyl ligands. The reversibility of the metal-to-benzo ring hydride transfer provides access to “masked”, highly reactive hydride derivatives and enables unusual H2-activation processes by these zirconocene complexes.

Results and Discussion Synthesis of a Cyclopentadienyl[η5:η3-dihydrononamethylfluorenediyl]zirconocene Hydride. The synthetic approach chosen for the formation of Flu′′- and Flu*-supported zirconium hydride species is based on β-H elimination of olefin from appropriate zirconocene dialkyl complexes.22 Thus, the zirconocene dichloride Cp′′Flu*ZrCl2 was treated with 2 equiv of i BuLi in anticipation of obtaining the alkyl hydride derivative Cp′′Flu*Zr(H)iBu. Spectroscopic observation of NMR-scale reactions established consumption of the starting materials and formation of isobutylene as expected. However, 1H NMR spectra suggested the presence of two zirconium-containing species, neither of which could be identified as the expected isobutyl hydride. Larger scale reactions allowed for isolation of one of these species, as brown crystals from cold pentane, which was identified as a zirconocene monohydride possessing a dianionic η5:η3-nonamethyl-1,2-dihydrofluorenediyl ligand (1,2-DHF*D) (19) (a) Bradley, C. A.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2003, 125, 8110. (b) Bradley, C. A.; Keresztes, I.; Lobkovsky, E.; Young, V. G.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126, 16937. (20) Bazinet, P.; Tilley, T. D. Organometallics 2008, 27, 1267. (21) Other metal complexes containing Flu′′ or Flu* are reported in. (a) Bazinet, P.; Tupper, K. A.; Tilley, T. D. Organometallics 2006, 25, 4286. (b) Moss, J.; Thomas, J.; Ashley, A.; Cowley, A. R.; O’Hare, D. Organometallics 2006, 25, 4279. (c) Moss, J.; Thomas, J.; Cowley, A. R.; O’Hare, D. J. Organomet. Chem. 2007, 692, 2071. (22) Pool, J. A.; Bradley, C. A.; Chirik, P. J. Organometallics 2002, 21, 1271.

Octa- and Nonamethylfluorenyl Complexes of Zr(IV)

Figure 1. Thermal ellipsoid plot for compound 1-syn-1,2-DHF*D. The methyl groups of the SiMe3 substituents along with most hydrogens have been omitted for clarity. Thermal ellipsoids are drawn at 50% probability.

through the combination of one- and two-dimensional NMR experiments, along with X-ray structural characterization (1syn-1,2-DHF*D; eq 1). This complex is an isomer of the corresponding zirconocene dihydride, which is the presumed precursor to 1-syn-1,2-DHF*D, formed by elimination of 2 equiv of isobutylene from Cp′′Flu*ZriBu2. Double β-H elimination is unexpected considering the strong precedent for formation of hydrido butyl species with bis(cyclopentadienyl) and bis(indenyl) complexes.19,22 The 1H NMR spectrum of 1-syn-1,2-DHF*D contains two peaks for the SiMe3 groups and three signals for the cyclopentadienyl ring protons, consistent with a nonsymmetric species. The DHF*D ligand gives rise to distinct signals in the 1H NMR spectrum, such as eight singlets (although two of the signals show small coupling values of 1.3 Hz), all integrating to three protons each, along with one doublet (0.71 ppm) and one quartet (2.72 ppm) integrating to three and one proton, respectively. The zirconium hydride signal appears as an upfield broad peak at 0.66 ppm, suggestive of a saturated zirconium center.23 Crosspeaks between the zirconium hydride and the methine proton were observed in the two-dimensional NOESY spectrum of 1-syn-1,2-DHF*D, confirming the endo configuration of the methine proton and the syn arrangement between the Zr-H and the sp3-hybridized carbon.

Single-crystal X-ray diffraction studies were performed to determine the molecular structure of 1-syn-1,2-DHF*D (Figure 1). Selected bond distances and angles are listed in Table 1. The structure of 1-syn-1,2-DHF*D confirms the presence of a (23) (a) Bercaw, J. E. In Transition Metal Hydrides; Advances in Chemistry Series 167; 1978; Chapter 10, p 136. (b) Hillhouse, G. L.; Bulls, A. R.; Santarsiero, B. D.; Bercaw, J. E. Organometallics 1988, 7, 1309.

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dianionic η5:η3-nonamethyl-1,2-dihydrofluorenediyl ligand, which can be viewed as possessing both an allyl and a cyclopentadienyl fragment. Coordination of the allyl fragment to the metal center results in a severe buckling of the fluorenyl structure. The large “hinge angle” of 45.4°, defined as the angle formed between the planes of the cyclopentadienyl and allyl fragments, illustrates the extent of this distortion. For comparison, the “hinge angle” between the five-membered ring and the other benzo ring in the ligand is only 9.7°. The corresponding “hinge angles” in the syn and anti isomers of the indenyl species (η5-1,3-iPr2C9H5)(η3:η5-1,3-iPr2-C9H6)ZrH were determined to be 44.7° and 44.5°, respectively.10 The sp3-hybridization of one of the carbons of the six-membered ring is evidenced by the C-C distances (C(2)-C(3) ) 1.505(5) and C(3)-C(4) ) 1.508(5) Å). The structure of 1-syn-1,2-DHF*D was of sufficient quality to allow the Zr hydride to be located and freely refined. The structure confirms the syn configuration of the compound, in which the sp3-hybridized carbon and the zirconium hydride are adjacent to one another. In essence, compound 1-syn-1,2-DHF*D may be regarded as the rearrangement product of a zirconocene dihydride intermediate, formed via a metal-to-benzo hydride transfer. Such a transformation has previously been observed for bis(indenyl)zirconium dihydride complexes, to generate zirconocene hydrides containing η5:η3-4,5-dihydroindenediyl ligand derivatives.10 However, to the best of our knowledge such a rearrangement has not been observed for fluorenyl ligands. Alternatively, the metal-to-benzo hydride transfer can be viewed as the insertion of an olefinic fragment of the fused six-membered ring into the Zr-H bond of the dihydride intermediate. The η5:η3-dihydrofluorenediyl ligand in 1-syn-1,2-DHF*D displays a coordination geometry that is strikingly similar to that reported for the dihydroindenediyl derivatives.10 Isomerization of Cp′′[η5:η3-1,2-DHF*D]ZrH and Trapping of the Dihydride Isomer. Monitoring benzene-d6 solutions of 1-syn-1,2-DHF*D over several days by NMR spectroscopy demonstrated that the compound undergoes a slow isomerization. The NMR spectroscopic features of the new isomer match those of the second product observed in the NMR-scale reactions. The most noticeable feature in the 1H NMR spectrum of the new isomer is a substantial downfield shift of the quartet for the methine proton of the sp3-hybridized carbon of the dihydrofluorenediyl ligand, to 3.76 ppm. The resonance for the zirconium hydride ligand is observed at 0.60 ppm, only slightly upfield of the corresponding signal for 1-syn-1,2-DHF*D. A sample monitored for three weeks displayed a final isomer ratio of 40:60 in favor of the newly formed isomer. No other products were observed in the 1H NMR spectrum, even after heating to 60 °C. The existence of syn and anti isomers for reported η5:η3-4,5dihydroindenediyl derivatives has been observed, and mechanistic studies suggested that isomerization proceeds via β-H elimination and formation of a bis(indenyl)zirconium dihydride intermediate, which can then undergo reinsertion of a CdC bond of the benzo ring into the Zr-hydride.10 These studies also determined that the syn isomer was thermodynamically more stable. Similar, reversible hydride migrations between the metal and a benzo ring in the case of 1-syn-1,2-DHF*D could in principle generate four isomers due to the nonsymmetric nature of the Flu* ligand (Chart 1). However, only two of the possible isomers are observed (eq 2). Cross-peaks between the zirconium hydride and the methine proton of the newly formed isomer were observed in the two-dimensional NOESY spectrum and allowed for identification of the second species as the syn-3,4-

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Bazinet and Tilley

Table 1. Selected Bond Distances and Angles for 1-syn-1,2-DHF*D Distances (Å) 2.505(3) 2.370(3) 2.301(3) 2.567(3) 2.602(3) 2.575(4) 2.511(3) 2.417(3) 3.080(4) 1.505(5) 1.411(5) 1.470(5)

Zr(1)-C(1) Zr(1)-C(2) Zr(1)-C(7) Zr(1)-C(8) Zr(1)-C(13) Zr(1)-C(4) Zr(1)-C(5) Zr(1)-C(6) Zr(1)-C(3) C(2)-C(3) C(4)-C(5) C(6)-C(7)

Zr(1)-C(23) Zr(1)-C(24) Zr(1)-C(25) Zr(1)-C(26) Zr(1)-C(27) Cp′′cent-Zr DHF*HC5-cent-Zr DHF*Hallyl-cent-Zr

2.515(3) 2.528(3) 2.487(3) 2.517(3) 2.544(3) 2.209 2.148 2.265

C(3)-C(4) C(5)-C(6) C(2)-C(7)

1.508(5) 1.432(5) 1.439(4)

Angles (deg) DHF*Hallyl-cent-Zr-DHF*HC5-cent DHF*HC5-cent-Zr-Cp′′cent

80.8 151.4

dihydrofluorenediyl isomer (1-syn-3,4-DHF*D). The absence of any anti isomers, although somewhat surprising, would result from either a strong thermodynamic preference for the syn isomers or large kinetic barriers for the formation of the anti isomers. Considering the absence of any detectable anti isomer, it is reasonable to assume that the transition states leading to formation of anti isomers are too high in energy for the transformation to occur.

The mechanism believed to allow for isomerization of 1-syn-1,2-DHF*D involves a reversible β-H elimination, which would form a putative zirconocene dihydride intermediate.10 Although not observed directly, evidence for the dihydride intermediate was obtained from reactions of 1-syn1,2-DHF*D with olefins. Compound 1-syn-1,2-DHF*D was used to catalyze the cyclization of 1,5-hexadiene to methylenecyclopentane (eq 3).24 Addition of 25 equiv of diene to 1-syn-1,2-DHF*D resulted in complete conversion to methylenecyclopentane within 20 h at room temperature. Addition of 250 equiv of diene led to approximately 45 turnovers after 24 h, but did not continue even after an additional day, suggesting the catalyst was deactivated during the reaction. At 60 °C, an increased cyclization rate was

DHF*Hallyl-cent-Zr-Cp′′cent

122.7

observed; however, at 90 °C the reaction led to multiple unidentified products in addition to methylenecyclopentane.

Additional evidence for an equilibrium involving 1-syn-1,2DHF*D and a zirconocene dihydride intermediate comes from reactions with isobutylene. Addition of an excess of isobutylene (3 equiv) to 1-syn-1,2-DHF*D followed by heating to 60 °C produced the cyclometalated zirconium isobutyl product [η5:η1-1-SiMe2CH2-C5H3-3-SiMe3]Flu*ZrCH2CHMe2 (2) (eq 4). Monitoring of NMR-scale reactions indicated that isobutane is produced during the reaction. The 1H NMR spectrum of compound 2 displays nine singlets between 2.11 and 2.83 ppm, indicating that the DHF*D ligand is transformed back into its original Flu* structure. Evidence for C-H activation of one SiCH3 group of the Cp′′ ligand comes from the presence of two doublets at -2.48 and -2.97 ppm, integrating to one proton each, for the diastereotopic methylene hydrogens, and two singlets for the inequivalent SiMe groups (0.47 and -0.04 ppm). The Zr-isobutyl ligand appears as an upfield set of doublets of doublets for the diasterotopic methylene protons, a multiplet for the methine proton, and two doublets for the inequivalent Me groups.

Chart 1

Presumably, the pathway by which 1-syn-1,2-DHF*D is converted to 2 involves initial β-H elimination from the DHF*D ligand to form the dihydride intermediate Cp′′Flu*ZrH2, followed by insertion of isobutylene to give a hydrido alkyl species (Scheme 1). Small-scale NMR tube reactions at room temperature indicated the presence of an intermediate tentatively identified as the zirconocene isobutyl hydride complex. This intermediate could not be generated cleanly due to its continued transformation via elimination of isobutane. Alkane elimination from such an intermediate could proceed through σ-bond metathesis to generate a cyclometalated monohydride, or alternatively via reductive elimination of isobutane followed by oxidative addition of a C-H bond. Finally, insertion of a second isobutylene into the Zr-H bond would generate compound 2.

Octa- and Nonamethylfluorenyl Complexes of Zr(IV)

Organometallics, Vol. 28, No. 7, 2009 2289 Scheme 1

Figure 2. Thermal ellipsoid plot for compound 2. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at 50% probability. Table 2. Selected Bond Distances and Angles for 2 Zr(1)-C(1) Zr(1)-C(2) Zr(1)-C(7) Zr(1)-C(8) Zr(1)-C(13) Zr(1)-C(23) Zr(1)-C(35) C(23)-Zr(1)-C(35)

Distances (Å) 2.507(4) Zr(1)-C(27) 2.594(4) Zr(1)-C(28) 2.630(4) Zr(1)-C(29) 2.698(4) Zr(1)-C(30) 2.578(4) Zr(1)-C(31) 2.278(4) Cp′′cent-Zr 2.305(4) Zr-Flu*cent Angles (deg) 98.66(16) Cp′′-Zr-Flu*cent

2.533(4) 2.634(4) 2.564(4) 2.465(4) 2.498(4) 2.235 2.294 135.42

The fact that compound 2 possesses an alkyl ligand containing a β-H is somewhat surprising considering the general instability of such compounds and the fact that isobutylene is readily eliminated during the formation of 1-syn-1,2-DHF*D. Excess isobutylene present during the preparation of compound 2 could be responsible for driving the equilibrium toward the olefin insertion product. Heating of a benzene-d6 solution of compound 2 to 90 °C in the absence of isobutylene led to product decomposition and generated mostly isobutane with only trace amounts of isobutylene, suggesting that 2 is relatively resistant to β-H elimination. The solid-state structure of compound 2 was determined by X-ray diffraction studies on crystals obtained from pentane at room temperature. The molecular structure of 2 is illustrated in Figure 2, and selected metrical parameters are listed in Table 2. The structure confirms that compound 2 does indeed possess an unmodified Flu* ligand, which exhibits typical η5-coordination to the metal center. The five Zr-C bond lengths range between 2.507(4) and 2.698(4) Å, with the shortest bond being between the metal center and the carbon at the 9-position. All other bonding parameters within the tricyclic core of Flu* resemble those of previously reported Flu*-containing Zr compounds.20 The structure also displays an η1:η5-coordinated Cp′′ ligand where one of the Si-CH3 groups has been C-H

activated and is now bound to the metal center. The resulting Zr(1)-C(35) bond length of 2.305(4) Å is only slightly elongated compared to the bond between the Zr atom and the methylene carbon of the isobutyl ligand (Zr(1)-C(23) ) 2.278(4) Å), despite the ring strain. Synthesis of an Octamethylfluorenyl-Supported Metallocene. To further investigate the role of the Flu* ligand in the unusual transformations observed in the formation of 1-syn1,2-DHF*D and 2, the synthesis of Flu′′-supported zirconocene analogues was explored. Treatment of Cp′′Flu′′ZrCl2 with 2 equiv of iBuLi in small-scale reactions, monitored via NMR spectroscopy, resulted in consumption the starting material, generation of isobutylene, and formation of mainly one compound, which was tentatively identified as the zirconocene alkyl hydride Cp′′Flu′′Zr(H)iBu. The 1H NMR spectrum contains two multiplets, at -1.32 and -1.68 ppm, due to the methylene protons of the isobutyl ligand along with a multiplet at 1.80 ppm for the methine proton and two doublets at 0.72 and 0.52 ppm for the Me groups. A broad resonance at 3.92 ppm is attributed to the hydride ligand. The spectrum also contains two distinct singlet resonances for the SiMe3 groups of the Cp′′ ligand, consistent with a nonsymmetric species. Observation of this intermediate suggests that the Flu′′ analogue undergoes β-H elimination at a slower rate compared to the Flu* species and that formation of the zirconocene dihydride is disfavored. Unfortunately, this compound could not be prepared cleanly from larger scale reactions due to its instability toward further transformations. Monitoring NMR-scale reactions over an extended period of time reveals that the alkyl hydride slowly undergoes further reaction, releasing isobutane and forming what appears to be a cyclometalated zirconocene isobutyl complex (approximately 50% conversion after 12 h). Formation of such a compound requires that the isobutylene initially generated reinsert into a Zr-H bond after cyclometalation. Preparativescale reactions performed in a closed vessel at 50 °C directly led to formation of the cyclometalated compound [η5:η1-1SiMe2CH2-C5H3-3-SiMe3]Flu′′ZrCH2CHMe2 (3; eq 5), which is the Flu′′ analogue of 2, obtained after reaction of 1-syn-1,2DHF*D with excess isobutylene. The NMR spectroscopic features of 3 closely resemble those of the Flu* analogue 2. The Flu′′ ligand appears as nine singlets (one for the ring proton and eight for the Me groups) in the 1H NMR spectrum. The cyclometalated Cp′′ ligand displays two upfield doublets for the diastereotopic methylene protons (-2.48 and -2.83 ppm) along with three singlets for the remaining SiMe groups and three multiplets for the ring protons. The isobutyl ligand exhibits two doublets of doublets for the diastereotopic methylene protons (-1.26 and -1.54 ppm) along with a multiplet for the methine proton (1.83 ppm) and two doublets for the inequivalent Me groups (0.78 and 0.72 ppm). The solid-state structure of compound 3 was determined by X-ray crystallography using crystals obtained from pentane at room temperature. The molecular structure of 3 is illustrated in Figure 3, and selected metrical parameters are listed in Table 3. The structure confirms the assignment of compound 3 as a

2290 Organometallics, Vol. 28, No. 7, 2009

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appeared to be the product of continued hydrogenation of 4, giving rise to a partially saturated Flu′′ ligand derivative (5). Neither compound could be isolated in pure form from the reaction mixture.

Figure 3. Thermal ellipsoid plot for compound 3. Hydrogen atoms have been omitted for clarity. Thermal ellipsoids are drawn at 50% probability.

cyclometalated zirconium isobutyl complex. The geometry of 3 closely resembles that of compound 2. The Flu′′ ligand exhibits η5-coordination to the metal, with the five Zr-C bond lengths ranging between 2.454(5) and 2.726(5) Å. The structure also displays an η5:η1-coordinated cyclometalated Cp′′ ligand along with an isobutyl group. The Zr-Cmethyelene bond lengths of 2.277(5) and 2.308(5) Å are similar to the corresponding values found for 2. Isolation of compound 3 directly from the reaction of Cp′′Flu′′ZrCl2 with isobutyl lithium, rather than a dihydrofluorenediyl derivative, suggests that the rates of olefin insertion, β-H elimination, and/or metal-to-benzo hydride transfer are significantly different for the Flu′′ derivatives. In an attempt to generate a Zr hydride species containing a Flu′′ ligand, hydrogenolysis of compound 3 was attempted. A benzene-d6 solution of 3 was exposed to an atmosphere of H2, and 1H NMR spectra taken over 20 h at room temperature indicated that the reaction proceeds very slowly. At 55 °C, this reaction was complete after 20 h and resulted in formation of a product identified as the η5:η3-octamethyl-3,4-dihydrofluorenediyl zirconium hydride derivative 4, based on the similarity of its NMR spectra to those of compound 1 (eq 6). Hydrogenolysis of 3 was performed on a preparative scale with an excess of H2; however, 1H NMR spectra of the crude material obtained after heating the reaction to 55 °C for 17 h indicated the presence of a major species in addition to compound 4. This species Table 3. Selected Bond Distances and Angles for 3 Zr(1)-C(1) Zr(1)-C(2) Zr(1)-C(7) Zr(1)-C(8) Zr(1)-C(13) Zr(1)-C(22) Zr-Flu′′cent C(22)-Zr(1)-C(36)

Distances (Å) 2.454(5) Zr(1)-C(26) 2.567(4) Zr(1)-C(27) 2.650(5) Zr(1)-C(28) 2.726(5) Zr(1)-C(29) 2.592(5) Zr(1)-C(30) 2.277(5) Zr(1)-C(36) 2.295 Cp′′cent-Zr Angles (deg) 98.02(19) Cp′′-Zr-Flu′′cent

2.523(5) 2.597(5) 2.561(5) 2.472(5) 2.498(5) 2.308(5) 2.227 134.09

In an attempt to efficiently generate compound 4, without significant further reaction to form 5, milder reaction conditions were employed. Reaction of the zirconocene dichloride Cp′′Flu′′ZrCl2 with iBuLi in the presence of 1 atm of H2 gas for 12 h at room temperature gave the zirconium monohydride species 4 as a crystalline yellow solid from cold pentane (43%, eq 7). The 1H NMR spectrum of compound 4 displays characteristic features that closely resemble those of the Flu* analogue 1-syn-3,4-DHF*D. The spectrum contains a quartet relatively downfield at 3.82 ppm for the methine proton of the sp3-hybridized carbon, along with a doublet at 0.84 ppm for the corresponding methyl group, supporting the assignment of 4 as the syn-3,4-dihydro isomer. The remaining methyl groups on the DHF′′D ligand appear as seven unique singlets, and the ring proton is observed as a singlet at 6.11 ppm. The metal hydride gives rise to a broad signal at 0.73 ppm. Interestingly, none of the other three possible isomers were observed in the NMR spectra of compound 4, suggesting that the metal-to-benzo ring hydride transfer occurs at a slower rate for the Flu′′ compared to the Flu* derivative or that the syn-3,4-dihydrofluorenediyl isomer is thermodynamically very strongly favored.

The hydrogenolysis of 3 was repeated with prolonged heating at 60 °C for 3 days, and this resulted in complete consumption of 3 and transformation of 4 to the partially saturated species 5, which was isolated in pure form after crystallization from cold pentane. Compound 5 was identified as a zirconocene monohydride possessing a dianionic η5:η3-octamethyl-1,2,3,4,5,6hexahydrofluorenediyl ligand (HHF′′D) through a combination of NMR experiments along with structural characterization. Further exposure of compound 5 to H2 at temperatures up to 100 °C did not lead to the formation any other products. The 1H NMR spectrum of 5 contains unique signals that confirm the hydrogenation of one of the six-membered rings of the Flu′′ ligand. The HHF′′D ligand in 5 appears in the 1H NMR spectrum as three singlets and five doublets, integrating for three protons each, along with one singlet and five multiplets integrating for one proton each. The multiplets for the methine protons are observed over a wide range of chemical shifts from 1.62 to 3.74 ppm. The zirconium hydride appears as a broad peak at 0.64 ppm, which is consistent with a saturated metal center. The quartet observed at 2.91 ppm was assigned to the sp3-hybridized carbon at the 5-position of the fluorene core,

Octa- and Nonamethylfluorenyl Complexes of Zr(IV)

Organometallics, Vol. 28, No. 7, 2009 2291

monohydride (Cp′′[η5:η3-C13Me9H5]ZrH) and the fully hydrogenated octahydrofluorenyl zirconium dihydride derivatives (Cp′′[C13Me9H8]ZrH2). Unfortunately, neither compound could be isolated in pure form due to their high solubility in nonpolar organic solvents. Observation of multiple hydrogenated products, including the fully reduced zirconocene dihydride species, suggests that the Flu*-containing species exhibit a higher reactivity toward H2 (vs the Flu′′ analogues).

Concluding Remarks

Figure 4. Thermal ellipsoid plot for compound 5. The methyl groups of the SiMe3 substituents along with most hydrogens have been omitted for clarity. Thermal ellipsoids are drawn at 50% probability.

adjacent to the allyl fragment. Cross-peaks in the twodimensional NOESY spectrum of 5 observed between the quartet at 2.91 and the zirconium hydride along with the methine proton appearing at 3.74 confirmed the syn arrangement between the Zr-H and the sp3-hybridized carbon at the 5-position. In order to confirm the assignment of 5, an X-ray diffraction study was undertaken. The solid-state structure of 5 is illustrated in Figure 4, and selected metrical parameters are listed in Table 4. The most salient feature in the molecular structure of 5 is the dianionic hexahydrofluorenediyl ligand derivative, which displays η5:η3-coordination to the Zr center. Similar to compound 1-syn-1,2-DHF*D, the ligand comprises both an allyl and a cyclopentadienyl-type moiety; however, the other sixmembered ring has been hydrogenated and exhibits a chairlike configuration. The four methyl groups of the saturated benzo ring all occupy exo positions (away from the metal), which indicates that ring hydrogenation occurs exclusively from the endo face of the ligand. This is contrary to what would be obtained from more common PtO2-catalyzed ring hydrogenations25 and suggests an intramolecular pathway as observed in the hydrogenation of bis(indenyl)zirconium species.10 Coordination of the allyl fragment to the Zr center causes severe buckling of the tricyclic core, resulting in a “hinge angle” of 42.9° between the planes of the allyl fragment and the five-membered ring. The structure also confirms that the carbon at the 5-position, adjacent to the allyl fragment, is sp3-hybridized and exhibits a syn configuration with the Zr-hydride ligand. Despite the drastically modified nature of the fluorenyl ligand derivative, compound 5 exhibits a metal coordination environment that is strikingly similar to that observed in 1-syn-1,2-DHF*D. The hydrogenolysis of the Flu* derivative 2 was also examined. Monitoring both NMR tube and preparative scale reactions by NMR spectroscopy indicated that reaction with hydrogen preceded slowly at room temperature, and heating to 55 °C produced multiple hydrogenated products. Prolonged hydrogenolysis reactions over several days led to formation of a mixture of products containing two major species, which are tentatively identified as the hexahydrofluorenediyl zirconium (24) (a) Bunel, E.; Burger, B. J.; Bercaw, J. E. J. Am. Chem. Soc. 1988, 110, 976. (b) Piers, W. E.; Shapiro, P. J.; Bunel, E.; Bercaw, J. E. Synlett 1990, 74. (25) Waymouth, R. M.; Bangerter, F.; Pino, P. Inorg. Chem. 1988, 27, 758.

The reaction of Cp′′Flu*ZrCl2 with iBuLi produces 1-syn1,2-DHF*D, which contains a η5:η3-dihydrofluorenediyl ligand formed by a metal-to-benzo ring hydride transfer. The formation of this compound provides an interesting example of a reaction type not available to zirconocene complexes containing only Cp-type ligands, as it involves intramolecular rearrangements that require participation of the fused-ring framework of Flu*. This transformation also reflects the high reactivity of the putative monomeric dihydride Cp′′Flu*ZrH2, presumably formed after double elimination of isobutylene, toward olefin insertion. Similar transformations involving hydride and polyhapto intermediates have been reported for homogeneous species that are catalysts for arene hydrogenation.26 For example, intramolecular hydride transfer to arene rings has been observed for aryloxidesupported Nb and Ta hydride species, which have shown the capacity to catalyze the hydrogenation of arenes and aryl phosphines.27,28 Stable Nb η5-cyclohexadienyl species generated after intermolecular hydride transfer to benzene or toluene have also been reported; however, these complexes were determined to be inactive as catalysts for the hydrogenation of arenes.29 Immobilized, cationic zirconium hydrides have demonstrated high catalytic activities for the hydrogenation of benzene, most likely via a mechanism involving insertion of the arene into a Zr-H bond.30 Compound 1-syn-1,2-DHF*D was shown to undergo reversible β-H elimination and exist in equilibrium with its 1-syn3,4-DHF*D isomer. Additionally, 1-syn-1,2-DHF*D inserts olefins such as 1,5-hexadiene and isobutylene, and this is consistent with reversible metal-to-benzo hydride transfer. Reversible intramolecular hydride transfer to a mesityl group to yield a η5-cyclohexadienyl complex was observed for an imido-amido supported Ta species that displayed modest activity for the hydrogenation of olefins and dienes.31 Exposure of the Flu*- and Flu′′-containing dialkyl species 2 and 3 to an atmosphere of hydrogen at moderate temperatures leads to intramolecular hydrogenations of the arene rings of the fluorenyl ligands, but complete hydrogenation of these ligand systems seems to be limited by the geometric constraints of ligand binding. Overall, the results presented here indicate that the Flu* and Flu′′ ligands support hydride complexes that are highly reactive toward olefin and arene insertions. Interestingly, these hydride species undergo reversible conversion to more (26) Dyson, P. J. Dalton Trans. 2003, 2964. (27) (a) Steffey, B. D.; Chesnut, R. W.; Kerschner, J. L.; Pellechia, P. J.; Fanwick, P. E.; Rothwell, I. P. J. Am. Chem. Soc. 1989, 111, 378. (b) Steffey, B. D.; Rothwell, I. P. J. Chem. Soc., Chem. Commun. 1990, 213. (c) Lockwood, M. A.; Potyen, M. C.; Steffey, B. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 1995, 14, 3293. (28) Rothwell, I. P. Chem. Commun. 1997, 1331. (29) Fryzuk, M. D.; Kozak, C. M.; Bowdridge, M. R.; Patrick, B. O. Organometallics 2002, 21, 5047. (30) Ahn, H.; Nicholas, C. P.; Marks, T. J. Organometallics 2002, 21, 1788. (31) (a) Gavenonis, J.; Tilley, T. D. J. Am. Chem. Soc. 2002, 124, 8536. (b) Gavenonis, J.; Tilley, T. D. Organometallics 2002, 21, 5549.

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Table 4. Selected Bond Distances and Angles for 5 Distances (Å) 2.423(2) 2.636(2) 2.594(2) 2.379(2) 2.269(2) 2.593(3) 2.547(3) 2.427(2) 3.068(3) 1.516(3) 1.538(4) 1.548(4) 1.540(4)

Zr(1)-C(1) Zr(1)-C(2) Zr(1)-C(7) Zr(1)-C(8) Zr(1)-C(13) Zr(1)-C(10) Zr(1)-C(11) Zr(1)-C(12) Zr(1)-C(9) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(5)-C(6)

Zr(1)-C(22) Zr(1)-C(23) Zr(1)-C(24) Zr(1)-C(25) Zr(1)-C(26) Zr-Cp′′cent Zr-HHF′′DC5-cent Zr-HHF′′Dallyl-cent

2.526(2) 2.546(2) 2.484(2) 2.493(2) 2.539(2) 2.208 2.145 2.290

C(8)-C(9) C(9)-C(10) C(10)-C(11) C(11)-C(12)

1.509(4) 1.510(4) 1.413(4) 1.423(4)

Angles (deg) HHF′′Dallyl-cent-Zr-HHF′′DC5-cent HHF′′DC5-cent-Zr-Cp′′cent

80.69 148.39

stable complexes containing partially reduced fluorenyl ligands. In this way, Flu* and Flu′′ may act as “reservoirs” for hydrogen, which may be available for delivery to various substrates. The latter aspect of the chemistry of Flu* and Flu′′ complexes is currently under investigation.

Experimental Section General Procedures. All air-sensitive manipulations were performed under an atmosphere of nitrogen using Schlenk techniques and/or a Vacuum Atmospheres glovebox. Dry, oxygen-free solvents were employed for all air-sensitive manipulations. Removal of thiophenes from benzene and toluene was accomplished by washing each with H2SO4 and saturated NaHCO3 followed by drying over MgSO4. All dried solvents were distilled from sodium benzophenone ketyl, with the exception of benzene-d6, which was purified by vacuum distillation from Na/K alloy, and dichloromethane-d2, which was purified by vacuum distillation from CaH2. The compounds Cp′′Flu′′ZrCl2 and Cp′′Flu*ZrCl2 were prepared according to literature procedures,20 and iBuLi was prepared from i BuBr and Li and crystallized from pentane at -35 °C. Other chemicals were purchased and used as received: H2 (Praxair), isobutylene (Aldrich). Elemental analyses were performed by the microanalytical laboratory at the University of California, Berkeley. All NMR spectra were recorded at ambient temperature unless otherwise noted, using either a Bruker AM-400, AMX-400, or AMX 300 instrument. Cp′′(η5:η3-C13Me9H)ZrH (1-syn-1,2-DHF*D). A 100 mL Teflon-stoppered Schlenk flask was charged with Cp′′(C13Me9)ZrCl2 (0.400 g, 0.60 mmol) and iBuLi (0.081 g, 1.2 mmol). The flask was evacuated, and toluene was vacuum transferred into the flask (15 mL) at -78 °C. The cold bath was removed, and the reaction mixture was allowed to warm to room temperature and stirred for 30 min. All volatiles were removed under vacuum, and the product was extracted with pentane (ca. 20 mL). The extract was filtered through Celite and concentrated under vacuum (to ca. 3 mL). The concentrated solution was cooled to -35 °C to yield the product as dark brown crystals (0.194 g, 54%). 1H NMR (benzene-d6, 400 MHz, 25 °C): δ 5.19 (m, 1H, CH), 4.57 (m, 1H, CH), 4.07 (m, 1H, CH), 2.86 (s, 3H, CH3), 2.72 (quart, J ) 6.6 Hz, 1H, CH), 2.68 (d, J ) 1.3 Hz, 3H, CH3), 2.67 (s, 3H, CH3), 2.45 (s, 3H, CH3), 2.22 (d, J ) 1.3 Hz, 3H, CH3), 2.07 (s, 3H, CH3), 2.06 (s, 3H, CH3), 1.71 (s, 3H, CH3), 0.71 (d, J ) 6.6 Hz, 3H, CH3), 0.66 (s, 1H, ZrH), 0.37 (s, 9H, CH3), 0.08 (s, 9H, CH3). 13C{1H} NMR (benzene-d6, 100 MHz, 25 °C): δ 131.4 (C), 130.9 (C), 128.8 (C), 122.1 (C), 121.5 (C), 120.4 (C), 118.0 (C), 114.7 (CH), 114.4 (CH), 111.6 (CH), 110.2 (C), 104.2 (C), 102.1 (C), 99.4 (C), 99.1 (C), 68.4 (C), 33.0 (CH), 24.7 (CH3), 24.0 (CH3), 21.2 (CH3), 19.7 (CH3), 17.03 (CH3), 17.00 (CH3), 16.9 (CH3), 16.5 (CH3), 16.3 (CH3), 1.0 (3 CH3), 0.4 (3 CH3). Anal. Calcd for C33H50Si2Zr: C, 66.71; H, 8.48. Found: C, 66.75; H, 8.64.

HHF′′Dallyl-cent-Zr-Cp′′cent

120.29

(η5:η1-C5H3-1-SiMe2CH2-3-SiMe3)(C13Me9)ZriBu (2). To a J-Young NMR tube was added Cp′′(η5:η3-C13Me9H)ZrH (0.110 g, 0.19 mmol) and C6D6 (1 mL). The resulting solution was degassed via three freeze-pump-thaw cycles. Three equivalents of isobutylene were added by condensing a known quantity of gas from a gas bulb into the NMR tube submersed in liquid nitrogen. The reaction mixture was allowed to slowly warm and was then heated to 60 °C for 10.5 h. The product was isolated by removing all volatile compounds under vacuum, and it was crystallized from a concentrated pentane solution at room temperature. The product was obtained as yellow crystals in two crops (total yield 0.059 g, 49%). 1H NMR (benzene-d6, 400 MHz, 25 °C): δ 6.67 (t, 1H, CH), 6.19 (t, 1H, CH), 5.34 (t, 1H, CH), 2.83 (s, 3H, CH3), 2.52 (s, 3H, CH3), 2.51 (s, 3H, CH3), 2.41 (s, 3H, CH3), 2.39 (s, 3H, CH3), 2.26 (s, 3H, CH3), 2.25 (s, 3H, CH3), 2.14 (s, 3H, CH3), 2.11 (s, 3H, CH3), 1.91 (m, 1H, CH), 0.80 (d, 3H, CH3), 0.69 (d, 3H, CH3), 0.47 (s, 3H, CH3), 0.26 (s, 9H, CH3), -0.04 (s, 3H, CH3), -1.07 (dd, 3JHH ) 9.2 and 13.3 Hz, 1H, CH2), -1.66 (dd, 3JHH ) 3.3 and 13.3 Hz, 1H, CH2), -2.48 (d, JHH ) 13.1 Hz, 1H, SiCH2), -2.97 (d, JHH ) 13.1 Hz, 1H, SiCH2). 13C{1H} NMR (benzene-d6, 100 MHz, 25 °C): δ 135.0 (C), 134.4 (C), 132.5 (C), 132.2 (C), 130.1 (C), 129.9 (C), 129.22 (C), 129.20 (C), 127.9 (C), 126.92 (C), 126.88 (C), 123.5 (C), 122.9 (CH), 122.5 (C), 120.9 (CH), 115.5 (CH), 112.6 (C), 88.4 (C), 71.0 (CH2), 32.7 (iBu-CH), 30.4 (iBuCH3), 26.8 (iBu-CH3), 25.3 (Si-CH2), 24.1 (CH3), 23.6 (CH3), 19.0 (CH3), 18.9 (CH3), 18.4 (CH3), 17.2 (CH3), 16.94 (CH3), 16.91 (CH3), 16.7 (CH3), 1.1 (1 CH3), 1.0 (3 CH3), -1.5 (1 CH3). Anal. Calcd for C37H56Si2Zr: C, 68.55; H, 8.71. Found: C, 68.72; H, 8.93. (η5:η1-1-SiMe2CH2-3-SiMe3-C5H3)(C13Me8H)ZriBu (3). A 100 mL Teflon-stoppered Schlenk flask was charged with Cp′′Flu′′ZrCl2 (0.580 g, 0.894 mmol) and iBuLi (0.120 g, 1.88 mmol). The flask was evacuated, and toluene (40 mL) was vacuum transferred into the flask at -78 °C. The flask was closed, the cold bath was removed, and the reaction mixture was allowed to warm to room temperature. The reaction mixture was then heated to 50 °C and stirred for 12 h. All volatile components were removed under vacuum, and the product was extracted into pentane (ca. 35 mL). The extract was filtered through Celite, and all volatile material was removed under vacuum. The product was purified by crystallization from a saturated hexanes solution at -35 °C and was obtained as yellow crystals (0.285 g, 50%). 1H NMR (benzene-d6, 400 MHz, 25 °C): δ 6.62 (m, 1H, CH), 6.37 (m, 1H, CH), 5.66 (s, 1H, CH), 5.19 (m, 1H, CH), 2.61 (s, 3H, CH3), 2.59 (s, 3H, CH3), 2.27 (s, 3H, CH3), 2.26 (s, 3H, CH3), 2.18 (s, 3H, CH3), 2.17 (s, 3H, CH3), 2.14 (s, 3H, CH3), 2.10 (s, 3H, CH3), 1.83 (m, 1H, CH), 0.78 (d, 3H, iBu-CH3), 0.72 (d, 3H, iBu-CH3), 0.45 (s, 3H, SiCH3), 0.22 (s, 9H, SiCH3), -0.10 (s, 3H, SiCH3), -1.26 (dd, 1H, ZrCH2), -1.54 (dd, 1H, ZrCH2), -2.48 (d, 1H, ZrCH2Si), -2.83 (d, 1H, ZrCH2Si). 13C{1H} NMR (benzene-d6, 100 MHz, 25 °C): δ 133.9 (C), 133.1 (C), 132.5 (C), 132.0 (C), 130.5 (C), 129.8 (C), 129.65

Octa- and Nonamethylfluorenyl Complexes of Zr(IV) (C), 129.58 (C), 126.7 (C), 126.4 (C), 125.7 (C), 122.9 (C), 122.6 (C), 122.0 (C), 117.5 (C), 114.8 (C), 111.8 (C), 74.4 (CH), 71.2 (ZrCH2CHMe2), 32.4 (CHMe2), 30.0 (CH(CH3)2), 27.2 (CH(CH3)2), 24.8 (ZrCH2SiMe2), 23.9 (CH3), 23.6 (CH3), 17.1 (CH3), 16.9 (CH3), 16.8 (CH3), 16.74 (CH3), 16.69 (CH3), 16.4 (CH3), 1.0 (SiCH3), 0.9 (3C, SiCH3), -1.7 (SiCH3) (Note: one aromatic signal is not observed due to overlap with C6D6 signals). Anal. Calcd for C36H54Si2Zr: C, 68.18; H, 8.58. Found: C, 68.28; H, 8.82. Cp′′(η5:η3-C13Me8H2)ZrH (4). A 250 mL Teflon-stoppered Schlenk flask was charged with Cp′′Flu′′ZrCl2 (0.500 g, 0.770 mmol) and iBuLi (0.103 g, 1.61 mmol). The flask was evacuated, and toluene (20 mL) was vacuum transferred into the flask at -78 °C. The flask was then backfilled with H2 and closed. The cold bath was then removed, and the reaction mixture was allowed to warm to room temperature and stirred for 12 h. All volatiles were removed under vacuum, and the product was extracted using pentane (20 mL). The extract was filtered through Celite, and all volatiles were removed under vacuum to give the crude product as a brown oily solid. The product was purified by crystallization from pentane solution (2 mL) at -35 °C and obtained as yellow crystals (0.194 g, 43%). 1H NMR (benzene-d6, 400 MHz, 25 °C): δ 6.11 (s, 1H, CH), 5.15 (m, 1H, CH), 5.06 (m, 1H, CH), 4.83 (m, 1H, CH), 3.82 (quart, 1H, CH), 2.53 (s, 3H, CH3), 2.43 (s, 3H, CH3), 2.29 (s, 3H, CH3), 2.23 (s, 3H, CH3), 2.14 (s, 3H, CH3), 2.10 (s, 3H, CH3), 1.56 (s, 3H, CH3), 0.84 (d, 3H, CH3), 0.73 (s, 1H, ZrH), 0.30 (s, 9H, SiCH3), 0.07 (s, 9H, SiCH3). 13C{1H} NMR (benzened6, 100 MHz, 25 °C): δ 131.5 (C), 129.6 (C), 128.3 (C), 122.9 (C), 121.9 (C), 121.7 (C), 119.7 (C), 119.0 (C), 117.0 (CH), 116.9 (C), 114.3 (CH), 113.9 (C), 113.1 (CH), 101.5 (C), 91.6 (C), 80.2 (CH), 68.2 (C), 35.5 (CH), 22.2 (CH3), 21.6 (CH3), 17.4 (CH3), 17.3 (CH3), 16.8 (CH3), 16.7 (CH3), 16.3 (CH3), 16.2 (CH3), 1.4 (SiCH3), 0.3 (SiCH3). Anal. Calcd for C32H48Si2Zr: C, 66.25; H, 8.34. Found: C, 66.38; H, 8.69. Cp′′(η5:η3-C13Me8H6)ZrH (5). A 100 mL Teflon-stoppered Schlenk flask was charged with compound 3 (0.400 g, 0.631 mmol) and hexanes (25 mL). The solution was degassed (3 × freeze/pump/ thaw), and the flask was then backfilled with H2. The reaction mixture was stirred at 60 °C for 3 days. All volatiles were removed under vacuum. A 1H NMR spectrum of the crude product indicated complete conversion of the starting material. The product was dissolved in hexanes (5 mL), and the resulting solution was filtered through Celite. All volatiles were removed under vacuum. The product was further purified by crystallization from a saturated pentane solution at -35 °C, yielding the pure product as yellow crystals (0.172 g, 47%). 1H NMR (benzene-d6, 400 MHz, 25 °C): δ 5.73 (s, 1H, CH), 5.20 (m, 1H, CH), 4.90 (m, 1H, CH), 4.75 (m, 1H, CH), 3.74 (m, 1H, CH), 2.91 (quart, 1H, CH), 2.53 (m, 1H, CH), 2.28 (s, 3H, CH3), 2.24 (m, 1H, CH), 2.15 (s, 3H,CH3), 1.62 (m, 1H, CH), 1.51 (s, 3H, CH3), 1.19 (d, 3H, CH3), 1.10 (d, 3H, CH3), 1.06 (d, 3H, CH3), 0.79 (d, 3H, CH3), 0.73 (d, 3H, CH3), 0.64 (s, 1H, ZrH), 0.47 (s, 9H, SiCH3), 0.23 (s, 9H, SiCH3). 13C{1H} NMR (benzene-d6, 100 MHz, 25 °C): δ 123.7 (C), 123.5 (C), 123.1 (C), 117.1 (C), 116.9 (C), 115.6 (CH), 113.4 (CH), 113.0 (C), 112.5

Organometallics, Vol. 28, No. 7, 2009 2293 Table 5. Selected Crystal Data and Data Collection Parameters for 1-syn-1,2-DHF*D, 2, 3, and 5 formula fw T (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z abs coeff (mm-1) final R indices R1 wR2 R indices (all) R1 wR2

1-syn-1,2-DHF*D

2

3

5

C33H50Si2Zr 594.13 125(2) 0.71073 triclinic P1j 9.926(2) 9.966(2) 16.895(4) 96.454(3) 92.927(3) 104.927(3) 1599.1(6) 2 0.438 0.0414 0.1036 0.0583 0.1106

C36H54Si2Zr 634.19 156(2) 0.71073 monoclinic P21/c 10.7773(14) 20.796(3) 15.510(2) 90 102.110(13) 90 3398.8(9) 4 0.416 0.0575 0.1297 0.0982 0.1448

C37H56Si2Zr 648.22 152(2) 0.71073 triclinic P1j 10.9001(17) 11.1535(17) 15.461(2) 73.957(2) 87.293(2) 76.215(2) 1754.1(5) 2 0.405 0.0570 0.1380 0.0870 0.1503

C32H52Si2Zr 584.14 148(2) 0.71073 monoclinic P21/c 9.7083(7) 16.2335(12) 20.6841(15) 90 98.2390(10) 90 3226.2(4) 4 0.433 0.0384 0.0924 0.0553 0.0981

(CH), 102.5 (C), 97.4 (C), 88.3 (CH), 63.9 (C), 39.5 (CH), 37.2 (CH), 35.9 (CH), 34.2 (CH), 33.3 (CH), 22.5 (CH3), 21.0 (CH3), 18.4 (CH3), 18.3 (CH3), 17.0 (CH3), 16.7 (CH3), 15.7 (CH3), 10.4 (CH3), 1.15 (SiCH3), 0.96 (SiCH3). Anal. Calcd for C32H52Si2Zr: C, 65.80; H, 8.97. Found: C, 65.68; H, 9.23. Crystallographic Structure Determinations. Crystallographic data for all compounds are summarized in Table 5. All crystals were mounted on a glass fiber using Paratone-N oil. The Laue symmetry of each crystal was photographically determined. No symmetry higher than triclinic was observed for 1-syn-1,2-DHF*D as well as 3, and solution in the centrosymmetric space group option for both compounds yielded computationally stable results of refinement. The space groups of 2 and 5 were assigned unambiguously from systematic absences. All structures were solved by direct methods, refined with anisotropic thermal parameters, and include idealized hydrogen atom contributions except for 1-syn-1,2-DHF*D and 5, where Zr-bonded hydrogen atoms were located and refined. All computations were performed using SHELXTL software (version 5.1, G. Sheldrick, Bruker Analytical X-ray Systems, Madison, WI).

Acknowledgment. We thank Jennifer McBee and Joe Escalada for assistance with the X-ray crystallography. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division, U.S. Department of Energy, under Contract No. DE-AC02-05CH11231, and P.B. thanks NSERC for a postdoctoral fellowship. Supporting Information Available: Crystallographic data (tables and CIF files) for 1-syn-1,2-DHF*D, 2, 3, and 5 are available free of charge via the Internet at http://pubs.acs.org. OM900047X