ansa-Zirconocenes with Aluminum or Gallium in Bridging Positions

Oct 19, 2010 - Synopsis. The first aluminum- and gallium-bridged ansa-metallocenes have been synthesized by the amine-elimination route...
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Organometallics 2010, 29, 6038–6044 DOI: 10.1021/om100788d

ansa-Zirconocenes with Aluminum or Gallium in Bridging Positions Clinton L. Lund,‡,^ Samuel S. Hanson,‡ Gabriele Schatte,§ J. Wilson Quail,§ and Jens M€ uller*,‡ ‡

Department of Chemistry, §Saskatchewan Structural Sciences Centre, University of Saskatchewan, 110 Science Place, Saskatoon, Saskatchewan S7N 5C9, Canada. ^ Current address: Department of Chemistry, University of Toronto, 80 St George St., Toronto, Ontario, M5S 3H6 Canada Received August 12, 2010

Bis(cyclopentadienyl)aluminum and -gallium species of the type (Pytsi)ECp2 [E = Al (2a), Ga (2b); Pytsi = C(SiMe3)2SiMe2(2-C5H4N)] have been prepared from NaCp and respective dichlorides (Pytsi)ECl2 in isolated yields of 65% (2a) and 83% (2b). In addition to applying standard methods of characterization (NMR spectroscopy, CHN analysis, and MS), the molecular structures of both compounds were solved by single-crystal X-ray crystallography. Reactions between 2a or 2b with Zr(NMe2)4 in benzene, followed by addition of an excess of ClSiMe3, gave the first ansa-zirconocene dichlorides with aluminum (4a) or gallium (4b) in bridging position. The gallium compound 4b was isolated in a yield of 40% and characterized by NMR spectroscopy, CHN analysis, and mass spectrometry. The aluminum species 4a could not be obtained in an analytically pure form; it was characterized by NMR spectroscopy, and its molecular structure was determined by single-crystal X-ray analysis. The angle between the two planes of the Cp ligands in 4a was found to be 57.17(11), which is larger than in Cp2ZrCl2 (R = 53.5) and smaller than in silicon-bridged ansa-zirconocene dichlorides (e.g., Me2Si(C5H4)2ZrCl2: R = 60.1). Crystallization attempts of 4a from DCCl3 solutions gave crystals of the new species (Pytsi)AlCl[(C5H4)ZrCp] (7), which was characterized by NMR spectroscopy and single-crystal X-ray analysis. In comparison to the Pytsi-containing species 2a and 2b, the less sterically protected compound [2-(Me2NCH2)C6H4]AlCp2 (6), which was prepared from NaCp and [2-(Me2NCH2)C6H4]AlCl2 in yields of 94% and characterized by standard methods (NMR, CHN, and MS), reacted with Zr(NMe2)4 to give the known Cp2Zr(NMe2)2. The targeted ansa-species was not formed, as judged by 1H NMR spectroscopy. The gallium-bridged compound 4b and Cp2ZrCl2, respectively, had been tested for polymerizations of ethylene utilizing MAO as an activator (300 equiv of MAO; 5 or 10 μmol of precatalyst; 15 or 70 psi), resulting in activities of 637 to 1080 for 4b and of 640 to 2601 kg PE (mol Zr)-1 h-1 atm-1 for Cp2ZrCl2. Introduction After the developments of low-pressure olefin polymerization catalysts by Ziegler and Natta, metallocene-based catalysts for homogeneous olefin polymerizations became the focus of attention.1 In particular, the finding that chiral ansa-metallocenes allow a control of the polymer tacticity was a major breakthrough for olefin polymerization.1 During the last three decades, non-metallocene catalyst systems including the half-sandwich, constrained-geometry compounds attracted a tremendous amount of research activity.2 Even though many ansa-metallocenes with different bridging elements are known, aluminum- and gallium-bridged species are still lacking.1 The following report addresses *To whom correspondence should be addressed. E-mail: jens.mueller@ usask.ca. (1) (a) Brintzinger, H. H.; Fischer, D.; M€ ulhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. 1995, 34, 1143–1170. (b) Bochmann, M. J. Chem. Soc., Dalton Trans. 1996, 255–270. (c) Kaminsky, W. J. Chem. Soc., Dalton Trans. 1998, 1413–1418. (d) Wang, B. Coord. Chem. Rev. 2006, 250, 242–258. (2) (a) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428–447. (b) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283–315. pubs.acs.org/Organometallics

Published on Web 10/19/2010

this deficiency. In 2005, our research group published the first aluminum-bridged sandwich compound;3 since then, we expanded the family of aluminum- and gallium-bridged metallacyclophanes from ferrocenophanes4 to vanadarenophanes,4b chromarenophanes,4b molybdarenophanes,5 and ruthenocenophanes.6 For all these strained sandwich species, a salt metathesis between a dilithiated sandwich compound and a respective group 13 element dichloride RECl2 had been employed, with R being a bulky, intramolecularly coordinating ligand (Chart 1). Even though the salt metathesis method is most widely used for strained sandwich compounds, the “flytrap” method offers an alternative approach (see Scheme 1).7 On the (3) Schachner, J. A.; Lund, C. L.; Quail, J. W.; M€ uller, J. Organometallics 2005, 24, 785–787. (4) (a) Schachner, J. A.; Lund, C. L.; Quail, J. W.; M€ uller, J. Organometallics 2005, 24, 4483–4488. (b) Lund, C. L.; Schachner, J. A.; Quail, J. W.; M€uller, J. Organometallics 2006, 25, 5817–5823. (c) Schachner, J. A.; Quail, J. W.; M€uller, J. Acta Crystallogr. 2008, E64, m517. (d) Bagh, B.; Gilroy, J. B.; Staubitz, A.; M€uller, J. J. Am. Chem. Soc. 2010, 132, 1794–1795. (5) (a) Lund, C. L.; Schachner, J. A.; Quail, J. W.; M€ uller, J. J. Am. Chem. Soc. 2007, 129, 9313–9320. (b) Lund, C. L.; Bagh, B.; Quail, J. W.; M€uller, J. Organometallics 2010, 29, 1977–1980. (6) Schachner, J. A.; Tockner, S.; Lund, C. L.; Quail, J. W.; Rehahn, M.; M€ uller, J. Organometallics 2007, 26, 4658–4662. r 2010 American Chemical Society

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Organometallics, Vol. 29, No. 22, 2010 Chart 1

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Scheme 2. Synthesis of ansa-Zirconocenes 4a (E = Al) and 4b (E = Ga)

Scheme 1. Common Synthetic Methods for ansa-Metallocenes

other hand, for the preparation of related ansa-metallocene dihalides, the flytrap method is used exclusively.8 In order to widen the synthetic options for aluminum- and galliumbridged sandwich compounds, we intended to develop a flytrap method for those species. After the first structural characterization of boron-bridged ansa-zirconocene dichloride in 1997 by Shapiro et al.,9 these ansa-metallocenes of the lightest group 13 element are now well known in the literature.10 In this report, we summarize our efforts to prepare the first ansa-zirconocenes with aluminum or gallium in bridging positions. Results of ethylene polymerizations using a gallium-bridged zirconocenophane, activated by MAO, are described.

Results and Discussion ansa-Metallocenes are usually prepared by the flytrap method (Scheme 1) either from a transition-metal halide and a respective dianionic ligand (method A) or through an amine elimination, in a one-step procedure, starting from the neutral ligand precursor ExRy(C5H5)2 (method B).8,11 Both methods start with neutral bis(cyclopentadienyl) ligand precursors, which are synthesized first in order to get access to aluminum- or gallium-bridged ansa-metallocenes. Scheme 2 illustrates the synthesis of the first aluminumand gallium-bridged ansa-zirconocenes, 4a and 4b, respectively, (7) Herbert, D. E.; Mayer, U. F. J.; Manners, I. Angew. Chem., Int. Ed. 2007, 46, 5060–5081. (8) Prashar, S.; Anti~ nolo, A.; Otero, A. Coord. Chem. Rev. 2006, 250, 133–154. (9) Stelck, D. S.; Shapiro, P. J.; Basickes, N.; Rheingold, A. L. Organometallics 1997, 16, 4546–4550. (10) (a) Shapiro, P. J. Eur. J. Inorg. Chem. 2001, 321–326. (b) Aldridge, S.; Bresner, C. Coord. Chem. Rev. 2003, 244, 71–92. (c) Braunschweig, H.; Breitling, F. M.; Gullo, E.; Kraft, M. J. Organomet. Chem. 2003, 680, 31– 42. (11) Other variations of the two paths outlined in Scheme 1 are known; see ref 8. (12) Eaborn, C.; Smith, J. D. J. Chem. Soc., Dalton Trans. 2001, 1541–1552. (13) (a) Al-Juaid, S. S.; Eaborn, C.; Hitchcock, P. B.; Hill, M. S.; Smith, J. D. Organometallics 2000, 19, 3224–3231. (b) Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Smith, J. D. Chem. Commun. 2000, 691–692. (c) AlJuaid, S. S.; Avent, A. G.; Eaborn, C.; El-Hamruni, S. M.; Hawkes, S. A.; Hill, M. S.; Hopman, M.; Hitchcock, P. B.; Smith, J. D. J. Organomet. Chem. 2001, 631, 76–86. (d) Al-Juaid, S. S.; Avent, A. G.; Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Patel, D. J.; Smith, J. D. Organometallics 2001, 20, 1223– 1229. (e) Eaborn, C.; Hill, M. S.; Hitchcock, P. B.; Smith, J. D. J. Chem. Soc., Dalton Trans. 2002, 2467–2472. (f) Howson, J.; Eaborn, C.; Hitchcock, P. B.; Hill, M. S.; Smith, D. J. J. Organomet. Chem. 2005, 690, 69–75.

by the amine-elimination method. From the known dichlorides 1a and 1b, the new bis(cyclopentadienyl) species were prepared in isolated yields of 65% (2a) and 83% (2b). We have chosen the bulky Pytsi ligand for the group 13 element in bridging position [Pytsi=C(SiMe3)2SiMe2(2-C5H4N)], as we had employed this ligand successfully in the synthesis of strained [1]metallacyclophanes3,4a,4c,6,14 and low-coordinate aluminum cations15 before. This trisyl-based ligand12 was first synthesized by Eaborn and Smith et al. and employed in subsequent chemistry.13 Species 2a and 2b show very similar NMR spectra that are consistent with Cs-symmetrical species. All 10 Cp protons appear as one sharp singlet in the 1H NMR spectra and, similarly, the Cp groups give rise to only one resonance in the 13C NMR measurements. It is well known that main group element Cp compounds are often highly fluxional,16 and one would expect that all Cp ligands in 2a and 2b are η1-coordinated, but fast haptotropic shifts should result in averaged NMR signals. This expectation is supported by the molecular structures of both species in the solid state (Figures 1 and 2; Table 1). Expectedly, both structures do not reveal any surprises. The group 13 element is distorted tetrahedrally surrounded with element-carbon bond lengths in the range 2.049(3) (Al1-C7) to 2.058(3) A˚ (Al1-C16) for 2a and 2.0353(18) (Ga1-C21) to 2.0867(18) A˚ (Ga1-C16) for 2b. The Al-N bond length (1.966(2) A˚) is shorter than the Ga-N bond length (2.0759(15) A˚), and both bonds are longer than those found in their respective starting dichlorides (1a: 1.9383(16) A˚;13f 1b 2.004(2) A˚),4a illustrating that the Lewis acidity decreases from 1a,b to 2a,b. As shown in Scheme 2, the new species of type 2 were used to prepare the ansa-zirconocenes 4a and 4b. The procedure was performed as a one-pot synthesis without the isolation of intermediates 3a,b. The conversions of the Zr diamides 3a,b to the desired Zr dichlorides 4a,b were carried out with Me3SiCl.17 Whereas the gallium-bridged species 4b was isolated in a moderate yield of 40%, the aluminum species 4a could not be obtained in an analytically pure form. However, 4a could be characterized by NMR spectroscopy, (14) Lund, C. L.; Stanga, O.; Quail, J. W.; M€ uller, J. Can. J. Chem. 2007, 85, 483–490. (15) Stanga, O.; Lund, C. L.; Liang, H.; Quail, J. W.; M€ uller, J. Organometallics 2005, 24, 6120–6125. (16) (a) Jutzi, P. Chem. Rev. 1986, 86, 983–996. (b) Jutzi, P.; Burford, N. Chem. Rev. 1999, 99, 969–990. (17) Diamond, G. M.; Jordan, R. F.; Petersen, J. L. J. Am. Chem. Soc. 1996, 118, 8024–8033.

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Figure 1. Molecular structure of 2a with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected atom-atom distances [A˚] and bond angles [deg] for 2a: Al1-N1 = 1.966(2), Al1-C7 = 2.049(3), Al1-C16 = 2.058(3), Al1-C21 = 2.055(3), C7-Al1-C16 = 119.32(11), C7-Al1-C21 = 113.92(11), C7-Al1-N1 = 97.46(9), N1Al1-C16 = 103.55(11), N1-Al1-C21 = 110.21(11), C16Al1-C21 = 110.68(12).

Figure 2. Molecular structure of 2b with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected atom-atom distances [A˚] and bond angles [deg] for 2b: Ga1-N1 = 2.0759(15), Ga1-C7 = 2.0512(17), Ga1-C16 = 2.0867(18), Ga1-C21 = 2.0353(18), C7-Ga1C16 = 116.95(7), C7-Ga1-C21 = 131.05(7), C7-Ga1-N1= 93.99(6), N1-Ga1-C16 = 100.48(7), N1-Ga1-C21 = 103.47(7), C16-Ga1-C21 = 104.41(7).

and we successfully picked one single crystal from impure samples that resulted in solving its molecular structure by X-ray crystallography (Figure 3). Unfortunately, we were not able to produce crystals of the gallium species 4b suitable for single-crystal analysis. NMR spectra for 4a and 4b are very similar, consistent with Cs-symmetrical species in solution. For example, in 1H NMR measurements, two singlets for the silicon-bound methyl groups (18:6 intensity ratio), four multiplets for Cp protons (2:2:2:2 intensity ratio), and four multiplets (1:1:1:1 intensity ratio) for the pyridyl moiety can be seen. Intramolecularly coordinated ligands like that in species 4a,b often show dynamic behavior in solution;18 for example, the galla[1]molybdarenophane equipped with the Pytsi ligand (18) (a) M€ uller, J.; Englert, U. Chem. Ber. 1995, 128, 493–497. (b) M€ uller, J.; Schr€ oder, R.; Wang, R. M. Eur. J. Inorg. Chem. 2000, 153–157.

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exhibits coalescences already at room temperature (1H NMR measurement at 500 MHz) caused by a fast breakage and re-formation of the Ga-N donor bond on the other side of the group 13 element.5a However, under similar conditions, 4a,b do not indicate any fluxional behavior. The molecular structure of 4a in the solid state is depicted in Figure 3; Table 1 compiles crystal and structural refinement data. The aluminum atom in 4a is similarly coordinated to that in the ligand precursor molecule 2a (Figure 1 and Scheme 2). Whereas the Al-N bond in 4a (1.986(3) A˚) is slightly longer than that in 2a (1.966(2) A˚), all other direct bonds around aluminum are slightly shorter, with the more significant changes found for those bonds where protons were replaced by the ZrCl2 moiety (Al1-C16: 2.058(3) to 1.986(3) A˚; Al1-C21: 2.055(3) to 1.996(3) A˚). Two Cp moieties in 4a, with a tilt angle R of 57.17(11) (Figure 4), are slightly more canted than those in Cp2ZrCl2 (R = 53.5)19 and, expectedly, less tilted compared to boron- (ERx = B(SMe2)Ph, R=65.9;9 ERx=BNiPr2, R = 65.520), carbon(ERx=CH2, R=70.0),21 and silicon-bridged (ERx= SiMe2, R = 60.1)21,22 ansa-zirconocene dichlorides. One part of the distortion at the two ipso-carbon atoms can be described by the angle φ (Figure 4), which was found to be 19.6(1) and 20.7(1) in species 4a. These values are higher than those measured for silicon-bridged ansa-zirconocene dichlorides (ERx = SiMe2, φ = 16.6).21 The θ angle in 4a (C16-Al1C21=91.52(12)) is significantly reduced compared to that in the free ligand precursor 2a (C16-Al1-C21 = 110.68(12); Figure 1). As mentioned before, the gallium-bridged ansa-species 4b was isolated as an analytically pure compound, whereas the aluminum homologue 4a could not be isolated with an acceptable purity. The main impurities in samples of 4a were the dichloride 1a (Scheme 2) and Cp2ZrCl2. However, their amounts varied from experiment to experiment. Organometallic aluminum compounds usually react with Brønsted acids, resulting in heterolytic cleavage of Al-C bonds. Obviously, HNMe2 acts as a Brønsted acid in the synthesis of 4a. Through reactions with HNMe2 species, 2a and intermediate 3a would yield CpH, (Pytsi)Al(NMe2)2, and Cp2Zr(NMe2)2. The latter two species would further be transformed into the detected dichlorides (Pytsi)AlCl2 (1a) and Cp2ZrCl2 through exposure to ClSiMe3 (Scheme 2). Aluminum-carbon bonds are much more polarized than respective gallium-carbon bonds, and hence, aluminum species are more susceptible to proton sources, which can explain why the gallium species 4b was isolatable in a pure form but the aluminum species 4a was not (Allred-Rochow electronegativities: Al = 1.47; Ga = 1.82).23 Consequently, HNMe2 should be removed as fast as possible from the reaction vessel to suppress these problematic side reactions. However, Jordan et al. showed that a certain amount of HNMe2 is required in the formation of the targeted (19) (a) Prout, K.; Cameron, T. S.; Forder, R. A. Acta Crystallogr., Sect. B 1974, B 30, 2290–2304. (b) Corey, J. Y.; Zhu, X. H.; Brammer, L.; Rath, N. P. Acta Crystallogr., Sec. C 1995, 51, 565–567. (20) Ashe, A. J.; Fang, X.; Kampf, J. W. Organometallics 1999, 18, 2288–2290. (21) Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B. M.; Parkin, G.; Brandow, C. G.; Bercaw, J. E.; Jardine, C. N.; Lyall, M.; Green, J. C.; Keister, J. B. J. Am. Chem. Soc. 2002, 124, 9525–9546. (22) Bajgur, C. S.; Tikkanen, W. R.; Petersen, J. L. Inorg. Chem. 1985, 24, 2539–2546. (23) Holleman-Wiberg. Inorganic Chemistry, 1st English ed.; Academic Press: San Diego, 2001; p 134.

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Table 1. Crystal and Structural Refinement Data for Compounds 2a, 2b, 4a, and 7 2a

2b

4a

7b

)

)

empirical formula C24H38AlNSi3 C24H38GaNSi3 C24H36AlCl2NSi3Zr C24H37AlCl3NSi3Zr fw 451.80 494.55 611.92 648.38 0.12  0.12  0.10 0.20  0.20  0.15 0.15  0.08  0.03 0.15  0.10  0.10 cryst size/mm3 monoclinic, P21/c (No. 14) monoclinic, P21/c (No. 14) monoclinic, P21/c (No. 14) cryst syst, space group monoclinic, P21/c (No. 14) Z 4 4 4 4 a/A˚ 19.1400(5) 16.2099(3) 9.6578(2) 16.851(3) b/A˚ 9.1740(2) 9.7061(2) 15.4967(3) 10.516(2) c/A˚ 15.4470(6) 19.5175(3) 19.8486(5) 17.334(4) β/deg 103.0300(18) 122.3580(10) 105.055(2) 103.36(3) 3 2642.51(14) 2593.95(8) 2868.66(11) 2988.6(10) volume/A˚ 1.136 1.266 1.417 1.441 Fcalc/mg m-3 temperature/K 173(2) 173(2) 173(2) 173(2) 0.224 1.210 0.739 0.801 μcalc/mm-1 θ range/deg 3.19 to 26.37 2.09 to 32.06 2.55 to 26.73 2.28 to 27.44 reflns collected/unique 10 309/3795 61 551/9029 39 263/6089 42 225/6820 absorp corr multiscan multiscan multiscan multiscan data/restraints/params 5400/0/270 9029/0/270 6089/0/298 6820/0/298 goodness-of-fit 1.034 1.027 1.038 1.045 0.0520 0.0410 0.0381 0.0417 R1 [I > 2σ(I )]a 0.1443 0.0921 0.0794 0.0967 wR2 (all data)a 0.568 and -0.677 0.366 and -0.559 0.375 and -0.391 0.454 and -0.619 largest diff peak -3 and hole, ΔFelect/A˚ P P P P a R1 = [ Fo| - |Fc ]/[ |Fo|] for [Fo2 > 2σ(Fo2)], wR2 = {[ w(Fo2 - Fc2)2]/[ w(Fo2)2]}1/2 [all data]. b Attempts to crystallize 4a from DCCl3 resulted in crystals of 7 as a monodeuterated species (see discussion and Experimental Section). The deuterium atoms were included as hydrogen atoms in the refinement. Chemical formula and derived quantities fw, Fcalc μcalc, and F(000) were not corrected.

Figure 3. Molecular structure of 4a with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected atom-atom distances [A˚] and bond angles [deg] for 4a: Al1-N1 = 1.986(3), Al1-C7 = 2.007(3), Al1-C16 = 1.986(3), Al1-C21 = 1.996(3), C7-Al1-C16 = 122.72(12), C7-Al1-C21 = 129.12(12), Zr1-Cl1 = 2.4502(8), Zr1Cl2 = 2.4472(8), C7-Al1-N1 = 97.50(11), N1-Al1-C16 = 111.72(12), N1-Al1-C21 = 103.21(12), C16-Al1-C21 = 91.52(12) (θ angle; see Figure 4), Cl1-Zr1-Cl2 = 97.52(3).

Figure 4. Angles to describe the geometry of single-atombridged ansa-metallocenes of C2v symmetry [R = 180 - β = 2τ - γ þ 180; angles defined according to ref 21]. Compound 4a: R = 57.17(11); φ(180 - Al1-C16-Cpcentroid) = 19.6(1); φ(180 - Al1-C21-Cpcentroid) = 20.7(1); γ(Cpcentroid-Zr1Cpcentroid) = 124.1(1).

ansa-zirconocenes. It has been shown that a complex with two zirconium atoms, RxE[(η5-C5H4)Zr(NMe2)3]2, is formed

as an intermediate and HNMe2 opens a path toward the desired product while regenerating the starting compound Zr(NMe2)4.17,24 The free amine is especially important for the preparation of chiral ansa-metallocenes, as it converts meso into rac species.17 We attempted to control the amount of HNMe2 present in the reaction vessel by changing the flow of inert gas passing by the top of the reflux condenser; however, this worked well only for 4b. The sensitivity of 4a toward protic compounds was also revealed during an attempt to grow single crystals of 4a from a NMR sample in CDCl3. After a few days of storing the NMR sample at -25 C, single crystals were obtained, but the X-ray structural determination showed that the ansabridge had been broken and instead of the expected species 4a, species 7 was found (Figure 5, Table 1). In addition to this structural characterization in the solid state, compound 7 was characterized by 1H and 13C NMR spectroscopy in solution. Proton NMR data revealed that 7 must have been formed from 4a by reaction of one equivalent of DCl. The singlet for the fast rotating Cp ligand in 7 at 6.61 ppm showed a relative intensity of four instead of five protons. Expectedly, the molecular structure of compound 7 does not reveal any surprises; for example, bond lengths are similar to those discussed before for species 2a and 4a. The tilt angle R for this unstrained zirconocene dichloride is 52.53(10), very similar to that for Cp2ZrCl2 (R = 53.5).19 The bulkiness of the Pytsi ligand also plays an important role in the accessibility of ansa-species. Similar to methods described for the Pytsi-containing compounds 2a and 2b (Scheme 2), we synthesized the less-protected bis(cyclopentadienyl) species 6 (Scheme 3). Reaction of 6 with Zr(NMe2)4 at room temperature for 16 h resulted in an exchange of Cp moieties. The Cp range in the 1H NMR spectrum showed only one singlet at 5.85 ppm (C6D6), (24) (a) Diamond, G. M.; Jordan, R. F.; Petersen, J. L. Organometallics 1996, 15, 4045–4053. (b) Christopher, J. N.; Diamond, G. M.; Jordan, R. F.; Petersen, J. L. Organometallics 1996, 15, 4038–4044. (c) Diamond, G. M.; Jordan, R. F.; Petersen, J. L. Organometallics 1996, 15, 4030–4037.

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Compound 4b and Cp2ZrCl2 showed similar activities at higher ethylene pressure and higher precatalyst loading (entry 1 compared to 4). However, precatalyst 4b is more sensitive than Cp2ZrCl2 to either a reduction of the amounts of the precatalyst (entry 2 compared to 5) or a reduction in precatalyst together with a reduction in ethylene pressure (entry 3 compared to 6). The highest activity of 1080 kg PE (mol Zr)-1 h-1 atm-1 (entry 6) was not even half of that of its competitor Cp2ZrCl2 (2601 kg PE (mol Zr)-1 h-1 atm-1; entry 3). Still, using Gibson’s scale to evaluate catalyst activities, the 4b/MAO system can be classified as highly active.2a Figure 5. Molecular structure of 7 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected atom-atom distances [A˚] and bond angles [deg] for 7: Al1-N1 = 1.949(3), Al1-C7 = 2.008(3), Al1C16 = 1.985(3), Al1-Cl3 = 2.1636(13), Zr1-Cl1 = 2.4466(9), Zr1-Cl2 = 2.4489(10), C7-Al1-C16=117.05(12), C7-Al1Cl3 = 119.18(9), C7-Al1-N1 = 98.52(11), N1-Al1-C16 = 100.59(12), N1-Al1-Cl3 = 102.52(8), C16-Al1-Cl3 = 113.97(10), Cl1-Zr1-Cl2 = 96.05(4), R = 52.53(10), γ(Cpcentroid-Zr1-Cpcentroid)=130.8(1) (see Figure 4). Scheme 3

indicating that Cp2Zr(NMe2)225 had been formed. Following this reaction by 1H NMR spectroscopy did not reveal that the targeted ansa-zirconocene was present. In another series of experiments, we intended to use the salt metathesis as shown in Scheme 1 (method A) to prepare new ansa-metallocenes. In the first step, ligand precursors are deprotonated to give salts of dianions. All experiments carried out to deprotonate ligand precursors 2a, 2b, and 6 failed, even though several bases had been applied.26 Herrmann et al. prepared the first tin-bridged ansa-zirconocene via the amine-elimination method and noted that “because of the facile cleavage of Sn-Cp bonds, the standard transmetalation strategy proved unfavorable for the synthesis of tin-functionalized metallocenes”.27 With respect to their electronegativities, tin (1.72), aluminum (1.47), and gallium (1.82) have comparable properties (Allred-Rochow values23). The group 13 elements are usually stronger Lewis acids, which makes the deprotonation of the flytrap precursors 2a,b even more difficult than for similar tin species, and one can assume that bases rather act as nucleophiles. Ethylene Polymerization. Some initial tests with compound 4b as a precatalyst for polymerization of ethylene with MAO as an activator were performed. In order to compare the activity of this new gallium-bridged zirconocene dichloride to a known precatalyst, a series of experiments with Cp2ZrCl2 was performed as well (Table 2). (25) Chandra, G.; Lappert, M. F. J. Chem. Soc. (A) 1968, 1940–1945. (26) MeLi, BuLi, tBuLi, LiNiPr2, lithium 2,20 ,6,60 -tetramethylpiperidide, and LiN(SiMe3)2 were used with thf or Et2O as main solvents. We expected that a dianionic flytrap would precipitate; however, significant amounts of precipitate were not obtained. 1H NMR spectra taken from aliquots of reaction mixtures did not indicate that deprotonation had occurred. (27) Herrmann, W. A.; Morawietz, M. J. A.; Herrmann, H.-F.; K€ uber, F. J. Organomet. Chem. 1996, 509, 115–117.

Summary and Conclusion Intramolecularly coordinated bis(cyclopentadienyl) compounds of aluminum and gallium (2a, 2b, and 6) have been synthesized starting from respective dichlorides and NaCp. Species 2a and 2b are monomeric in the solid state with η1-coordinated Cp ligands. Jutzi et al. had used intramolecular coordination to prepare monomeric Al and Ga species containing Cp moieties that exhibit η1-coordinated Cp.28 In contrast to species described here, the donor moiety was anchored to the Cp ligand. Only the sterically protected species equipped with the bulky Pytsi ligand could be successfully employed to obtain the first aluminum- and gallium-bridged ansa-metallocenes (4a and 4b) using the amine-elimination method. While the aluminum species 4a could not be obtained in an analytically pure form, its molecular structure was determined in the solid state using single-crystal X-ray analysis. The syntheses of the new ansa-zirconocenes have proven more challenging than expected. The main difficulty is the presence of HNMe2, which can act as a Brønsted acid to cleave element-carbon bonds of species involved. The resulting byproducts hinder the isolation of pure products, and only the gallium-bridged ansa-zirconocene dichloride 4b was isolated analytically pure. The sensitivity of 4a toward proton sources was manifested in a serendipitous discovery that crystallization attempts of 4a from CDCl3 solutions resulted in the opened species 7 (Figure 5). The gallium species 4b can be activated with MAO and successfully applied to polymerize ethylene. Its activity is similar to or below that of the nonbridged Cp2ZrCl2.

Experimental Section General Procedures. Manipulations were done using standard Schlenk and glovebox techniques (O2 level