Synthesis, Structures, and Ethylene Polymerization Behavior of Bis

Jul 13, 2010 - Changle Chen, Han Lee, and Richard F. Jordan*. Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, ...
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Organometallics 2010, 29, 5373–5381 DOI: 10.1021/om1004034

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Synthesis, Structures, and Ethylene Polymerization Behavior of Bis(pyrazolyl)borate Zirconium and Hafnium Benzyl Complexes† Changle Chen, Han Lee, and Richard F. Jordan* Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637 Received May 1, 2010

The alkyl hydrido bis(pyrazolyl)borate reagent Tl(MeHB(3-mesitylpyrazolyl)2) (Tl(MeBpMs), 1) has been prepared and used to generate group 4 metal MeBpMs complexes. The reaction of 1 with ZrCl4 affords (MeBpMs*)2ZrCl2 (2, MeBpMs* = MeHB(3-mesitylpyrazolyl)(5-mesitylpyrazolyl)-). The reaction of 1 with M(CH2Ph)4 yields (MeBpMs)M(CH2Ph)3 (M = Zr (3), Hf (4)), bibenzyl, and Tl0, via initial MeBpMs/benzyl exchange to produce 3 or 4 and Tl(CH2Ph), followed by thermal decomposition of Tl(CH2Ph). The reaction of 3 with [Ph3C][B(C6F5)4] proceeds by one-electron oxidation of a zirconium benzyl bond to yield [(MeBpMs)Zr(CH2Ph)2][B(C6F5)4] (5) and a series of organic products derived from coupling and H atom transfer reactions of trityl and benzyl radicals. The reaction of 4 with [Ph3C][B(C6F5)4] yields [(MeBpMs)Hf(CH2Ph)2][B(C6F5)4] (6) along with Ph3CCH2Ph (86%) as the major organic product. The high yield of Ph3CCH2Ph in this reaction suggests that electrophilic benzyl abstraction competes with oxidative M-CH2Ph bond cleavage in this case. The reaction of 3 or 4 with 1 equiv of [H(OEt2)2][B(C6F5)4] affords [(MeBpMs)M(CH2Ph)2(OEt2)][B(C6F5)4] (M = Zr (7), Hf (8)). Zirconium complexes 3, 5, and 7 exhibit a stronger tendency for η2-benzyl bonding than hafnium analogues 4, 6, and 8. The cationic species 5 and 6 polymerize ethylene.

Introduction The activation of group 4 metal Tp0 MCl3 complexes that contain sterically bulky tris(pyrazolyl)borate ligands (Tp0 ) with methylalumoxane (MAO) generates highly active olefin polymerization catalysts.1,2 These Tp0 MCl3/MAO catalysts display interesting properties, including the production of ultrahigh molecular weight polyethylene (PE) and high 1-hexene incorporation in ethylene/hexene copolymerization. The catalytically active species in these systems have not been identified, and the chemistry of group 4 metal Tp0 M

alkyls has not been extensively explored.3-5 However, based on results for other MAO-activated group 4 metal catalysts, cationic Tp0 MRnX2-nþ (n = 1, 2; X = Cl or anionic ligand derived from MAO) alkyl complexes may play a key role in Tp0 MCl3/MAO catalysts. We recently reported the synthesis of Tp*Zr(CH2Ph)3 (Tp* = HB(3,5-Me2-pz)3) by the reaction of K[Tp*] with Zr(CH2Ph)4 (Scheme 1).6,7 The reaction of Tp*Zr(CH2Ph)3 with Ph3Cþ yields the cationic species Tp*Zr(CH2Ph)2þ, which rearranges to the bis-pyrazolyl species {(PhCH2)(H)B(μ-Me2pz)2}Zr(η2-Me2pz)(CH2Ph)þ at 0 °C via exchange of pyrazolyl and benzyl groups between boron and zirconium. A similar rearrangement of Tp*2Sm(CtCPh) to

† Part of the Dietmar Seyferth Festschrift. *To whom correspondence should be addressed. E-mail: rfjordan@ uchicago.edu. (1) (a) Murtuza, S.; Casagrande, O. L., Jr.; Jordan, R. F. Organometallics 2002, 21, 1882. (b) Michiue, K.; Jordan, R. F. Macromolecules 2003, 36, 9707. (c) Michiue, K.; Jordan, R. F. Organometallics 2004, 23, 460. (d) Michiue, K.; Jordan, R. F. J. Mol. Catal. A 2008, 282, 107. (2) (a) Gil, M. P.; Casagrande, O. L., Jr. Appl. Catal., A 2007, 332, 110. (b) Gil, M. P.; dos Santos, J. H. Z.; Casagrande, O. L., Jr. J. Mol. Catal. A 2004, 209, 163. (c) Furlan, L. G.; Gil, M. P.; Casagrande, O. L., Jr. Macromol. Rapid Commun. 2000, 21, 1054. (d) Gil, M. P.; Casagrande, O. L., Jr. J. Organomet. Chem. 2004, 689, 286. (e) Gil, M. P.; dos Santos, J. H. Z.; Casagrande, O. L., Jr. Macromol. Chem. Phys. 2001, 202, 319. (f) Nakazawa, H.; Ikai, S.; Imaoka, K.; Kai, Y.; Yano, T. J. Mol. Catal. A 1998, 132, 33. (g) Karam, A.; Jimeno, M.; Lezama, J.; Catari, E.; Figueroa, A.; de Gascue, B. R. J. Mol. Catal. A 2001, 176, 65. (h) Karam, A.; Casas, E.; Catari, E.; Pekerar, S.; Albornoz, A.; Mendez, B. J. Mol. Catal. A 2005, 238, 233. (i) Karam, A.; Pastran, J.; Casas, E.; Mendez, B. Polym. Bull. 2005, 55, 11. (j) Casas, E.; Karam, A.; Diaz-Barrios, A.; Albano, C.; Sanchez, Y.; Mendez, B. Macromol. Symp. 2007, 257, 131. (3) (a) Reger, D. L.; Tarquini, M. E.; Lebioda, L. Organometallics 1983, 2, 1763. (b) Ipaktschi, J.; Sulzbach, W. J. J. Organomet. Chem. 1992, 426, 59.

(4) For group 4 metal alkyl complexes containing tridentate bis(pyrazolyl)phenoxide ligands see: (a) Cuomo, C.; Milione, S.; Grassi, A. Macromol. Rapid Commun. 2006, 27, 611. (b) Milione, S.; Cuomo, C.; Grassi, A. Top. Catal. 2006, 40, 163. (c) Milione, S.; Bertolasi, V.; Cuenca, T.; Grassi, A. Organometallics 2005, 24, 4915. (d) Howe, R. G.; Tredget, C. S.; Lawrence, S. C.; Subongkoj, S.; Cowley, A. R.; Mountford, P. Chem. Commun. 2006, 223. (5) For recent work on other group 4 metal scorpionate complexes see: (a) Dunne, J. F.; Su, J.; Ellern, A.; Sadow, A. D. Organometallics 2008, 27, 2399. (b) Cai, H.; Lam, W. H.; Yu, X.; Liu, X.; Wu, Z.; Chen, T.; Lin, Z.; Chen, X.; You, X.; Xue, Z. Inorg. Chem. 2003, 42, 3008. (c) Gazzi, R.; Perazzolo, F.; Sostero, S.; Ferrari, A.; Traverso, O. J. Organomet. Chem. 2005, 690, 2071. (d) Bigmore, H. R.; Dubberley, S. R.; Kranenburg, M.; Lawrence, S. C.; Sealey, A. J.; Selby, J. D.; Zuideveld, M. A.; Cowley, A. R.; Mountford, P. Chem. Commun. 2006, 436. (e) Nomura, K.; Hasumi, S.; Fujiki, M.; Itagaki, K. J. Chem. Soc., Dalton Trans. 2009, 9052. (f) Itagaki, K.; Kakinuki, K.; Katao, S.; Khamnaen, T.; Fujiki, M.; Nomura, K.; Hasumi, S. Organometallics 2009, 28, 1942. (g) Takeuchi, D. J. Chem. Soc., Dalton Trans. 2010, 311. (6) Lee, H.; Jordan, R. J. Am. Chem. Soc. 2005, 127, 9384. (7) For a similar synthesis of Tp0 AlR2 compounds see: Looney, A.; Parkin, G. Polyhedron 1990, 9, 265.

r 2010 American Chemical Society

Published on Web 07/13/2010

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Scheme 1a

a

Figure 1. Molecular structure of Tl(MeBpMs) (1). Hydrogen atoms except the B-H---Tl hydrogen are omitted. Selected bond lengths (A˚) and angles (deg): Tl(1)-N(1) 2.635(5), N(1)-N(2) 1.368(7), B(1)N(2) 1.583(8), B(1)-C(13) 1.607(13), N(1)-Tl(1)-N(1A) 79.1(2), N(2)-B(1)-N(2A) 106.4(7), N(2)-B(1)-C(13) 111.0(5), N(2)N(1)-Tl(1) 119.0(4), N(1)-N(2)-B(1) 119.0(5).

Anion = B(C6F5)4-.

Scheme 2a

and initial studies of the ethylene polymerization behavior of the cationic species.

Results and Discussion

{H(PhCtC)B(μ-Me2pz)2}(Tp*)Sm(η2-Me2pz) was reported by Takats.8 Both Tp*Zr(CH2Ph)2þ and {(PhCH2)(H)B(μ-Me2pz)2}Zr(η2-Me2pz)(CH2Ph)þ polymerize ethylene at -78 to -60 °C to linear polyethylene (PE) without significant chain transfer.9 These results suggest that similar rearrangements of Tp0 ligands to bis(pyrazolyl)borate (Bp0 ) ligands may be a general reaction for d0 metal Tp0 complexes and that Bp0 metal alkyl species may be important in group 4 Tp0 MCl3/MAO catalysts. The bis-pyrazolyl ligand in {(PhCH2)(H)B(μ-Me2pz)2}Zr(η2-Me2pz)(CH2Ph)þ is an unusual example of a mixed alkyl hydrido bis-pyrazolyl ligand (RBp0 ).10 The objective of the present work is to expand this class of ligand and to investigate the properties of group 4 metal RBp0 complexes, with the ultimate aim of understanding the mechanism of Tp0 MX3/MAO catalysts. We describe the synthesis and structures of the Tl salt of the new ligand MeHB(3-mesitylpyrazolyl)2([MeBpMs]-), neutral (MeBpMs)M(CH2Ph)3, and cationic (MeBpMs)M(CH2Ph)2þ zirconium and hafnium complexes

Tl(MeBpMs) (1). The reaction of Li[MeBH3] with 2 equiv of 3-mesitylpyrazole (HpzMs) at 60 °C in THF, followed by reaction with TlOAc at room temperature, affords Tl(MeBpMs) (1) in 73% yield (Scheme 2). Pure 1 was isolated as a white solid by simply washing the crude product with hexanes. NMR monitoring experiments in THF-d8 using Cp2Fe as an internal standard showed that the formation of 1 is accompanied by reversible 1,2-borotropic shifts that interconvert 3-Ms-pz (3mesitylpyrazolyl) and 5-Ms-pz isomers. As shown in Scheme 2, the reaction of Li[MeBH3] with HpzMs yields a 2:1 mixture of Li[MeB(3-Ms-pz)H2] and Li[MeB(5-Ms-pz)H2] (100%, isomers not distinguished) after 1 day at 23 °C. Further reaction at 23 °C gives less than 30% of bis(pyrazolyl)borate species after 2 days. However, heating the mixture at 60 °C for 2 days yields a 3:1 mixture of Li[MeBpMs] and Li[MeBpMs*] (MeBpMs* = MeHB(3-mesitylpyrazolyl)(5-mesitylpyrazolyl)-) in quantitative yield. The reaction of this mixture with TlOAc at 23 °C for 2 days yields a 6:1 mixture of Tl(MeBpMs) and Tl(MeBpMs*). The latter isomer is much more soluble than Tl(MeBpMs) in hexanes and hence is removed by washing with hexanes. Molecular Structure of 1. The molecular structure of 1 was determined by X-ray diffraction and is shown in Figure 1. The chelate ring of 1 adopts a boat conformation. The B-H hydrogen is located in an axial position of the chelate ring. The BH-Tl distance (3.06 A˚) is slightly shorter than the sum of van der Waals radii of Tl and H (3.16 A˚), consistent with the presence of a weak B-H---Tl interaction. A variety of thallium bis(pyrazolyl)borate complexes, including Tl(H2B(pz)2) and Tl(H2B(pzTrip)2) (pzTrip = 3-(2,4,6-iPr3-phenyl)-pz), have been synthesized and structurally characterized by Parkin.11 These complexes also exhibit weak BH---Tl interactions.

(8) Lin, G.; McDonald, R.; Takats, J. Organometallics 2000, 19, 1814. (9) Nienkemper, K.; Lee, H.; Jordan, R. F.; Ariafard, A.; Dang, L.; Lin, Z. Organometallics 2008, 27, 5867. (10) H2B(pz0 )2- and R2B(pz0 )2- ligands (R = alkyl, aryl, pz0 = generic pyrazolyl) have been studied extensively. Trofimenko, S. Scorpionates: The Coordination Chemistry of Polypyrazolylborate Ligands; Imperial College Press: London, 1999.

(11) (a) Dowling, C.; Ghosh, P.; Parkin, G. Polyhedron 1997, 16, 3469. (b) Fillebeen, T.; Hascall, T.; Parkin, G. Inorg. Chem. 1997, 36, 3787. (c) Fleming, J. S.; Psillakis, E.; Couchman, S. M.; Jeffery, J. C.; McCleverty, J. A.; Ward, M. D. J. Chem. Soc., Dalton Trans. 1998, 537. (d) Ghosh, P.; Hascall, T.; Dowling, C.; Parkin, G. J. Chem. Soc., Dalton Trans. 1998, 3355. (e) Ghosh, P.; Rheingold, A. L.; Parkin, G. Inorg. Chem. 1999, 38, 5464.

a

Ms = 2,4,6-trimethylphenyl.

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Figure 2. Molecular structure of (MeBpMs*)2ZrCl2 (2). Hydrogen atoms (except the BH---Zr hydrogens) are omitted. Selected bond lengths (A˚) and angles (deg): Cl(1)-Zr(1) 2.4086(8), N(1)Zr(1) 2.2785(19), N(3)-Zr(1) 2.3517(19); N(1)-Zr(1)-N(1A) 82.94(10), N(1)-Zr(1)-N(3) 77.53(7); N(1A)-Zr(1)-N(3) 132.51(6), N(3)-Zr(1)-N(3A) 144.26(9), N(1)-Zr(1)-Cl(1A) 138.75(5), N(1A)-Zr(1)-Cl(1A) 86.88(5), N(3)-Zr(1)-Cl(1A) 80.49(5), N(3A)-Zr(1)-Cl(1A) 83.08(5), Cl(1A)-Zr(1)-Cl(1) 124.54(3).

(MeBpMs*)2ZrCl2 (2). One possible route to group 4 metal (MeBpMs)MR3 compounds is generation and alkylation of (MeBpMs)MCl3 complexes. However, the reaction of 1 with 1 equiv of ZrCl4 in CH2Cl2 affords (MeBpMs*)2ZrCl2 (2), which was isolated as a white solid (43% based on 1, eq 1). NMR monitoring of this reaction showed that 2 is formed in 80% yield based on 1 and is the only major product. It is likely that steric crowding drives the isomerization of MeBpMs to MeBpMs* ligands in this reaction. Similar results were obtained for the reaction of 1 and ZrCl4 in toluene and THF, the reaction of 1 with the soluble complex ZrCl4(THF)2 in THF, and the reaction of 1 with 0.5 equiv of ZrCl4 in THF. The mono-Bp0 species (MeBpMs)ZrCl3 was not identified in any of these reactions.

The solid-state structure of 2 was determined by X-ray diffraction and is shown in Figure 2. Complex 2 features a C2-symmetric distorted dodecahedral coordination geometry at Zr, which is common for eight-coordinate Zr(IV) complexes.12 The short BH-Zr (2.22 A˚) and B-Zr (3.011 A˚) distances in 2 indicate the presence of a B-H---Zr agostic (12) (a) Evason, W.; Matthews, M. L.; Patel, B.; Reid, G.; Webster, M. Dalton Trans. 2004, 20, 3305. (b) Solari, E.; Maltese, C.; Franceschi, F.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C. J. Chem. Soc., Dalton Trans. 1997, 2903. (c) Baggio, R.; Garland, M. T.; Perec, M.; Vega, D. Inorg. Chem. 1995, 34, 1961.

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interaction. This interaction is weaker than that in [{(PhCH2)(H)B(μ-Me2pz)2}Zr(η2-Me2pz)(CH2Ph)][B(C6F5)4] (Zr-H 2.03 A˚, Zr-B 2.870 A˚)6 and similar to that in {H(μ-H)B(pz)2}CpZrCl2 (Zr-H 2.27 A˚, Zr-B 2.957 A˚).13 (MeBpMs)Zr(CH2Ph)3 (3). An alternative possible route to Bp0 MR3 species is Bp0 /R exchange, similar to the approach used in the synthesis of Tp*Zr(CH2Ph)3 and {HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)}Hf(CH2Ph)3 (Scheme 1).6,14 The reaction of 1 with Zr(CH2Ph)4 in benzene affords (MeBpMs)Zr(CH2Ph)3 (3) in 86% isolated yield (eq 2). NMR monitoring of this reaction in CD2Cl2 showed that 3 is formed quantitatively along with 0.5 equiv of bibenzyl within 10 min at 23 °C. A Tl0 mirror is produced in this reaction. These results are consistent with a mechanism involving initial ligand exchange to produce 3 and the thermally unstable species Tl(CH2Ph) and subsequent decomposition of Tl(CH2Ph) to Tl0 and bibenzyl.15 Compound 3 is stable in CD2Cl2 for at least 2 h and as a solid for at least 6 months at -35 °C under nitrogen.

The molecular structure of 3 was determined by X-ray diffraction and is shown in Figure 3. Compound 3 exhibits pentagonal-bipyramidal geometry at Zr with N(2), C(39) C(40), C(32), and H(47) located in equatorial positions and C(25) and N(4) located in axial sites. Two benzyl groups (C(25) and C(32)) are bound in an η1 manner (Zr-Cipso distances = 3.340, 3.374 A˚; Zr-C-Cipso angles = 122.2°, 126.6°), while the third (C(39)) displays a short Zr-Cipso distance (Zr(1)-C(40) 2.571 A˚) and an acute Zr-C-Cipso angle (Zr(1)-C(39)-C(40) 84.6°) indicative of η2-coordination. The short BH-Zr (2.22 A˚) and B-Zr (2.939 A˚) distances in 3 indicate the presence of a B-H---Zr agostic interaction in the solid state. Dynamic Properties of 3. At -95 °C, the 1H NMR spectrum of 3 contains one set of 3-mesitylpyrazole resonances and two sets of -CH2Ph resonances in a 2:1 intensity ratio, which is consistent with time-averaged Cs symmetry. The low-field region of the 1H NMR spectrum of 3 in CD2Cl2 at -95 °C (Figure 4) contains two sets of -CH2Ph resonances in a 2:1 intensity ratio, doublets for the pyrazolyl H4 and H5 hydrogens (JHH = 2 Hz), and singlets for the mesityl H3 and H5 hydrogens. The high-field region of the spectrum contains three singlets for the mesityl methyl groups along with two doublets (2H each, JHH = 8 Hz) and a singlet (2H) for the -CH2Ph methylene hydrogens. These results are most easily explained by assuming that 3 retains the solid-state structure (13) Reger, D. L.; Mahtab, R.; Baxter, J. C.; Lebioda, L. Inorg. Chem. 1986, 25, 2046. (14) Lee, H.; Nienkemper, K.; Jordan, R. F. Organometallics 2008, 27, 5075. (15) (a) Lee, A. G. Q. Rev. Chem. Soc. 1970, 24, 310. (b) Schwerdtfeger, P.; Boyd, P. D. W.; Bowmaker, G. A.; Mack, H. G.; Oberhammer, H. J. Am. Chem. Soc. 1989, 111, 15. (c) McKillop, A.; Elsom, L. F.; Taylor, E. C. J. Am. Chem. Soc. 1968, 90, 2423. (d) Kimblin, C.; Bridgewater, B. M.; Hascall, T.; Parkin, G. J. Chem. Soc., Dalton Trans. 2000, 1267.

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Figure 3. (a) Molecular structure of (MeBpMs)Zr(CH2Ph)3 (3). Hydrogen atoms except the B-H hydrogen H(47) are omitted. (b) The core structure of 3. The mesityl rings and hydrogen atoms except the B-H hydrogen H(47) are omitted. Selected bond lengths (A˚) and angles (deg): C(25)-Zr(1) 2.254(2), C(32)-Zr(1) 2.304(2), C(39)-Zr(1) 2.265(2), N(2)-Zr(1) 2.3562(19), N(4)-Zr(1) 2.322(2); C(25)-Zr(1)-C(39) 102.86(9), C(25)-Zr(1)-C(32) 89.41(8), C(39)-Zr(1)-C(32) 123.14(8), C(25)-Zr(1)-N(4) 139.36(7), C(39)-Zr(1)-N(4) 112.04(8), C(32)-Zr(1)-N(4) 88.53(8), C(25)-Zr(1)-N(2) 84.11(8), C(39)-Zr(1)-N(2) 83.16(8), C(32)-Zr(1)-N(2) 153.70(7), N(4)-Zr(1)-N(2) 80.16(6).

Figure 4. Variable-temperature 1H NMR spectra of 3 in (CD2Cl2, 500 MHz). The aromatic region and the CH3/CH2 region are shown.

in solution but that rotation around the Zr-CH2Ph bond of the η2-benzyl group (Zr(1)-C(39)) is fast on the NMR time scale. In this case, the η2-CH2Ph hydrogens are rapidly permuted, giving rise to the singlet at δ 1.57, the η1-CH2Ph groups are rapidly permuted, and the methylene hydrogens on a given η1-CH2Ph unit remain diastereotopic, giving rise to the doublets at δ 1.29 and 0.66. When the temperature is raised, the two sets of benzyl resonances collapse to a single set of resonances due to exchange of the η1- and η2-benzyl groups (Figure 4). The 1 JCH value (125 Hz) for the ZrCH2Ph unit at -10 °C is

intermediate between the values expected for η1 (ca. 115 Hz) and η2 (ca. 145 Hz) coordination,16 consistent with the presence of two η1-benzyls and one η2-benzyl that exchange rapidly.17 The free energy barrier for the benzyl exchange process in 3, ΔGq = 9.3 kcal/mol (-75 °C), was determined from the coalescence of the o-Ph resonances.18 (16) Bei, X.; Swenson, D. C.; Jordan, R. F. Organometallics 1997, 16, 3282. (17) (a) Qian, B.; Scanlon, W. J.; Smith, M. R., III. Organometallics 1999, 18, 1693. (b) Pellecchia, C.; Grassi, A.; Immirzi, A. J. Am. Chem. Soc. 1993, 115, 1160.

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Figure 5. Molecular structure of (MeBpMs)Hf(CH2Ph)3 (4). Hydrogen atoms except the B-H hydrogen are omitted. Selected bond lengths (A˚) and angles (deg): Hf(1)-C(1) 2.212(3), Hf(1)C(15) 2.227(3), Hf(1)-C(8) 2.250(3), Hf(1)-N(1) 2.259(2), Hf(1)-N(3) 2.329(2), C(1)-Hf(1)-C(15) 105.81(10), C(1)Hf(1)-C(8) 95.86(10), C(15)-Hf(1)-C(8) 105.03(10), C(1)Hf(1)-N(1) 145.30(8), C(15)-Hf(1)-N(1) 107.53(9), C(8)-Hf(1)N(1) 84.62(8), C(1)-Hf(1)-N(3) 87.19(9), C(15)-Hf(1)-N(3) 98.28(9), C(8)-Hf(1)-N(3) 154.63(8), N(1)-Hf(1)-N(3) 78.85(7).

(MeBpMs)Hf(CH2Ph)3 (4). The reaction of 1 with Hf(CH2Ph)4 in benzene yields (MeBpMs)Hf(CH2Ph)3 (4, 90%, eq 2). NMR analysis of this reaction in CD2Cl2 shows that 5 is formed quantitatively along with 0.5 equiv of bibenzyl and a Tl0 mirror within 10 min at 23 °C. 4 is stable in CD2Cl2 for at least 4 h and as a solid for at least 6 months at -35 °C under nitrogen. The molecular structure of 4 is shown in Figure 5 and is very similar to that of the Zr analogue 3. The short BH-Hf (2.32 A˚) and B-Hf (3.019 A˚) distances indicate the presence of a B-H---Hf agostic interaction. The biggest difference between 4 and 3 is that all three benzyl groups in 4 are bound in an η1 manner, while, as noted above, 3 contains one η2 and two η1 benzyl groups. Generation of [(MeBpMs)Zr(CH2Ph)2][B(C6F5)4] (5). The reaction of 3 and [Ph3C][B(C6F5)4] in CD2Cl2 at -60 °C generates [(MeBpMs)Zr(CH2Ph)2][B(C6F5)4] (5) quantitatively, along with a series of organic products including bibenzyl, toluene, triphenylmethane, and several coupling products of trityl and benzyl radicals (A-D), as shown in Scheme 3. The 1H and 13C NMR spectra of 5 at -60 °C contain one set of pzMs resonances and two sets of benzyl resonances. The two methylene hydrogens within each benzyl group are equivalent. The 1JCH values for the benzyl methylene groups (134, 145 Hz) are indicative of η2-coordination. These results show that 5 has time-averaged Cs symmetry in CD2Cl2 under these conditions. Complex 5 likely adopts a trigonal-bipyramidal structure in which the pzMs groups and one benzyl (18) (a) The free energy of activation at the coalescence temperature (-75 °C) was determined using the method of Shanan-Atidi and Bar-Eli, setting the relative population ratio at 1:2. The chemical shifts of the o-Ph resonances at -95 °C were used to estimate the chemical shift difference (Δν) in the slow exchange limit, assuming that Δν at -95 °C is the same as in the slow exchange limit. This assumption was required because 1H NMR spectra could not be obtained below -95 °C due to freezing of the solution. (b) Shanan-Atidi, H.; Bar-Eli, K. H. J. Chem. Phys. 1970, 74, 961.

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group occupy equatorial sites and the agostic B-H and the other benzyl group occupy axial sites, and in which rotation around the Zr-benzyl bonds is fast on the NMR time scale. This proposed structure is analogous to that established by X-ray crystallography for {(PhCH2)(H)B(μ-Me2pz)2}Zr(η2Me2pz)(CH2Ph)þ.6 Complex 5 is unstable above 0 °C in CD2Cl2.19 The organic products in Scheme 3 are indicative of a mechanism involving initial one-electron oxidation of a zirconium benzyl bond of 3 by Ph3Cþ to yield 5 along with a trityl radical and a benzyl radical, followed by radical coupling and H atom transfer reactions. Interestingly, under these conditions, the trityl dimer A (Gomberg’s dimer) isomerizes to 1-(diphenylmethyl)-4-(trityl)benzene (B), and the trityl/benzyl coupling product C isomerizes to 1-(diphenylmethyl)-4-(benzyl)benzene (D). These isomerizations are slow at -60 °C, and the yields indicated in Scheme 3 were recorded after 10 min at this temperature.20 However, when the reaction mixture was warmed to 0 °C, A and C isomerized completely to B and D, respectively. The A/B isomerization was observed previously in the reactions of trityl chloride and triphenylcarbinol with Zn/SnCl2 in acetic acid,21 and it has been shown that this process can be catalyzed by acids and bases.22,23 Several examples of one-electron oxidation reactions of d0 organometallics have been observed previously, including, for example, the reaction of Cp2Zr(CH2Ph)2 with Cp2Feþ to generate Cp2Zr(CH2Ph)þ and benzyl radical.24-26 Generation of [(MeBpMs)Hf(CH2Ph)2][B(C6F5)4] (6). The reaction of 4 and [Ph3C][B(C6F5)4] in CD2Cl2 at -60 °C generates [(MeBpMs)Hf(CH2Ph)2][B(C6F5)4] (6, 100%) and Ph3CCH2Ph (86%), along with toluene (12%), B (13%), and trace amounts of other organic compounds (eq 3).27 The high yield of Ph3CCH2Ph in this reaction suggests that electrophilic benzyl abstraction competes with the oxidative M-CH2Ph bond cleavage process implicated for 3. The 1H (19) The reaction of 3 with [Ph3C][B(C6F5)4] in C6D5Cl or Cl2DCCDCl2 generates 5 cleanly at -30 °C, but 5 decomposes at 0 °C in these solvents. (20) The total yield of organic products in Scheme 3 corresponds to quantitative conversion of one -CH2Ph group of 3 and the Ph3Cþ cation to these products. (21) Ullmann, F.; Borsum, W. Chem. Ber. 1902, 35, 2877. (22) Takeuchi, H.; Nagai, T.; Tokura, N. Bull. Chem. Soc. Jpn. 1971, 44, 753. (23) Ashby, E. C.; Park, W. S.; Goel, A. B.; Su, W. Y. J. Org. Chem. 1985, 50, 5184. (24) Jordan, R. F.; Lapointe, R. E.; Bajgur, S. C.; Echols, S. F.; Willett, R. J. Am. Chem. Soc. 1987, 109, 4111. (25) (a) Borkowsky, S. L.; Baenziger, N. C.; Jordan, R. F. Organometallic 1993, 12, 486. (b) Burk, M. J.; Tumas, W.; Ward, M. D.; Wheeler., D. R. J. Am. Chem. Soc. 1990, 112, 6133. (c) Hayashi, Y.; Osawa, M.; Wakatsuki., Y. J. Organomet. Chem. 1997, 542, 241. (d) Burk, M. J.; Staley, D. L.; Tumas, W. J. Chem. Soc., Chem. Commun. 1990, 809. (e) Jordan, R. F. J. Chem. Educ. 1988, 65, 285. (f) Jordan, R. F.; Bajgur, C. S.; Willett, R.; Scott, B. J. Am. Chem. Soc. 1986, 108, 7410. (g) Straus, D. A.; Zhang, C.; Tilley, T. D. J. Organomet. Chem. 1989, 369, C13. (h) Lee, J. B.; Ott, K. C.; Grubbs, R. H. J. Am. Chem. Soc. 1982, 104, 7491. (i) BrownWensley, K. A.; Buchwald, S. L.; Cannizzo, L.; Clawson, L.; Ho, S.; Meinhardt, D.; Stille, J. R.; Straus, D.; Grubbs, R. H. Pure Appl. Chem. 1983, 55, 1733. (j) Grubbs, R. H.; Tumas, W. Science 1989, 243, 907. (26) For other reactions in which Ph3Cþ effects one-electron oxidation of organometallic species see: (a) Cheng, T. Y.; Szalda, D. J.; Zhang, J.; Bullock, R. M. Inorg. Chem. 2006, 45, 4712. (b) Eide, E. F.; Piers, W. E.; Parvez, M.; McDonald, R. Inorg. Chem. 2007, 46, 14. (c) Brownie, J. H.; Baird, M. C.; Laws, D. R.; Geiger, W. E. Organometallics 2007, 26, 5890. (d) Inoue, S.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2008, 130, 6078. (27) The total yield of organic products corresponds to quantitative conversion of one -CH2Ph group of 4 and the Ph3Cþ cation to these products.

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Chen et al. Scheme 3a

a

Anion = B(C6F5)4-.

and 13C NMR spectra of 6 at -60 °C are indicative of timeaveraged Cs symmetry at -60 °C. However, in contrast to the Zr analogue 5, 6 contains one η1 and one η2 benzyl group (1JCH = 117, 148 Hz). Complex 6 decomposes at 0 °C in CD2Cl2.28

Generation of [(MeBpMs)M(CH2Ph)2(OEt2)][B(C6F5)4] ((M = Zr (7), Hf (8)). The synthesis of base-stabilized (MeBpMs)M(CH2Ph)2Lþ species was investigated to provide thermally more stable derivatives of 5 and 6. The reaction of 3 or 4 with 1 equiv of [H(OEt2)2][B(C6F5)4] at -60 °C in CD2Cl2 generates the cationic ether adducts [(MeBpMs)M(CH2Ph)2(OEt2)][B(C6F5)4] ((M = Zr (7), Hf (8)) in quantitative yield, along with 1 equiv of toluene and 1 equiv of free Et2O (eq 4). The NMR data for 7 and 8 are consistent with C1-symmetric structures and the presence of one η1 and one η2 benzyl group. Complexes 7 and 8 likely have octahedral structures with the Et2O ligand trans to a 3-mesityl-pz group, as indicated in eq 4. An analogous structure was established for {(PhCH2)(H)B(μ-Me2pz)2}Zr(η2-Me2pz)(CH2Ph)(PMe3)þ (28) The reaction of 4 with [Ph3C][B(C6F5)4] in C6D5Cl or Cl2DCCDCl2 generates 6 cleanly at -30 °C, but 6 decomposes at 0 °C in these solvents.

by X-ray crystallography.6 The 1JCH value for the η2-benzylmethylene unit in 7 (142 Hz) is larger than that in 8 (129 Hz), and the 2JHH value for the η2-benzylmethylene unit in 7 (9 Hz) is smaller than that in 8 (12 Hz). These data indicate that the M---Ph interaction is stronger in 7 than 8.16 This difference is consistent with stronger preference for η2-benzyl bonding observed for Zr complexes 3 and 5 versus Hf complexes 4 and 6. Both 7 and 8 are stable at 0 °C but decompose slowly at room temperature in CD2Cl2 or C6D5Cl.

Ethylene Polymerization Behavior of 3/[Ph3C][B(C6F5)4] and 4/[Ph3C][B(C6F5)4]. The ethylene polymerization behavior of 3 (20 psi ethylene) was investigated in toluene at 23 °C with [Ph3C][B(C6F5)4] as a cocatalyst in the presence of 100 equiv of MeAl(BHT)2 (MAD; BHT = 2,6-di-tert-butyl-4methylphenoxide), which was added as an impurity scavenger.29,30 (29) No polymerization activity was observed in the absence of MAD. (30) (a) Stapleton, R. A.; Galan, B. R.; Collins, S.; Simons, R. S.; Garrison, J. C.; Youngs, W. J. J. Am. Chem. Soc. 2003, 125, 9246. (b) Stapleton, R. A.; Al-Humydi, A.; Chai, J.; Galan, B. R.; Collins, S. Organometallics 2006, 25, 5083.

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3/[Ph3C][B(C6F5)4] produces linear PE (Tm = 134.6 °C) with a weight average molecular weight (Mw) of 2.0  105. The PE exhibits a bimodal MWD (Mw/Mn = 49) comprising a major low molecular weight fraction (Mp = 8.6  103; Mp = peak molecular weight) and a minor high molecular weight fraction (Mp = 3.2  105). The broad molecular weight distribution suggests that several active species may form from the initially generated (MeBpMs)Zr(CH2Ph)2þ cation, e.g., by isomerization via 1,2 borotropic shifts, reaction with MAD, or other processes. In contrast, under the same conditions, the Hf catalyst 4/[Ph3C][B(C6F5)4] generates linear PE (Tm = 134.6 °C) with an Mw of 2.4  105 and a unimodal molecular weight distribution (Mw/Mn = 3.7). The ethylene polymerization activity of 3/[Ph3C][B(C6F5)4] (2.5 (kg) (mmol)-1 (atm)-1 h-1 at 23 °C) is lower than that of the analogous tris(pyrazolyl)borate catalysts TpMsZrCl3/MAO ([TpMs]-=HB(3-mesitylpyrazolyl)3-, 3.8-21 (kg) (mmol)-1 (atm)-1 h-1 at 60 °C) and TpMs*ZrCl3/MAO ([TpMs*]- =HB(5-mesitylpyrazolyl)(3-mesitylpyrazolyl)2-, 5.2-16 (kg) (mmol)-1 (atm)-1 h-1 at 60 °C), although direct comparison is difficult due to the apparent miltisite character of 3/[Ph3C][B(C6F5)4], differences in polymerization conditions, and the sensitivity of the performance of TpMsZrCl3/MAO and TpMs*ZrCl3/MAO to the MAO level.1c,d However, the polymerization activity of 5/[Ph3C][B(C6F5)4] (0.80 (kg) (mmol)-1 (atm)-1 h-1 at 23 °C) is much lower than that of the directly comparably discrete catalyst TpMs*Hf(CH2Ph)3/[Ph3C][B(C6F5)4], in which TpMs*Hf(CH2Ph)2þ is the initially formed species (56 (kg) (mmol)-1 (atm)-1 h-1 at 23 °C).14 The ether adducts 7 and 8 exhibit only very low ethylene polymerization activity at 0 and 23 °C, due to the strong Et2O coordination.

Conclusion The alkyl hydrido bis(pyrazolyl)borate reagent Tl(MeHB(3mesitylpyrazolyl)2) (Tl(MeBpMs), 1) has been prepared and used to generate group 4 metal MeBpMs complexes. Compound 1 reacts with M(CH2Ph)4 to afford (MeBpMs)M(CH2Ph)3 (M = Zr (3), Hf (4)), bibenzyl, and Tl0. These reactions proceed by initial MeBpMs/benzyl exchange to produce 3 or 4 and Tl(CH2Ph), followed by thermal decomposition of Tl(CH2Ph). The reaction of 3 with [Ph3C][B(C6F5)4] proceeds by oneelectron oxidation of a zirconium benzyl bond to yield [(MeBpMs)Zr(CH2Ph)2][B(C6F5)4] (5) and organic products derived from coupling and H atom transfer reactions of trityl and benzyl radicals. The reaction of 4 with [Ph3C][B(C6F5)4] yields [(MeBpMs)Hf(CH2Ph)2][B(C6F5)4] (6) along with Ph3CCH2Ph (86%) as the major organic product. The high yield of Ph3CCH2Ph suggests that electrophilic benzyl abstraction competes with oxidative M-CH2Ph bond cleavage in this case. The reaction of 3 or 4 with 1 equiv of [H(OEt2)2][B(C6F5)4] affords [(MeBpMs)M(CH2Ph)2(OEt2)][B(C6F5)4] (M = Zr (7), Hf (8)). Zirconium complexes 3, 5, and 7 exhibit a stronger tendency for η2-benzyl bonding than hafnium analogues 4, 6, and 8. The cationic species 5 and 6 polymerize ethylene.

Experimental Section General Procedures. All manipulations were performed using standard vacuum line, Schlenk, or glovebox techniques under a purified N2 atmosphere. THF was distilled from sodium benzophenone ketyl. Toluene, benzene, pentane, and hexanes were dried by passage through activated alumina and BASF R3-11 oxygen scavenger. CH2Cl2 was dried over CaH2 and distilled. Solvents were stored under N2 or vacuum prior to use. TlOAc was purchased from Aldrich and used as received. ZrCl4 was

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purchased from CERAC and sublimed prior to use. The compounds 3-mesitylpyrazole, Li[MeBH3], Zr(CH2Ph)4, and Hf(CH2Ph)4 were prepared by literature procedures.31-33 NMR spectra were recorded on Bruker DMX-500 and DMX-400 spectrometers in Teflon-valved NMR tubes. Chemical shifts are reported versus SiMe4 and were determined by reference to the residual 1H and 13C solvent peaks. 11B chemical shifts are reported relative to external BF3 3 Et2O. Coupling constants are reported in hertz. Gel permeation chromatography was performed on a Polymer Laboratories PL-GPC 220 using 1,2,4-trichlorobenzene solvent (stabilized with 125 ppm BHT) at 160 °C. A set of three PLgel 10 μm Mixed-B or Mixed-B LS columns was used. Samples were prepared at 160 °C and filtered through 2 or 5 μm stainless steel frits prior to injection. Electrospray mass spectra (ESI-MS) were recorded on freshly prepared samples (ca. 1 mg/mL in CH2Cl2) using an Agilent 1100 LC-MSD spectrometer incorporating a quadrupole mass filter with an m/z range of 0-3000. Typical instrumental parameters were drying gas temperature 350 °C, nebulizer pressure 35 psi, drying gas flow 12.0 L/min, and fragmentor voltage 0, 70, or 100 V. Elemental analyses were performed by Midwest Microlab, LLC. Tl(MeBpMs) (1). A mixture of Li[MeBH3] (0.278 g, 7.76 mmol) and HpzMs (2.89 g, 15.5 mmol) in THF was stirred at 23 °C for 3 h and then at 60 °C for 2 days. The mixture was cooled to 23 °C, Tl(OAc) (2.98 g, 11.3 mmol) was added, and the mixture was stirred for 2 days at 23 °C. The long reaction time is necessary to ensure that most of the Tl(MeBpMs*) is isomerized to Tl(MeBpMs). The mixture was filtered through Celite, and the colorless filtrate was dried under vacuum to give a white solid. The solid was washed with hexanes (3  10 mL) and dried under vacuum to yield a white solid (3.45 g, 73%). Anal. Calcd for C25H30BN4Tl: C, 49.90; H, 5.03; N, 9.31. Found: C, 49.92; H, 5.17; N, 9.37. 1H NMR (CD2Cl2): δ 7.67 (d, J = 2, 2H, pz), 6.87 (s, 4H, 3- and 5-Ms), 6.04 (d, J = 2, 2H, pz), 5.20 (br, 1H, BH), 2.27 (s, 6H, Me), 1.98 (s, 12H, Me), 0.50 (d, J = 5, 3H, BCH3). 11B NMR (CD2Cl2): δ -4.8 (br). 13C{1H} NMR (CD2Cl2): δ 137.8, 137.7, 133.4, 131.1, 128.4, 128.3, 105.2, 21.1, 20.7, The CH3B signal could not be detected due to the proximity of the quadrupolar B. NMR Monitoring of the Reaction of Li[MeBH3] and 2 equiv of HpzMs. A mixture of Li[MeBH3] (5.4 mg, 0.15 mmol) and HpzMs (55.8 mg, 0.298 mmol) in THF-d8 was stirred at 23 °C for 1 day. NMR analysis showed that a 2:1 mixture of Li[MeB(3-Mspz)H2] and Li[MeB(5-Ms-pz)H2] was present. 1H NMR (THFd8) Li[MeB(3-Ms-pz)H2] (major isomer): δ 7.25 (d, J = 2, 1H, pz), 6.75 (s, 2H, Ms), 5.76 (d, J = 2, 1H, pz), 2.09 (s, 3H, Me), 1.96 (s, 6H, Me), -0.52 (t, J = 8, 3H, BCH3); Li[MeB(5-Mspz)H2]: δ 7.43 (d, J = 2, 1H, pz), 6.82 (s, 2H, Ms), 5.86 (d, J = 2, 1H, pz), 2.25 (s, 3H, Me), 2.04 (s, 6H, Me), -0.05 (t, J = 8, 3H, BCH3), B-H resonances were broad at 4.00-6.00 and were not assigned. ESI-MS: Li[MeB(3-Ms-pz)H2]2- and Li[MeB(5-Mspz)H2]2- calcd m/z 433.3, found 433.2. Further reaction at 60 °C for 2 days gave a 3:1 mixture of Li[MeBpMs] and Li[MeBpMs*]. 1H NMR (THF-d8): Li[MeBpMs]: δ 7.58 (d, J = 2, 2H, pz), 6.79 (s, 4H, Ms), 5.85 (d, J = 2, 2H, pz), 2.22 (s, 6H, Me), 1.98 (s, 12H, Me), 0.46 (d, J = 5, 3H, BCH3); Li[MeBpMs*]: δ 7.46 (d, J = 2, 1H, pz), 7.38 (d, J = 2, 1H, pz), 6.89 (s, 2H, Ms) 6.85 (s, 1H, Ms), 6.83 (s, 1H, Ms), 5.85 (d, J = 2, 1H, pz), 5.83 (d, J = 2, 1H, pz), 2.30 (s, 3H, Me), 2.28 (s, 3H, Me), 2.27 (s, 3H, Me), 2.07 (s, 3H, Me), 1.97 (s, 3H, Me), 1.82 (s, 3H, Me), -0.03 (d, J = 5, 3H, BCH3), B-H resonances were broad at 4.006.00 and were not assigned. ESI-MS: MeBpMs- and MeBpMs*calcd m/z 397.3, found 397.2. (31) Rheingold, A. L.; White, C. B.; Trofimenko, S. Inorg. Chem. 1993, 32, 3471. (32) Singaram, B.; Cole, T. E.; Brown, H. C. Organometallics 1984, 3, 774. (33) Zucchini, U.; Albizzati, E.; Giannini, U. J. Organomet. Chem. 1971, 26, 357.

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(MeBpMs*)2ZrCl2 (2). A flask was charged with ZrCl4 (0.510 g, 2.19 mmol) and 1 (2.64 g, 4.38 mmol), and methylene chloride (100 mL) was added by vacuum transfer at -78 °C. The mixture was warmed to room temperature over 6 h, and the resulting white suspension was stirred for 2 days. The volatiles were removed under vacuum, yielding a white solid. Toluene (100 mL) was added to the solid, and the mixture was filtered through Celite. The clear filtrate was concentrated to ca. 50 mL under vacuum. White crystals were obtained by slow diffusion of pentane into the concentrated toluene solution at -35 °C for 3 days (0.970 g, 43%). Anal. Calcd for C50H60B2N8ZrCl2: C, 62.76; H, 6.32; N, 11.71. Found: C, 63.04; H, 6.28; N, 11.44. 1H NMR (CD2Cl2): δ 7.81 (d, J = 2, 2H, pz), 7.42 (d, J = 3, 2H, pz), 6.92 (s, 2H, Ms), 6.90 (s, 2H, Ms), 6.89 (s, 2H, Ms), 6.74 (s, 2H, Ms), 5.97 (d, J = 3, 2H, pz), 5.95 (d, J = 3, 2H, pz), 4.85 (br, 2H, BH), 2.33 (s, 6H, Me), 2.32 (s, 6H, Me), 1.76 (s, 6H, Me), 1.75 (s, 6H, Me), 1.71 (s, 6H, Me), 1.69 (s, 6H, Me), -0.08 (d, J = 4, 6H, BCH3). 11B NMR (CD2Cl2): δ -3.5 (br). 13C{1H} NMR (CD2Cl2): δ 153.3, 146.3, 141.4, 139.0, 138.9, 138.0, 137.9, 137.7, 137.6, 131.9, 130.9, 128.5, 128.3, 128.1, 128.0, 127.2, 106.7, 106.6, 21.3, 21.2, 20.9, 20.3, 19.8, 19.4, -3.6 (br, BMe). 2/MAO produces PE with low activity (0.38 (kg) (mmol 2)-1 (atm)-1 (h)-1 at 60 °C, 1000 equiv of MAO and 20 psi ethylene pressure; MAO was obtained as a 13.5 wt % Al solution in toluene from Albemarle). The PE prepared by 2 could not be analyzed by GPC because it could not be dissolved in 1,2,4tricholorobenzene. (MeBpMs)Zr(CH2Ph)3 (3). A solution of Zr(CH2Ph)4 (0.30 g, 0.66 mmol) in benzene (20 mL) was added dropwise to a solution of 1 (0.40 g, 0.66 mmol) in benzene (20 mL). The mixture was stirred for 20 min and filtered through Celite. The yellow filtrate was dried under vacuum to yield a yellow solid. The solid was washed with cold pentane (3  10 mL) and dried under vacuum (0.43 g, 86%). Anal. Calcd for C46H51BN4Zr: C, 72.51; H, 6.75; N, 7.35. Found: C, 72.28; H, 6.79; N, 7.50. 1H NMR (CD2Cl2, -10 °C): δ 7.66 (d, J = 2, 2H, pz), 6.99 (s, 2H, Ms), 6.90 (s, 2H, Ms), 6.87 (t, J = 7, 6H, m-Ph), 6.82 (t, J = 7, 3H, p-Ph), 6.18 (d, J = 2, 2H, pz), 5.84 (d, J = 7, 6H, o-Ph), 4.94 (br, 1H, BH), 2.32 (s, 6H, Me), 2.19 (s, 6H, Me), 1.77 (s, 6H, Me), 1.44 (s, 6H, ZrCH2), -0.03 (d, J = 4, 3H, BCH3). 1H NMR (CD2Cl2, -95 °C): 7.61 (d, J = 2, 2H, 5-pz), 7.33 (br, 1H, p-Ph), 7.01 (br, 2H, p-Ph), 6.95 (s, 2H, Ms), 6.86 (s, 2H, Ms), 6.73 (br t, J = 7, 4H, m-Ph), 6.53 (br t, J = 6, 2H, m-Ph), 6.17 (d, J = 2, 2H, 4-pz), 5.73 (d, J = 6, 4H, o-Ph), 5.52 (br, 2H, o-Ph), 4.90 (br, 1H, BH), 2.27 (s, 6H, Me), 2.14 (s, 6H, Me), 1.65 (s, 6H, Me), 1.57 (s, 2H, ZrCH2), 1.29 (br d, J = 8, 2H, ZrCHaHb), 0.66 (br d, J = 8, 2H, ZrCHaHb), -0.32 (br, 3H, BCH3). 11B NMR (CD2Cl2, -10 °C): δ -5.5 (br). 13C{1H} NMR (CD2Cl2, -10 °C): δ 152.7, 145.0, 138.6, 137.8, 137.7, 132.8, 129.5, 128.9, 128.2, 127.9, 126.8, 122.3, 107.0, 78.4 (1JCH = 125, ZrCH2), 21.2 (two Me resonances overlapped), 20.9, -3.3 (br, BMe). (MeBpMs)Hf(CH2Ph)3 (4). A solution of Hf(CH2Ph)4 (1.09 g, 2.00 mmol) in benzene (30 mL) was added dropwise to a solution of 1 (1.20 g, 2.00 mmol) in benzene (30 mL). The mixture was stirred for 30 min and filtered through Celite. The green filtrate was dried under vacuum to yield a pale green solid. The solid was washed with cold pentane (3  10 mL) and dried under vacuum to yield spectroscopically pure product (1.54 g, 91%). Pale green crystals of 4 were obtained by slow diffusion of pentane into a concentrated toluene solution at -35 °C. Anal. Calcd for C46H51BN4Hf: C, 65.06; H, 6.05; N, 6.60. Found: C, 65.12; H, 6.10; N, 6.51. 1H NMR (CD2Cl2): δ 7.72 (d, J = 2, 2H, pz), 7.05 (s, 2H, 3- or 5-Ms), 6.95 (s, 2H, 3- or 5-Ms), 6.89 (t, J = 8, 6H, m-Ph), 6.68 (t, J = 7, 3H, p-Ph), 6.26 (d, J = 2, 2H, pz), 5.91 (d, J = 7, 6H, o-Ph), 4.34 (br, 1H, BH), 2.34 (s, 6H, Me), 2.23 (s, 6H, Me), 1.91 (s, 6H, Me), 1.63 (s, 6H, HfCH2), 0.18 (d, J = 4, 3H, BMe). 11B NMR (CD2Cl2): δ -5.5 (br). 13C{1H} NMR (CD2Cl2, -10 °C): δ 154.0, 146.5, 139.5, 138.3, 138.2, 134.3, 129.0, 128.9, 127.9, 127.2, 126.2, 121.9, 107.9, 93.1 (1JCH = 116, HfCH2), 21.4, 21.2, 20.8, -3.5 (br, BMe).

Chen et al. Generation of [(MeBpMs)Zr(CH2Ph)2][B(C6F5)4] (5). A valved NMR tube was charged with 3 (15.0 mg, 0.0195 mmol) and [Ph3C][B(C6F5)4] (18.3 mg, 0.0195 mmol), and CD2Cl2 (0.6 mL) was added by vacuum transfer at -78 °C. The tube was shaken at this temperature to give an orange solution and placed in an NMR probe that had been precooled to -60 °C. A 1H NMR spectrum was obtained and showed that [(MeBpMs)Zr(CH2Ph)2][B(C6F5)4] had formed quantitatively. 1H NMR (CD2Cl2, -60 °C): δ 7.85 (d, J = 2, 2H, pz), 7.41 (t, J = 7, 1H, p-Ph), 7.13 (t, J = 8, 2H, m-Ph), 7.08 (t, J = 7, 2H, m-Ph), 7.05 (s, 2H, Ms), 6.89 (s, 2H, Ms), 6.30 (d, J = 2, 2H, pz), 5.90 (br, 2H, o-Ph), 5.85 (br, 2H, o-Ph), 4.30 (br, 1H, BH), 2.51 (s, 2H, ZrCH2), 2.28 (s, 2H, ZrCH2), 2.27 (s, 6H, Me), 2.02 (s, 6H, Me), 1.79 (s, 6H, Me), 0.65 (d, J = 4, 3H, BCH3), one p-Ph resonance is obscured by the Ph3CCH2Ph resonances (7.23-7.15). 11 B NMR (CD2Cl2, -60 °C): δ -2.8 (br). 13C{1H} NMR (CD2Cl2, -60 °C): δ 155.1, 143.5, 139.9, 137.4, 136.5, 135.6, 130.3, 129.0, 128.9, 128.7, 128.4, 128.2, 128.1, 127.4, 125.7, 124.7, 108.8, 92.3 (1JCH = 134, ZrCH2), 87.6 (1JCH = 145, ZrCH2), 20.8, 20.4, 20.4, -2.4 (BMe). The organic products bibenzyl, toluene, Gomberg’s dimer (A), Ph3CH, and Ph3CCH2Ph were identified by comparison of the 1H NMR spectrum of the product mixture with the NMR spectra of authentic materials. 1-(Diphenylmethyl)-4-(trityl)benzene (B) and 1-(diphenylmethyl)-4-(benzyl)benzene (D) were identified by comparison of the 1H NMR spectrum of the product mixture with literature data.34 Compound C was not conclusively identified, but its structure was assigned on the basis of the similarity of its NMR data to those for related compounds and by the conversion of C to D, which parallels the conversion of A to B. Compound D was previously observed as a minor product of the coupling of the Ph2CH 3 radical.34b,c The presence of bibenzyl, toluene, Ph3CH, Ph3CCH2Ph, and D was confirmed by GC-MS. Selected 1H NMR data for organic products (CD2Cl2, -60 °C): A: δ 6.41 (d of d, J = 10, 2; 2H, vinylic), 5.75 (d of d, J = 10, 4; 2H, vinylic), 5.03 (br m, 1H, allylic). B: δ 5.55 (s, 1H, CH). C: δ 5.80 (d of d, J = 10, 4; 2H, vinylic; the other vinyl H resonances is obscured), 3.30 (br m, 1H, allylic), 2.71 (d, J = 8, 2H, CH2). D: δ 5.50 (s, 1H, CH), 3.85 (s, 2H, CH2). Generation of [(MeBpMs)Hf(CH2Ph)2][B(C6F5)4] (6). A valved NMR tube was charged with 4 (20.0 mg, 0.0235 mmol) and [Ph3C][B(C6F5)4] (21.7 mg, 0.0235 mmol), and CD2Cl2 (0.6 mL) was added by vacuum transfer at -78 °C. The tube was shaken at this temperature to give a green solution and placed in an NMR probe that had been precooled to -60 °C. A 1H NMR spectrum was obtained and showed that [(MeBpMs)Hf(CH2Ph)2][B(C6F5)4] had formed quantitatively. The spectrum also contained resonances for Ph3CCH2Ph (86%), toluene (12%), B (13%), and trace amounts of Ph3CH and D. Data for [MeBpMs)Hf(CH2Ph)2][B(C6F5)4]: 1H NMR (CD2Cl2, -60 °C): δ 7.92 (d, J = 2, 2H, pz), 7.28 (t, J = 8, 2H, m-Ph), 6.74 (t, J = 7, 1H, p-Ph), 7.06 (s, 2H, 3- or 5-Ms), 6.94 (s, 2H, 3- or 5-Ms), 6.38 (d, J = 2, 2H, pz), 6.18 (d, J = 8, 2H, o-Ph), 5.89 (br, 2H, o-Ph), 4.24 (br, 1H, BH), 2.27 (s, 6H, Me), 2.11 (s, 6H, Me), 1.95 (s, 2H, HfCH2), 1.84 (s, 6H, Me), 1.81 (s, 2H, HfCH2), 0.69 (d, J = 2, 3H, BCH3), the other p-Ph and the other m-Ph were obscured by the Ph3CCH2Ph resonances (7.23-7.15). 11B NMR (CD2Cl2, -60 °C): δ -4.4 (br). 13C{1H} NMR (CD2Cl2, -60 °C): δ 155.7, 144.8, 140.0, 137.5, 136.6, 136.5, 136.1 (pz), 131.3, 129.1, 128.4, 128.3, 128.2, 128.1, 127.1, 125.4, 123.8, 109.4 (pz), 93.9 (1JCH = 117, HfCH2), 88.1 (1JCH = 148, HfCH2), 20.8, 20.7, 20.6, -3.2 (BMe). Generation of [(MeBpMs)Zr(CH2Ph)2(OEt2)][B(C6F5)4] (7). A valved NMR tube was charged with 3 (15.0 mg, 0.0195 mmol) and [H(OEt2)2][[B(C6F5)4] (16.2 mg, 0.0195 mmol), and CD2Cl2 (0.6 mL) was added by vacuum transfer at -78 °C. The tube was shaken at this temperature to give an orange solution and placed in an NMR probe that had been precooled to -60 °C. A 1H (34) (a) Ashby, E. C.; Park, W. S.; Goel, A. B.; Su, W. Y. J. Org. Chem. 1985, 50, 5184. (b) Szunerits, S.; Utley, J. H. P.; Nielsen, M. F. Perkin Trans. 2 2000, 669. (c) Okada, T.; Okamoto, Y.; Sakurai, H. Bull. Chem. Soc. Jpn. 1974, 47, 2251.

Article NMR spectrum was obtained and showed that [(MeBpMs)Zr(CH2Ph)2(OEt2)][B(C6F5)4] (7) had formed quantitatively. Free Et2O and toluene were also formed. 1H NMR (CD2Cl2, -60 °C): δ 7.91 (d, J = 2, 1H, pz), 7.88 (d, J = 2, 1H, pz), 7.31 (t, J = 7, 1H, p-Ph), 7.22 (t, J = 8, 2H, m-Ph), 7.15 (s, 1H, Ms), 7.14 (s, 1H, Ms), 7.11 (s, 1H, Ms), 7.03 (s, 1H, Ms, obscured by m-Ph resonance), 7.03 (t, J = 8, 2H, m-Ph), 6.90 (d, J = 7, 2H, o-Ph), 6.81 (t, J = 7, 1H, p-Ph), 6.50 (d, J = 2, 1H, pz), 6.29 (d, J = 2, 1H, pz), 6.04 (br d, J = 7.5, 2H, o-Ph), 4.50 (br, 1H, BH), 3.67 (d of q, J = 12, 7; 2H, coordinated O(CH2CH3)2), 3.22 (d, J = 10, 1H, ZrCHaHb), 2.87 (d, J = 11, 1H, ZrCHcHd), 2.29 (s, 3H, Me), 2.27 (s, 3H, Me), 2.25 (s, 3H, Me), 2.12 (d of q, J = 12, 7; 2H, coordinated O(CH2CH3)2), 1.98 (s, 3H, Me), 1.87 (s, 3H, Me), 1.80 (s, 3H, Me), 1.75 (d, J = 10, 1H, ZrCHaHb), 1.64 (d, J = 11, 1H, ZrCHcHd), 0.73 (t, J = 7, 6H, coordinated O(CH2CH3)2), 0.35 (br, 3H, B-Me). 11B NMR (CD2Cl2, -60 °C): δ -4.6 (br). 13C{1H} NMR (CD2Cl2, -60 °C): δ 155.9, 153.2, 145.6, 140.5, 139.0, 137.4, 137.4, 137.1, 136.6, 136.2, 134.1, 133.7, 131.0, 129.7, 128.8, 128.7, 128.5, 128.2, 128.0, 127.8, 127.6, 126.2, 124.6, 123.2, 109.2 (pz), 108.1 (pz), 100.2 (1JCH = 113, Zr-CH2), 85.6 (1JCH = 142, ZrCH2), 67.4 (coordinated O(CH2CH3)2), 20.87, 20.85, 20.81, 20.77, 20.72, 19.96, 12.8 (coordinated O(CH2CH3)2, -1.3 (BMe3). Generation of [(MeBpMs)Hf(CH2Ph)2(OEt2)][B(C6F5)4] (8). A valved NMR tube was charged with 4 (20.0 mg, 0.0235 mmol) and [H(OEt2)2][[B(C6F5)4] (19.5 mg, 0.0235 mmol), and CD2Cl2 (0.6 mL) was added by vacuum transfer at -78 °C. The tube was shaken at this temperature to give a green solution and placed in an NMR probe that had been precooled to -60 °C. A 1H NMR spectrum was obtained and showed that [(MeBpMs)Zr(CH2Ph)2(OEt2)][B(C6F5)4] (8) had formed quantitatively. Free ether and toluene were also formed. 1H NMR (CD2Cl2, -60 °C): δ 7.97 (d, J = 2, 1H, pz), 7.89 (d, J = 2, 1H, pz), 7.21 (t, J = 7, 1H, p-Ph), 7.15 (t, J = 7, 2H, m-Ph), 7.11 (s, 1H, Ms), 7.06 (s, 1H, Ms), 7.04 (t, J = 7, 2H, m-Ph), 6.95 (s, 1H, Ms), 6.92 (s, 1H, Ms), 6.75 (t, J = 7, 1H, p-Ph), 6.54 (d, J = 2, 1H, pz), 6.33 (d, J = 2, 1H, pz), 6.09 (d, J = 7, 2H, o-Ph), 5.78 (d, J = 7, 2H, o-Ph), 4.02 (br, 1H, BH), 3.76 (d of q, J = 12, 7; 2H, coordinated O(CH2CH3)2), 2.59 (d, J = 12, 1H, HfCHaHb), 2.58 (d of q, J = 12, 7; 2H, coordinated O(CH2CH3)2), 2.29 (s, 3H, Me), 2.25 (s, 3H, Me), 2.25 (s, 3H, Me), 2.03 (d, J = 13, 1H, HfCHcHd), 1.99 (s, 3H, Me), 1.98 (d, J = 12, 1H, HfCHaHb), 1.93 (s, 3H, Me), 1.89 (s, 3H, Me), 1.26 (d, J = 13, 1H, HfCHcHd), 0.64 (t, J = 7, 6H, coordinated O(CH2CH3)2), 0.46 (br, 3H, B-Me). 11B NMR (CD2Cl2, -60 °C): δ -2.1 (br). 13C{1H} NMR (CD2Cl2, -60 °C): δ 155.6, 154.2, 144.6, 140.6, 139.7, 137.8, 137.5, 137.4, 137.3, 137.1 135.5, 134.7, 131.0, 130.3.7, 128.8, 128.4, 128.2, 128.0, 127.7, 127.3, 127.2, 125.9, 125.0, 123.2, 109.5 (pz), 109.0 (pz), 97.4 (1JCH = 129, HfCH2), 94.9 (1JCH = 111, HfCH2), 69.3 (coordinated O(CH2CH3)2), 21.2, 21.0, 20.8, 20.8, 20.7, 20.1, 12.4 (coordinated O(CH2CH3)2). -3.7 (B-CH3). Ethylene Polymerization. Ethylene polymerization reactions were performed in a 200 mL Fischer-Porter bottle equipped with a magnetic stir bar and a stainless steel pressure head

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equipped with inlet and outlet needle valves, a septum-capped ball valve for injections, a check valve for safety, and a pressure gauge. In a glovebox, the bottle was charged with MAO or MAD/[Ph3C][B(C6F5)4]. Dry toluene (70 mL) was added to the mixture, and the bottle was sealed. The bottle was removed from the glovebox and attached to a stainless steel double manifold (vacuum/ethylene) line. The nitrogen atmosphere was removed under vacuum, and the solution was saturated with ethylene and thermally equilibrated at a specified temperature for 10 min. The pressure of ethylene was set to 20 psi during saturation and polymerization. The polymerization was started by injection of a solution of 3 or 4 in toluene (10 mL). The total volume of the reaction mixture was 80 mL for all polymerization reactions. The total pressure was kept constant by feeding ethylene on demand. After the specified reaction time, the polymerization was stopped by venting the reaction vessel followed by the addition of excess methanol. The polymer was washed with acidic methanol and then methanol and dried under vacuum for 12 h. X-ray Crystallography. Crystallographic data are provided in the Supporting Information. Data were collected on a Bruker Smart Apex diffractometer using Mo KR radiation (0.71073 A˚). Non-hydrogen atoms were refined with anisotropic displacement coefficients. All terminal hydrogen atoms were included in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative isotropic displacement coefficients. All ORTEP diagrams are drawn with 50% probability ellipsoids. Specific comments for each structure are as follows. Tl(MeBpMs) (1): Single crystals of 1 were obtained by slow diffusion of pentane into a concentrated toluene solution at -35 °C. The residual electron density between Zr and B was assigned to an H atom (H14), and its position was isotropically refined. (MeBpMs*)2ZrCl2 (2): Single crystals of 2 were obtained by slow diffusion of hexanes into a concentrated toluene solution at -35 °C. One disordered toluene molecule is present, and its H atoms were not found. The residual electron density between Zr and B was assigned to an H atom (H26), and its position was isotropically refined. (MeBpMs)Zr(CH2Ph)3 (3): Single crystals of 3 were obtained by slow diffusion of pentane into a concentrated toluene solution at -35 °C. The residual electron density between Zr and B was assigned to an H atom (H47), and its position was isotropically refined. (MeBpMs)Hf(CH2Ph)3 (4): Single crystals of 4 were obtained by slow diffusion of pentane into a concentrated toluene solution at -35 °C. The residual electron density between Zr and B was assigned to an H atom (H47), and its position was isotropically refined.

Acknowledgment. This work was supported by the National Science Foundation (Grant CHE-0911180). Supporting Information Available: This material is available free of charge via the Internet at http://pubs.acs.org.