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crowded tris(pyrazolyl)borate complexes TpMsZrCl3 (2a; TpMs ) ... catalysis. 2a/MAO is more active for ethylene polymerization than the less crowded a...
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Organometallics 2004, 23, 460-470

Synthesis, Structures, and Olefin Polymerization Behavior of Sterically Crowded Tris(pyrazolyl)borate Zirconium and Hafnium Complexes Kenji Michiue and Richard F. Jordan* Department of Chemistry, The University of Chicago, 5735 South Ellis Avenue, Chicago, Illinois 60637 Received January 10, 2003

The synthesis, molecular structures, and olefin polymerization behavior of the sterically crowded tris(pyrazolyl)borate complexes TpMsZrCl3 (2a; TpMs ) HB(3-mesitylpyrazolyl)3-), TpMs*ZrCl3 (2b; TpMs* ) HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)-), (TpMs**ZrCl2)2(µ-O) (2c; TpMs** ) HB(3-mesitylpyrazolyl)(5-mesitylpyrazolyl)2-), and TpMs*HfCl3 (3b) are described. The molecular structures of TlTpMs (1a) and TlTpMs* (1b), the reagents used to prepare 2a-c and 3b, are also described. The core structures of 1a,b, and of 2a-c and 3b, are very similar, but the steric properties of these compounds differ markedly due to the different placements of the mesityl substituents on the tris(pyrazolyl)borate ligands. Under MAO activation conditions at low catalyst concentrations, 2a exhibits extremely high activity for ethylene polymerization and ethylene/hexene copolymerization. 2a/MAO produces ultrahigh-molecular-weight polyethylene and ethylene/hexene copolymers (up to 27% hexene) with narrow molecular weight distributions (Mw/Mn ) 1.8-2.3), characteristic of single-site catalysis. 2a/MAO is more active for ethylene polymerization than the less crowded analogues 2b/MAO and 2c/MAO. Introduction The discovery of highly active group 4 metallocene olefin polymerization catalysts and the general activator MAO (methylalumoxane)1 triggered extensive studies of new single-site catalysts based on well-defined organometallic and coordination complexes.2 Tris(pyrazolyl)borate ligands (Tp′ ) generic substituted tris(pyrazolyl)borate) are attractive candidates for ancillary ligands for single-site catalysts.3 Tp′ ligands are formally analogous to cyclopentadienyl (Cp) ligands in that both are six-electron-donor uninegative ligands.4 However, Tp′ ligands are 3-fold-symmetric σ-N donors and tend to form fac-octahedral complexes, while Cp ligands are typically 5-fold-symmetric π-C donors and tend to form tetrahedral complexes. Several studies have demonstrated that Tp′TiCln(OR)3-n/MAO catalysts (n ) 1-3) containing the simple Tp′ ligands HB(pyrazolyl)3- (Tp) or HB(3,5-dimethylpyrazolyl)3- (Tp*) polymerize ethylene, ethylene/R-olefins, and styrene.5 However, these catalysts exhibit poor activity and produce polymers with broad molecular weight distributions (MWDs).

In previous work, we examined the ethylene polymerization behavior of a set of Tp′TiCl3 and Tp′TiCl2(OR) complexes containing Tp′ ligands with diverse steric properties (Tp′ ) HB(3-mesitlpyrazolyl)3- (TpMs), HB(3-mesitylpyrazolyl)2(5-mesitylpyrazolyl)- (TpMs*), Tp*, Tp, BuB(pyrazolyl)3- (BuTp)) under MAO activation conditions.6 These studies showed that the activity of Tp′TiX3/MAO catalysts is strongly influenced by the Tp′ steric properties. In particular, the bulky TpMs- and TpMs*-based catalysts exhibit high activity and produce linear polyethylene (PE). The predominant chain transfer mechanism in ethylene polymerization by TpMs*TiCl3/MAO is chain transfer to MAO and the AlMe3 contained therein, and therefore this system produces saturated PE chains. This chain transfer mechanism results in broad MWDs under most conditions, due to the change of the AlMe3 and MAO concentrations as the polymerization reaction progresses.7 The use of high [AlMe3] leads to low-molecular-weight PE with a narrow MWD, whereas the use of low [AlMe3] leads to ultrahigh-molecular-weight PE with a broad MWD. Also, the

(1) (a) Kaminsky, W. Macromol. Chem. Phys. 1996, 197, 3907. (b) Sinn, H.; Kaminsky, W. Adv. Organomet. Chem. 1980, 18, 99. (c) Sinn, H.; Kaminsky, W.; Vollmer, H. J.; Woldt, R. Angew. Chem., Int. Ed. Engl. 1980, 19, 390. (d) Andersen, A.; Cordes, H. G.; Herwig, J.; Kaminsky, W.; Merck, A.; Mottweiler, R.; Pein, J.; Sinn, H.; Wolmer, H. J. Angew. Chem., Int. Ed. Engl. 1976, 15, 630. (2) (a) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283. (b) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428. (c) Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169. (3) (a) Trofimenko, S. Scorpionates: The Coordination Chemistry of Polypyrazolylborate Ligands; Imperial College Press: London, 1999. (b) Trofimenko, S. Chem. Rev. 1993, 93, 943. (4) Tellers, D. M.; Skoog, S. J.; Bergman, R. G.; Gunnoe, T. B.; Harman, W. D. Organometallics 2000, 19, 2428.

(5) (a) Karam, A.; Jimeno, M.; Lezama, J.; Catari, E.; Figueroa, A.; de Gascue, B. R. J. Mol. Catal. A 2001, 176, 65. (b) Ikai, S.; Kai, Y.; Murakami, M.; Nakazawa, H. Jpn. Kokai Tokkyo Koho 11 228 614, 1999; Chem. Abstr. 1999, 131, 200267. (c) Matsunaga, P. T.; Rinaldo, S. PCT Int. Appl. WO 99 29739, 1999; Chem. Abstr. 1999, 131, 32260. (d) Nakazawa, H.; Ikai, S.; Imaoka, K.; Kai, Y.; Yano, T. J. Mol. Catal. A 1998, 132, 33. (e) Jens, K. J.; Tilset, M.; Heuman, A. PCT Int. Appl. WO 97 17379, 1997; Chem. Abstr. 1997, 127, 51112. (f) Aoki, T.; Kaneshima, T. Eur. Pat. Appl. 0 617 052, 1994; Chem. Abstr. 1994, 122, 315348. (g) Newman, T. H. Eur. Pat. Appl. 0 482 934, 1992; Chem. Abstr. 1992, 117, 172275. (h) Obara, T.; Ueki, S. Jpn. Kokai Tokkyo Koho 01 095 110, 1989; Chem. Abstr. 1989, 111, 233883. (6) Murtuza, S.; Casagrande, O. L., Jr.; Jordan, R. F. Organometallics 2002, 21, 1882.

10.1021/om0300171 CCC: $27.50 © 2004 American Chemical Society Publication on Web 01/06/2004

Tris(pyrazolyl)borate Zirconium and Hafnium Complexes

use of high [MAO] strongly inhibits the activity of TpMs*TiCl3/MAO. More recently, we showed that the closely related TiIII catalyst K[TpMs*TiIIICl3] behaves somewhat differently than TpMs*TiIVCl3 under MAO activation conditions.8 The K[TpMs*TiIIICl3]/MAO catalyst is somewhat more active and produces slightly lower MW PE than the TpMs*TiIVCl3/MAO system, and is not inhibited by high [MAO]. Most strikingly, K[TpMs*TiIIICl3]/MAO readily incorporates hexene into ethylene/hexene copolymers, whereas TpMs*TiIVCl3/MAO does not. These differences show that different active species are formed from the K[TpMs*TiIIICl3] and TpMs*TiIVCl3 precursors. As TiIV catalysts can be reduced under polymerization conditions,9 the observation of different behaviors of TpMs*TiIII and TpMsTiIV catalysts suggests that the formation of both TiIII and TiIV active species may contribute to the broad MWDs observed for Tp′TiCl3/ MAO catalysts. It is also possible that isomerization of the Tp′ ligand,3a,6 or slippage of the κ3-Tp′ ligand to κ2 or κ1 binding modes,3,5c,10 could lead to multiple active species, since such reactions can occur under mild conditions. As part of a continuing effort to understand the chemistry of Tp′TiCl3/MAO catalysts, we have investigated the analogous Zr and Hf systems. As Zr and Hf are not likely to be reduced under polymerization conditions, these studies provide a means of probing the factors that influence the performance of Tp′MCl3/MAO catalysts and the MWDs of Tp′MCl3/MAO-produced PEs in the absence of catalyst redox chemistry. Previous investigators reported that TpZrCl3/MAO and Tp*ZrCl3/ MAO exhibit poor activity and yield broad MWD PEs.5d However, Casagrande et al. showed that TpMs*ZrCl3/ MAO exhibits high activity for ethylene polymerization.11 Here we describe the molecular structures of the thallium compounds TlTpMs (1a) and TlTpMs* (1b), the use of these reagents to prepare Tp′MCl3 (M ) Zr, Hf) complexes, the molecular structures of TpMsZrCl3 (2a), TpMs*ZrCl3 (2b), (TpMs**ZrCl2)2(µ-O) (2c), and TpMs*HfCl3 (3b), and the olefin polymerization behavior of these catalysts under MAO activation conditions. A particularly active catalyst (2a) has been discovered. Results and Discussion Synthesis and Isolation of 1a and 1b. As described by Trofimenko, the reaction of 3-mesitylpyrazole with KBH4 in boiling anisole, followed by transmetalation to thallium, yields an isomeric mixture of 1a and 1b from which 1a is isolated by crystallization.12 However, as it is difficult to isolate 1b in pure form in large quantities (7) For similar behavior in other catalyst systems see: (a) Britovsek, G. J. P.; Bruce, M.; Gibson, V. C.; Kimberley, B. S.; Maddox, P. J.; Mastroianni, S.; McTavish, S. J.; Redshaw, C.; Solan, G. A.; Stro¨mberg, S.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc. 1999, 121, 8728. (b) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998, 120, 4049. (c) Przybyla, C.; Tesche, B.; Fink, G. Macromol. Rapid Commun. 1999, 20, 328. (8) Michiue, K.; Jordan, R. F. Polym. Mater. Sci. Eng. 2002, 86, 295. (9) Chen, E. Y-X.; Marks, T. J. Chem. Rev. 2000, 100, 1391 and references therein. (10) Kime-Hunt, E.; Spartalian, K.; DeRusha, M.; Nunn, C. M.; Carrano, C. J. Inorg. Chem. 1989, 28, 4392. (11) Furlan, L. G.; Gil, M. P.; Casagrande, O. L., Jr. Macromol. Rapid Commun. 2000, 21, 1054. (12) Rheingold, A. L.; White, C. B.; Trofimenko, S. Inorg. Chem. 1993, 32, 3471.

Organometallics, Vol. 23, No. 3, 2004 461

Figure 1. Molecular structures of 1a and 1b. Hydrogen atoms are omitted. Selected bond lengths (Å) and angles (deg) for 1a: Tl(1)-N(1) ) 2.522(6), Tl(1)-N(3) ) 2.527(4), Tl(1)-N(5) ) 2.582(3); N(2)-N(1)-Tl(1) ) 124.7(4), N(4)-N(3)-Tl(1) ) 125.6(3), N(6)-N(5)-Tl(1) ) 123.2(2), N(1)-Tl(1)-N(3) ) 73.0(2), N(1)-Tl(1)-N(5) ) 73.6(1), N(3)-Tl(1)-N(5) ) 71.7(1), N(2)-B(1)-N(4) ) 110.1(3), N(2)-B(1)-N(6) ) 109.7(3), N(4)-B(1)-N(6) ) 108.8(3). Selected bond lengths (Å) and angles (deg) for 1b: N(1)-Tl(1) ) 2.54(1), N(3)-Tl(1) ) 2.588(9), N(5)-Tl(1) ) 2.509(9); N(2)-N(1)-Tl(1) ) 121.7(7), N(4)-N(3)-Tl(1) ) 121.8(6), N(6)-N(5)-Tl(1) ) 120.9(6), N(5)-Tl(1)-N(1) ) 73.7(3), N(5)-Tl(1)-N(3) ) 77.1(3), N(1)-Tl(1)-N(3) ) 72.8(3), N(4)-B(1)-N(6) ) 110.8(9), N(4)-B(1)-N(2) ) 109.4(9), N(6)-B(1)-N(2) ) 109.7(9).

by this method, we developed a more efficient procedure for the isolation of 1b by column chromatography (NEt3neutralized SiO2 with toluene eluent; yield 24%). Molecular Structures of 1a and 1b. The molecular structures of 1a and 1b were determined by X-ray diffraction and are very similar (Figure 1). The Tl-N distances in 1a (average 2.54 Å) and 1b (average 2.55 Å) are similar and are typical for TlTp′ complexes (2.502.68 Å).13 The N-Tl-N angles in 1a (average 72.8°) are also similar to those in 1b (average 74.5°) and are somewhat more acute than the angles in typical TlTp′ compounds (76-78°). Similar acute N-Tl-N angles were observed in TlTpTol (75.2°; TpTol ) HB(3-ptolylpyrazolyl)3-).14 In both 1a and 1b, the Tl and B substituents on a given pyrazole ring are slightly (13) Rheingold, A. L.; Liable-Sands, L. M.; Trofimenko, S. Chem. Commun. 1997, 17, 1691. (14) Ferguson, G.; Jennings, M. C.; Lalor, F. J.; Shanahan, C. Acta Crystallogr., Sect. C 1991, 47, 2079.

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Scheme 1a

Scheme 2a

a

Ms ) mesityl.

Scheme 3a

a

Ms ) mesityl.

staggered, such that the Tl-N-N-B dihedral angles in 1a are in the range 3.99-7.42° (average 6.13°), and those in 1b are in the range 4.46-17.51° (average 10.48°). The pyrazole and mesityl rings are staggered by ca. 70° (range of angles between pyrazole and mesityl planes: in 1a, 63.3-84.2°; in 1b, 61.6-87.4°). Much smaller dihedral angles were observed in TlTpTol (1.8 and 29.6° for two independent molecules in the unit cell).14 Synthesis and Characterization of Tris(pyrazolyl)borate Zr and Hf Complexes. Tris(pyrazolyl)borate Zr and Hf complexes were synthesized by the reaction of the appropriate MCl4 salts with 1a or 1b (Scheme 1).6,8,11,15 The reaction of ZrCl4 with 1a in toluene yields a 72/28 mixture of TpMsZrCl3 (2a) and TpMs*ZrCl3 (2b), from which 2a is isolated by crystallization in 35% yield based on 2a; this is currently the best method for the synthesis of 2a. The reaction of ZrCl4 with 1a in the presence of THF (e.g. ZrCl4(THF)2 in THF) yields a 35/65 mixture of 2a and 2b. A similar increase in the extent of isomerization of TpMs to TpMs* was observed in the reaction of TiCl4 with 1a in the presence of THF.6 The reaction of ZrCl4 with 1b in toluene yields 2b quantitatively, whereas the reaction of ZrCl4 with 1b in THF caused minor isomerization to 2a. Exposure of a toluene solution of 2b to a small amount of air at room temperature resulted in precipitation of the dinuclear µ-oxo compound (TpMs**ZrCl2)2(µ-O) (2c; TpMs** ) HB(3-mesitylpyrazolyl)(5-mesitylpyrazolyl)2-), which was identified by X-ray diffraction (Scheme 2).16 The reaction of HfCl4 with either 1a or 1b in toluene yields 3b as the major product (56% and 85%, respec(15) Gil, M. P.; dos Santos, J. H. Z.; Casagrande, O. L., Jr. Macromol. Chem. Phys. 2001, 202, 319. (16) NMR spectra show that 2b is not contaminated with a TpMs**ZrCl3 impurity. It is presumed that 2c is formed by hydrolysis, although this issue was not investigated. For similar reactivity see: (a) Flores, J. C.; Wood, J. S.; Chien, J. C. W.; Rausch, M. D. Organometallics 1996, 15, 4944. (b) Flores, J. C.; Chien, J. C. W.; Rausch, M. D. Organometallics 1994, 13, 4140.

a

Ms ) mesityl.

tively; Scheme 3).17 Compound 3b was isolated in 64% yield from the latter reaction. The reaction of HfCl4 and 1b in THF yields a complex mixture and was not studied further. The observation of the TpMs/TpMs* rearrangements in Schemes 1-3 indicates that the B-N bonds in these Zr and Hf systems are labile (i.e. 1,2-borotropic shifts can occur easily) and raises the possibility that similar isomerizations may occur under polymerization conditions to generate multiple active species. Molecular Structures of 2a-c and 3b. The molecular structures of 2a,b and 3b are illustrated in Figure 2. The core structures of 2a and 2b are very similar (Tables 1 and 2). The Zr-N distances (2a, average 2.33 Å; 2b, average 2.32 Å), Zr-Cl distances (2a, average 2.38 Å; 2b, average 2.37 Å), N-Zr-N angles (2a, average 80.79°; 2b, average 80.20°), and Cl-Zr-Cl angles (2a, average 95.38°; 2b, average 97.86°) are almost identical for the two cases. The core structure of 3b is also very similar to that of 2a,b (Table 3). However, the steric properties of these compounds differ markedly. As illustrated by the space-filling diagrams in Figure 2, the three mesityl rings in 2a form a deep pocket that shields the three chloride ligands, whereas in 2b and 3b the two chloride ligands that flank the 5-mesitylpyrazole ring are sterically more accessible. The structure of 2c is less symmetrical than those of 2a and 2b due to the µ-oxo ligand (Figure 3). The Zr-N distance for the pyrazole ligand trans to oxygen (17) Other known Tp′Hf compounds include TpCpHfCl2 and Tp*HfCl3. See: (a) Kresinski, R.; Jones, C. J.; McCleverty, J. A. Polyhedron 1990, 9, 2185. (b) Oshiki, T.; Mashima, K.; Kawamura, S.; Tani, K.; Kitaura, K. Bull. Chem. Soc. Jpn. 2000, 73, 1735.

Tris(pyrazolyl)borate Zirconium and Hafnium Complexes

Organometallics, Vol. 23, No. 3, 2004 463

Figure 2. Molecular structures and space-filling diagrams of 2a,b and 3b. Hydrogen atoms are omitted from the ORTEP views. Table 1. Selected Bond Lengths (Å) and Angles (deg) for TpMsZrCl3 (2a)

Table 2. Selected Bond Lengths (Å) and Angles (deg) for TpMs*ZrCl3 (2b)

N(1)-Zr(1) N(3)-Zr(1) N(3A)-Zr(1)

N(1)-Zr(1) N(3)-Zr(1) N(5)-Zr(1)

N(2)-N(1)-Zr(1) N(4)-N(3)-Zr(1) N(3)-Zr(1)-N(1)

2.353(3) 2.313(2) 2.313(2) 118.3(2) 119.4(1) 80.86(7)

Cl(1)-Zr(1) Cl(2)-Zr(1) Cl(2A)-Zr(1) N(3)-Zr(1)-Cl(1) N(3)-Zr(1)-Cl(2) N(1)-Zr(1)-Cl(2)

2.370(1) 2.3876(7) 2.3876(7) 92.51(5) 91.47(5) 90.42(5)

(2.38(3) Å) is longer than the other two Zr-N distances (both 2.29(3) Å), but the average Zr-N distance (average 2.32 Å) is similar to those in 2a and 2b. Influence of Tp′ Ligand Structure on the Ethylene Polymerization Behavior of Tp′ZrCl3/MAO Catalysts. The ethylene polymerization performances of 2a/MAO and 2c/MAO were compared under condi-

N(2)-N(1)-Zr(1) N(4)-N(3)-Zr(1) N(6)-N(5)-Zr(1) N(5)-Zr(1)-N(1) N(5)-Zr(1)-N(3) N(1)-Zr(1)-N(3) N(5)-Zr(1)-Cl(1) N(1)-Zr(1)-Cl(1) N(3)-Zr(1)-Cl(1)

2.334(4) 2.337(4) 2.297(5) 118.7(3) 118.9(3) 122.9(3) 80.3(2) 80.6(2) 79.7(2) 167.7(1) 89.4(1) 91.1(1)

Cl(1)-Zr(1) Cl(2)-Zr(1) Cl(3)-Zr(1) N(5)-Zr(1)-Cl(2) N(1)-Zr(1)-Cl(2) N(3)-Zr(1)-Cl(2) Cl(1)-Zr(1)-Cl(2) N(5)-Zr(1)-Cl(3) N(1)-Zr(1)-Cl(3) N(3)-Zr(1)-Cl(3) Cl(1)-Zr(1)-Cl(3) Cl(2)-Zr(1)-Cl(3)

2.353(2) 2.378(2) 2.391(2) 89.6(1) 167.2(1) 90.9(1) 99.7(7) 88.0(1) 92.8(1) 167.3(1) 99.12(6) 94.75(8)

tions of medium catalyst concentration (6.2 µM in toluene) and low ethylene pressure (1.4 atm) at 60 °C.

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Figure 4. Kinetic profiles of ethylene polymerization by 2a/MAO and 2c/MAO. Entry designations refer to Table 4. Figure 3. Molecular structure of 2c. Hydrogen atoms are omitted. Selected bond lengths (Å) and angles (deg): N(2)Zr(1) ) 2.381(3), N(4)-Zr(1) ) 2.285(3), N(6)-Zr(1) ) 2.287(3), Cl(1)-Zr(1) ) 2.395(1), Cl(2)-Zr(1) ) 1.9246(5); N(1)-N(2)-Zr(1) 119.9(2), N(5)-N(6)-Zr(1) ) 125.6(2), Zr(1)#1-O(1)-Zr(1) ) 175.9(2), O(1)-Zr(1)-N(4) ) 92.9(1), O(1)-Zr(1)-N(6) ) 84.71(9), N(4)-Zr(1)-N(6) ) 79.0(1), O(1)-Zr(1)-N(2) ) 160.50(8), N(4)-Zr(1)-N(2) ) 75.4(1), N(6)-Zr(1)-N(2) ) 77.8(1), O(1)-Zr(1)-Cl(1) ) 97.0(1), N(4)-Zr(1)-Cl(1) ) 166.23(8), N(6)-Zr(1)-Cl(1) ) 92.42(8), N(2)-Zr(1)-Cl(1) ) 92.35(7), O(1)-Zr(1)Cl(2) ) 95.66(6), N(4)-Zr(1)-Cl(2) ) 85.87(8), N(6)Zr(1)-Cl(2) ) 164.84(8), N(2)-Zr(1)-Cl(2) ) 98.96(8), Cl(1)-Zr(1)-Cl(2) ) 102.55(4). Table 3. Selected Bond Lengths (Å) and Angles (deg) for TpMs*HfCl3 (3b) Hf(1)-N(1) Hf(1)-N(3) Hf(1)-N(5) N(5)-Hf(1)-N(3) N(5)-Hf(1)-N(1) N(3)-Hf(1)-N(1) N(5)-Hf(1)-Cl(2) N(3)-Hf(1)-Cl(2) N(1)-Hf(1)-Cl(2) N(5)-Hf(1)-Cl(1) N(3)-Hf(1)-Cl(1) N(1)-Hf(1)-Cl(1)

2.331(2) 2.293(2) 2.262(2) 80.80(8) 79.35(8) 80.36(8) 168.85(6) 90.27(6) 92.64(6) 89.80(6) 168.45(6) 91.37(6)

Hf(1)-Cl(1) Hf(1)-Cl(2) Hf(1)-Cl(3) Cl(2)-Hf(1)-Cl(1) N(5)-Hf(1)-Cl(3) N(3)-Hf(1)-Cl(3) N(1)-Hf(1)-Cl(3) Cl(2)-Hf(1)-Cl(3) Cl(1)-Hf(1)-Cl(3) N(2)-N(1)-Hf(1) N(4)-N(3)-Hf(1) N(6)-N(5)-Hf(1)

2.3665(7) 2.3457(7) 2.3689(8) 98.20(3) 88.43(6) 92.61(6) 166.70(6) 98.71(3) 93.84(3) 119.2(2) 119.8(2) 123.4(2)

The results are summarized in Table 4. Under these conditions, injection of 2a into a reactor charged with MAO, ethylene, and toluene resulted in immediate filling of the reactor with solid polymer (entries 1 and 2), and the reaction was stopped within 1-2 min. In contrast, 2c/MAO produced PE comparatively slowly, and smooth polymerization was continued for 6 min. The productivity of 2c/MAO based on polymer yield is much lower than that of 2a/MAO. Additionally, ethylene uptake curves clearly indicate that the rate of polymerization (Rp) by 2a/MAO is much higher than that of 2c/MAO (Figure 4). The PEs produced by 2a/MAO and 2c/MAO are highly linear, with Tm ) 132-133 °C. The molecular weight

Figure 5. 1H NMR spectrum (110 °C, 1,2-C6D4Cl2) of polyethylene (Table 6, entry 9): (a) olefinic region; (b) polymer main chain (i) and end methyl end group (ii) resonances.

(MW) of the PE from 2a/MAO is much higher than that from 2c/MAO, and in both cases narrow MWDs (Mw/ Mn ) 2.6) are observed at low MAO loadings. The 1H NMR spectra of the PEs produced by both catalysts contain methyl end group resonances, but olefin end group resonances are unobservable.18 The 1H NMR spectrum of a typical PE produced by 2a/MAO (vide infra, Table 6, entry 9) is shown in Figure 5. These observations show that the predominant chain transfer mechanism is alkyl exchange with MAO and/or the AlMe3 therein, as observed for TpMs*TiCl3/MAO.6 The initial studies of 2a/MAO described above, and Casagrande’s study of 2b/MAO,11 show that both catalysts are quite active. To obtain a meaningful comparison of 2a/MAO and 2b/MAO, ethylene polymerizations were performed under conditions of low catalyst con-

Table 4. Ethylene Polymerization Behavior of 2a,ca entry 1 2 3 4

precat. 2a 2a 2c 2c

amt of MAO (equiv) 400 1000 400 2000

ethylene press. (atm) 1.4 1.4 1.4 1.4

time (min)

yield (g)

Pb

Mwc (×103)

Mw/Mn

Al/chaine

Tm (°C)

2.8d

0.500 0.495 0.311 0.423

15.7 44.0 4.6 6.3

652 301 100 72

2.6 2.3 2.6 4.0

131 173 32 56

133 133 133 132

1d 6 6

a Polymerization conditions: glass Fischer-Porter bottle, 80 mL of toluene, 0.5 µmol of precatalyst, polymerization temperature 60 °C. The precatalyst/toluene solution was injected into the MAO/toluene solution in a Fischer-Porter bottle at 1.4 atm of ethylene pressure to start the reaction. b P ) productivity in units of kg of polymer/((mmol of Zr) atm h). c Weight average molecular weight determined by GPC, reported using polyethylene calibration. d Stirring was stopped and the reaction terminated at this time due to polymer precipitation. e In units of (mol of total initial Al-Me)/(mol of chains produced).

Tris(pyrazolyl)borate Zirconium and Hafnium Complexes Table 5. Ethylene Polymerization Behavior of 2a,ba

entry

precat.

1 2 3 4 5 6 7 8 9 10

2a 2a 2a 2a 2b 2b 2b 2b Cp2ZrCl2 (PhIm)2ZrCl2g

amt of P20min/ MAOb (103 yield equiv) (g) Pyieldc P5mind P20mine P5minf 50 100 300 500 50 100 300 500 300 500

0.060 0.145 0.088 0.109 0.039 0.054 0.043 0.043 0.118 0.120

70 169 103 127 46 63 51 50 138 141

55 97 63 84 45 62 44 27 82 133

46 91 50 63 27 40 29 21 84 80

0.84 0.95 0.79 0.75 0.60 0.64 0.67 0.80 1.02 0.60

a Polymerization conditions: Argonaut parallel pressure reactor, 5 mL of toluene, PC2H4 ) 4.1 atm, 0.000 625 µmol of precatalyst, polymerization temperature 75 °C. The precatalyst/toluene solution was injected into the MAO/toluene solution in the Argonaut parallel pressure reactor at 4.1 atm ethylene pressure to start the reaction. b Dried MAO. c Pyield ) productivity based on polymer yield. All productivities are in units of kg of polymer/((mmol of Zr) atm‚h). d P5min ) productivity based on accumulated ethylene uptake at 5 min. e P20min ) productivity based on accumulated ethylene uptake at 20 min. f P20min/P5min ) ratio of productivity at 20 and 5 min. g Bis[N-(3-tert-butylsalicylidene)anilinato]zirconium dichloride; temperature 50 °C.

Figure 6. Kinetic profiles of ethylene polymerization by 2a,b, (PhIm)2ZrCl2, and Cp2ZrCl2, with MAO activation. Entry designations refer to Table 5.

centration (0.125 µM in toluene) and at several MAO concentrations, using dried MAO (which is substantially free of AlMe3). Higher ethylene pressure (4.1 atm) and temperature (75 °C) and longer polymerization times (20 min) compared to the conditions of Table 4 were used in these experiments. The results are summarized in Table 5 and Figure 6. It is clear that 2a/MAO is more active than 2b/MAO. The activity of 2a/MAO is initially higher and decays more slowly than that of 2b/MAO. Both 2a/MAO and 2b/MAO exhibited the highest productivity at [MAO] ) 12.5 mM (MAO/Zr ) 100 × 103, Table 5, entries 2 and 6).19 These results show that the activity of the Tp′ZrCl3 catalysts varies in the order of the degree of steric crowding of the catalyst precursor: 2a/MAO > 2b/MAO, 2c/MAO > TpZrCl3/MAO, Tp*ZrCl3/MAO.5d The data available to date do not allow identification of the active species in these catalysts, and therefore, it is difficult to provide a detailed rationale for this trend. While it is reasonable to propose that activation occurs by alkylation of the Zr precatalyst followed by abstraction

Organometallics, Vol. 23, No. 3, 2004 465

of Cl- or Me- to generate an active Zr cation, attempts to prepare alkyl derivatives of 2a-c for chemical and polymerization studies using non-MAO activators have been unsuccessful to date.20 It is likely that the MAO abstracts the µ-oxo group from 2c to generate an active species that is similar to those generated in 2a,b/MAO.21 It is interesting to compare the activity of 2a,b/MAO to the activities of other highly active single-site catalysts, such as bis(phenoxyimine) zirconium catalysts22 and metallocene systems. For this purpose, ethylene polymerizations using (PhIm)2ZrCl2/MAO (PhIm ) N-(3tert-butylsalicylidene)anilinato) and Cp2ZrCl2/MAO were performed under conditions similar to those used to evaluate 2a/MAO and 2b/MAO.22c As shown in Table 5 and Figure 6, the activities of (PhIm)2ZrCl2/MAO and Cp2ZrCl2/MAO are similar to that of 2a/MAO and higher than that of 2b/MAO under similar conditions. Ethylene uptake curves show that (PhIm)2ZrCl2/MAO is initially the most active of these catalysts but its activity decays markedly over the 20 min polymerization time. Effect of Polymerization Conditions on the Performance of 2a/MAO. To obtain a better understanding of the polymerization behavior of 2a/MAO, the influence of the reaction conditions was investigated. As discussed by Bochmann et al.,23 productivity in olefin polymerization reactions is affected by many factors, including mass transport effects, the efficiency of catalyst activation, and the stability of the active species. Mass transport efficiency is influenced by stirring rate, viscosity of the mixture, polymer precipitation, temperature, and polymer molecular weight. If a catalyst exhibits adequate activity and stability under polymerization conditions, lowering the catalyst concentration may improve productivity and enable assessment of true activity trends. Accordingly, the influence of catalyst concentration on the behavior of 2a/MAO was investigated. Ethylene polymerizations by 2a/MAO were performed at several catalyst concentrations (Table 6). The use of dilute catalyst concentration (0.12 µM) and MAO/Zr ratios of 3000 and 5000 results in smooth polymerization, high activity, and the production of ultrahighmolecular-weight PE (entries 3 and 4). Ethylene uptake curves indicate that Rp reaches ca. 70 kg of polymer/ ((mmol of Zr) atm h) under these conditions and that higher MAO/Zr ratios lead to faster activation (Figure (18) Hansen, E. W.; Blom, R.; Bade, O. M. Polymer 1997, 38, 4295. (19) (a) For comparison, Casagrande et al. observed optimum productivity for 2b/MAO at a MAO/Zr ratio of 500.11 This optimum MAO loading corresponds to [MAO] ) 5.6 mM, which is very similar to the optimum [MAO] found here. The difference in optimum [MAO] may result from differences in the MAO properties as well as other factors. (b) The PEs produced by 2a/MAO and 2b/MAO in Table 5 have ultrahigh MWs (Mw > ca. 2 × 106) due to the use of dried MAO, which significantly reduces the chain transfer to AlMe3. (20) Tp*Ti and Tp*Zr alkyl complexes that contain alkoxide ligands have been prepared: (a) Reger, D. L.; Tarquini, M. E.; Lebioda, L. Organometallics 1983, 2, 1763. (b) Ipaktschi, J.; Sulzbach, W. J. Organomet. Chem. 1992, 426, 59. (21) For comparison, the reaction of (Cp2ZrCl)2(µ-O) with AlMe3 affords Cp2ZrMeCl cleanly: Wailes, P. C.; Weigold, H.; Bell, A. P. J. Organomet. Chem. 1971, 33, 181. (22) (a) Makio, H.; Kashiwa, N.; Fujita, T. Adv. Synth. Catal. 2002, 344, 477. (b) Matsui, S.; Mitani, M.; Saito, J.; Tohi, Y.; Makio, H.; Matsukawa, N.; Takagi, Y.; Tsuru, K.; Nitabaru, M.; Nakano, T.; Tanaka, H.; Kashiwa, N.; Fujita, T. J. Am. Chem. Soc. 2001, 123, 6847. (c) A lower temperature was used in polymerizations with (PhIm)2ZrCl2/ MAO due to the known thermal sensitivity of this catalyst.22a,b (23) Zhou, J.; Lancaster, S. J.; Walker, D. A.; Beck, S.; ThorntonPett. M.; Bochmann, M. J. Am. Chem. Soc. 2001, 123, 223.

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Michiue and Jordan

Table 6. Effect of Polymerization Conditions on the Performance of 2a/MAOa entry 1 2 3 4 5 6 8 9 10

TOFc

Mvd Al/ Tm Mwe (×103) (×103) Mw/Mn chainf (°C)

138.8 1856 8.7 117 74.8 1000 99.7 1333 135.1 1806 1199.2 16032 4.8 65 0.3 11 0.5 19

132 132 133 132 133 133 133 132 133

amt of precat. amt of MAO amt of MAO ethylene time yield (µmol) (µmol) (equiv) press. (atm) (min) (mg) 0.005 0.01 0.01 0.01 0.01 0.01 0.1 6.5 6.5

50 10 30 50 100 200 20 1300 3100

10000 1000 3000 5000 10000 20000 200 200 470

1.4 1.4 1.4 1.4 1.4 1.4 1.4 4.3 4.2

6 6 6 6 6 0.55 6 6 6

94 12 101 135 182 148 65 724 1274

Pb

8448 3590 936 895 805 567 261

2.2 2.2 2.1 11.4 11.9

297 701 150 122 72

a Polymerization conditions: glass Fischer-Porter bottle, 80 mL of toluene, polymerization temperature 60 °C. The precatalyst/toluene solution was injected into the MAO/toluene solution in a Fischer-Porter bottle at 1.4 atm of ethylene pressure to start the reaction. b P ) productivity, in units of kg of polymer/((mmol of Zr) atm h). c Turnover frequency, in units of mol of ethylene/((mol of Zr) sec). d Viscosity average molecular weight. e Weight average molecular weight determined by GPC, reported using polyethylene calibration. f In units of (mol of total initial Al-Me)/(mol of chains produced).

Table 7. Ethylene/1-Hexene Copolymerization by 2a/MAOa

Figure 7. Kinetic profiles of ethylene polymerization by 2a/MAO. Entry designations refer to Table 6.

7). At these dilute catalyst concentrations, the absolute MAO concentration is quite low, even though the MAO/ Zr ratio is high; thus, catalyst activation is slow and the observed activity may be limited by solvent impurities that are not completely scavenged by MAO. In fact, lowering the MAO/Zr ratio to 1000 resulted in a significant decrease in PE yield (entry 2). In contrast, at [MAO] ) 1.2 mM (MAO/Zr ) 10000, entry 5), a high Rp is attained within 2 min and high-MW, narrow-MWD PE is produced, characteristic of a single-site catalyst. Increasing [MAO] further to 2.5 mM (MAO/Zr ) 20000, entry 6) resulted in immediate activation, fast ethylene uptake, and gelation of the reaction mixture, necessitating termination of the reaction after ca. 1 min. As a result, a very high net productivity of 1199 kg of polymer/((mmol of Zr) atm h), corresponding to a turnover frequency of 16032 (mol of ethylene inserted)/ ((mol of Zr) sec) was obtained. The productivity of 2a/ MAO is comparable to that of other highly active singlesite catalysts, but 2a/MAO is noteworthy in that it produces exceptionally high MW PE with narrow MWD under high activity conditions.23 Further dilution of [2a] to 0.062 µM did not substantially increase the productivity (Table 6, entry 1). At higher catalyst loadings and higher ethylene pressures, the polymerizations are strongly mass transport limited (entries 9 and 10). Under these conditions, broad MWDs are observed, due to nonuniform diffusion of monomer and alkylaluminum to the active species.24 Ethylene/1-Hexene Copolymerization by 2a/ MAO. Ethylene/1-hexene copolymerization by 2a/MAO (24) As noted in Table 6, the ratio (mol of total initial Al-Me)/(mol of PE chains produced) is >70; therefore, chain transfer to Al probably does not make a large contribution to the broadening of the MWD in these cases.

entry

hexene (M)

Pb

1 2 3

0 2 5

182 240 101

hexenec (mol %) rErH 18.3 26.8

Tg (°C)

DPd

15367 0.17 -1.0 21290 0.28 -8.2 12469

Mwe Mw/ (×103) Mn 936 1589 930

2.2 2.0 1.8

a Polymerization conditions: glass Fischer-Porter bottle, total volume of toluene/1-hexene 80 mL, PC2H4 ) 1.4 atm, 0.01 µmol of 2a, 100 µmol of MAO (Al/Zr ) 10 000), polymerization temperature 60 °C, polymerization time 6 min. The precatalyst/toluene solution was injected into the MAO/(toluene + 1-hexene) solution at 1.4 atm of ethylene pressure to start the reaction. b P ) productivity based on yield, in units of kg of polymer/((mmol of Zr) atm h). c Hexene incorporation determined by 13C NMR. d DP ) average degree of polymerization; Mn ) (% hexene/100)(DP)(FWhexene) + (1 - (% hexene/100))(DP)(FWethylene). e Weight average molecular weight determined by GPC and reported using polyethylene calibration.

was evaluated, and the results are summarized in Table 7. This catalyst exhibits good activity and excellent comonomer incorporation at low catalyst concentration (0.12 µM). Copolymerization with 1-hexene does not significantly decrease the MW or broaden the MWD under the studied conditions. The copolymers produced by 2a/MAO are amorphous (entry 2, Tg ) -1.0 °C; entry 3, Tg ) -8.2 °C). The 13C NMR spectrum of a representative copolymer produced by 2a/MAO (entry 3) is shown in Figure 8. A trace level of methyl branches is present, but no ethyl or propyl branches are detectable.25 These observations, and the absence of NMRdetectable branches in PE homopolymers produced by 2a/MAO, suggest that the methyl branches arise by 1,2hexene insertion into Zr-CH3 species at chain initiation and/or by chain transfer to Al following 1,2-hexene insertion, rather than by chain walking. The rErH values (entry 2, 0.17; entry 3, 0.28) indicate that 2a/MAO has some tendency toward alternating ethylene and hexene insertions. No evidence for regioirregular HH placements was found from the 13C NMR spectra.26 The level of hexene incorporation by 2a/MAO is comparable to that of K[TpMs*TiIIICl3]/MAO (up to 26 mol % hexene) under similar copolymerization condi(25) (a) Galland, G. B.; de Souza, R. F.; Mauler, R. S.; Nunes, F. F. Macromolecules 1999, 32, 1620. (b) Randall, J. C. JMS-Rev. Macromol. Chem. Phys. 1989, C29, 201. (c) Hsieh, E. T.; Randall, J. C. Macromolecules 1982, 15, 1402. (26) For regioirregular polyhexene, see: Saito, J.; Mitani, M.; Matsui, S.; Kashiwa, N.; Fujita, T. Macromol. Rapid. Commun. 2000, 21, 1333 and references therein.

Tris(pyrazolyl)borate Zirconium and Hafnium Complexes

Organometallics, Vol. 23, No. 3, 2004 467

Figure 8. 13C NMR spectrum (110 °C, 1,2-C6D4Cl2) of ethylene/1-hexene copolymer produced by 2a/MAO (Table 7, entry 3). For assignments, see ref 25. Table 8. Ethylene Polymerization by 3b/MAOa entry

precat.

amt of MAO (equiv)

time (min)

yield (g)

Pc

Mvd (×103)

Mwe (×103)

Mw/Mn

Al/chainf

Tm (°C)

1 2 3 4 5b

3b 3b 3b 3b 3b

50 100 200 1000 1000

6 6 6 6 6

0.48 1.84 2.90 0.89 0.74

1.2 4.4 7.0 2.1 5.5

3146 2427 1495

1298 836 760 678 28

2.3 4.4 9.5 22.3 9.1

192 33 18 111 12

133 135 134 132 129

a Polymerization conditions: glass Fischer-Porter bottle, toluene 80 mL, P C2H4 ) 4.2 atm, 1 µmol of precatalyst, polymerization temperature 60 °C. The precatalyst/toluene solution was injected into the MAO/toluene solution in a Fischer-Porter bottle at 1.4 atm of ethylene pressure, and then the ethylene pressure was immediately increased to 4.2 atm. b PC2H4 ) 1.4 atm. c Productivity based on yield, in units of kg of polymer/((mmol of Hf) atm h). d Viscosity average molecular weight. e Weight average molecular weight determined by GPC, reported using polyethylene calibration. f In units of (mol of total initial Al-Me)/(mol of chains produced).

tions.8 However, the latter catalyst produces low-MW copolymer (Mw ) 17 × 103, MWD ) 2.9), with a more random comonomer distribution (rErH ) 0.52) and a low level of regioirregular HH placements. The differences in the structures of the ethylene/hexene copolymers produced by 2a/MAO and K[TpMs*TiIIICl3]/MAO result in markedly different physical properties: the former is amorphous, whereas the latter is semicrystalline.27 The amount of hexene incorporated by 2a/MAO is much larger than that of TpMs*TiIVCl3/MAO (up to 3.3 mol % hexene).15 For comparison, moderate levels of propylene incorporation in ethylene/propylene copolymerization were observed for the sterically open Tp′ZrCl3 catalysts TpZrCl3/MMAO (8 mol % propylene, MWD ) 2.4) and Tp*ZrCl3/MMAO/[Ph3C][B(C6F5)4] (21 mol % propylene, MWD 4.2).5d (27) For studies of the influence of MW on the crystallinity and melting behavior of ethylene/hexene copolymers see: Alamo, R. G.; Chan, E. K.; Mandelkern, L. Macromolecules 1992, 25, 6381.

Ethylene Polymerization by 3b/MAO. The ethylene polymerization behavior of the hafnium complex 3b was briefly investigated in toluene ([3b] ) 12 µM) with MAO as the cocatalyst, and the results are summarized in Table 8. Under these conditions, polymer yields are high and the polymerizations are mass transport limited. The 1H NMR spectrum of a low-molecular-weight polymer (entry 5) shows that methyl end groups but no olefin end groups are present. Additionally, the MWs decrease as the [MAO] is raised. These results indicate that the primary chain transfer mechanism is chain transfer to aluminum, as observed for the Ti and Zr analogues. The PEs produced by 3b/MAO under the conditions of Table 8 have broad MWDs, except for entry 1, where the [MAO] was the lowest. The broad MWDs result from the significant change in [Al-Me] during the polymerization (see Al/chain ratios) and the heterogeneity of the polymerization medium (see high polymer yields).

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Organometallics, Vol. 23, No. 3, 2004

Conclusions These studies provide a phenomenological picture of the olefin polymerization behavior of Tp′MX3/MAO catalysts (M ) Zr, Hf). The ethylene polymerization activity of Tp′ZrX3/MAO varies in the order of the degree of steric crowding of the Tp′Zr unit: 2a/MAO > 2b/ MAO, 2c/MAO > TpZrCl3/MAO, Tp*ZrCl3/MAO.5d Similar activity trends were observed for Tp′TiIII and Tp′TiIV catalysts.6,8 2a/MAO exhibits particularly high activity and produces ultrahigh-MW PE at low catalyst concentrations. 2a/MAO exhibits good hexene incorporation and high activity in ethylene/hexene copolymerization and produces ultrahigh-MW, amorphous copolymer.28 The predominant chain transfer mechanism for 2a,c/ MAO and 3b/MAO is chain transfer to aluminum, which leads to lower MWs with increasing [MAO].29 2a/MAO and 3b/MAO produce PE and ethylene/hexene copolymers with narrow MWDs (Mw/Mn ) 1.8-2.3), characteristic of single-site catalysis, under conditions where the polymerizations are not mass transport limited and the initial [MAO] is sufficiently high that it does not change significantly during the polymerization reaction due to chain transfer to Al. Efforts to identify the active species in these Tp′MX3/MAO single-site catalysts and to develop a molecular-level understanding of these systems are in progress. Experimental Section General Procedures. All manipulations were performed using drybox or Schlenk techniques under a purified N2 atmosphere or on a high-vacuum line unless otherwise indicated. Nitrogen was purified by passage through columns containing activated molecular sieves and Q-5 oxygen scavenger. Solvents were distilled from appropriate drying/deoxygenating agents (Et2O and THF, sodium benzophenone ketyl; CH2Cl2, CaH2; CDCl3 and CD2Cl2, P4O10). Pentane, hexanes, benzene, and toluene were purified by passage through columns of activated alumina and BASF R3-11 oxygen removal catalyst. Solvents were stored under N2 or vacuum prior to use. ZrCl4 and HfCl4 were purchased from CERAC (99.9%). ZrCl4(THF)230 and TpMs*ZrCl3 (2b)11 were prepared by literature procedures. MAO was obtained from Albemarle as a 13.71 wt % Al solution (AlMAO/AlTMA ) 87/13 (mol/mol)) in toluene and was stored at -36 °C prior to use. For polymerization at low Al/M ratios, the MAO stock solution was diluted to 1.37 or 0.137 wt % and stored at -36 °C. Dried MAO was prepared by evaporating MAO (1.2 M solution in toluene) to remove AlMe3. A solution of dried MAO in toluene (0.16 M) was prepared and used in the experiments in Table 5. All other chemicals were purchased from Aldrich and used without further purification. NMR spectra were recorded on a Bruker DRX400 spectrometer, in flame-sealed tubes, at 23 °C. 1H and 13C chemical shifts were determined by reference to the residual 1H and 13C solvent signals. Coupling constants are reported in Hz. Field desorption mass spectra (FD-MS) were recorded on a JEOL SX-102A instrument. The observed isotope patterns closely matched calculated isotope patterns. The listed m/z value corresponds to the most intense peak in the isotope pattern. Gel permeation chromatography (GPC) was performed on a Polymer Laboratories PL-GPC 220 instrument using 1,2,4(28) For ultrahigh-MW ethylene/propylene copolymer, see: Ishii, S.; Saito, J.; Matsuura, S.; Suzuki, Y.; Furuyama, R.; Mitani, M.; Nakano, T.; Kashiwa, N.; Fujita, T. Macromol. Rapid Commun. 2002, 23, 693. (29) Similar results were observed for 2b/MAO. (30) Manzer, L. E. Inorg. Synth. 1982, 21, 135.

Michiue and Jordan trichlorobenzene solvent (stabilized with 125 ppm BHT) at 150 °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. Polyethylene molecular weights were determined by GPC using polystyrene standards; the molecular weights in Tables 4-8 are reported versus polyethylene standards and were calculated by the universal calibration method using MarkHouwink parameters (K ) 17.5 × 10-5, R ) 0.670 for polystyrene; K ) 40.6 × 10-5, R ) 0.725 for polyethylene). Intrinsic viscosity values [η] were determined in decalin at 135 °C using an Ubbelohde viscometer. Viscosity average molecular weights (Mv) were determined by the following equation: [η] ) (6.2 × 10-4)Mv0.7.31 Elemental analyses were performed by Midwest Microlab or Galbraith Laboratories, Inc. TlTpMs (1a) and TlTpMs* (1b). A solution of 1a and 1b in CH2Cl2 was prepared by the method of Trofimenko,12 starting with 3-mesitylpyrazole (4.00 g, 21.5 mmol) and potassium borohydride (0.294 g, 5.45 mmol). The mixture was concentrated and filtered, yielding pure 1a (0.96 g, 21% based on potassium borohydride). The mother liquor was evaporated to dryness, and the residue was washed with methanol. From the methanol wash, 3-mesitylpyrazole (1.27 g, 6.82 mmol) was recovered as fine needles. The methanol-insoluble solid was dissolved in hot toluene, cooled to room temperature, and filtered to give a second crop of 1a (0.15 g, 3%). The toluene filtrate was dried under vacuum to give a 7/93 mixture of 1a and 1b (1.45 g, 35%). Compound 1b was isolated by flash chromatography. Dry silica gel (400 mL) was stirred in 1% triethylamine/toluene solution, loaded into a glass column, and washed with toluene. A 7/93 1a/1b mixture (1.92 g), obtained as described above, was suspended in toluene (40 mL) and loaded into the column. Compound 1b eluted with toluene as the first band and was isolated by removal of the toluene under vacuum (1.8 g, 100% recovery). TpMsZrCl3 (2a). A Schlenk flask was charged with 1a (300 mg, 0.389 mmol) and ZrCl4 (91 mg, 0.389 mmol). Toluene (30 mL) was added at 20 °C, and the resulting white suspension was stirred at this temperature. The reaction was monitored by 1H NMR analysis of small aliquots, which showed that 1a was consumed slowly. After 62 h, 1a was completely consumed and a 72/28 mixture of 2a and 2b was present. The reaction mixture was filtered through a fine glass frit, and the insoluble white powder was thoroughly washed with 30 mL of toluene. The filtrate and the wash were combined, concentrated to 3 mL, and cooled to -37 °C. A white powder precipitated and was collected by filtration, washed with toluene, and dried to yield 2a (105 mg, 35.2%). Anal. Calcd for C36H40BCl3N6Zr: C, 56.51; H, 5.27; N, 10.98. Found: C, 57.31; H, 5.59; N, 10.27. 1H NMR (CD Cl ): δ 7.98 (d, 3J 2 2 HH ) 2.2, 3H, pz 5-H), 6.84(s, 6H, Ms m-H), 6.14 (d, 3JHH ) 2.2, 3H, pz 4-H), 2.29 (s, 9H, Ms p-Me), 1.93 (s, 18H, Ms o-Me). 13C{1H} NMR (CDCl3): δ 158.3 (pz C-3), 138.9 (Ms C-4), 138.7 (Ms C-2 and C-6), 138.6 (pz C-5), 129.2 (Ms C-1), 127.9 (Ms C-3 and C-5), 108.4 (pz C-4), 21.3 (Ms o- and p-Me). FD-MS: 764 (M+). TpMs*ZrCl3 (2b). 2b was prepared in 60.2% yield by the previously reported method11 starting with ZrCl4 (0.302 g, 1.30 mmol) and 1b (1.00 g, 1.30 mmol). Anal. Calcd for C36H40BCl3N6Zr: C, 56.51; H, 5.27; N, 10.98. Found: C, 55.90; H, 5.24; N, 10.73. 1H NMR (CD2Cl2): δ 8.23 (d, 3JHH ) 2.2, 1H, pz 3-H), 7.73 (d; 3JHH ) 2.1, 2H, pz 5-H), 7.02 (s; 2H, Ms m-H), 6.88 (s; 2H, Ms m-H), 6.86 (s; 2H, Ms m-H), 6.24 (d; 3JHH ) 2.2, 1H, pz 4-H), 6.11 (d; 3JHH ) 2.1, 2H, pz 4-H), 2.39 (s; 3H, Ms p-Me), 2.32 (s; 6H, Ms p-Me), 1.95 (s; 6H, Ms o-Me), 1.93 (s; 6H, Ms o-Me), 1.88 (s; 6H, Ms o-Me). 13C{1H} NMR (CD2Cl2): δ 158.0 (pz C-3), 148.3 (pz C-3), 145.9, 139.5, 139.1, 138.9 (2C overlapped), 138.7, 137.9, 128.7, 128.4, 128.1, 128.0, 127.6, 108.1 (pz C-4), 107.2 (pz C-4), 21.4 (2C overlapped), 21.3, 21.2, 19.9. FD-MS: 764 (M+). (31) Chiang, R. J. Polym. Sci. 1959, 36, 91.

Tris(pyrazolyl)borate Zirconium and Hafnium Complexes (TpMs**ZrCl2)2(µ-O) (2c). A solution of 2b (622 mg, 0.81 mmol) in toluene (5 mL) was stored in a sample tube that was sealed with a rubber septum that allowed slow leakage of air. Granular crystals of 2c formed over the course of 1 month at room temperature. The supernatant was removed from the mixture by decantation, and the crystals were dried under vacuum to give a white powder (114 mg, 77.1 µmol, 9.5%). Anal. Calcd for C72H80B2Cl4N12OZr2: C, 58.61; H, 5.47; N, 11.39. Found: C, 58.62; H, 5.54; N, 11.24. 1H NMR (CD2Cl2): δ 7.84 (d, 3JHH ) 2.0, 4H, pz 3-H), 7.53 (d; 3JHH ) 2.1, 2H, pz 5-H), 6.91 (s; 4H, Ms m-H), 6.84 (s; 4H, Ms m-H), 6.80 (s; 4H, Ms m-H), 6.02 (d; 2H, 3JHH ) 2.2; pz 4-H), 5.95 (d; 3JHH ) 2.2, 4H, pz 4-H), 2.36 (s; 6H, Ms p-Me), 2.28 (s; 12H, Ms p-Me), 1.93 (s; 12H, Ms o-Me), 1.87 (s; 12H, Ms o-Me), 1.60 (s; 12H, Ms o-Me). 13C{1H} NMR (CD2Cl2): δ 157.0 (pz C-3), 148.5 (pz C-3), 143.8, 139.2, 139.0, 138.9, 138.4, 137.9, 137.0, 129.1, 128.2, 128.0, 127.9, 127.5, 107.1 (pz C-4), 107.0 (pz C-4), 21.4, 21.2, 21.1 (2C overlapped), 19.6 (Ms Me). TpMs*HfCl3 (3b). A Schlenk flask was charged with 1b (730 mg, 0.95 mmol), HfCl4 (303 mg, 0.95 mmol), and toluene (50 mL), and the resulting white suspension was stirred for 55 h at 20 °C. The mixture was filtered through Celite, and the white solid was washed with toluene (30 mL). The clear toluene filtrate and wash were combined and concentrated to give a cloudy solution (ca. 15 mL), which was then stored at -37 °C. The white fine needle crystals that formed were collected by filtration, washed with hexanes, and dried under vacuum, yielding a white powder (519 mg, 64.4%). NMR spectra showed that this material contained 0.12 equiv of toluene. Anal. Calcd for C36H40BCl3N6Hf‚0.12C7H8: C, 51.25; H, 4.78; N, 9.73. Found: C, 51.64; H, 5.16; N, 9.79. 1H NMR (CD2Cl2): δ 8.27 (d, 3JHH ) 2.3, 1H, pz 3-H), 7.77 (d; 3JHH ) 2.0, 2H, pz 5-H), 7.05 (s; 2H, Ms m-H), 6.90 (s; 2H, Ms m-H), 6.88 (s; 2H, Ms m-H), 6.28 (d; 3JHH ) 2.4, 1H, pz 4-H), 6.15 (d; 3J HH ) 2.3, 2H, pz 4-H), 2.41 (s; 3H, Ms p-Me), 2.33 (s; 6H, Ms p-Me), 1.97 (s; 6H, Ms o-Me), 1.95 (s; 6H, Ms o-Me), 1.90 (s; 6H, Ms o-Me). 13C{1H} NMR (CD2Cl2): δ 158.5 (pz C-3), 148.5 (pz C-3), 146.3, 139.5, 139.1, 139.0, 138.8, 138.7, 137.8, 128.6, 128.4, 128.1, 127.9, 127.4, 108.5 (pz C-4), 107.5 (pz C-4), 21.4, 21.2, 19.9 (Ms Me). FD-MS: 852 (M+). X-ray Structure Determinations. Crystal data, data collection details, and solution and refinement procedures are summarized in Tables 9 and 10, and full details are provided in the Supporting Information. The ORTEP diagrams were drawn with 50% probability ellipsoids. All non-hydrogen atoms were refined with anisotropic displacement coefficients unless otherwise indicated. All hydrogen atoms were included in the structure factor calculation at idealized positions and were allowed to ride on the neighboring atoms with relative anisotropic displacement coefficients. Specific comments for each compound follow. 1a: crystals of 1a were grown by slow evaporation of a CH2Cl2 solution. 1b: crystals of 1b‚CH2Cl2 were grown by slow evaporation of a CH2Cl2 solution. The CH2Cl2 solvent molecule of 1b‚CH2Cl2 was disordered about a center of symmetry and modeled as such. No anomalous bond lengths or thermal parameters were noted, with the exception of the solvent molecule. 2a: crystals of 2a‚2CH2Cl2 were grown from a saturated CH2Cl2 solution at 0 °C. 2b: crystals of 2b‚2C6H6 were obtained from a saturated benzene solution of 2b. 2c: crystals of 2c were obtained from a saturated toluene solution of 2c. Most of the residual electron density present after final refinement was not associated with the 2c but was presumed to be due to the toluene of crystallization; however, the toluene could not be adequately modeled. The program SQUEEZE was applied to the data and found and removed 438 electrons/cell containing a solvent volume of 2006 Å3. 3b: Crystals of 3b were obtained from a saturated diethyl ether solution at 0 °C. Ethylene Polymerizations (Tables 4, 6, and 8). Polymerization reactions were performed in a 200 mL FischerPorter bottle equipped with a magnetic stir bar, a stainless

Organometallics, Vol. 23, No. 3, 2004 469 Table 9. Summary of Crystallographic Data for Compounds 1a,b TlTpMs (1a)

TlTpMs* (1b)

formula cryst size (mm) cryst syst space group a, Å b, Å c, Å R, deg β, deg γ, deg V, Å3 Z T (K) diffractometer radiation, λ (Å) 2θ range (deg) index ranges: h; k; l

C36H39BN6Tl C36H39BN6Tl‚CH2Cl2 0.35 × 0.20 × 0.20 0.30 × 0.10 × 0.10 triclinic triclinic P1 P1 h 8.911(2) 8.925(5) 11.080(2) 11.072(6) 17.320(4) 18.098(9) 102.948(3) 98.190(8) 94.331(3) 90.595(8) 93.909(3) 103.808(8) 1655.5(6) 1717(2) 2 2 135 100 Bruker SMART APEX Mo KR, 0.710 73 2.42-56.60 3.84-50.04 -11 to +11; -10 to +10; -14 to +14; -13 to +13; -21 to +22 -21 to +21 no. of rflns collected 16 176 12 383 no. of unique rflns 13 202 5938 no. of obsd rflns I > 4σ(Fo), 12 704 I > 4σ(Fo), 5585 Rint 0.0168 0.0400 µ, mm-1 4.912 4.816 max/min transmn 1.0, 0.636 1.0, 0.605 a structure soln Patterson methods direct methodsa refinement method full-matrix least squares on F2 no. of data/restraints/ 13202/3/812 5938/0/430 params abs cor SADABS based on redundant diffractions R indices (I > 2σ(I))b R1 ) 0.0227, R1 ) 0.0643, wR2 ) 0.0554 wR2 ) 0.1744 R indices (all data)b R1 ) 0.0236, R1 ) 0.0676, wR2 ) 0.0562 wR2 ) 0.1760

a SHELXTL-Version 5.1, Bruker Analytical X-ray Systems, Madison, WI. b R1 ) ∑||Fo| - |Fc||/∑|Fo| and wR2 ) [∑[w(Fo2 Fc2)2]/∑[w(Fo2)2]]1/2, where w ) q/[σ2(Fo2) + (aP)2 + bP].

steel pressure head 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 solution and toluene (70 mL) and sealed. The MAO solution in toluene was first diluted to 1.37 wt % Al for runs in which the MAO loading was in the range 100-500 µmol and was further diluted to 0.137 wt % Al for runs in which the MAO loading was less than 100 µmol. The bottle was sealed, removed from the glovebox, and attached to a stainless steel double-manifold vacuum/ethylene line. The nitrogen atmosphere was removed by vacuum, and the solution was saturated with ethylene at the polymerization pressure and thermally equilibrated at the polymerization temperature for 10 min. The ethylene pressure was decreased to 1.4 atm, the polymerization was started by addition of a solution of the precatalyst complex in 10 mL of dry toluene, and the ethylene pressure was immediately increased to the experimental pressure. 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 of the reaction vessel and addition of excess methanol. The polymer was washed with acidic methanol and then washed with methanol and dried under vacuum for 12 h. Ethylene/1-Hexene Copolymerizations (Table 7). The procedure was identical with that for the ethylene homopolymerizations described above, except that 1-hexene was charged in the Fischer-Porter bottle with MAO and dry toluene. The total volume of toluene and 1-hexene was 80 mL for all polymerization reactions. Workup Procedure for High-Hexene-Content Copolymer. After the reaction mixture was quenched as described

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Organometallics, Vol. 23, No. 3, 2004

Michiue and Jordan

Table 10. Summary of Crystallographic Data for Compounds 2a-c and 3b TpMsZrCl3 (2a)

TpMs*ZrCl3 (2b)

(TpMs**ZrCl2)2(µ-O) (2c)

formula

C36H40BCl3N6Zr‚2CH2Cl2

C36H40BCl3N6Zr‚2C6H6

cryst size (mm) cryst syst space group a, Å b, Å c, Å β, deg V, Å3 Z T (K) diffractometer radiation, λ (Å)

0.04 × 0.01 × 0.05 monoclinic P21/m 8.031(2) 22.662(6) 11.791(3) 96.181(4) 2133.5(9) 2 130 Bruker SMART APEX Mo KR, 0.710 73

2θ range (deg) index ranges: h; k; l

5.00-50.06 -9 to +9; -26 to +26; -14 to +14 16 770 3861 I > 4σ(Fo), 3434 0.0435 0.0732 1.0, 0.569 direct methodsa 3861/0/264

0.20 × 0.02 × 0.01 monoclinic P21 12.27(1) 14.33(1) 14.182(8) 105.47(1) 2402(3) 2 200 CCD diffractometerc synchrotron radiation, 0.68888 3.34-45.08 4.36-50.06 -13 to +13; -15 to +15; -13 to +14; -26 to +19; -12 to +12 -39 to +39 11 261 21 376 5965 7556 I > 4σ(Fo), 4946 I > 4σ(Fo), 6281 0.0428 0.0457 0.434 0.410 1.0, 0.849 1.0, 0.594 direct methodsa Patterson methodsa full-matrix least squares on F2 5965/1/546 7556/0/430

R1 ) 0.0368, wR2 ) 0.0987 R1 ) 0.0408, wR2 ) 0.1006

SADABS based on redundant diffractions R1 ) 0.0419, R1 ) 0.0608, wR2 ) 0.0912 wR2 ) 0.1586 R1 ) 0.0582, R1 ) 0.0706, wR2 ) 0.0982 wR2 ) 0.1666

no. of rflns collected no. of unique rflns no. of obsd rflns Rint µ, mm-1 max/min transmn structure soln refinement method no. of data/restraints/ params abs cor R indices (I > 2σ(I))b R indices (all

data)b

C36H39BCl2N6O0.5Zr‚ (toluene) 0.40 × 0.16 × 0.12 monoclinic C2/c 11.796(2) 22.295(4) 33.087(6) 99.233(4) 8589(3) 8 100 Bruker SMART APEX Mo KR, 0.710 73

TpMs*HfCl3 (3b) C36H40BCl3HfN6 0.40 × 0.25 × 0.07 monoclinic P21/n 11.908(2) 14.791(3) 21.255(4) 100.561(3) 3680(1) 4 100 Bruker SMART APEX Mo KR, 0.710 73 4.58-56.58 -15 to +15; -19 to +19; -28 to +28 35 013 8769 I > 4σ(Fo), 8463 0.0294 3.086 1.0, 0.687 direct methodsa 8769/0/437 R1 ) 0.0298, wR2 ) 0.0786 R1 ) 0.0310, wR2 ) 0.0795

a SHELXTL-Version 5.1, Bruker Analytical X-ray Systems, Madison, WI. b R1 ) ∑||F | - |F ||/∑| F | and wR2 ) [∑[w(F 2 - F 2)2]/ o c o o c ∑[w(Fo2)2]]1/2, where w ) q/[σ2(Fo2) + (aP)2 + bP]. c CCD single-crystal diffractometer, Beamline 15, ChemMetCARS Sector 15, Advanced Photon Source, Argone National Laboratory.

above, the mixture was poured into 400 mL of hexane and washed with aqueous HCl (2 mL of concentrated HCl in 50 mL of water) and then with water (3 × 50 mL). The organic phase was separated and evaporated to dryness under vacuum for 12 h, yielding the copolymer product. Small-Scale Ethylene Polymerizations (Table 5). Smallscale polymerization reactions were performed in an parallel pressure reactor (Argonaut Endeavor catalyst screening system) containing eight reaction vessels (10 mL), each equipped with a mechanical stirrer. A toluene solution of dried MAO (4 mL) was loaded in each reaction vessel, and a stainless steel manifold equipped with ethylene line was attached. The nitrogen atmosphere was replaced with ethylene, and the solution was saturated with ethylene at the polymerization pressure and thermally equilibrated at the polymerization temperature. The polymerization was started by addition of a toluene solution of the precatalyst (0.25 mL of toluene solution of precatalyst followed by 0.75 mL of toluene wash). The total volume of the reaction mixture was 5 mL for all polymerizations. The pressure was kept constant by feeding ethylene on demand. After the specified reaction time, the polymerization was stopped by addition of excess methanol. The polymer was washed with acidic methanol, washed with methanol, and dried under vacuum for 10 h.

Acknowledgment. This work was supported by the U.S. Department of Energy (DE-FG-02-00ER15036) and

Mitsui Chemicals, Inc. (Japan). We thank Mitsui Chemicals, Inc. (Japan), for Mv and FD-MS measurements and the polymerization experiments in Table 5. X-ray diffraction analyses were performed by Ian Steele at the X-ray Crystallographic Laboratory of the University of Chicago Department of Chemistry. ChemMatCARS Sector 15 is principally supported by the National Science Foundation/Department of Energy under Grant Nos. CHE9522232 and CHE0087817 and by the Illinois board of higher education. The Advanced Photon Source is supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science, under Contract No. W-31-109-Eng-38. We also thank Dr. Antoni Jurkiewicz (University of Chicago) for valuable discussions on NMR issues. Supporting Information Available: Text giving crystal data and data collection and solution and refinement details and tables giving atomic coordinates, isotropic displacement parameters, anisotropic displacement parameters, and bond distances and bond angles for 1a,b, 2a-c, and 3b. This material is available free of charge via the Internet at http://pubs.acs.org. OM0300171