Alkyl Complexes as Single-Component Ethylene Polymerization

Organometallics 2010, 29, 767–774 DOI: 10.1021/om900546p

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Diamido-Ether Uranium(IV) Alkyl Complexes as Single-Component Ethylene Polymerization Catalysts Cassandra E. Hayes and Daniel B. Leznoff* Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6 Received June 23, 2009

The synthesis and characterization of two new uranium(IV) dialkyl complexes supported by two different diamido ether ligands are reported. The reaction of [{({2,6-iPr2C6H3}N{CH2CH2})2O}UCl2] ([{dippNCOCN}UCl2]) with 2 equiv of KCH2Ph generates the organoactinide [{dippNCOCN}U(CH2Ph)2] (3), which shows an η1-, η2-benzyl arrangement of the alkyl ligands in the solid state. Reaction of [({tBuN(SiMe2)}2O)UCl2] ([{tBuNON}UCl2]) with 2 equiv of LiCH(SiMe3)2 yields the alkyl-bridged uranium dimer [{tBuNON}U{CH(SiMe3)(SiMe2CH2)}]2 (4), which forms via γ C-H activation of the CH(SiMe3)2 substituent. Reaction of 3 and 4 as well as previously reported [{tBuNON}U(CH2SiMe3)2] (1) and [{dippNCOCN}U(CH2SiMe3)2] (2) with 1 atm of ethylene under ambient conditions produced high molecular weight polymers, demonstrating that diamido ether actinide complexes can act as ethylene polymerization catalysts; activities up to 560 g/mol 3 h 3 atm using 1 were observed. Reaction of 1 and 2 with 1 atm of ethylene and 1 equiv of B(C6F5)3 demonstrated a 4-fold decrease in activity of the catalyst. Reaction of 1 or 2 with an excess of MMAO or Et2AlCl did not yield any polymer formation upon the addition of ethylene. After reaction of 1 with Et2AlCl, removal of volatiles in vacuo yielded a dark red liquid that, upon standing, formed green crystals of [{tBuNON}U{(μ-Cl)2AlEt2}2] (5), indicating a route for catalyst deactivation by aluminum-based cocatalysts.

Introduction In recent years the study of actinide-based complexes has received considerable attention as a means of expanding the knowledge of their diverse chemistry and their potential for facilitating demanding chemical transformations.1 In particular, research has focused on the use of cyclopentadienyltype, especially pentamethylcyclopentadienyl (Cp*)-based, actinide complexes and their reactivity.2,3 The catalytic potential of organoactinide (An = Th(IV), U(IV)) complexes has been relatively less explored compared to analogous early transition-metal metallocene complexes,3,4 but they have demonstrated an ability to facilitate several catalytic cycles centered around alkynyl-actinide precursors including hydroamination,5-7 hydrosilylation,8,9 and recently

even hydrothiolation.10 In addition to this, Cp*An-based complexes can effect the oligomerization and polymerization of olefins11-13 and the polymerization of more delicate monomers, including diesters.14 As alkene polymerization catalysts these Cp*An complexes have been studied in comparison with analogous group IV metallocene systems. However, their activities are on average an order of magnitude lower than those of the early transition-metal systems.15 Modification of the Cp* ligand or selective enhancements of added cocatalyst have improved the activity of these organoactinide systems,12 as was the case in the transition-metal systems.16 However, the need to find other potentially active and diverse catalyst systems has led to the exploration of other ligand frameworks aside from those based on carbocyclic ligands. In particular, transition-metal complexes containing multidentate amido ligands in place of Cp-type ligands have been explored in terms of their efficiency as active catalysts.17 Amido ligands are typically easy to substitute, allowing for

*To whom correspondence should be sent. Tel: 778-782-4887. Fax: 778-782-3765. E-mail: [email protected]. (1) Ephritikhine, M. Dalton Trans. 2006, 2501. (2) Marks, T. J. Science 1982, 217, 989. (3) Andrea, T.; Eisen, M. S. Chem. Soc. Rev. 2008, 37, 550. (4) Barnea, E.; Eisen, M. S. Coord. Chem. Rev. 2006, 250, 855. (5) Stubbert, B. D.; Stern, C. L.; Marks, T. J. Organometallics 2003, 22, 4836. (6) Straub, T.; Haskel, A.; Neyroud, T. G.; Kapon, M.; Botoshansky, M.; Eisen, M. S. Organometallics 2001, 20, 5017. (7) Haskel, A.; Straub, T.; Eisen, M. S. Organometallics 1996, 15, 3773. (8) Dash, A. K.; Wang, J. Q.; Eisen, M. S. Organometallics 1999, 18, 4724. (9) Dash, A. K.; Gourevich, I.; Wang, J. Q.; Wang, J.; Kapon, M.; Eisen, M. S. Organometallics 2001, 20, 5084. (10) Weiss, C. J.; Wobser, S. D.; Marks, T. J. J. Am. Chem. Soc. 2009, 131, 2062.

(11) Jia, L.; Yang, X.; Stern, C.; Marks, T. J. Organometallics 1994, 13, 3755. (12) Jia, L.; Yang, X.; Stern, C. L.; Marks, T. J. Organometallics 1997, 16, 842. (13) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. Angew. Chem., Int. Ed. Engl. 1997, 36, 774. (14) Barnea, E.; Moradove, D.; Berthet, J.-C.; Ephritikhine, M.; Eisen, M. S. Organometallics 2006, 25, 320. (15) Yang, X.; Stern, C.; Marks, T. J. Organometallics 1991, 10, 840. (16) Bochmann, M. J. Organomet. Chem. 2004, 689, 3982. (17) Britovesk, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428.

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Organometallics, Vol. 29, No. 4, 2010

Hayes and Leznoff

Figure 2. Diamido ligand precursors used in the synthesis of complexes 1-4. Scheme 1. Synthesis of Diamido Uranium(IV) Complexes 1-4

Figure 1. Selected amido-transition-metal complexes (M = group IV, X = alkyl) used in the polymerization of ethylene and R-olefins.

a wide range of steric and electronic modifications via the amido R-group18 to target improved reactivity and higher catalyst activity. Indeed, amido ligands coupled with early transition metals and lanthanides have produced highly active olefin polymerization catalysts.17 The introduction of ligands with multiple amido donors onto group IV transition metals has also produced catalysts with impressive activities: for example, diamido(silyl) ether [NON] zirconium-based complexes19,20 (Figure 1, A and B) demonstrate activities in the range of 100 kg/mol 3 h,21 and group IV diamido alkyl-bridged and pyridine-bridged complexes (Figure 1, C and D) perform as very active catalysts for the polymerization of other R-olefins, although their ethylene polymerization capability is unreported.22,23 The use of a diamido ligand bridged by a silicocarbon-based backbone (Figure 1, E) in conjunction with zirconium generates activities that rival the most active group IV metallocene complexes, with values near 103 kg/mol 3 h.24 While there has been an extensive and comprehensive survey of transition-metal amido complexes and their ability to catalyze olefin polymerization,17 the extension of this use of amido ligands to actinide chemistry is still underdeveloped. This fact remains despite recent developments that show amidoactinides can activate a variety of small, typically inert molecules including CO225,26 and dinitrogen27,28 as well (18) Bradley, D. C.; Chisholm, M. H. Acc. Chem. Res. 1976, 9, 273. (19) Elias, A. J.; Schmidt, H. G.; Noltemeyer, M.; Roesky, H. W. Eur. J. Solid State Inorg. Chem. 1992, 29, 23. (20) Aizenberg, M.; Turculet, L.; Davis, W. M.; Schattenmann, F.; Schrock, R. R. Organometallics 1998, 17, 4795. (21) Male, N. A. H.; Thornton-Pett, M.; Bochmann, M. J. Chem. Soc., Dalton Trans. 1997, 2487. (22) Scollard, J. D.; McConville, D. H.; Vittal, J. J.; Payne, N. C. J. Mol. Catal. A 1998, 128, 201. (23) Guerin, F.; McConville, D. H.; Vittal, J. J.; Yap, G. A. P. Organometallics 1998, 17, 5172. (24) Gibson, V. C.; Kimberley, B. S.; White, A. J. P.; Williams, D. J.; Howard, P. Chem. Commun. 1998, 313. (25) Korobkov, I.; Gambarotta, S. Organometallics 2004, 23, 5379. (26) Castro-Rodriguez, I.; Nakai, H.; Zakharov, L. N.; Rheingold, A. L.; Meyer, K. Science 2004, 305, 1757. (27) Roussel, P.; Scott, P. J. Am. Chem. Soc. 1998, 120, 1070. (28) Korobkov, I.; Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int. Ed. 2003, 42, 4958. (29) Castro-Rodriguez, I.; Nakai, H.; Gantzel, P.; Zakharov, L. N.; Rheingold, A. L.; Meyer, K. J. Am. Chem. Soc. 2003, 125, 15734. (30) Korobkov, I.; Gambarotta, S.; Yap, G. P. A. Organometallics 2001, 20, 2552. (31) Arunachalampillai, A.; Crewdson, P.; Korobkov, I.; Gambarotta, S. Organometallics 2006, 25, 3856.

as supporting C-H,29,30 C-O, and C-N bond activations.31 In addition to this small molecule reactivity amidouranium complexes are also well suited to unique reactivity due to their potentially wide range of accessible oxidation states.32 Despite this track record of reactivity and the success of amido-transition-metal alkene polymerization catalysts, there are no reported examples of an amidoactinide olefin polymerization catalyst. Recently, we reported the synthesis of a series of actinide(IV) dialkyl species supported by two types of diamido ligands.33,34 The first of these, O(SiMe2NtBu)22- ({tBuNON}2-),19,21,35,36 contains a silyl ether backbone as shown in Figure 2; the R groups on the amido donors can be easily altered for basicity and steric requirements, while the neutral silyl ether acts as a hemilabile donor,37 binding when required by the metal center. The second framework, ({2,6-iPr2C6H3}N{CH2CH2})2O2({dippNCOCN}2-, Figure 2),20 contains a carbon backbone and also a neutral ether donor. Herein we report the synthesis of two new, related diamido-ether uranium(IV) dialkyl complexes and the ability of these and some of our previously reported analogues to act as ethylene polymerization catalysts.33,34 Indeed, several of these amidoactinide alkyl complexes proved to be active single-component ethylene polymerization catalysts under ambient conditions, yielding high molecular weight polymers. Upon addition of common cocatalyst activating agents (32) Monreal, M. J.; Carver, C. T.; Diaconescu, P. L. Inorg. Chem. 2007, 46, 7226. (33) Jantunen, K. C.; Batchelor, R. J.; Leznoff, D. B. Organometallics 2004, 23, 2186. (34) Jantunen, K. C.; Haftbaradaran, F.; Katz, M. J.; Batchelor, R. J.; Schatte, G.; Leznoff, D. B. Dalton Trans. 2005, 3083. (35) Mund, G.; Batchelor, R. J.; Sharma, R. D.; Jones, C. H. W.; Leznoff, D. B. J. Chem. Soc., Dalton Trans. 2002, 136. (36) Mund, G.; Vidovic, D.; Batchelor, R. J.; Britten, J. F.; Sharma, R. D.; Jones, C. H. W.; Leznoff, D. B. Chem. Eur. J. 2003, 9, 4757. (37) Slone, C.; Weinberger, D.; Mirkin, C. Prog. Inorg. Chem. 1999, 48, 233.

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Figure 3. Molecular structure and numbering scheme for 3 (ORTEP 40% probability ellipsoids). Hydrogen atoms and isopropyl substituents of the amide phenyl groups are omitted for clarity. Selected bond distances (A˚) and angles (deg): U1-N10, 2.223(10); U1-N11, 2.197(10); U1-O10, 2.485(8); U1-C10, 2.483(14); U1 3 3 3 C11, 3.384(18); U1-C20, 2.544(13); U1-C21, 2.717(15); C10-C11, 1.44(2); C20-C21, 1.44(2); N10-U1-N11, 130.0(4); N10-U1-O10, 67.3(3); U1-C10C11, 116.6(10); U1-C20-C21, 80.8(8); C10-C11-C12, 123.3(16); C20-C21-C22, 124.4(18); data pertaining to the second molecule in the unit cell are available in Table SI-1.

such as B(C6F5)3, Et2AlCl, and MMAO, polymer formation was either drastically reduced or shut down entirely.

Results Synthesis of Diamido-ether Uranium(IV) Dialkyl Complexes. The preparation of all diamido-ether uranium(IV) dialkyls followed standard salt metathesis protocols as shown in Scheme 1; 1 and 2 were previously reported.33,34 As an example, reaction of a slurry of [{dippNCOCN}UCl2] in hexanes with 2 equiv of KCH2Ph yielded, after stirring for 12 h and workup, bright red [{dippNCOCN}U(CH2Ph)2] (3) in 61% yield. The attempted use of (PhCH2)MgCl to synthesize the same product proved unsuccessful, yielding only intractable materials. Red block-shaped crystals of 3 suitable for X-ray diffraction were grown by slow evaporation of a pentane solution; the crystal structure contains two molecules in the unit cell, each with slightly different arrangements of the benzyl ligands. One of these molecules is shown in Figure 3. For both molecules the uranium is found in a distorted six-coordinate environment, bound by the tridentate {dippNCOCN} ligand and both an η1and an η2-coordinated benzyl group. The U1-N10, U1N11, and U1-O10 distances of 2.223(10), 2.197(10), and 2.485(8) A˚, respectively, are generally unremarkable, comparing well with those found in 2.34 The U1-N10 and U1-N11 lengths are shorter than in similar monodentate amido uranium(IV) complexes by approximately 0.05 A˚.38,39 The U1-O10 distance is comparable to that in the silyl ether complex [{tBuNON}UCl2]2.33 Considering first the η1-benzyl group in 3, the U1-C10 bond length is 2.483(14) A˚, typical of other U-C single bond lengths such as 2.424 A˚ in [Cp*2UMe2],40 and the U1-C10-C11 bond angle is an unremarkable 116.6(10)°. The U1 3 3 3 C11 distance is 3.384(18) A˚, much too long for (38) Andersen, R. A.; Zalkin, A.; Templeton, D. H. Inorg. Chem. 1981, 20, 622. (39) McCullough, L. G.; Turner, H. W.; Andersen, R. A.; Zalkin, A.; Templeton, D. H. Inorg. Chem. 1981, 20, 2869. (40) Jantunen, K. C.; Burns, C. J.; Castro-Rodriguez, I.; Da Re, R. E.; Golden, J. T.; Morris, D. E.; Scott, B. L.; Taw, F. L.; Kiplinger, J. L. Organometallics 2004, 23, 4682.

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any interaction to be occurring between the metal and the ipso-carbon. By comparison, the analogous U1-C20 and U1-C21 distances in the η2-benzyl fragment are 2.544(13) and 2.717(15) A˚, both within the range of a U-C single bond and comparable with both the U-C bond length in 2 (U-CH2SiMe3, 2.40(2) and 2.44(2) A˚)34 and the Th-C bond lengths of 2.503(3) and 2.826(3) A˚ for the η2-benzyl in the diamidoactinide benzyl compound [{XA2}Th(CH2Ph)2] (XA2 = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tert-butyl9,9-dimethylxanthene).41 Notably, the U1-C20 length for 3 is slightly longer and the U1-C21 length is slightly shorter than the analogous distances in [{XA2}Th(CH2Ph)2], consistent with stronger η2-coordination in 3. In addition, the U1-C20-C21 bond angle in 3 is 80.8(8)°, substantially more acute than for the η1-benzyl fragment, further indication of a stronger η2-coordination. Transition-metal η2-benzyl complexes show similarly tight bond angles ranging between 83° and 89°; the angle in 3 is, to our knowledge, the most acute reported.42,43 The room-temperature 1H NMR spectrum of 3 displays 15 broadened, paramagnetically shifted resonances, and due to its complexity it could not be completely assigned; several of the resonances clearly overlap. Notably, the η1-benzyl protons are present at -125.92 ppm. This highly upfield shifted resonance is characteristic of other uranium(IV) alkyl complexes, including 1 and 2, which both show resonances assigned to the η1-alkyl-hydrogens that are shifted past -120 ppm in their 1H NMR spectra. Similarly, the η2-U-CH2Ph protons can be assigned to the resonance at -73.46 ppm based on both the integration of 2H and the premise that these protons are likely to be second-closest to the uranium center after the η1-CH2 benzyl protons and so should also be shifted considerably from their diamagnetic position. These two resonances remain distinctive in the 1H NMR spectrum as the temperature is changed from -50 to 100 °C. At -50 °C the spectrum has 20 resonances, as would be expected for a system with Cs symmetry. Such a structure would have restricted rotation of the isopropyl phenyl groups but would allow for fluctuation in the U-η1-benzyl, η2-benzyl mirror plane. Seven resonances can be fully assigned, with one more partially assigned. There are four resonances at 33.17, 22.96, 2.58, and -12.48 ppm, which all have an integration of 6H and so are likely the four isopropyl phenyl substituents, consistent with Cs symmetry. The resonances at -187.72 and -137.19 ppm are likely attributable to the η1-CH2Ph protons of the η1-benzyl and the η2-CH2Ph protons of the η2-benzyl, respectively. The para-protons of the η1-benzyl and η2-benzyl are found at -23.22 and 2.19 ppm with an integration of 1H and 3H, respectively; this latter resonance overlaps with another 2H resonance (note that the spectrum should have 21 resonances, but only 20 were observed at the low-temperature limit), which could not be assigned to a particular hydrogen. Upon increasing the temperature, the resonances migrate toward the diamagnetic region and broaden considerably; coalescence occurs at 50 °C. At the high-temperature limit of 100 °C six resonances can be clearly resolved, while the two most shifted resonances (those that originated at -187.72 and 120.09 ppm at -50 °C) (41) Cruz, C. A.; Emslie, D. J. H.; Harrington, L. E.; Britten, J. F. Organometallics 2008, 27, 15. (42) Fryzuk, M. D.; Leznoff, D. B.; Rettig, S. J. Organometallics 1995, 14, 5193. (43) Dryden, N. H.; Legzdins, P.; Phillips, E. C.; Trotter, J.; Yee, V. C. Organometallics 1990, 9, 882.

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Hayes and Leznoff Table 1. Ethylene Polymerization Results for Complexes 1-4 entry

complexa

1 2 3 4 5 6 7 8

1 2 3 4 1 1 1 1

yield of PE (g)

activityb

time (h)

0.384 0.036 0.036 0.031 0.022 0.007 0.734 0.800

383.7 54.6 50.6 30.8 40.3 24.0 560.3 458.0

12 12 12 12 8 4 18 24

a Conditions: 0.05 g of catalyst, 250 mL flask, 1 atm of ethylene, hexanes (30 mL), 25 °C. b Activities are measured in g/mol 3 h 3 atm.

broaden into the baseline. The high-temperature limit for free rotation of the isopropyl phenyl substituents and exchange of the U-CH2Ph protons should yield 10 resonances; this limit is not reached even at 100 °C. In an effort to determine how much steric pressure the large uranium center could withstand, the synthesis of the “superbulky” dialkyl [{tBuNON}U{CH(SiMe3)2}2] was targeted. Reaction of 2 equiv of LiCH(SiMe3)2 with [{tBuNON}UCl2] in hexanes resulted in a bright yelloworange solution within 5 min of addition. After workup an orange solid (4) was isolated (82% yield). Single orange, plate-like crystals of 4 suitable for X-ray diffraction were grown by slow evaporation of a hexanes solution. The structure of [{tBuNON}U{CH(SiMe3)(SiMe2CH2)}]2 (4) (Figure 4) revealed a dinuclear uranium complex, with each metal center in a distorted octahedral environment. Each uranium is coordinated by the tridentate {tBuNON} backbone, two bridging CH2 carbon units formed through the γ C-H activation of individual CH(SiMe3)2 ligands, and a terminal carbon unit bonded through the CH of one CH(SiMe3)(SiMe2CH2) moiety. The overall symmetry of the structure is C2 since the right and left SiMe3 groups point in the same direction out of the molecular plane. In addition the R-carbon of the CH(SiMe3)(SiMe2CH2) substituent is unusually planar, contrary to other CH(SiMe3)2 complexes, which demonstrate a tetrahedral geometry at the R-carbon center. The only reported example of an actinide complex coordinated to the CH(SiMe3)2 ligand is the homoleptic U(III) alkyl [U{CH(SiMe3)2}3].44 The U1-O11 distance and U1-N11 distances are 2.603(8) and 2.261(8) A˚, respectively, both considerably longer (by nearly 0.1 A˚) than in comparable diamidouranium(IV) dialkyl structures.33 The U1 3 3 3 U2 distance of 3.965(2) A˚ is too long to be considered as a metal-metal interaction.30,45 The NMR of 4 has a large

number of paramagnetically shifted resonances that due to their broadness could not be assigned. The U-C distances in 4 are 2.435(9) A˚ for U1-C3 (the terminal carbon), and 2.608(12) and 2.654(10) A˚ for U1-C1 and U1-C2, respectively (the bridging carbons). The U1-C3 distance is similar to other U-C single bonds supported by the {tBuNON} ligand.33 On the other hand, the U1-C1 and U1-C2 distances, while still in the range for a U-C single bond,34 are substantially longer, as expected for a bridging ligand. The U1-C1-U2 bond angle is 97.6(3)°, suggesting there is some strain on the tetrahedral carbon resulting from maintaining the bridging environment. In addition, the U1-C3-Si1 angle of 93.7(3)° is quite acute in comparison with [U{CH(SiMe3)2}3], which has a U-CH-SiMe3 angle of 122°.44 This value is also more acute than early transition-metal and lanthanide complexes containing the bulky CH(SiMe3)2 group. Complete cleavage of this C-H bond, as observed for 4, has not been reported for any other system.46,47 Ethylene Polymerization. Solutions of diamido-ether uranium(IV) dialkyl complexes 1 through 4 were placed under 1 atm of ethylene at room temperature. For all four complexes, high molecular weight polyethylene was produced, illustrating that diamidoactinide alkyl complexes can act as active ethylene polymerization catalysts, with a range of activities as described in Table 1. Unless otherwise specified, activities are based on a standard set of reaction conditions that specify time and catalyst loadings. Two important trends can be determined from this data. The first is the difference in catalytic activity between dialkyl complexes of the {tBuNON} and {dippNCOCN} ligands. For example, 1 has an activity of 384 g/mol 3 h after a 12 h period, while 2 is considerably lower at 55 g/mol 3 h. It is difficult to determine if this change in activity is the result of altering the ligand backbone or the result of varying the substituent on the amido-R group. The second important trend is the difference in activity based on alkyl substituent. Complex 1 is the more active of the two {tBuNON}-based catalysts, while 4 has an activity that is much lower. There are several differences between the structures of 1 and 4 that may explain this drastic difference in activity. Clearly, the alkyl substituents are not the same (R = CH2SiMe3 in 1 versus R = CH(SiMe3)(SiMe2CH2) in 4); this results in a change in nuclearity (monomer versus dimer) and also a more congested coordination sphere in 4. In addition, the alkyl substituent for 1 is monodentate and 1 is mononuclear, while in 4 the alkyl substituent is bidenate by virtue of the C-H activation and also bridges two metal centers,

(44) Van Der Sluys, W. G.; Burns, C. J.; Sattelberger, A. P. Organometallics 1989, 8, 855. (45) Korobkov, I.; Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int. Ed. 2002, 41, 3433.

(46) Coughlin, E. B.; Henling, L. M.; Bercaw, J. E. Inorg. Chim. Acta 1996, 242, 205. (47) Schumann, H.; Esser, L.; Loebel, J.; Dietrich, A.; Van der Helm, D.; Ji, X. Organometallics 1991, 10, 2585.

Figure 4. Molecular structure and numbering scheme for 4 (ORTEP 40% probability ellipsoids). Hydrogen atoms and amido tert-butyl groups are omitted for clarity. Disorder among silyl methyl groups is also omitted for clarity. Selected bond distances (A˚) and angles (deg): U1-N11, 2.261(8); U1-N12, 2.248(10); U1-O11, 2.603(8); U1-C1, 2.608(12); U1-C2, 2.654(10); U1-C3, 2.435(9); U1 3 3 3 U2, 3.965(2); N11-U1N12, 120.3(4); N11-U1-O11, 61.2(3); C1-U1-C2, 82.1(4); C1-U2-C2, 82.2(4); C1-U1-C3, 73.9(3); U1-C1-Si1, 87.0(5); U1-C1-U2, 97.6(3); additional bond angles and distances are listed in Table SI-2.

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Table 2. Ethylene Polymerization Results Using Various Activating Agents for Complexes 1 and 2 entry

complexa

1 2 3 4 5

1 2 1 2 1

catalyst (mmol) 0.073 0.059 0.073 0.059 0.044

activating agent (mmol) B(C6F5)3 (0.073) B(C6F5)3 (0.059) Et2AlCl (0.727) Et2AlCl (0.598) MMAO (0.440)

amount of polymer (g)

activityb

0.085 0.012 0 0 0

93.4 17.1 0 0 0

a Conditions: 0.050 g of precatalyst, 250 mL flask, toluene (30 mL) for B(C6F5)3 or hexanes (30 mL) for Et2AlCl or MMAO, 1 atm of ethylene, 12 h, 25 °C. b Activities measured in g/mol 3 h 3 atm.

making 4 dinuclear. Any combination of these factors could be responsible for the change in activity. The activities expressed by these compounds are most remarkable when compared with other diamido actinide species such as [{XA2}Th(CH2Ph)2],41 which showed no ethylene polymerization in toluene. To the best of our knowledge, 1-4 are the first reported examples of amidoactinide ethylene polymerization catalysts and are also among the few examples of a single-component actinide-based catalyst system.48 When compared with other nonamido actinide catalysts for olefin polymerization, nearly all of which are cationic, 1-4 are less active by about a factor of 103-104. With the use of Cp*-based catalysts, such as [Cp*2ThMe2], no polymerization takes place in the absence of an activating agent and the overall activity is strongly dependent on the choice of noncoordinating anion.15 When BPh4- is used, the activity ranks near 103 g/mol 3 h, but when B(C6F5)4- is used, the activity begins to approach the values that are seen with some early transition-metal metallocene complexes at about 106 g/mol 3 h.15 Similarly, comparing 1-4 with early transition-metal amido ethylene polymerization catalysts (nearly all of which are cationic) shows that 1-4 are still considerably less active, with activities in some cases nearly 103 times lower.17,49 Nevertheless, since it is clear that 1-4 do indeed act as ethylene polymerization catalysts, the opportunity now exists to adjust the steric and electronic properties of the donating amido substituents to increase their activity. To ensure that catalyst death did not occur during the time period in which the polymerization was run, several timed reactions were performed, as indicated in Table 1. It was noted that with increasing reaction time, the activity actually increased; by doubling the reaction time (entries 5 and 6) the activity nearly doubled. The activity further increased by nearly 10-fold with longer times (entry 1) and does not begin to decrease until after 18 h (entries 7 and 8). This suggests that no significant catalyst death is occurring over the course of the polymerization reaction time studied, and as such 1 (as an example of the reactivity of complexes 1-4) can be considered quite robust for the processes of polymerization. Ethylene polymerization was also attempted by adding typical boron-based and alkylaluminum activating agents, the data for which are presented in Table 2. Interestingly, these common cocatalysts are found to hinder the ability of 1 and 2 to act as ethylene polymerization catalysts, lowering their activity considerably. Complex 1 is a considerably active single component catalyst by comparison with other actinide-based systems; however upon the addition of B(C6F5)3 a 4-fold decrease in activity occurs, dropping from 384 g/mol 3 h to 93 g/mol 3 h. A similar drop in activity is also observed for 2 upon addition of B(C6F5)3. (48) Straub, T.; Haskel, A.; Eisen, M. S. J. Am. Chem. Soc. 1995, 117, 6364. (49) Scollard, J. D.; McConville, D. H. J. Am. Chem. Soc. 1996, 118, 10008.

Addition of an excess of aluminum-based reagents, either Et2AlCl or MMAO, resulted in complete catalyst deactivation and zero polymer formation, as seen in Table 2. Thus, when an excess of MMAO is added to 1 under ethylene, no polymeric products were formed, and only intractable materials were produced. Similarly, when polymerization was attempted with Et2AlCl, no polymer product formed; however after 12 h of stirring, the solvents were removed in vacuo, yielding a dark red oil. Upon standing, the oil separated to form plate-like green crystals of 5 and a remnant, unidentifiable red oil. The crystal structure of 5 portrays the uranium(IV) center in a seven-coordinate distorted pentagonal-bipyramidal geometry, coordinated by the tridentate {tBuNON} ligand and four chlorides that act as bridges to two aluminum centers (Figure 5). The two amido donors of the {tBuNON} ligand act as the axial ligands of the bipyramid with the silyl ether and chloride ligands within the pentagonal plane. As such, the N1-U1-N10 angle is 129.3(3)°; all angles between adjacent donor atoms in the pentagonal plane are between 63.9(3)° and 79.33(16)°. Each of the two aluminum atoms are tetrahedral, with two bridging chlorides and two ethyl substituents; notable bond lengths and angles are reported in Table SI-3. Complexes similar to that of 5 have been demonstrated with a number of early metal catalyst systems, including those that contain metallocene and amido frameworks.50-53 The U1-N1 and U1-O1 bond lengths of 5 are 2.176(6) and 2.435(8) A˚, respectively, similar to {tBuNON}UCl2 and other {tBuNON}uranium(IV) complexes.33 The U1-Cl11 and U1-Cl21 bond lengths are 2.865(6) and 2.881(7) A˚, respectively, comparing well with the Sm-Cl lengths of 2.8230(6) A˚ in [Cp*2Sm(μ-Cl)2AlEt2].50 The Al11-Cl11 bond length of 2.243(7) A˚ is also comparable to the analogous Al-Cl distance of [Cp*2Sm(μ-Cl)2AlEt2] (2.2638(11) A˚) and also those in bis(imino)pyridine chromium catalysts.50,52 In addition, the Cl11Al11-Cl21 angle of 93.4(3)° is smaller than that in the chloride-bridged dimer [AlCl2CH3]2 (the Cl-Al-Cl angle in this case is 98°); this distortion is likely due to the crowded pentagonal plane observed in complex 5.54

Discussion Formation of Complex 4. The anticipated dialkyl product of the reaction of [{tBuNON}UCl2] and LiCH(SiMe3)2 was (50) Evans, W. J.; Champagne, T. M.; Giarikos, D. G.; Ziller, J. W. Organometallics 2005, 24, 570. (51) Vidyaratne, I.; Scott, J.; Gambarotta, S.; Duchateau, R. Organometallics 2007, 26, 3201. (52) Feghali, K.; Harding, D. J.; Reardon, D.; Gambarotta, S.; Yap, G.; Wang, Q. Organometallics 2002, 21, 968. (53) Iovu, H.; Hubca, G.; Racoti, D.; Hurst, J. S. Eur. Polym. J. 1999, 35, 335. (54) Cesari, M.; Pedretti, U.; Zazzetta, A.; Lugli, G.; Marconi, W. Inorg. Chim. Acta 1971, 5, 439.

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Figure 5. Molecular structure and numbering scheme for 5 (ORTEP 40% probability ellipsoids). Hydrogen atoms and rotational disorder among the Et2AlCl2 groups have been excluded for clarity. Selected bond lengths (A˚) and angles (deg): U1-N1, 2.176(6); U1-O1, 2.435(8); U1-Cl11, 2.865(6); U1-Cl21, 2.881(7); Al11-Cl11, 2.243(7); Al11-Cl21, 2.233(7); Al11-C21, 1.996(8); Al11-Cl31, 1.989(8); N1-U1N10 , 129.3(3); N1-U1-O1, 64.67(16); Cl11-U1-Cl21, 69.07(16); U1-Cl11-Al11, 98.8(2); Cl11-Al11-Cl21, 93.4(3).

Hayes and Leznoff

polymers are completely soluble at 150 °C in trichlorobenzene with 0.01% butylhydroxytoluene for GPC analysis. By comparison with [Cp*2ThMe]þ both the Mw and Mn of polymer generated by 1 are considerably lower and the polydispersity index from polymer of 1 is higher than that from [Cp*2ThMe]þ as well.11 Comparison with {CyNON}Zr(CH2Ph)2 shows the Mw of polymer generated by 1 is lower by an order of magnitude, but the polydispersity is better (the PDI of polymer generated by {CyNON}Zr(CH2Ph)2 is 2.8). Information for the polymer prepared using the analogous {tBuNON}Zr(CH2Ph)2 system is unreported.21 The use of B(C6F5)3 cocatalyst with 1-4 resulted in significant drops in activity (Table 1). With many early transition-metal based catalyst systems and actinide Cp* systems the anionic B(C6F5)3R- (R = abstracted alkyl ligand) competes for the active polymerization site on the cationic metal, occasionally slowing the activities of these catalysts considerably, as is possibly the case here.16 The use of B(C6F5)3 with precatalysts in nonaromatic hydrocarbon solvents leads to complete abstraction of the alkyl substituent and an interaction between the newly formed alkyl-boron species and the cationic metal, as is the case with [{XA2}Th][PhCH2B(C6F5)3]2, which also prevents ethylene polymerization.41 Another potential issue with the use of this fluoro-boron reagent arises from the use of aromatic hydrocarbon solvents; here the solvent may interact with the cationic species that is produced by the abstraction of an alkyl ligand, reducing or halting the catalyst’s ability to polymerize a substrate. This is the case for the amidoactinide cation [{XA2}Th(CH2Ph)(η6-C6H5Me)][(B(C6F5)4)], where a toluene molecule blocks off the open coordination site on the metal to prevent ethylene polymerization.56 Any of these situations may be responsible for the reduction in catalyst activity for compounds 1-4 with this activator. The structure of 5 provides a clear reason for catalyst deactivation with the use of aluminum-chloride-based reagents. Complete replacement of the U-C bonds with U-Cl bonds eliminates the requisite metal-alkyl site for ethylene insertion and chain growth, thereby deactivating the catalyst.

not formed. Instead, in order to relieve the extreme steric pressure provided by the additional bulk from the bis(trimethylsilyl)methyl ligands, the complex underwent γC-H activation of one silyl methyl substituent to yield an alkyl-bridged uranium dimer (Scheme 1a). The impact of the remaining steric bulk around the uranium is still evident in the extended bond lengths in the resulting compound 4: The U-N and U-O lengths in 4 are longer than in other {tBuNON}U-based compounds, as are both the terminal and bridging U-C bond lengths. While this is the first example of full C-H activation of the γ-hydrogens in the CH(SiMe3)2 ligand, there is precedence for γ-C-H agostic interactions when this ligand is used in conjunction with group 3 and lanthanide metals, particularly when other bulky ancillary ligands are employed.46,47 Other actinide complexes have been known to undergo similar C-H activations to form four-membered metallocycles, such as the formation of Cp*2Th(CH2)2SiMe2, which results from the heating of Cp*2Th(CH2SiMe3)2.55 Ethylene Polymerization. The activity of the catalyst systems presented in Table 1 indicates two trends in reactivity. The first is the difference in activity between different diamido ligands, and the second is the difference in activity between complexes containing the same backbone but different alkyl substituents. In addition, with longer reaction times the activity of complex 1 (as a representative example) increased substantially. A sample of polyethylene produced by catalyst 1 was analyzed by GPC in 1,3,5-trichlorobenzene and indicates a high molecular weight polymer with an Mp of 2.7 104 and a low molecular weight shoulder (Figure SI-1). The Mw of 2.4 104 and Mn of 8.9 103 suggest a PDI of 2.7 but are calculated from the entire molecular weight distribution and so are not indicative of the majority of the polymer sample. This information is consistent with the insolubility of the polymers produced by catalysts 1-4 in the majority of organic solvents, including 1,2-dichlorobenzene and chloroform even at elevated temperatures; fortunately these

Uranium(IV) dialkyl complexes 1-4 were prepared from the corresponding uranium(IV) dichloride precursors by reaction with LiCH2SiMe3, KCH2Ph, or LiCH(SiMe3)2. Complex 3 demonstrates an η1-benzyl, η2-benzyl arrangement of the alkyl substituents, while 4 provides the first example of γ-C-H activation of a CH(SiMe3)2 ligand. All four complexes catalyze the polymerization of ethylene as single-component catalysts but are less effective when cocatalysts are added. The isolation of coordinately saturated, metal-alkyl-free 5 from the reaction of 1 with an excess of Et2AlCl provides an explanation for the drop in catalytic activity upon addition of this particular reagent. Now that it is clear that amidoactinide alkyl complexes can act as ethylene polymerization catalysts, modifications to the diamido ligands are worth exploring as a means of improving the catalytic activity of these complexes. The preparation of the thorium analogues of compounds 1-4 is underway, in anticipation that their higher Lewis acidity will yield more active catalysts.

(55) Bruno, J. W.; Smith, G. M.; Marks, T. J.; Fair, C. K.; Schultz, A. J.; Williams, J. M. J. Am. Chem. Soc. 1986, 108, 40.

(56) Cruz, C. A.; Emslie, D. J. H.; Robertson, C. M.; Harrington, L. E.; Jenkins, H. A.; Britten, J. F. Organometallics 2009, 28, 1891.

Conclusions

Article


Experimental Section General Procedures. All techniques and procedures were carried out under a nitrogen atmosphere either with an Mbraun Labmaster 130 glovebox or using standard Schlenk and vacuumline techniques. All glassware was dried overnight at 160 °C prior to use. Toluene, tetrahydrofuran (THF), and diethyl ether were distilled from a sodium/benzophenone under nitrogen. Hexanes were distilled from sodium under nitrogen. Pentane was dried over sodium under a nitrogen atmosphere. Deuterated solvents were freeze-pump-thawed three times and dried over sodium. Uranium tetrachloride,57 {tBuNHSiMe2}2O,19,21 {(2,6-iPr2C6H3)NH(CH2CH2)}2O,20 [{tBuNON}UCl2]2,33 [{tBuNON}U(CH2SiMe3)2] (1),33 [{dippNCOCN}UX2] (X = Cl, CH2SiMe3 (2)),34 KCH2Ph,58 and LiCH(SiMe3)259 were prepared in accordance with published literature procedures. Pentane was removed in vacuo from LiCH2SiMe3 (1.0 M, Aldrich) prior to use. B(C6F5)3 was purchased from Strem and used without further purification. MMAO (10% w/w in toluene) and Et2AlCl (1.0 M in hexanes) were purchased from Aldrich and used without further purification. UHP grade ethylene was purchased from Matheson and used without additional purification for all polymerization reactions. All other chemicals were purchased from commercial sources and used without further purification. NMR spectra were recorded at 294 K, unless otherwise stated, on a 500 MHz Varian Unity spectrometer or a 600 MHz Bruker Avance spectrometer; variable-temperature NMR was performed on a 500 MHz Bruker Ultrashield Plus. All 1H NMR shifts are reported in ppm relative to the impurity of internal solvent: specifically, d6-benzene at δ 7.15 or d8-toluene at δ 2.06. Elemental analyses (C, H, N) were performed at Simon Fraser University by Mr. Farzad Haftbaradaran employing a Carlo Erba EA 1110 CHN elemental analyzer. Analysis of the polyethylene produced by catalyst 1 was performed by PolyAnalytik Inc. of London, Ontario, using triple detection GPC in 1,3,5-trichlorobenzene and butylhydroxytoluene at 150 °C. [{dippNCOCN}U(CH2Ph)2] (3). [{dippNCOCN}UCl2] (0.150 g, 0.20 mmol) was suspended in 20 mL of hexanes and stirred at room temperature. Two equivalents of KCH2Ph (0.053 g, 0.41 mmol) was suspended in 10 mL of hexanes and transferred quantitatively to the [{dippNCOCN}UCl2] slurry, upon which the mixture immediately turned dark orange-red; stirring was continued at room temperature for 14 h. During this time the reaction became dark red-brown in color. The mixture was then filtered through a Celite-padded medium-porosity glass frit to yield a clear dark red solution. Excess hexanes were removed in vacuo to yield a bright red powder of [{dippNCOCN}U(CH2Ph)2] (3). Yield: 0.106 g (61%). Dark red, block-shaped crystals were grown by slow evaporation of a pentane solution. Anal. Calcd for C42H56N2OU: C, 59.84; H, 6.70; N, 3.32. Found: C, 59.85; H, 7.00; N, 3.01. 1H NMR (d6-benzene, 295 K): δ 79.83 (br s, 2H), 56.50 (br s, 2H), 46.09 (br s, 2H), 28.30 (br s, 2H), 21.31 (s, 8H, CH(CH3)2), 15.25 (s, 6H, CH(CH3)2), 3.00 (s, 9H), -3.73 (s, 10H, CH(CH3)2), -6.37 (s, 4H), -14.01 (s, 1H), -22.85 (br s, 2H), -42.75 (br s, 2H), -49.21 (br s, 2H), -73.46 (br s, 2H, η2-CH2Ph), -125.92 (br s, 2H, η1-CH2Ph). 1H NMR (d6-benzene, 223 K): δ 120.09 (br s, 2H), 84.54 (br s, 2H), 68.02 (br s, 2H), 41.11 (s, 2H), 33.17 (s, 6H, CH(CH3)2), 24.83 (s, 2H), 22.96 (s, 2H, CH(CH3)2), 2.58 (s, 6H, CH(CH3)2), 2.19 (s, 3H), -4.41 (s, 2H), -9.30 (s, 2H), -11.17 (s, 2H), -12.48 (s, 2H, CH(CH3)2), -20.38 (s, 2H), -23.22 (s, 2H), -29.54 (br s, 2H), -61.68 (s, 2H), -68.40 (s, 2H), -137.19 (br s, 2H, η2-CH2Ph), -187.72 (br s, 2H, η1-CH2Ph). 1H NMR (d6-benzene, 313 K): δ 71.22 (br s, 2H), 49.97 (br s, 2H), 41.01 (br s, 2H), 25.58 (br s, 2H), 18.40 (br, s, 6H), 13.39 (br s, 6H), (57) Hermann, J. A.; Suttle, J. F. Inorg. Synth. 1957, 5, 143. (58) Bailey, P. J.; Coxall, R. A.; Dick, C. M.; Fabre, S.; Henderson, L. C.; Herber, C.; Liddle, S. T.; Lorono-Gonzalez, D.; Parkin, A.; Parsons, S. Chem. Eur. J. 2003, 9, 4820. (59) Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 10358.

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-1.72 (br s, 8H), -23.29 (br s, 2H), -39.13 (br s, 2H), -44.13 (br s, 2H), -59.56 (br s, 2H), -111.98 (br s, 2H). 1H NMR (d6-benzene, 373 K): δ 35.26 (br s), 18.02 (br s), 8.22 (v br s), 5.53 (v br s), -3.07 (v br s), -32.04 (br s); integrations at 373 K are not reported due to extreme broadness limiting their reliability. [{tBuNON}U{CH(SiMe3)(SiMe2CH2)}]2 (4). [{tBuNON}UCl2] (0.150 g, 0.45 mmol) was suspended in 20 mL of hexanes and stirred at room temperature. Two equivalents of LiCH(SiMe3)2 (0.085 g, 0.90 mmol) was dissolved in hexanes and transferred quantitatively to the [{tBuNON}UCl2] slurry. The mixture immediately turned bright yellow; stirring was continued at room temperature for 14 h. During this time the reaction turned bright orange-yellow. The mixture was then filtered through a Celitepadded medium-porosity glass frit to yield a clear orange-yellow solution. Excess hexanes were removed in vacuo to yield a bright orange powder of [{tBuNON}U{CH(SiMe3)(SiMe2CH2)}]2 (4). Yield: 0.141 g (82%). Orange, plate-like crystals were grown by slow evaporation of a hexanes solution. Anal. Calcd for C38H96N4O2Si8U2: C, 34.01; H, 7.21; N, 4.18. Found: C, 34.23; H, 7.59; N, 3.87. 1H NMR (d6-benzene): δ 43.60 (br s, 9H), 40.69 (s, 2H, SiMe2CH2), 21.95 (s, 3H), -10.37 (s, 3H), -14.04 (s, 3H), -18.50 (s, 3H), -23.52 (s, 9H), -29.53 (s, 3H), -31.05 (s, 3H), -47.83 (s, 9H), -139.29 (br s, 1H, CH(SiMe3)(SiMe2CH2)). [{tBuNON}U{(μ-Cl)2AlEt2}2] (5). [{tBuNON}U(CH2SiMe3)2] (0.057 g, 0.073 mmol) was dissolved in 20 mL of hexanes, and Et2AlCl (0.80 mL, 0.727 mmol, 1.0 M in hexanes) was added via syringe to the solution at room temperature. The resulting solution remained brown in color and was freeze-pumpthawed for one cycle to remove excess nitrogen before being exposed to 1 atm of ethylene for 12 h. The reaction still appeared dark brown in color, and no precipitates were formed during this time period. Excess hexanes were removed in vacuo to yield a dark red oil. Overnight the oil separated into pale green crystals and remnant red oil. The crystals were isolated by quickly redissolving them in cold pentane, decanting, and allowing the pentane solution to evaporate. Anal. Calcd for C20H50N2Al2Cl4OSi2U 3 1/2(C5H12): C, 31.40; H, 6.56; N, 3.25. Found: C, 31.38; H, 6.85; N, 3.85. 1H NMR (d6-benzene): δ 95.59 (s, 20H, C(CH3)3), -8.05 (br s, 13H, -CH2CH3), -13.19 (br s, 8H, -CH2CH3), -16.99 (s, 18H, Si(CH3)2). Ethylene Polymerization. In a typical reaction without activating agent the selected uranium complex was dissolved in 30-40 mL of hexanes. The mixture was degassed via one freeze-pump-thaw cycle and polymerization initiated by introducing ethylene at 1 atm to the solution at room temperature. After 12 h the reaction was quenched by venting excess ethylene and addition of 20 mL of acidified methanol (10% concentrated hydrochloric acid in methanol). All precipitates were collected on a frit filter, washed with methanol, and dried in vacuo. For reactions where Et2AlCl was used as the activating agent, the alkyl aluminum was added via syringe to the initial solution of uranium dialkyl. For reactions that used B(C6F5)3 as the activating agent, the boron reagent was added to a solution of the uranium complex in toluene to allow complete dissolution of both reagents. Polymerization reactions were then carried out as for all other reactions. X-ray Crystallography. Crystallographic data for all structures are collected in Table SI-4. The crystals were sealed in capillaries under nitrogen before being mounted in a metallic pin with epoxy adhesive. Crystal descriptions for each compound are as follows. 3 is a red rectangular block with dimensions of 0.20 0.35 0.50 mm3; 4 is an orange triangular plate with dimensions of 0.13 0.45 0.65 mm3; 5 is a green rectangular plate with dimensions of 0.5 0.4 0.05 mm3. In the case of 4 the crystal was then transferred to the cold stream of the X-ray diffractometer. All measurements were collected on a Bruker SMART APEX II diffractometer using monochromated Mo KR radiation (λ = 0.71073 nm). 4 was collected at -150 °C. Data reduction and absorption correction details can be found in the crystal information file (Supporting Information).

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The structures were solved using direct methods (SIR 92) and refined by least-squares procedures using CRYSTALS.60 Hydrogen atoms on carbon atoms were included at geometrically idealized positions (C-H bond distance 0.95) and were not refined. The isotropic thermal parameters of the hydrogen atoms were fixed at 1.2 times that of the preceding carbon atom. The plots for the crystal structures were generated using ORTEP 3.61 The thermal ellipsoids in the ORTEP drawings are shown at the 40% probability level. For 3, coordinates and anisotropic displacement parameters for all non-hydrogen atoms were refined. For 4, coordinates and anisotropic displacement parameters for all non-hydrogen atoms, with the exception of two tert-butyl groups of the {tBuNON} ligand and the two unactivated silyl-methyl groups of the CH(SiMe3)2 ligand, were refined. These groups were found to be rotationally disordered and treated accordingly. The Flack enantiopole parameter62 (0.306(9)) was included in the final cycles of refinement of 4. Due to the geometry around (60) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Prout, K.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487. (61) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565. (62) Flack, H. Acta Crystallogr. Sect. A 1983, 39, 876.

Hayes and Leznoff C3 and C4 of 4 hydrogen atoms were placed at 50% occupancy on both sides of the plane of the carbon atom. For 5, coordinates and anisotropic displacement parameters for all non-hydrogen atoms, with the exception of the aluminum ethyl groups, were refined. The (μ-Cl)2AlEt2 groups are 45:55 disordered over two positions.

Acknowledgment. This work was supported by NSERC of Canada and SFU. C.E.H. is grateful to Natural Resources Canada for an Internship and to the province of British Columbia for a Pacific Century Graduate Scholarship. The authors thank Dr. Michael J. Katz (Simon Fraser) for assistance with several X-ray structural determinations. Supporting Information Available: Complete crystallographic data in cif format for all three reported crystal structures, selected bond length and angle tables, and table of crystallographic data summary, and the GPC diagram for the molecular weight distribution of polyethylene produced by catalyst 1. This material is available free of charge via the Internet at http://pubs.acs.org.

Alkyl Complexes as Single-Component Ethylene Polymerization

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